A “Thematic Network”, funded by the EC under Framework 5 – EVR1-CT-2001-20009

GAMBLEGlobal Altimeter Measurements By Leading Europeans

Requirements for Future Satellite Altimetry:Recommendations for Missions and Research Programmes

Principal AuthorsDavid Cotton, Tom Allan, Satellite Observing Systems, UK

Yves Menard, Centre National d´Études Spatiales, FrPierre-Yves le Traon, Collecte Localisation Satellites, Fr

Luigi Cavaleri, Istituto per lo Studio della Dinamica delle Grandi Masse, ItEelco Doornbos, Delft University of Technology, NL

Peter Challenor, Southampton Oceanography Centre, UK

http://www.altimetrie.net

GAMBLEGlobal Altimeter Measurements By Leading Europeans

The GAMBLE Executive Summary is published as a separate document

This report should be referred to as Cotton et al (2004)

February 2004

Requirements forAltimeter DataSea Surface HeightOperational oceanography systems beingdeveloped within the European GMESprogramme (MERSEA, MFSTEP), will requireaccurate measurements of ocean eddiesand associated currents - the ocean“mesoscale”. The TOPEX/Poseidon+ERS(Jason-1+ENVISAT) configuration is aminimum configuration for observing thismesoscale. For the post Jason-1 period(2007 >), dual satellite coverage is aminimum requirement. For full resolution of eddies, measurements must resolvevariability on the scale of the “Rossby” radius(~25 km). Three/four interleaved Jason-1 (orthree ERS/ENVISAT) will allow a very goodmapping of sea level and velocity. Comparedto the T/P+ERS combination, improvementby a factor of 3 (more than 10 compared toT/P) would be achieved. Velocity mappingerrors will remain, however, at the level of~20% of the signal variance because ofvariability on small temporal and spatialscales. Much higher sampling rates in time(<5 days) and space (~50km) are requiredto map high frequency signals such as these.This would require constellations of six ormore microsats and/or the application ofwide swath techniques.

Why GAMBLE?Measurements from satellite radar altimetershave revolutionised our knowledge of theocean, through studies in sea level, oceancirculation and climate variability. Satellitealtimetry is recognised as an essentialcomponent of global ocean observingsystems under programmes such asGODAE, CLIVAR and GOOS, and throughits ability to provide stable and accuratelong term monitoring of sea-level change.

Altimetry also lies at the centre of recentEuropean developments in operationaloceanography, such as MERCATOR,MERSEA, FOAM, TOPAZ, and MFSTEP. In addition altimeter wave data are routinelyassimilated into wave forecast models,providing support to offshore operationsaround the world.

The GAMBLE thematic network has broughttogether European experts in ocean altimetryto consider future developments in satellitealtimetry - with the aim of providingrecommendations for research activities,and future altimeter missions, that arenecessary to support and build on recentdevelopments in operational oceanographyand to maintain ocean monitoringprogrammes.

Over a period of 21 months GAMBLE helda series of workshops to identify andprioritise requirements of users, establishthe state of the art in altimetric methodsand technology, and so arrive at well-founded recommendations for futuredevelopments.

3

4

5

6

7

8

9

10

11

12

13

14

15

2

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

Sea StateBoth research and operational applicationsrequire higher sampling to allowmeasurements of severe events. Becauseof the lack of observations, models are notable to provide accurate forecasts, orrepresent the true variability of wave fieldsgenerated by rapidly developing systemssuch as polar lows.

Studies have demonstrated that animprovement in forecasts can be expectedif data from more than one satellite areassimilated into the wave models.

Accurate wave information continues to bethe major requirement for offshore operators.Wave height and period information areparticularly important in new and challengingoperating conditions. Global measurementsof the wave spectrum would also be ofsignificant value to both researchers andoffshore operators.

A better understanding is required ofconditions in which “Rogue” waves aregenerated.

Current designs for wide swath altimetersare able to measure sea surface height butnot wave height or wind speed across theswath. The SWIMSAT concept offers acapability to provide wave spectrummeasurements.

What are theStakes?The launch of an altimeter-carrying ERS-1in 1991, followed in 1992 by the dedicatedaltimeter mission TOPEX/Poseidon,confirmed the remarkable precisiondemonstrated by the pioneering but short-lived NASA spacecraft, Seasat, launched in1978. The radar altimeter record may lackthe glamour of the imagery provided by the synthetic aperture radar, or the vividcontrasts displayed in the imagery of oceancolour scanners, but its relative simplicityof operation coupled with an uninterruptedflow of all-weather, global data, make it oneof the most revealing and versatile of allsensors for monitoring the oceans. Its pulsesreflected from the sea surface can beanalysed to provide accurate informationon the Earth’s gravity field, surface currentsand eddies, wave height and wind speed,and, as we shall see, its performance over river estuaries and lakes is also proving useful in studies of the globalhydrology budget.

What led to the series of GAMBLEWorkshops sponsored by the Commissionwas the realisation by the user communitiesin Europe – research climatologists,oceanographers, forecasters, servicecompanies and marine operators – that thefuture of altimetry was not entirely secure.To some extent it became a victim of itsown success. TOPEX/Poseidon waslaunched in 1992 and was still working wellwhen its successor JASON was launchedin 2001. Likewise, the altimeter carried onERS-2, launched in 1995, was still generatinguseful data when it was replaced byENVISAT in 2002. It almost seemed thataltimeters could go on working forever. That,of course, is not the case.

It was becoming increasingly obvious thatalthough the precision of the altimeter’ssignal along its nadir track proved sufficientfor most practical purposes, its sampling ofocean features was quite inadequate whereconsecutive passes could be over 3,000kmapart. At the same time came the realisationthat not only were there no firm plans toincrease the number of platforms, but therewas a chance that that number could soonreduce to one (JASON-2) with all theconsequential risks, both to global researchprogrammes and operational opportunities,should it fail.

The user communities became seriouslyalarmed and suggested a series ofWorkshops to discuss cost-effective options.This GAMBLE was funded by the researchdirectorate of the European Commissionthrough a Framework V Thematic Network,and this is a report of their findings and mainrecommendations.

Dissemination of resultsThe GAMBLE recommendations for researchwill be taken forward in a number of projects,under the auspices of EC Frameworkprojects, and the EC/ESA GMESprogramme. The mission recommendationswill be presented to a wide range oforganizations and key players in theEuropean ocean and space communities.The authors would welcome comments andsuggestions. Please refer to the project website – http://www.altimetrie.net for furtherinformation, and for access to the full reports.

Keywords: Satellite altimetry, currents, seastate, climate monitoring.

3

4

5

6

7

8

9

10

11

12

13

14

15

2

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47AcknowledgementsThe authors would like to thank all membersof the GAMBLE team for their enthusiasticparticipation in this activity. They would alsolike to record their appreciation to themembers of the Steering Group for theirvaluable comments, and to the other expertsexternal to GAMBLE who providedcontributions to workshops and discussions.

The GAMBLE coordinators also wish toexpress their thanks to the EuropeanCommission for supporting this initiativeand, in particular, to the officer assigned tothe project (Alan Edwards) without whosepatient, helpful, and sensible guidance theGAMBLE project would not have been thesuccess it has proved to be.

The GAMBLE teamName Affiliation Role

Tom Allan, David Cotton Satellite Observing Systems, UK Contractor

Yves Menard, Patrick Vincent, Eric Thouvenot Centre National des Etudes Spatiales, France Contractor

Eelco Doornbos, Ron Noomen, Pieter Visser Delft University of Technology, The Netherlands Contractor

Luigi Cavaleri Istituto per lo Studio della Dinamica delle Grandi Masse, Italy Contractor

Peter Challenor, Christine Gommenginger, David Woolf Southampton Oceanography Centre/James Rennell Division, UK Member

Pierre-Yves Le Traon, Fabien Lefevre, G Dibarboure Collecte Localisation Satellites Space Oceanography Division, France Member

Philip Moore University of Newcastle, UK Member

Laurent Phalippou ALCATEL Space Industries, France Member

Daniele Hauser Centre d’Etudes des Environnements Terrestre et Planetaires, France Member

Pierre Bahurel, Laurent Kerleguer Service Hydrographique et Oceanographique de la Marine, France Member

Jacques Verron. J Branckart, F Debost Laboratoire des Ecoulements Geophysiques et Industriels, France Member

Christian Le Provost, Baptise Mourre. Laboratoires d’Etudes en Geophysique et Oceanographie Spatiale, France Member

Phil Woodworth Proudman Oceanographic Laboratory, UK Member

Alex da Silva Curiel, Max Meerman Surrey Satellite Technology Ltd. , UK Member

Detlev Mueller Max-Planck-Institut für Meteorologie, Germany Member

Keith Haines Environmental Systems Science Centre/The University of Reading, UK Member

Johnny Johannessen Nansen Environmental and Remote Sensing Center, Norway Member

Philippe Dandin Météo France Steering Group

Phillipe Gaspar Collecte Localisation Satellites, France Steering Group

Colin Grant BP, UK Steering Group

Trevor Guymer IACMST / EuroGOOS Steering Group

Giuseppe Manzella ENEA, It Steering Group

Chris Shaw Shell, UK Steering Group

John Thompson Oceanroutes, UK Steering Group

C. K. Shum University of Ohio, USA Presentation

Drazen Svehla Technical University of Munich, Germany Presentation

Johannes Guddall, Bjorn Age Hjollo Meteorologisk Institutt, Norway Presentation

Peter Janssen ECMWF Presentation

Jean-Michel Lefevre Météo France Presentation

Susanne Lehner DLR, Germany Presentation

Francisco Ocampo Torres CICESE, Mexico Presentation

Mark Calverly Fugro GEOS, UK Presentation

Jim Gunson UK Met. Office Presentation

Mark White Nowcasting International, Ireland Presentation

Jerome Benveniste ESA/ESRIN Presentation

Eric Lindstrom NASA, USA Presentation

Philippa Berry De Montfort University, UK Presentation

Ernesto Rodriguez Jet Propulsion Laboratory, NASA, USA Presentation

Olivier Germain STARLAB, Spain Presentation

It is with deep regret that we record the passing away of Christian Le Provost, a key member of the GAMBLE team, which happened

just as this report was being finalized. Christian has been a major actor in ocean altimetry for the past 25 years, and made unique

contributions to TOPEX/Poseidon and Jason-1 projects. Christian's warm and friendly personality will be sorely missed.

3

4

5

6

7

8

9

10

11

12

13

14

15

2

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47Table of contents

Glossary 8

1 Introduction 101.1 GAMBLE Overview 101.2 The Past 101.3 Future Prospects 111.4 European Social and Policy issues 111.5 The “GAMBLE” Approach 13

2 User Requirements for Satellite Altimeter Data 142.1 Sea Surface Height Features 142.2 Sea State Error Budgets and Feature Detectability 192.3 Precise Orbit Determination and Range Corrections 242.4 Offshore Operator Requirements for Altimeter Derived Data 272.5 Framework for Future Research 30

3 Future Strategy 353.1 Capabilities of Available and New Altimeter Technology 303.2 Recommendations for Future Altimeter Missions 38

4 Conclusions 444.1 General 444.2 Links to The European GMES programme 444.3 Research 444.4 Future Missions to Increase Sampling 454.5 The Last Word 45

5 References 46

Appendix - Links to GAMBLE Documents 47

GlossaryAltiKa Ka band altimeter, proposed by CNES

ASAR Advanced Synthetic Aperture Radar

(carried on ENVISAT)

CETP Centre d’Etude des Environnements

Terrestre et Planétaires, France

CHAMP Challenging Mini-Satellite Payload,

Research satellite

CICESE Centro de Iinvestigacion Cientifica

y de Educacion Superior de

Ensenada (Mexico)

CLIVAR International Programme on Climate

Variability and Predictability

CLS Collecte Localisation Satellites,

France

CNES Centre National D’ Etudes Spatiales,

France

CNRS Centre National de la Recherche

Scientifique, France

CRYOSAT A satellite altimeter mission to monitor

ice sheet thickness

DGFI Deutsches Geodätisches

Forschungsinstitut, Munich, Germany

DIODE “Détermination Immédiate

d'Orbite par Doris Embarqué”

immediate onboard orbit determination

by DORIS

DLR Deutsches Zentrum für Luft und

Raumfart (German Space Agency)

DORIS Precise Orbit Determination system,

on TOPEX and Jason

DUACS “Developing Use of Altimetry for

Climate Studies” – A Near Real time

altimeter data processing system

based at CLS, France

EC European Commission

ECMWF European Centre for Medium-Range

Weather Forecasts

ENEA Italian National Agency for New

Technologies, Energy and the

Environment

ENVISAT European Environment Monitoring

Satellite, launched 2002

ERA ECMWF Re-Analysis. 15 year

atmospheric hind-cast

ERS-1 1st European Remote Sensing Satellite

(launched 1991)

ERS-2 2nd European Remote Sensing

Satellite (launched 1995)

ESA European Space Agency

ESF European Science Foundation

ESRIN ESA Space Research Institute.

(Frascati, Italy)

ESSC Environmental Systems Science

Centre, Reading, UK

EUMETSAT European Agency for operational

meteorological satellites

EuroGOOS European Contribution to the Global

Ocean Observing System

FOAM Forecasting Ocean Assimilation Model.

Ocean circulation model at the UK

Met. Office

GANDER Proposal for a constellation of wave

measuring Micro-satellite altimeters

Geosat US Navy Altimeter Satellite

(1985-90)

GFO Geosat Follow-On - Follow on to

Geosat (1998-)

GIM Global Ionosphere Map

GMES Global Monitoring for Environment

and Security – A major joint EC/ESA

programme

GNSS Global Navigation Satellite System

GOCE Gravity Field and Steady-State

Ocean Circulation Mission, ESA

mission planned launch 2006

GODAE Global Ocean Data Assimilation

Experiment

GOOS Global Ocean Observing System

GPS Global Positioning System

GRACE Satellite mission to improve mapping

of the Earth’s gravity field, launched

in 2002

Hmax Maximum Wave Height

Hs Significant Wave Height

IACMST (UK government) Inter-Agency

Committee on Marine Science and

Technology

IAS – (PG) International Altimeter Service –

(Planning Group)

ILRS International Laser Ranging Service

IPCC Intergovernmental Panel on

Climate Change

IRI International Reference Ionosphere

ISDGM Istituto per lo Studio della Dinamica

delle Grandi Masse

Jason-1 Ku/C band altimeter launched in 2001

by CNES/NASA

Jason-2 Successor to Jason-1, Planned launch

2007 (also OSTM)

JCOMM Joint WMO-IOC Technical Commission

for Oceanography and Marine

Meteorology

JPL Jet Propulsion Laboratory (NASA).

LEGI Laboratoire des Ecoulements

Geophysiques et Industriels, France

LEGOS Laboratoires d’Etudes en

Geophysique et Oceanographie

Spatiale, France

MERCATOR A French operational oceanography

programme

MFS Mediterranean Forecasting System.

MPI Max-Planck-Institut für Meteorologie,

Germany

NASA (USA) National Aeronautics and Space

Administration

NCEP (USA) National Centers for

Environmental Prediction

NERSC Nansen Environmental and Remote

Sensing Centre, Bergen, Norway

NOAA (USA) National Oceanographic and

Atmospheric Administration

NPOESS National Polar-Orbiting Operational

Environmental Satellite System (USA)

OGP International Association of Oil and

Gas Producers

OSDR Operational Sensor Data Record (refers

to Jason data)

OSTM Ocean Surface Topography from Space

Mission – see Jason-2

PDF Probability distribution function

POL Proudman Oceanographic

Laboratory (UK)

POD Precise Orbit Determination

Poseidon A solid state Ku band altimeter carried

as an experimental instrument on

TOPEX.

Poseidon2 Upgrade of POSEIDON adopted as the

main altimeter on Jason.

PRARE Precise range and range rate

equipment. Orbit determination

instrument on ERS-1 and 2

PROTEUS ALCATEL mini satellite platform

RMS Root Mean Square (a measure of data

scatter)

SAR Synthetic Aperture Radar

Scatterometer Satellite radar instrument to measure

ocean surface wind

SHOM Service Hydrographique et

Oceanographique de la Marine

SIRAL Advanced interferometric, synthetic

aperture radar altimeter, developed

for Cryosat

SLA Sea Level Anomaly

SLR Satellite Laser Ranging

SOC Southampton Oceanography Centre

SOS Satellite Observing Systems (UK)

σ0 Surface Radar Backscatter

SSH Sea Surface Height

SSTL Surrey Satellite Technology Limited

(UK)

SWH, Hs Significant Wave Height.

SWIMSAT A proposal for a satellite borne Wave

Measuring Radar.

TMR TOPEX Microwave Radiometer

Tz Zero Up-crossing Wave Period

TOPAZ “Towards an Operational Prediction

system for the North Atlantic European

coastal Zones” – operational ocean

modelling system for the North Atlantic

and Nordic Seas (Norway)

TOPEX Ku/C band altimeter launched in 1992

by CNES/NASA

TU Delft Delft University of Technology, The

Netherlands

WSOA Wide Swath Ocean Altimeter – a

proposed swath measuring altimeter

UKMO United Kingdom Meteorological Office

U10 Wind speed referenced to 10m above

the ocean surface

WAM A widely used computer model for

wave generation, propagation and

dissipation

WITTEX “Water Inclination Topography and

Technology Experiment” – (USA)

proposal for microsatellite borne radar

altimeter using delay Doppler

technique

3

4

5

6

7

8

9

10

11

12

13

14

15

2

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

1 Introduction1.1 GAMBLE OverviewMeasurements from satellite altimeters overthe past ten years have enabled significantadvances in oceanographic research andforecasting, in particular:• A dramatic and unprecedented

improvement in our knowledge of theglobal ocean circulation.

• mm precision measurement of year onyear global sea level change (recognisedas one of the key techniques in themonitoring of climate change) .

• Significant improvements in the accuracyof short-term sea state forecasts.

• An understanding of the characteristicsof global wave climate variability.

Satellite altimetry is now recognised as anessential component of global observingsystems under programmes such asGODAE, CLIVAR and GOOS. Altimetry isalso central to the recent developments inocean modelling such as MERCATOR andTOPAZ, and so lies at the heart of all systemsbeing developed to provide operationalocean forecasting.

Each of these highly important applicationsrequires a reliable and continuous provisionof data from satellite altimeters. Anagreement between NASA, NOAA, andEUMETSAT has secured the future for amission to follow Jason-1, for the 2007-2011 period. However, the long-termprovision of altimetry data is still not assured,and the needs of operational oceanographysystems now being developed will not bemet by present mission plans.

The GAMBLE (Global AltimeterMeasurements By Leading Europeans)thematic network was established toaddress this highly important issue.GAMBLE was supported by the EuropeanCommission’s Framework 5 programmeover a 21 months period.

1.2 The PastAltimeters had flown on satellites before1978 but it was the Seasat mission in thatyear which demonstrated that a radaraltimeter could measure its own heightabove the sea surface to a precision ofaround 10cm from an orbiting height of800km. This meant that if due account weretaken of contributions to the signal fromatmospheric effects and biases at the seasurface – especially from uncertainties inthe orbit, and variations in the gravity fieldover the oceans – the measured sea surfaceslope could be converted to estimates of acomponent of surface geostrophic currents.

On a number of subsequent missionsleading up to Envisat and Jason the precisionof the altimeter was increased to reveal acomplex pattern of sea level anomaliesproduced by the meso-scale eddies(responsible for much of the ocean’s heattransport ) as well as western boundarycurrents and large scale events such asEl Niño.

These are all well documented and thecombined effectiveness and reliability of a sensor that works day and night in allweathers has ensured a continuous seriesof altimeter measurements since 1991.

Satellite altimeters have also provided a continuous time-series of globalmeasurements of sea state, throughmeasurements of wave height and windspeed. These measurements have allowedfor the first time, studies of inter-annualvariability in global wave climate and led toa better understanding of the character ofbetween year variability. These data are nowwidely made use of by the offshore industry(shipping and offshore exploration) throughstatistical climate analyses and assimilationinto met-ocean forecast models.

1.3 Future ProspectsAlready national and international spaceagencies are making plans for the nextgeneration of altimeters and altimeter-carrying spacecraft. For the near future(2007-2011), CNES, NASA, EUMETSAT, andNOAA are planning a joint OSTM/Jason-2mission, which includes a core missionbased on Jason-1 follow on instrumentation,and a demonstration mission involving thenew concept of an Interferometric WideSwath Altimeter.

However, no missions are planned as“follow-ons” to GFO or ENVISAT, until thefirst US “NPOESS” altimeter, due in 2011(labelled as M2 in Figure 1.1). Indeed thereare no plans for the “Precursor” missionindicated in Figure 1.1 This situation haspotentially serious consequences with regardto our ability to monitor long-term oceansignals, and for the viability of “OperationalOceanography” systems now beingdeveloped.

At the time of writing, plans for the NPOESS“M2” altimeter mission are not finalised. Twooptions are being considered: a “free-flyer”or one instrument among several on a large,multi-instrumented, polar orbiting platform.

With regard to other future missions, anumber of proposals are at varying stagesof preparation (see also section 2.5). Theyinclude, from Europe:• Cryosat – An ESA altimeter mission to

measure ice mass balance and variability.Due for launch in 2005.

• SWIMSAT – A wave measuring satelliteradar, which would provide direction andwavelength information (Hauser, 2001).

• AltiKa – Ka band altimeter on a micro-satellite platform (Verron et al. 2001).

• GANDER – A constellation of wave-measuring altimeters on micro-satelliteplatforms (Jolly and Allan, 2000).

1.4 European Social andPolicy issuesEuropean scientists, supported by advancedEuropean technology are playing anincreasingly important role in the search forevidence of a changing environment. Clearly,the protection of European coastlines is oneof the most fundamental social objectivesof the Community, but of equal importancein the shorter-term is the need to increasesafety at sea while also making the Europeanmarine industry more competitive byreducing damage and delays inflicted onships and offshore operations by adverseconditions (e.g. bad weather, unfavourablecurrents). Programmes to improve marinesafety have been a recurring theme in EUpolicy for many years.

Monitoring key features of the environmentin order to provide an ‘early warning’ ofglobal climate change has always been atop priority for the EU. Many would rate thisone of the most important services thatEuropean marine research workers cancontribute to the world. Within 50 – 100years changing sea levels and surfacetemperatures could have a profound effecton Europe’s coastal defences, fisheries,aquaculture, tourism and weather patterns.A recent IPCC report (Gitay et al., 2002)highlights the serious implications of presentclimate trends.

Figure 1.1 Satellite altimeter missions – operational, planned andproposed 1997-2012

Altimetric measurements: SSH, SWH, wind speed at nadirpossible scenario for operational convergence between the USA & Europe

In orbit Approved Planned/pending approval Proposed scenario

3

4

5

6

7

8

9

10

11

12

13

14

15

2

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

Over 90% of the transport of goods in and out of Europe is by sea. Despiteprogress made in forecast models andcommunication systems, storms at sea stilltake their toll in lives, damage and delays.• Some 150 ships of over 500 tonnes are

lost every year; the marine insuranceindustry pays out $2.5 billion, and 24,000lives are lost.

• A recent report by the Food andAgricultural Organisation (FAO) identifiesfishing at sea as the most dangerousoccupation in the world. The causes aretwo-fold - inadequate boats and ignoranceof the conditions that await them.

• The environmental cost of cleaning-up oil spills following avoidable weatherrelated incidents at sea can also be high.In recent years hundreds of million ofEuros have been spent in the clean upoperations following major oil spills inEuropean waters.

Many European countries have highlydeveloped meteorological forecastingcentres and through the use of detailedmodels and high-speed computers,warnings of gales and their progressionacross the oceans are forecast in advance.Weather forecasts may be reliable for mostof the time but the development of thestrongest storms that do most of the damageis difficult to predict accurately byconventional means. Unfortunately, nomatter how good the models or fast thecomputers their accuracy depends on thequality of the input data and this may besparse, especially in bad weather where ithas been shown that quality is diminished,particularly for data from in situinstrumentation.

There has been rapid development inoperational ocean circulation models overthe last 5 years to the point that a numberof organisations now routinely produceweekly forecasts of ocean parametersthroughout the water column. As well asproviding important new insights into meso-scale ocean processes, products from thesemodels have many practical applications –identification and location of ocean eddies,oceanic fronts and areas of up-welling,predictions of oil spill drift and, when coupledwith ecosystem models, predictions ofdevelopment of harmful algal blooms.

A number of questions may be asked. Howcan ocean satellites be made more relevantto the daily routine of marine operations? If they can be designed for a wider usercommunity, is that community prepared topay for information? If so, is there enoughof a commercial market to encourage privateinvestment into what was previously anexclusive public domain? Or shouldinformation on sea state and other featuresof operational value remain the province ofnational meteorological centres and be paidfor from the public purse? GAMBLE did notdirectly address the economic issues (theremay be a case for doing so), but broughttogether scientific and commercial user ofaltimeter derived products with the aim ofidentifying common requirements and soproviding recommendations of realisticoptions for future altimeter missions thatwould serve the needs of both communities.

The need to make industry and academiawork together is placed first on a list ofsocio-economic issues identified in a reportby the European Science Foundation’sMarine Board1. This need was recognised,and addressed, within GAMBLE by thepresence of industry representatives in theSteering Group, and the inclusion of aworkshop to identify the operational needsof industry.

Near real-time operation can only beachieved by a constellation of severalobserving platforms and the only way toafford the number required to (say) track theprogress of storms at sea is to use small,special-purpose platforms. Recent studiescarried out in European centres haveconfirmed that dedicated altimeters canachieve a good performance within theconstraints of power, weight, size andpointing angle imposed by an 80kg micro-satellite. The means are at hand then tomonitor ocean sea state and topography atthe frequency required by marine operations.In themselves, these routine applicationsrepresent a considerable market whichencompasses container ships, tankers,fishing boats, naval vessels, offshoreplatforms and leisure yachts as well as the routing and meteorological forecastingcentres which serve them.

1“Towards a European Marine

Research Area”, European

Science Foundation, Marine

Board, December 2000,

http://www.esf.org/marineboard

The issues addressed by GAMBLE -understanding processes governing globalcurrents, improving short term predictionsof sea state and currents, enhancing globalobserving systems – form the backbone ofthe first scientific challenge identified in theESF Marine Board report. This report alsocalls for the development of infrastructureto support operational oceanography fromsatellites, and an improvement in co-ordination of space based activities toprovide an enhancement of complementarityand competitiveness.

The GAMBLE programme reflects the newawareness in Europe that space policy mustbe shaped by the requirements of a wideruser community. Science - especially thataspect aimed at improving our understandingof the changes taking place in our ownenvironment - will retain a high priority. Butsatellite observations of the oceans havebeen almost exclusively directed in the pastto climate/marine research. By investigatinghow science missions may now be combinedwith operational services that will providebulletins on the sea conditions to supportoffshore operations, Europe will be makingsignificant progress.

1.5 The “GAMBLE”ApproachWithin GAMBLE, SOS and CNES convenedmeetings of some 17 European laboratoriesinvolved in different aspects of satellitealtimetry. First, the team consulted with usersof altimeter data through a series of workshopsin Venice, Southampton, Delft and Stavanger,designed to identify and investigate therequirements that must be met to allowsatellite-borne, polar-orbiting altimeters toresolve key features in the measurement ofboth sea surface height and sea state. Next,the project partners reviewed the technologiesand mission opportunities expected to beavailable over the next 10 years and discussedhow this technology may be best used tosatisfy user requirements. A number ofpossible solutions were considered, includingsupplementing the precise long-term Jason-class missions with a number of relativelyinexpensive micro-satellites, together withthe development of technologicaladvancements such as swath altimetry andwave spectrum measuring radar. Conclusionsand recommendations were then presented

and discussed during a 1-day session whenEuropean researchers were joined byAmerican colleagues at the start of a meetingof the Jason Science Working Team held inNovember 2003 at Arles.

The approach was aligned upon six“themes”, given below with the name of thelead organisation:Theme 1 - Sea Surface height (CLS),at TU Delft, The NetherlandsTheme 2 - Sea State (ISDGM),at ISDGM, Venice, ItalyTheme 3 - Orbit determination (TUD),at TU Delft, The NetherlandsTheme 4 - Marine Operators Requirements(SOS), at NPD, Stavanger, NorwayTheme 5 - Research Programme (SOC)Theme 6 - Constellation Optimisation (CNES)

In the next two sections of this documentwe summarise the findings from each ofthese six themes. Section 2 includessummary reports from Themes 1-5,addressing users requirements and providesrecommendations for future researchactivities. Section 3 reviews the capabilitiesof available and planned technology andprovides recommendations for future satellitealtimeter missions.

Each Working Group prepared a report ofits investigations. Readers who desiregreater detail are referred to the original fullreports, listed below:

Report Title Section Lead Author

Theme 1 Final Reports on Error Budgets /Feature Detectability in Sea Surface Height. 2.1 CLS

Theme 1 Subsidiary Report on TOPEX/Poseidon,Jason-1 Tandem Mission 2.1.4 CLS

Theme 2 Final Report on sea-state error budget/Impact of GAMBLE in sea-state analysisand forecasting 2.2 ISDGM

Theme 3 Final Recommendations for OrbitDetermination and Tracking 2.3 TUD

Theme 4 Report on Marine Operators’ Requirements 2.4 SOS

Theme 1-3 Report on Error Budgets andPotential Solutions (3.1)2 SOS

Theme 5 Framework for RecommendedResearch Programme 2.5 SOC

Theme 6 Orbit RecommendationsSatellite and payload specificationrecommendations 3.2 CNES

Table 1.1 Table of key scientific reports from GAMBLE

2Section 3.1 of this report

summarises the capabilities of

planned and proposed future

missions, but does not re-cap SSH

and sea state error budgets

3

4

5

6

7

8

9

10

11

12

13

14

15

2

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

2.1.1 Objectives and ApproachThe objectives of this work package were tosummarize work that has been carried outto provide recommendations for futuremissions on:• Sampling requirements for SSH (and

surface current) measurements.• SSH error budget (orbit, noise, corrections,

repetitivity, geoid).• Merging methodologies.

Main activities were the presentation anddiscussion of recent research involving theuse of combined data from different altimetermissions with relation to applications (scientificand operational) of ocean sea surface heightmeasurements. These discussions reviewedand built on the work presented at suchmeetings as Jason Science Working Team,and the High Resolution Ocean Topographymeeting held at the University of Marylandin March 2001 (Chelton, 2001).

Aspects discussed included:• Simulation of new altimeter

missions/concepts.• Theoretical analyses of sea level and

velocity mapping capabilities of existingmultiple altimeter missions.

• Analysis of the Wide Swath Ocean Altimetersystem and constellations of 3/4 satellites(AltiKa, Wittex) – (Chelton, 2001).

• Analysis of the potential contribution ofGPS reflected signals.

• Merging of data from existing missions(TOPEX/Poseidon, ERS, GFO, Jason andENVISAT).

In addition contributions were taken fromGAMBLE partners on data assimilation (LEGI,SHOM, NERSC and MPI), feature detection(SOC), and coastal regions (POL, LEGOS).

A joint workshop to consider sea surfaceheight features and orbits and trackingrequirements/capabilities was held at TUDelft. Presentations made at that meetingare available at the GAMBLE web site. Issuesdiscussed at the workshop included:• Overview of new altimeter concepts (Jason-

2, Wide Swath Altimetry, Cryosat, AltiKa,GANDER) – CNES, ALCATEL, SOS, OhioState University.

• Simulations of the contribution of presentand future missions - LEGI, LEGOS, SOC.

2 User Requirements for Satellite Altimeter Data2.1 Sea Surface Height Features(For full report refer to http://www.altimetrie.net/docs/GAMBLE_finalreport_theme1.pdf )

• Refined requirements for sea levelmeasurements, open ocean features, tidal features, and coastal features(particularly addressing sampling issues)– CLS, LEGOS.

• Requirements for Sea Surface HeightMeasurement Errors (general issues, orbitsconsiderations, and issues related tomeasurements from micro-satelliteplatforms) - CLS, CNES, SOC, SOS, DUT.

In addition, CLS compiled a report on theTOPEX/Poseidon – Jason tandem missionto discuss the implications for multi-satellitesampling of features in sea surface height.

2.1.2 Measurement IssuesMean Dynamic TopographyCurrently available geoid models are notsufficiently accurate to provide a usefulestimation of mean dynamic topography.New gravity missions (CHAMP, GRACE andGOCE) will improve the situation. After GOCE(launch 2006), an independent estimation ofthe geoid with an accuracy of 1-2 cm rmsfor scales larger than 100 km should beavailable. Meanwhile, the only solution is toestimate the mean dynamic topography froma combination of in situ and model data, andglobal geoids. The error on the resulting meandynamic topography is typically of 5-10 cmrms; this should improve in the future withthe Argo global array of profiling floats andwith improved data assimilationmethodologies as planned by GODAE.

Mapping and merging of multiplealtimeter missionsMerged multi-satellite altimeter data setsmust be produced to map the meso-scalevariability. An effective methodology is touse the more precise missions (e.g. T/P,Jason-1) as a reference. This reduces theorbit error for the other satellites to a fewcm rms, even if the initial orbit errors are aslarge as 1 m.

Following the homogenization and inter-calibration of altimetric data, the Sea LevelAnomaly (SLA) for the different missions mustbe extracted. These should be calculatedrelative to the same ocean mean using acommon reference surface. The final step isto merge the SLAs from the different missionsvia a mapping or assimilation technique.

0

2

4

6

8

10

12

16

18

20

22

24

Mean error

Standard deviation

T/P GEOSAT ERS T/P ERS T/PGEOSAT

T/PJASON

T/P ERSGEOSAT

T/P ERSJASON

T/P ERSGEOSATJASON

2.1.3 Sea Level MeasurementRequirementsClimate and meso-scale applicationsA generally agreed baseline requirement forfuture altimeter missions is for at least two(preferably three) altimeter missions withone very precise long-term altimeter system.The long-term altimeter system providesthe low frequency and large-scale climaticsignals and a reference for the other altimetermissions. The TOPEX/Poseidon and Jasonseries were designed to meet theseobjectives. The other missions will measurethe higher frequency signals, in particularthe meso-scale signal, which cannot be wellobserved with a single altimeter mission.This does not require precise altimetersystems as most of the altimetric errors(in particular the orbit error) are at longwavelengths and do not significantly impactthe meso-scale signal.

Studies have quantified the meso-scalemapping capability when combining variousexisting or future altimeter missions in termsof sea level anomaly (SLA) and zonal (U) andmeridional (V) velocity. The main results are:• There is a large improvement in sea level

mapping when two satellites are included.Compared to T/P alone, the combinationof T/P and ERS reduces mean mappingerror by a factor of 4 and standarddeviation by a factor of 5. (see Figure 2.1)

• Mapping of the velocity field places higherdemands on sampling. The U and V meanmapping errors are 2 to 4 times largerthan the SLA mapping error. Only acombination of three satellites can provide a velocity field mapping errorbelow 10% of the signal variance.

Data Assimilation PerspectiveData assimilation experiments have beenconducted to examine how well the meso-scale ocean circulation can be determinedfrom multi-satellite configurations.

The comparison between error statisticscalculated for various scenarios and othermission parameters such as altitude, allowsa number of conclusions to be drawn:

• The addition of a second altimeter improvesthe reconstruction of the meso-scalecirculation by 18-28%, depending on themission parameters. The addition of a thirdsatellite makes an additional improvementof 10-16% depending again on the flightconfiguration.

• A 3-day interval between successiveanalyses appears to reveal meso-scalefeatures better than other assimilationperiods (intervals of 1, 3, 10 and 20 dayswere tested).

• With regard to meso-scale features, thescenarios which optimised spatial sampling(interleaved tracks, or space offset)performed very well when compared tothose which optimised temporal sampling(time offset). In contrast, the case of twosatellites flying in parallel (with an offset of0.5° between tracks) seemed less effective.

• The “best” observing scheme may bedependant on whether the variable ofinterest is related to the surface circulation,or to deep ocean fields.

• Determination of dynamical modes withintensifications at intermediate depths will require more than two satellites. In adynamical context, the lack of informationconcerning these modes might affect theglobal 3-D flow field, and therefore mayalso limit the quality of the estimation nearthe surface. The launch of 3000 Argoprofilers will also make a significantcontribution.

In a three satellite constellation at giveninclination, higher altitude orbits support aless effective mapping capability.

Figure 2.1 Mean and standard

deviation of Sea Level Anomaly

(SLA) mapping error for single

and multiple altimeter missions

(Le Traon and Dibarboure, 1999).

Units are in % of signal variance.

The calculation assumes a space

scale of 150 km and a time scale

of 15 days and a noise/signal ratio

of 2%.

3

4

5

6

7

8

9

10

11

12

13

14

15

2

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

Overall, these results confirmed other studieswhich had indicated that a third satellite isrequired to capture the meso-scale featuressatisfactorily. The identification of the optimalconfiguration is not trivial. Assimilationexperiments are being extended over longerperiods to refine these conclusions.

To further improve mapping (required forsome scientific and operational applicationswhich have been proposed), it is necessaryto resolve the high frequency and high wave-number signals, i.e. sample the ocean witha time sampling below 10 days and 100 km.Such a sampling density would require aconstellation of altimeter satellites and/orthe development of different concepts forsatellite altimetry (e.g. wide swathtechniques).

Measurement ErrorsAssuming the Jason series continues toprovide a long-term reference, additionalmeasurement systems do not have toprovide very precise measurements. Resultsderived from these systems will not besensitive to very long wavelengths errors(wavelengths > 5000 km/ 10 000 km) if theJason satellites are used to constrain thelarge-scale (climatic) signals.

The typical amplitude of the meso-scalesignal is 4 to 8 cm rms in the open ocean.A 2 to 4 cm measurement noise (1 secondaverage) is thus satisfactory but a smallernoise will allow a better estimation of thevelocity fields and a detailed analysis of theeddy structure in the along-track direction.Wet troposphere and ionospheric correctionsimpact on medium and large-scale signals.In high precision missions these aremeasured by dedicated instrumentation(radiometer, dual frequency). Studies shouldbe undertaken to quantify the degradationof results for altimeter missions without aradiometer or dual frequency capability.

Coastal ApplicationsMost of the SSH signal associated withenergetic coastal currents have heightdifferences ranging from 2 to 10 cm or more.1-2 cm precision would thus provide a goodresolution of these signals. Degrading that resolution to 5 cm would make theobservation of many coastal features verydifficult. The most demanding constraint

results from the temporal resolutions neededto resolve most of the coastal features: thetemporal revisit of a site must be 1-2 daysfor rapidly changing currents. Sampling at10 km intervals will allow marginal detectionof these features. Thus, it is in the coastaldomain that we find the most strenuousrequirements in terms of space/timeresolution and accuracy; but it is in theseregions that we also find such requirementsmost difficult to satisfy, particularly due tothe contamination of land in the altimeterand radiometer footprints. It is concludedtherefore that altimetry should be combinedwith other technologies that provide moredetailed fields over shelves (SST and oceancolour observation from space, coastalradars, other in situ observation techniques),and through assimilation into coastalcirculation models.

Tidal Studies• Sun Synchronous Orbits

With a sun-synchronous orbit, a satellitealtimeter always observes the solar tidesat the same phase of their period. Thecontribution to the altimeter signal is thenjust an unknown constant. This affects themain solar tide components S1 and S2.

S2 is now better known after analysis of T/P and ERS data, and progress in hydrodynamic modelling and dataassimilation. For S2 the accuracy of recentsolutions is at the cm level over the deepocean. However, these solutions need tobe improved over coastal and shelf regionswhere their accuracy is only of the orderof 20 cm. Complementary studies arerequired for coastal areas, includingsatellite altimetry observation with higher space resolution on a non sun-synchronous orbit.

• Tides over mid ocean ridgesTo fully observe the 2D structure of shortwavelength tidal characteristics due tomid ocean ridges, high-resolution altimetryis needed. Measurements must be at thelevel of accuracy of the on-going altimetermissions T/P and ERS, with a (provisional)space resolution of the order of 10 km.The time resolution for tidal applicationsis not so crucial, so long as the tidalaliasing problem is considered with care(ERS and T/P have known problems).

• Barotropic tides over continental shelvesand near the coastsTidal amplitudes can increase by several metres over shelves andapproaching the coast. Also, horizontalgradients can reach up to several cm per km; and the horizontal patterns inamplitude and phase of the main tidalcomponents are strongly reduced.

High-resolution altimetry can thus help to improve the mapping of the tidalcharacteristics in coastal areas, byresolving their 2D spatial structures andobserving the strong horizontal gradientsin amplitude and phase experienced alongthe coastlines. As above, T/P and ERSlevel of accuracy is required. Spaceresolution must be typically of the orderof 5 km. The time resolution for tidalapplications is not crucial, so long as tidal aliasing is considered.

• Non-linear tides over continental shelvesand near the coastsIn coastal areas, tides are also morecomplex because of non-linear dynamicalprocesses, which distort the tidal waves.These non-linear constituents can be ofthe order of 10 cm. Their patterns are alsomore complex as their frequency is higher.

Although long altimeter records now allowthe amplitude and phase of theseconstituents to be extracted with anacceptable level of accuracy (2.5 cm), theshort wavelength of these higher harmonictidal waves is difficult to resolve by intertrack interpolations. High-resolutionaltimetry is needed to fully map thesenon-linear high frequency tidal waves.The required accuracy and precision arethe same as above (~cm height, 5 kmhorizontal resolution, no major constrainton the time sampling, bearing in mindaliasing requirements). Large swathaltimeter satellite missions, like WSOA onT/P-Jason track, with 13 km resolutionand 150 km swath will allow full coverageof areas such as the North Sea. Althoughthe space resolution will be marginal, suchmeasurements will help to map these non-linear features.

2.1.4 TOPEX/Poseidon /Jason-1 Tandem Mission(For full report see http://www.altimetrie.net/docs/tandem_report_GAMBLE.pdf).

In September 2002 the TOPEX/Poseidonsatellite was moved into an orbit so that thedual orbit configuration of Jason-1 andTOPEX/Poseidon was optimized to measuremeso-scale oceanographic features.

In this configuration both TOPEX/Poseidonand Jason-1 are on a 10-day repeat orbitwith interleaved ground tracks. This providestwice the spatial resolution achieved byTOPEX/Poseidon alone. The ability of thisconfiguration to map meso-scale oceanfeatures is an important consideration whendiscussing possible multi-satellite orbitconfigurations for future missions. To thisend, CLS carried out a study for GAMBLEto analyze the ability of the tandem missionto provide improved mapping of sea leveland ocean circulation variability.

Figure 2.2: Absolute dynamic

topography in the Gulf Stream

region on December 11, 2002

derived from the combination

of Jason-1 and

TOPEX/Poseidon (T/P) (top

figure). Jason-1 and T/P

tracks are superimposed in

white. The middle figure is

from Jason-1 data only. The

bottom figure is the difference

between the Jason-1+T/P

and Jason-1 map.

Comparison with the sea level

observed along an ERS track

(red track) (below).

SS

H (m

)

0.4

0.504

0.608

0.712

0.816

0.92

1.024

1.128

1.232

1.336

1.44

1.544

1.648

2002/12/11 – Jason-1 & T/P

290 295 300 305 310 315

44

42

40

38

36

34

32

30

290 295 300 305 310 315

44

42

40

38

36

34

32

30

2002/12/11 – Jason-1

2002/12/11 –Difference (Jason-1 & TP – Jason-1)

290 295 300 305 310 315

44

42

40

38

36

34

32

30

ERS2 Cycle 078 - Pass 336

30 32 34 36 38 40 42 44Latitude ( ˚ )

0.0

0.5

1.0

1.5

2.0

SS

H (

m )

ERS-2

Jason-1 & T/P

Jason-1

3

4

5

6

7

8

9

10

11

12

13

14

15

2

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

Table 2.1 Indicative measurement requirements for applications of seasurface topography

Application Parameter Indicative Measurement Requirements

Spatial Time Latency AccuracyResolution Resolution

Meso-scale Sea surface 25-50 km 5 day 3 day 2-4 cmvariability topography

Global mean sea Sea surface 100 km 10 day 10 day 0.5 mm yr-1

level change topography

ENSO – seasonal to Sea surface 200 km 5 day 5 day 4 cminterann. prediction topography

Large scale Sea surface 200+ km 10 day 3 day 2 cmvariability topography

Tides Tidal constants 100 km non sun Not 2 cm(Sun synchronous) – sea surface synch. Orbit applicable

height >100 visitsto eachlocation

Coastal features Sea surface 10 km 1-2 day 1 day 1-2 cmtopography

Tides near coasts & Tidal constants 10 km > 100 visits Not 1-2 cmtopography – sea surface applicable

height

Barotropic tides Tidal constants 5 km > 100 visits Not 2 cm– sea surface applicableheight

Non-linear tides Tidal constants 5 km > 100 visits Not 1 cm– sea surface applicableheight

Several months of the TOPEX/PoseidonJason-1 tandem mission were analyzed.The first requirement was to estimate amean track for TOPEX/Poseidon consistentwith a Jason-1 7-year mean. Results fromthe tandem mission were then comparedwith those derived from Jason-1 alone andan external verification was performed withERS-2 data (Figure 2.2). These preliminaryresults are very encouraging anddemonstrate the potential of an optimizedtwo-satellite constellation for the meso-scale circulation monitoring. They alsoconfirmed the results of earlier theoreticalanalyses by Le Traon and Morrow (2001)and Le Traon and Dibarboure (2002). Futurestudies should now be performed over alonger time series (at least one year) andshould also analyze the contribution of thetandem mission together with ERS-2 /ENVISAT and GEOSAT Follow On data.Detailed analysis of velocity mapping error(through the comparison with surface driftersand current-meter moorings) and eddy/meanflow interaction estimations should becarried out. External comparison with veryhigh resolution Sea Surface Temperature orOcean Color images should also beperformed to quantify the capability ofmultiple altimeter configurations to capturesmall space and time scales of the meso-scale variability.

2.1.5 Summary of SSHRequirementsBased on the discussions on requirementsfor Sea Surface Height measurementspresented above it was recommended thata minimum requirement for an operationalmonitoring system would be 2 (preferably3) altimeters, one of which would provide“Jason” class accuracy to provide thereference for others which could be lessprecise and could, for instance, be hostedon micro-satellites. A demonstration of wideswath altimetry would be required beforesuch an instrument could be included in anoperational configuration.

In considering a constellation of altimeters,priority issues are to:• Identify preferred sampling regimes.• Establish how to ensure satisfactory

accuracy from micro-satellite swath rangemeasurements (ionosphere, tropospherecorrections, accurate orbit knowledge).

• Identify preferred orbits.

Table 2.1 provides indicative SSHmeasurement requirements, based on theidentified characteristics of key features.“Latency” refers to the maximum desiredtime lag between the time of themeasurement, and the availability of thedata product to the user.

2.2 Sea State Error Budgets and Feature Detectability(For full report http://www.altimetrie.net/docs/GAMBLE_finalreport_theme2_030126.pdf)

2.2.1 Objectives and ApproachThe objectives of this aspect of the GAMBLEproject were to bring in expert opinion and hold discussions to providerecommendations for future missions on:• Sampling requirements for sea state: wave

parameters including significant waveheight, wave period and direction; oceansurface wind speed; and other air sea fluxparameters including rain rate, air sea gastransfer velocity and wind stress.

• Accuracy and reliability requirements forthese parameters.

• Studies that may be necessary to ensurethat best use is made of altimeter seastate data.

The general approach was to review andassess state of the art knowledge from mostrecent workshops and literature. Membersof the GAMBLE team and external expertshave offered written contributions andworkshop presentations. A workshop washeld at the premises of ISDGM, in Venice.

Contributions from GAMBLE partnersincluded:ALCATEL – Recent and proposeddevelopments in altimeter instrumentation.CETP – SWIMSAT mission proposal andresults from a simulation of assimilation ofwave spectra information into a global wavemodel.CNES – Jason missions, and results fromstudies into Ka band altimetry.ISDGM – Applications of sea state data near the coast and in the Mediterranean Sea.SOC – Presentation of recent studies atSOC, including wave climate variability,investigations to develop new altimeteralgorithms for wind speed and wave period.SOS – Presentation of requirements for seastate data from the operational offshore usercommunity (near real time and climatology).

Contributions from invited experts included:CICESE (Mexico) – Requirements for satellitederived sea state data from countries withoutthe resources to support large integratedmonitoring systems.

ECMWF - Results from studies ofassimilating significant wave height datainto global wave models (estimates of errorin altimeter, buoy and model measurements,the benefits to forecast accuracy ofassimilating data from more than onealtimeter mission), sea state bias theory.Météo France – Studies in comparingperformance of different wave models, and comparing the affect of assimilatingaltimeter significant wave height data, and wave spectra data (as would beavailable from SWIMSAT).Meteorologisk Institutt, Norway –Emphasised the importance of improvingthe forecasting of extreme events that affectvulnerable coastal populations.DLR –MAXWAVE, which investigated the physics and occurrence of very high“Rogue” waves.UK Met. Office – Operational modellingand research at the UKMO. A key problemwas identified as the accurate representationof the generation and propagation of swell.

2.2.2 Feature CharacteristicsThe distribution of wind and waves on theoceans is mainly characterised by the sharpdistinction between the storm belt, atlatitudes poleward of 40°-45°, and thetropical-equatorial zone. North/South of thiszone a permanent west to east atmosphericflow brings with it a steady flow of storms.The border between the two areas dependson the season, with a shift to a morepoleward position during the warmer monthsof the year.

The overall pattern reflects this distribution,with the strongest winds present at latitudesbetween 60°-70°. Local patterns exist closeto the continents. Exceptions to the abovepattern are the hurricanes, which aregenerated only in the sub-tropical belt. Their dimension is limited, at least withrespect to the extra-tropical storms, but theextremely high winds still lead to extremelylarge waves.

3

4

5

6

7

8

9

10

11

12

13

14

15

2

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

The distribution of wave heights closelyfollows the wind climate. The storm belt ischaracterised by frequently occurring activesea conditions, with wind generated, steepbreaking waves. In the tropical-equatorialzone the climate is characterised mainly byswell, when the long waves generated bythe northerly (and southerly) stormspropagate out of the stormy area. Becauseof the very limited attenuation with distance,an energetic swell can propagate forthousands of kilometres.

These different characteristics are relevantfor satellite altimetry. The different physicsassociated with the different wind and waveconditions affect the altimeter output andthe quality of the results. The higher andsteeper the waves, the higher is the sea-state bias affecting the accuracy of seasurface height. There are strong doubtsabout the validity of altimeter wind data at extremely high wind speeds. In theseconditions the physics of the surfacechanges dramatically, and is still largely not understood.

The different time scales and spatialgradients present in different situations setdifferent constraints on the desired altimetersampling strategy. For climate purposes,there is no special constraint on the accuracyof the time and location of a pass, or ontime interval between passes and thespacing between adjacent passes. The scaleof the processes is fairly wide, with variationson the scale of hundreds of kilometres. Thechoice is a compromise between the numberof data at each location and the spatialdensity of the information. For individualstorms, particularly hurricanes, the time andspace scales shrink dramatically. However,no particular optimum satellite orbitconfiguration can be chosen in advancebecause of the almost random (within thestorm belt) distribution of the storms.

All this holds for the open oceans. Close to the coasts we find much larger spatialgradients. In these conditions a much higherdensity of information would be required,most likely beyond the practical possibilitiesof satellite altimetry alone. As for a detaileddescription of the open ocean climate, a steptowards a solution lies in the parallel use ofthe information from the numerical models.

2.2.3 Priority issues• Sampling

Altimeter data are still relatively scarce ona global scale, with large gaps in spaceand time. An orbit with a return period of,e.g., ten days implies each location isvisited on less than forty occasions peryear, with a gap of 2.5° between adjacenttracks. Most applications place the highestpriority on increased sampling in time andspace; some users have indicated anorder of magnitude increase in samplingis desirable.

• AccuracyThe accuracy of measurements is still anissue, more for wind speed than for waveheight. It has not been possible to verifyeither of the parameters against in situdata for extreme conditions (wind speed> 20 ms-1. Hs > 15m - see Cotton, 1998).It is for just such conditions that usershave the most urgent need for accurateinformation, particularly for offshoreoperations.

• Assimilation into ModelsIn application to forecast models, thealtimeter has been the basic instrumentfor the supply of wave height information.However, the impact of altimeter data islimited to a relatively narrow band eitherside of the ground track (see Figure 2.3).The main drawback has been the inabilityto correct the individual wave systemsthat compose the two-dimensionalspectrum at a given location. SAR datahave been useful in this respect, boostingthe development of techniques for theassimilation of the measured spectra. To date the assimilation of SAR data has required a priori knowledge of thespectrum, obtained from the model inwhich the data are to be assimilated.

Recent studies at Météo France havedemonstrated improved forecastingaccuracy when data from more than one altimeter are assimilated.

Figure 2.3. Analysis Increments

(Analysis minus First Guess) from

the assimilation of ERS2 altimeters

wave height valid for 8 March 2001

at 00 UT: only increments larger

than half a metre are represented.

Increments d’assimilation H1/3 du 08/03/2001 á 00h utc

3

4

5

6

7

8

9

10

11

12

13

14

15

2

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

-90˚

-60˚

-30˚

0˚

30˙

60˚

90˚

180˚ 210˚ 240˚ 270˚ 300˚ 330˚ 0˚ 30˚ 60˚ 90˚ 120˚ 150˚ 180˚

Other Wave ParametersMany offshore operational users haveindicated that an accurate wave periodparameter would be of significant benefit.In addition users have asked for wavedirection, separate estimates of height,period and direction for wind-sea and swell.This leads to a need for full directional wave spectra.

• Understanding of Wave ProcessesOne of the key issues for the improvementof numerical wave models is a betterknowledge, hence formulation, of thephysics of waves, and of their interactionwith the atmosphere. Waves form theinterface that controls fluxes between theocean and atmosphere systems, that inturn control the earth climate. A betterknowledge of the processes involved ishighly desirable. However, the basicdifficulty in studying the physics of waves,e.g. their generation by wind, is tocharacterise the processes taking placeas a sequence of single, highlyconcentrated, events, but sparse in spaceand time. A satellite can only detect theintegrated effect, and be used as theverification tool of a numerical model tryingto represent the physical truth. In additionit is also clear that the application ofaltimeter data to this problem is limitedbecause Hs is the only wave parameterthat is routinely provided, therefore theavailability of the two-dimensional wavespectrum is highly desirable.

• Coastal StudiesKnowledge of the conditions in coastalareas present a particular challenge. Todate the altimeter has not been able toprovide information very close to thecoasts, the minimum distance being inprinciple half the diameter of the areasampled by the radar. Besides, when flyingoffshore, a few seconds are required forthe instrument to lock again on the seasurface. In this case data are availableonly from 20-30 km away from the coast.(N.B. Early results from ENVISAT promiseimproved performance in this respect.) Inaddition it is clear that a single altimetercannot adequately sample the spatialvariability that characterises the coastalenvironment.

• Air-Sea Flux ClimatologiesClimatologies of air-sea fluxes (momentum,heat, gas, freshwater) are centrallyimportant in climate studies. Dual frequency(or multi-frequency) altimeters offer a uniquepossibility to make direct measurementsof surface wind stress and air-sea gastransfer velocities. The development ofalgorithms to derive estimates for theseparameters from the altimeter is the subjectof ongoing research.

Presently available estimates of globalmean air-sea gas transfer velocities varyby a factor of two. Any improvement onthis resolution would be beneficial. Anaccuracy to aim for (in a climatology) couldbe 50% (or 10 cmhr-1) on a monthly 2° x2° grid.

-5.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.5

1.0

1.5

2.0

2.5

3.0

5.0

m

Also an estimate of rain rate can be derivedfrom the differential radar backscatter fora dual frequency altimeter. Whilst other,specifically designed, satellite missionsprovide better sampling; altimeter estimatesprovide a valuable independent verification.

• Event and Process StudiesThere is a growing interest in improvingour understanding of the physics ofextremely severe conditions. Of course itis in these conditions when human andenvironmental safety are most at risk, butit is exactly in such situations when in situinstrumentation are most vulnerable.Although the physical processes aredifferent, satellite measurements are notvulnerable in the same way and so satellitesoffer an opportunity to measure extremeconditions.There are two areas of specific interest.• The study of wind and wave fields

associated with events such as tropicalstorms or cyclones.

• The study of occurrences andcharacteristics of so-called “Rogue” waves.

Joint use of altimeter/SAR and perhapsaltimeter/optical data sets could provideimportant information by allowing profiles of individual severe events.

For detailed studies of ocean wave processesmore sophisticated processing of thealtimeter signal could be attempted.

2.2.4 Summary of UserRequirementsTable 2.2 provides a summary of userrequirements, from the research and offshoreoperator communities, for “Sea State”information that can be derived from altimetermeasurements. In this instance “Sea State”is defined to include wind and wavemeasurements, and air sea fluxes (includingrain). As before, “latency” is the delay fromthe time the measurement is taken to theavailability of this measurement to the user.Note that the resolution requirements foroffshore and coastal near-real time data (30km and 1 hour) are requirements for a finalproduct which may be derived from a numberof sources, not only altimeters. For theGANDER constellation, SOS proposed analtimeter sampling regime of 400 km every6 hours (equivalent to 100km each day).

Thus, from the perspective of researchersand commercial users, priorities are:• Improved sampling in space and time

(uniformly distributed, so far as is possible).• Availability of wave directional and wave

period information – also separate windsea and swell parameters (height, periodand direction), wave steepness and jointdistributions (e.g. of wave height / period,Hmax/Hs).

• Better combination of ocean / coastalmonitoring resources (satellite, in situ,models) to provide improved warning ofsevere events with impacts on vulnerablepopulations.

• Implementation of a (new) wind speedalgorithm valid for higher wind speeds.

• Validation of measurements at higher waveheights and wind speeds.

• Improved performance at coasts (higheralong track resolution, quicker gain ofocean surface when the track comes fromland to sea).

• Near real time availability of data ( < 3 hours).

• Joint climatologies of altimeter and SARdata - e.g. groupiness, expectedmaximum wave heights, crest length.

• Climatologies of air-sea fluxes(momentum, heat, gas, freshwater) areimportant for climate studies. Dualfrequency altimeters offer the possibilityfor direct measurements of surface windstress, and air-sea gas transfer velocities.

• Improved estimation of sea state bias(information from the wave spectrum could help).

In terms of satellite missions, the interimrecommendations were:• Ensure continuity of coverage by at least

2 satellites, but ideally improve samplingfrequency.

• To plan for a post-2011 capability tomeasure wave spectra, note that theanticipated 5 year lifetime of the ENVISAT(with reference to the ASAR), ends in 2007.

• Depending on the success of earliermissions, to consider a further wavespectra measuring radar and/or asignificant (order of magnitude) increasein sampling.

Application Altimeter Parameter Indicative measurement requirements

Spatial resolution(km) Time resolution latency accuracy

Climate Research Sig. wave ht (Hs)1 100 1 mon 3 months 0.1 m

Offshore appls. (climate) Hs1 1° x 1° 1 mon Years 0.1 m

assimilation into models Hs1 100 6 hr 3 hr 0.25 m

near real time - offshore Hs1 30 1 hr 1-3 hr 0.25 m

near real time - coastal Hs1 10 1 hr 1-3 hr 0.25 m

Joint U10/ Hs pdfs U10/Hs 1° x 1° 1 mon Years 0.1 m /2 ms-1

Climate Research wave period1 100 1 mon 3 months 0.5 s

Offshore appls. (climate) wave period1 1° x 1° 1 mon Years 0.2 s

near real time- offshore wave period1 30 1 hr 1-3 hr 0.5 s

near real time- coastal wave period1 10 1 hr 1-3 hr 0.1 s

Joint Hs / period pdfs Hs/T 1° x 1° 1 mon Years 0.1 m / 0.2 s

Offshore appls. (climate) wave dir.1 1° x 1° 1 mon Years ±10°near real time- offshore wave dir. 1 30 1 hr 1-3 hr ±10°near real time- coastal wave dir.1 10 1 hr 1-3 hr ±10°

Offshore appls. (climate) dir. wave spectrum 100 km 1 mon Years 15° dirn,10% wavelen.

Near real time dir. wave spectrum 30 1 hr 1-3 hrs 10% max energy

Surface wind stress (U*) ∆σ02 100 1d 1d 0.1 m s-1

Air sea gas transfer vel. ∆σ02 100 1d 1d 10 cm hr-1

Rain rate ∆σ02 250 30d 90d 10 mm mon-1

Event and feature studies Hs 200 1d 3 hr 0.1m / 10%

U10 200 1d 3 hr 2 ms-1 / 10%

σ0 200 1d 3 hr 0.1 dB

Global Mapping of Joint Alt/SAR data sets. Requirements to be determined through research“Extreme” Waves

Extreme Event Combined Hs/ sea level: kms hours hours threshold to beWarning Systems alt/model/in-situ systems determined

Sea State Bias Hs, (∆)σ0, wave spectrum Various, according to sea surface height requirements < 1 cm

This implied:1. Basic requirement for 2007-2011Jason-2 + 1-3 single frequency micro-satellite altimeters (e.g. Altika, GANDER).

2. Further development for 2011 timescaleJason-2 + NPOESS + SWIMSAT + 1-3 (ormore) single freq. micro-sat altimeters.

3. Possible long term scenario for 2015+timescaleJason series + NPOESS series + singlefrequency microsatellite constellation ORGPS reflectometry system + SWIMSATsuccessor.

Table 2.2. Indicative User requirements for altimeter derived sea state information.

1Separate wind sea and swell

information desirable2Dual frequency altimeter required.

N.B. Altimeter wave heights and

wind speed measurements should

ideally be accurate to the

maximum of the expected range.

Wind speeds are known to be

inaccurate above 15 ms-1.

3

4

5

6

7

8

9

10

11

12

13

14

15

2

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

2.3 Precise Orbit Determination and Range Corrections(For full report see http://www.altimetrie.net/docs/GAMBLE_finalreport_theme3.pdf)

2.3.1 Objectives and ApproachThe range observation made by the altimeterinstrument must be related to the terrestrialreference frame through precise orbitdetermination; and account must be takenof the ionospheric and tropospheric effectson the travel time of the radar pulse. Forthese purposes, current altimetry systemssuch as Jason-1 and Envisat carry additionalhardware: precise tracking capabilities fororbit determination, an altimeter with dual-frequencies for ionosphere corrections anda microwave radiometer to measure the wettroposphere correction. While thesecomponents are essential for the currentand future reference class missions, themeasurement requirements of several otherpossible future altimeter systems might leadto less strict hardware specifications. Thesealternatives may lead to a reduction in costsby allowing for an easier realization of themission objectives using a micro-satelliteplatform or as a passenger payload on an existing platform. This section will discuss the various technical issues andavailable solutions.

The main objective is to providerecommendations for optimum orbits, orbitmaintenance and satellite tracking and orbiterror budgets that may be expected forvarious proposed satellite altimeter missions.

Contributions from GAMBLE partnersincluded:TU Delft – Orbit solutions, precise orbitdetermination, satellite laser ranging, andgravity field modelling.CLS – Gravity field modelling.CNES – The DORIS satellite orbitdetermination system.SSTL – GNSS/GPS micro-satelliteapplications.U Newcastle – Precise orbit determination,cross-over orbit reduction.SOS – Sea state and GANDER micro-satellite issues.

Contributions from invited experts included:Technical University of Munich – GPSOrbit Determination.Ohio State University – Orbit Choices andOrbit Issues.

2.3.2 Precise Orbit DeterminationAny error in the radial component of thecomputed orbit of an altimetry satellitedirectly affects the accuracy of the oceantopography science product. Precise OrbitDetermination (POD) is traditionally basedon a combination of force modelling andhigh-accuracy tracking data.

Tracking systemsThere are currently three tracking systemsavailable for cm-level orbit determination:Satellite Laser Ranging (SLR), DORIS, andgeodetic GPS, each with its own specificcharacteristics.

SLR is the only system providing a directand unambiguous highly accurate rangemeasurement and is therefore essential forthe calibration and verification of radiometricPOD and altimeter instruments. In addition,the global network of around 30 laserstations is used to establish the positionand motion of the Earth’s centre of massand rotation axis, which are of importancefor altimetry-based research as well. TheSLR community, organized under theInternational Laser Ranging Service (ILRS),puts a high priority on obtaining SLRmeasurements from altimetry satellites.These SLR measurements are usually usedin combination with geodetic GPS or DORIS,ensuring an excellent stability of thereference frame.

The French DORIS system consists of aground network of nearly 60 beacons, whichare distributed evenly over the world for anexcellent global coverage. The Doppler shiftin the radio signals supplies range-rate datathat can be used in dynamic or reduced-dynamic POD.

The GPS system was originally designedfor ground-based positioning and navigation,but a number of special techniques haveenabled the use of these signals for satellitepositioning. Low cost GPS receivers arefrequently flown on micro-satellite platforms,supplying an accuracy of several metres. Aspecial receiver was first developed at JPL,enabling cm-level POD. Research on howto make optimal use of the various GPSsignals for POD is ongoing.

In the absence of a radiometric trackingsystem, the SLR measurements alone mightbe too sparse for robust POD. After thefailure of the PRARE tracking system onERS-1 and the GPS receiver on GFO, theuse of the altimetry data itself for orbitdetermination has been investigated. Byusing altimeter height differences overcrossover locations, the orbit determinationfor these missions has been thoroughlyimproved. A careful parameterizationscheme and weighting of the data ensuresthat the absorption of sea level variationsinto the orbit is limited.

POD schemesThe use of force models allows for therestitution of accurate orbits even when thetracking data contains gaps or outliers, whileon the other hand, uncertainties in the forcemodel can be solved for using the trackingdata. This is the dynamic POD scheme. Inreduced-dynamic POD, more and moreforce model parameters are estimated forhigher accuracy. In the extreme case ofkinematic POD, the force models themselveshave become irrelevant, and the computedorbit completely follows the tracking data.Kinematic POD is only possible using GPStracking, however the presence ofsystematic errors, outliers and data gapscurrently make the dynamic and reduced-dynamic POD schemes more suitable foraltimeter missions.

Force models, orbit choice and orbitmaintenanceThe two most important force model errorsources, gravity and atmospheric drag,decrease drastically with the orbital altitude.However requirements on altimetertransmitter power and launcher restrictionsgenerally favour lower orbits. Fortunately,gravity field models have dramaticallyimproved in recent years, and the CHAMPand GRACE missions have brought downthe gravity-induced orbit error for currentaltimetry satellites to below 1 cm.Atmospheric drag will quite likely remain amajor error source at lower altitudes,especially during peaks in the 11-year solaractivity cycle. The next high solar-activityperiod is expected to take place from 2009-2015.

The perturbation forces will also cause theorbit to drift so that corrective manoeuvresare required to maintain a repeating groundtrack. Again, the above arguments aboutthe orbital altitude and solar activity cycleapply, so that the maximum number ofmanoeuvres is required for low satellitesduring high solar-activity.

Tracking system recommendationsBased on a mission’s sea surface heightmeasurement requirements, a PODrequirement can be formulated, which willlead to specific hardware configurations,based on experience with previous missions.This is illustrated by the five cases identifiedbelow:1.Primary SSH reference missions (the

Jason-1 case): For the highest achievablelevel of POD accuracy (<2 cm radial RMSat 1336 km altitude) and additionalredundancy, the combination of SLR,DORIS and Geodetic GPS is therecommended configuration.

2.Secondary SSH reference missions (theEnvisat case): For <3-4 cm radial RMSaccuracy at 800 km altitude, and orbitsthat are completely independent fromaltimetry, SLR plus either DORIS orGeodetic GPS are required.

3.Meso-scale SSH missions (wide swath,or small constellations): For <10 cm radialaccuracy, an SLR retro-reflector issufficient (but minimal) hardware. PODcan then be performed using altimetryas additional tracking data. This approachhas been proven using ERS-1, ERS-2and GFO.

4.Meso-scale SSH/wind/wave largeconstellations (four or more satellites): Alarge constellation might burden the SLRtracking stations too much. There will bemany more altimeter crossovers for radialorbit precision however. A non-geodeticGPS receiver instead of SLR will alsoprovide stability in the along-/cross-trackposition components.

5.Wind/wave-only missions: There is noPOD requirement for these missions.

3

4

5

6

7

8

9

10

11

12

13

14

15

2

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

When orbits are required in (near) real-time,the DORIS/DIODE navigator system is aflight-proven technology, delivering a radialaccuracy of below 30 cm rms. Onboard orbitdetermination using geodetic GPS is morecomplicated due to the need for auxiliarydata. However, accurate GPS orbits can becomputed on the ground within a few hoursof data acquisition.

2.3.3 Ionosphere and wettroposphere correctionsAltimetric systems such as Jason-1 andEnvisat carry a dual-frequency altimeter andmicrowave radiometer, in order to give adirect and independent measure of the delayon the radar echo caused by free electronsin the ionosphere and water vapour andcloud liquid water droplets in thetroposphere. Estimates of these propertiescan also be obtained by other means.

The ionosphere correction can also beobtained from models such as Bent and IRIor from analysis of dual-frequency GPS andDORIS data. The GPS-derived GlobalIonosphere Map (GIM) correction is closestto the altimeter-derived values (see Figure2.4), and is accurate enough for manyapplications. For the proposed Ka-bandaltimeter, the ionosphere delays will be muchless than for the present Ku-band systems,and might be considered negligible underlow solar activity conditions.

An alternative to the radiometer wettroposphere correction is available fromatmospheric model fields supplied byECMWF and NCEP. Since changes in theprocessing at these centres are known tocause jumps or drifts in the time series,these data are not suitable for applicationssuch as sea level change. Moreover, thesemeteorological models are generallyinsufficiently accurate at the meso-scale(see Figure 2.4), so that model errors maybe confused for actual meso-scale oceanicfeatures. However, neither of these concernsplays a role in the application of the altimeterdata for orbit determination, where the large-scale (>2000 km) behaviour dominates;smaller scale errors will be effectively filteredout in the POD process.

Figure 2.4 Along-track comparisons of ionospheric delay (top) and

wet tropospheric delay (bottom) along a TOPEX/Poseidon track.

The equator crossing is at 0 seconds on the x-axis. The values

from the dedicated on-board instruments (dual-frequency, TMR)

are compared with those derived from alternative measurements

and based on models.

Iono

sphe

ric p

ath

del

ay (m

m)

150

0

50

100

-1500 -1000 -500 0 500 1000 1500Time (sec)

Dual-frequencyBent

DORISJPL-GIM

Dual-frequency / DORIS / Bent / JPL-GIM Ionosphere Corrections

Wet

tro

pos

phe

ric p

ath

del

ay (m

m)

400

0-1500 -1000 -500 0 500 1000 1500

Time (sec)

TMR wet tropoECMWF analysis

ECMWF re-analysisNCEP wet tropo

TMR / ECMWF / NCEP Wet Tropospheric Corrections

50

100

150

200

250

300

350

2.4 Offshore Operator Requirements forAltimeter Derived Data(For full report see: http://www.altimetrie.net/docs/GAMBLE_OWS_Frep.pdf)

2.4.1 Objectives and ApproachOffshore operators are working tospecifications that become more challengingyear by year. These specifications can relateto operational activities, e.g. the need tooperate within precise sea state limits, orfor advance knowledge of “weatherwindows”, or they can form part of thedesign procedure (planning for operationsor vessel and structure design).

A priority of GAMBLE was to consider inputfrom offshore operators, so that therequirements for future satellite altimetermissions are driven as much by the needsof offshore operators (taking into accountcommercial, safety or environmentalconsiderations) as they are by the needs ofthe scientific community. Thus an operators’workshop was convened to allow operatorsto communicate their priorities, and to initiatea dialogue between GAMBLE partners andoffshore operators to find the best way inwhich these requirements can be satisfied.

The workshop was planned with the supportof the Met-Ocean Committee of theInternational Oil and Gas ProducersAssociation (OGP), and hosted in Stavanger,Norway in the offices of the NorwegianPetroleum Directorate. Twelverepresentatives of the OGP community (fromShell, BP, Total, Chevron/Texaco,ConocoPhilips, and ExxonMobil) attendedthe workshop, in addition to members ofthe GAMBLE team and four invited speakersfrom the Met-Ocean information supplyindustry. We are grateful to the chair of theOGP Met-Ocean Committee, Chris Shaw,for his support in arranging this workshop.

2.4.2 Available Data Products –and Expected DevelopmentsAt the workshop, presentations from thosedeveloping new products derived fromsatellite altimetry helped to define the “stateof the art”. These included presentationson monitoring and forecasting oceancurrents and ocean circulation by CLS andMERCATOR, operational wave modellingby the UK Meteorological Office, and adiscussion of EO measurements of a recentsevere storm to the North of Scotland bySatellite Observing Systems.

Key points were:Sea Surface Topography / Ocean Currents• Improved ocean current products are

becoming available from operationalprogrammes such as MERCATOR.

• Interpolated and gridded sea levelanomalies and surface current fields.Available as 5-7 day averages, atresolutions of up to 1/3°.

• Forecasts and analyses of surface andsubsurface currents are available fromcoupled ocean circulation models.Resolutions range from 1/3° (global) to1/8° or better in regional models.

(Figure 2.5 provides an example of outputfrom the MERCATOR system.)

Ocean Waves• Forecasts and analyses (hind-casts) are

available from global and regional wavemodels which routinely assimilate satellitealtimeter data (e.g. the UK Met. Office,Météo France. Resolutions range from60km (global) to 12km (regional). Figure2.6 show a UKMO forecast for a severewave event to the North of Scotland inJanuary 2003.

• Research is ongoing, aiming to provideimproved representation of swell andidentification of conditions favourable for“rogue waves”.

• A case study shows that in situ andsatellite altimeter data are consistent forsignificant wave heights to 12m.

• Severe events have the greatest impact, but are often the most difficult to predict.

3

4

5

6

7

8

9

10

11

12

13

14

15

2

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

2.4.3 User Priorities for oceancurrent and sea state data overthe next decade.A recent overview of user requirements fromthe perspective of the OGP community wasgiven in a report for EuroGOOS (Grant andShaw, 1999). They indicated that the mostimportant parameter for offshore operationswas wave height followed by wave period,wind speed and ocean currents. Other met-ocean parameters required are waterelevation, air and sea temperature, visibility,cloud height and sea ice.

A presentation by Fugro GEOS discussedexpected developments in the offshorerenewables sector. The offshore renewablesmarket is expected to grow significantly inthe coming years (a predicted tenfold increasebetween 2010 and 2020). The largest potentialwind power resource in Europe lies in thecoastal waters around the UK.

Nowcasting International argued that whilst improvements in models andmeasurements in recent years have led to higher resolution forecasts with betteraccuracy, the presentation and delivery ofdata to users has been slower to evolve.Data should be formatted and systemsestablished so that a specific set of requiredinformation is easily transmitted under thecontrol of the customer.

Figure 2.5 Example of Mercator

1/15° model output showing

upwelling off the North-West

African Coast, temperature at 3m

depth and surface currents. For

18 April 2001

Figure 2.6 Hindcast (left panel) and

forecast (right panel) of a severe

wave event to the NW of Scotland

on 15 Jan 2003). From the UK

Met. Office.

The 1/15 ̊Mercator modelAnalysed temperature: T on 16-04-2001 near 3m

28

27

26

25

24

23

22

21

20

-20 -18 -16 -14 -12

15.00

16.88

16.75

17.62

18.50

19.38

20.26

21.12

22.00

min = 14.89 max = 21.75deg C

10W 0 10E

Hs (m) at 18z 15/1/2003 from 00z 14-1-2003

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

50N

55N

60N

10W 0 10E

Hs (m) at 18z 15/1/2003 from 00z 16-1-2003

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

50N

55N

60N

DiscussionDiscussions highlighted the following issues:

Ocean Currents• Although wave heights continued to be

the “most important” parameter forplanning, design and operations, thebiggest uncertainties lay in gainingaccurate knowledge of ocean currents.Participants were aware of the potentialof satellite altimeter data to improve ourknowledge of ocean currents, and someUS based users had made use of productson sites such as University of Colorado(http://www-ccar.colorado.edu/research/gom/html/gom_nrt.html) and the US Naval Research Laboratory(http://www.ocean.nrlssc.navy.mil/altimetry/). However, there was still felt tobe a deficiency in the quality and amountof ocean current information.

• Currents derived from altimetry oftenseemed to be lower than those directlyobserved offshore.

• Daily to weekly surface current chartswere regarded as providing sufficienttemporal resolution – spatial resolutionwas seen as a bigger problem.

• One contribution suggested that astatistical approach may yield usefulinformation on the amount and characterof variability present in small scales forwhich there are limited directmeasurements, and which are not wellrepresented in models.

Ocean Waves• Improved spatial and temporal resolution

in waves is important.• Wave period measurements are important

even at low wave heights. Long, low swellscan cause operational problems. Areasof particular interest in this respect areWest Africa and North East Asia(specifically Sakhalin).

• Wave direction is important for design aswell as for operational use.

• A particular problem at higher latitudes isthe sudden development of polar lows.These can generate severe waveconditions which models are unable topredict accurately. More satellitemeasurements at latitudes pole-ward of60° could contribute to a significantimprovement.

• Swell - there is a problem in the advectionof swell over long distances, which isconnected to the resolution and to thedispersion intrinsic in the advectionalgorithms used in the wave models.

• Extremes – a large part of the difficultiesexperienced by wave models in fore-casting extreme events is connected tosimilar problems in the input wind fields.The problem appears especially withstorms which are characterized by largespatial gradients, in which cases peaks,of both wind and waves, are smoothedout and missed. The physics relevant inthese extreme cases has a role. It isquestionable whether the same physicsapply to moderate and extreme conditions.

• Gustiness has a relevant role. The physicalbasis is properly understood, but thepracticalities are difficult to handle for tworeasons. ECMWF has implemented therole of wind gustiness in enhancing themaximum wave heights. However,gustiness is poorly understood from themeteorological point of view, and thepresent meteorological models do notsuccessfully generate the high levels ofgustiness found in nature. The secondreason involves statistics. Gustinessintroduces a strong element ofrandomness in the wind and wave fields,and there is no way, except by nowcast,to anticipate “when” and “where” ananomalous maximum will hit.

• Freak, or “rogue” waves - a theory for anestimate of their probability has beendeveloped and recently implemented intothe wave model at ECMWF. Obviously,as for gustiness, it is only possible toestimate the probability of freak waves ofa given wave height.

3

4

5

6

7

8

9

10

11

12

13

14

15

2

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

2.5.1 Objectives and approachThe purpose of this aspect of the GAMBLEproject was to look at the scientific researchquestions that could be addressed by aconstellation of altimeters and what furtherresearch is needed to fully utilise such asystem. For the purposes of this report,scientific research was defined to includeresearch into pre-operational oceanography,but not the real time supply of data tooperational entities. Three groups ofapplications were considered: sea surfacetopography, sea state and non-oceanapplications. Scientific research in generalis driven by one of two motivations;requirements from some user or thescientist’s curiosity. Satellite remote sensing,with its large capital requirements is generallydriven by user requirements which willprovide the main focus for this report. Insome cases however we will introduce someideas motivated purely by curiosity. AlthoughGAMBLE has primarily focussed onoceanographic applications it wasrecognised that there are many interestingand important applications of altimeter dataover land and ice.

Southampton Oceanography Centre drewup an original draft report. A half-dayworkshop was held in Toulouse inSeptember 2003, and comments from the GAMBLE team incorporated in the final report. We present a summary of thatreport here.

2.5.2 User RequirementsThe reports from the earlier parts of theGAMBLE project were used as the basis toset out the requirements for future research.

Clearly the area where constellations ofaltimeters are going to have the most impactis in mapping the oceans. In general we arelooking for research areas where progresscan be made from improved spatial/temporalsampling. However there are other areasthat will support the use of constellations.For example the identification of conditionslikely to produce rogue waves in altimeterproducts or the development of cheap dualfrequency altimeters. In addition, manyclimatologies (e.g., of monthly mean windspeed and wave height) will be improvedsimply by virtue of higher sampling, sincesparse sampling can often be identified asthe principal source of uncertainty in thesample mean (e.g., Woolf and Challenor,2002). In some cases (notably wind speed)present-day sampling is inadequate for acomplete monthly climatology at a scaleappropriate to the autocorrelation lengthscale of the measured variable.

Areas where altimeter constellations willhave a significant impact include: -• Ocean mesoscale• Ocean barotropic signals• Coastal studies• Mapping the wave climate• Better extreme waves for design• Conditions likely to produce Freak waves• Air-sea gas transfer (Kyoto monitoring)• Land hydrology• Land and sea ice monitoring

We also need to carry out research intoaltimeter constellations themselves. Areasthat need investigation include: -• Cost savings for batch production• Constellation optimisation• Should we mix wide swath and wave

spectrum (SWIMSAT) altimeters in theconstellation? How?

• How does losing one (or two) satellitesdegrade the constellation?

• How reliable will cheap altimeter micro-satellites be?

2.5 Framework for Future Research(For full report see http://www.altimetrie.net/docs/GAMBLE_WP7.pdf)

2.5.3 Sea Surface TopographyMany research topics involving mappingthe ocean are presented in the High-Resolution Ocean Topography report byChelton (2001). However that report onlyconsiders wide swath altimeters and smallconstellations of a few satellites. In GAMBLEwe have been considering constellationsfrom a few satellites to the 12 or so in theGANDER concept. One of the major issuesfor any large constellation of altimeters,where cost and mass considerations maynot allow sophisticated tracking systems,is orbit determination. The orbits and trackingstudies for GAMBLE (Section 2.3) haveshown that satisfactory orbits can beprovided through a combination ofcrossovers with well tracked altimeters andrelatively cheap tracking systems such aslaser retro-reflectors.

The main requirements for improvedcoverage in space and time are the need tolook at coastal processes, mesoscale andbarotropic fields.

Altimeter sea surface topography gainsgreatly in utility if it is assimilated into anumerical model of the ocean. There are anumber of research questions related toassimilation. So far, studies have,understandably, concentrated on the effectan additional one or two satellites has onassimilation. There remain fundamentalquestions about the number of satellitesand assimilation. What is the optimal numberof altimeters to get the large-scale flowcorrect? In an eddy-resolving (permitting)model do we need to constrain every eddywith observations or is it possible to describethe eddy field statistically only constrainingthe occasional eddy, as at present? Coastalprocesses, in general, require a much higherspace-time sampling than even 12 altimeterscould provide. However the constraints fromassimilating such data will be much strongerthan for a small number of satellites.

The problems of identifying features andthese assimilation problems are related. If we can identify a feature in the altimeterdata then, crudely, the assimilation processwill be able to constrain the model to ‘fit’that feature. Assimilation schemes havebeen proposed that explicitly identify

features and ‘move’ them to the correctplace in the model. With the altimetercoverage from two satellites it is possibleto identify features such as baroclinic Rossby waves, and the major currentsystems. When we come to the mesoscalehowever, it is almost impossible to tracksingle eddies in time. Constellations willsolve this problem and allow new researchareas such as the tracking of Agulhas eddiesacross the South Atlantic and, in conjunctionwith sea surface temperature data,estimation of the heat transport from the Indian to the Atlantic Oceans.

Mesoscale features are generally hard todetect in altimeter data because of the poorspatial resolution of current configurations.This is perhaps most severe in coastalregions where both the spatial and thetemporal variability are high. Barotropicsignals can have large spatial signals buttravel very rapidly and so are difficult todetect. They are clearly seen in altimeterdata. In the past they were removed by aninverse barometer correction, recently thetrend has been to use the output frombarotropic models to make thesecorrections. These barotropic models arenot only used as a correction for altimetrybut also for time dependent gravity datafrom satellites such as GRACE. The inputto these barotropic models is usually surfacepressure and there is no assimilation of data.Because of the difficulty of observingbarotropic signals in the ocean they haveproved hard to verify. Frequent data from aconstellation of altimeters would enable usfirst to verify how good these models are,and secondly to assimilate the data into themodels themselves.

There are other phenomena, for exampletsunamis and storm surges, where thesampling of one or two altimeters is notsufficient to detect the signal at the seasurface. Large constellations offer thepromise of detecting such signals.Operationally this could be very important as both tsunamis and stormsurges cause considerable amounts ofdamage and loss of life, particularly in the developing countries.

3

4

5

6

7

8

9

10

11

12

13

14

15

2

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

2.5.4 Sea State and OtherOcean ParametersA constellation of altimeter satellites givesus much better spatial and temporalsampling to examine the wave climate. Theresearch possibilities follow from this. Wecan divide the research opportunities intothree areas: wave climate, extremes andnew parameters.

Wave ClimateAt present using two altimeters in theTOPEX/Poseidon and ERS orbits we candefine monthly mean wave climate fairlyaccurately on a 2° x 2° square grid over theglobe. If we are to conduct research into waveclimatology and its changes over time andspace in semi-enclosed areas we need a finergrid than is delivered by two satellites. In theopen ocean 2° squares a finer grid would beuseful for a number of research topics, forexample wave height in storms (particularlyassociated with highly concentratedmeteorological features such as Polar Lows)or looking at the interaction of waves withcurrents, in particular the AgulhasRetroflection. Having more passes per monthper square will also make for superiorclimatologies, as the variability of the estimateswill be reduced. As the number of satellitesis increased it will become possible not onlyto define the monthly mean but also the withinmonth variance, a parameter it is impossibleto measure at present.

One of the current deficiencies in wavemodels is the way they deal with swell.Altimeters can in principle track swell as itpropagates across the ocean, though atpresent this is difficult because of the poorsampling. With a constellation it would bepossible to track the progress of a swellsystem simultaneously in both the wavemodel and in the satellite data. Such studieswould increase our knowledge of the wayswell propagates across the ocean and howit is represented in models. The prospect ofmeasuring both wind speed and significantwave height (SWH) for these tracked swellsystems also opens new avenues ofresearch into atmosphere-oceaninteractions, e.g. the role of sea state forthe propagation of atmospheric systemsand for air-sea momentum transfer, whichwould benefit weather forecasting models.

Such research is already under way forhurricanes using SAR images, from whichboth wind and sea state information can beextracted, although the present samplingby a single satellite (even if using SAR inwide scan mode) allows only for occasionaland brief monitoring of the features ofinterest. Of course the presence of heavyrain in the centre of such storms causessignificant difficulties for radar.

The calibration of wave measurements fromsatellites and the verification of wave modelsare closely connected. The number ofcoincidences between models andaltimeters is so much higher than thecoincidence of either with wave buoys thatit makes sense to calibrate one against theother with the buoys being used as anadditional constraint. We know that bothwave models and altimeters work reasonablywell for the majority of waves. In the extremeranges there is more doubt. At very lowwave heights (SWH ~ < 0.5 m) and above~20m SWH the altimeter is expected tohave difficulty in providing reliable estimates.Little is known of the behaviour of wavemodels in high sea state regimes. Having aconstellation of altimeters, all calibrated toeach other and the global buoy networkshould provide enough coincidences tocollect data in this important (but sparselypopulated) regime of large significant waveheight.

ExtremesOne important potential application ofaltimeter data is the calculation of extremewave heights for design conditions. Theadvantage of a satellite-based approachis that it should be possible to estimatethe extreme conditions in a consistent wayacross the globe. However the applicationof altimeter data to these problems suffersfrom the drawback that the sampling is so infrequent that it is not possible toguarantee that we have observed theextreme conditions in a month. Thiscontradicts some of the assumptions made in the methods used to estimate the return values. Increasing the numberof observations by increasing the numberof satellites will reduce this problem.Research is needed into this problem and into ways of solving it.

Another aspect of extreme value theory thatneeds research is the problem of estimatingextremes of a field of data rather than at asingle point. Although this could be lookedat with existing data the improvedidentification of extremes from a constellationwill make this a much more tractableproblem.

There has been a lot of recent interest inso-called ‘rogue waves’. A rogue waveoccurs when an individual wave is muchhigher than Gaussian statistics would predictfor that sea state. An interesting researchtopic would be to see if any information onthe non-linearity of the sea (Challenor andSrokosz, 1989 for skewness from altimeter)or on the highest waves could be extractedfrom the altimeter return signal. If it could,it would become much easier to map whereand when in the world we could expect tosee rogue waves. Assuming linear wavemapping under range propagatingconditions (still controversial) investigationof individual waves in SAR imagery hasshown that rogue waves may be much moreprevalent that previously believed (Rosenthalet al., 2003).

Other Ocean ParametersIn recent years the estimation of newparameters from altimetry has been thesubject of considerable interest. Of particularinterest are wave period (Gommenginger etal, 2003), precipitation (Quartly, 1998), windstress (Elfouhaily et al., 1998) and gastransfer velocity (Glover et al, 2002). Muchof this work is still at the development stageand further research is needed into theproperties of these parameters. Wave periodcan be estimated from a single frequencyradar while precipitation and a proposedmethod of deriving gas transfer velocitiesneed dual frequency altimeters. Wind stressrequires one or two frequencies dependingon the method used (Gommenginger et al.,2002). It may also be possible to make goodestimates of wave breaking from satellites.Srokosz (1986) proposed a relationshipbetween the fourth moment of the wavespectrum and the frequency of wavebreaking. Since this moment is theoreticallyclosely related to altimeter backscatter,altimetry provides a method of retrievingwave breaking frequency.

Precipitation and gas transfer involve theuse of dual frequency altimeters. Thisadditional capability would have to befactored into the cost of any constellationbut would add significantly to the capability(as well as allowing the ionosphericcorrection to be easily calculated).Precipitation changes on very small scalesand the very narrow swath of the altimeterhas meant that it is difficult to producereliable climatologies from a single altimeter(TOPEX). A large enough constellation wouldbe able to produce climatologies with smallenough uncertainty to allow the study ofprecipitation changes over the ocean fromyear to year and month to month.

Air-sea gas transfer is a very important topicbut gas transfer velocities are very difficultto measure. In a ground-breaking paperGlover et al (2002) produce an algorithm toestimate the gas transfer coefficient fromdual frequency altimeter data. Knowledgeof the geographical distribution of gastransfer (and its integrals over various oceanbasins) is vital if we are to understand theflux of carbon in and out of the ocean.Mapping such a parameter is very difficult,if not impossible, with only one or twosatellites. A constellation would help us toproduce credible maps and hence obtainintegrated estimates of the fluxes of gasessuch as CO2 over the oceans.

2.5.5 Non-Ocean ApplicationsAlthough GAMBLE is specifically concernedwith the oceans, there are a number ofresearch and operational aspects over bothice and land surfaces that should beconsidered. Altimetry over land has beencarried out experimentally for a long time(e.g. Berry 1995 and 1999). In the past fewyears, research on interpreting individualland echoes has advanced to the pointwhere most echo shapes can be interpreted.These advances have opened up a rangeof novel applications for satellite altimeterdata, including mapping, surface hydrologyand surface texture studies.

One of the key applications is the global landhydrology budget. As water resources inmany parts of the world become increasinglyscarce, the ability to provide independentglobal measurement and monitoring of waterusage and distribution across national

3

4

5

6

7

8

9

10

11

12

13

14

15

2

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

boundaries and from source to estuary ofthe strategic ‘river corridors’ becomes evermore important. Lake systems integrateprecipitation: lake time series contain uniqueclimate indicators and with a decadal timeseries already gathered and several furtheryears from current missions, the continuationof this data stream is required to allowidentification of climate shifts.

The current altimeter-based datasets canonly identify seasonal and inter-annualvariation. For most human-orientedapplications, measurements are reallyrequired every 2-4 days. With such asystem, returning data in near real time,drought prediction and mitigation, famineforecasting, flood prediction, hydropowerapplications and global monitoring of the earth’s land water resources all become achievable.

The additional ability of such a constellationto return near-real-time data over thecryosphere should also not be neglected.Combination of the land surface hydrology,snow cover, and land ice mapping wouldallow the earth’s freshwater resources to beeffectively monitored globally.

2.5.6 Research into ConstellationsSo far we have considered research topicsthat can be tackled with a constellation ofaltimeters. Research is also required toprovide an objective and scientific basis forrecommendations on the specifications ofany proposed altimeter constellation (i.e.number of satellites, orbits, instrumentation).First there is the question of the optimalnumber for a constellation.

A priori we would expect the added utilityof each satellite to fall as the number ofsatellites increases. However we need to accurately calculate this dependency,in particular we need to know whatadditional utility is gained from increasingthe number in the constellation. This clearlydepends upon the spatial and temporalscales of the parameter being studied. We would not expect to get the sameanswer for significant wave height as for sea surface height.

A twist on this problem is the inclusion ofnew technology in the constellation: a wide-swath altimeter (WSOA) is due to fly onJason-2; and SWIMSAT, which measuresthe wave spectrum, has been proposed toESA. How would the inclusion of suchsystems in a constellation improve ourunderstanding of the oceans? Theproponents of such systems naturallyconcentrate on the use of a single system.However it is likely that these newtechnologies would be much more effectivewhen used in combination with the improvedsampling provided by a constellation. Oncea constellation was established the riskinvolved in flying a new altimeter designwould be reduced since the failure of a singlecomponent would have less effect than theloss of a stand-alone mission.

2.5.7 ConclusionsThe last decade has seen radar altimetrymoving from a research topic towards anoperational system. The next step is to gofrom a few satellites launched for researchpurposes to planned constellationsoptimised for operational applications. In this review we have considered whatresearch can be carried out with such aconstellation and what research is neededto optimise a system of altimeters foroperational purposes. If altimetry is going tocomplete the transition from a research toolto an operational system to be used insupport of EU policy the issues addressedhere will have to be tackled.

3.1.1 Mission OptionsNew concepts for altimeters address theissues of improving sampling and improvingdata provision through a wider range ofocean surface geophysical parameters, orimproved reliability/accuracy in presentlyavailable parameters.

Briefly these new concepts are:• Constellation of micro-satellite altimeters

– the AltiKa and GANDER proposals existas possible options. AltiKa has beendesigned as a range-measuring altimeterwith built in dual frequency radiometer;GANDER was designed as a Ku bandwave measuring radar (no radiometer).

• Wide Swath altimetry would provide arange measurement in a ~100km swathon either side of the satellite, but doesnot provide sea-state information off-nadir.

• Use of GPS reflections. Potentiallyprovides an order of magnitudeimprovement in sampling, but technologyis not mature and further studies arerequired before it is possible to beconfident that this technique offers apractical option. Significant averagingmay be required to bring down random errors.

• “Wittex” - Micro-satellites employing delayDoppler altimeters (these effectively sub-sample the altimeter echo). Possible bi-static operation increases the groundtracks to 2n-1 (for n satellites).

• SWIMSAT – A wave measuring realaperture radar, with a possible heightmeasuring capability at nadir.

Of course, these new options exist inaddition to presently mature and operationalaltimeter missions, and those at advancedplanning stages:

Operational Missions:• Jason-1 – Accurate sea surface

topography altimeter. Dual frequency(Ku/C band) Poseidon-2 solid-statealtimeter with radiometer and accurateorbit positioning. 10-day repeat, 66°inclination orbit. Continues the 10+ yeartime series started with TOPEX/Poseidon.Launched December 2001. 5 year planned lifetime.

• ENVISAT RA2 – Accurate dual frequency(Ku/ S band) altimeter with radiometerand accurate orbit positioning. 35-dayrepeat, 98° inclination orbit. Continuesthe series started with ERS-1 (1991). Someenhanced tracking capabilities – betterperformance close to coasts. LaunchedMarch 2002, 5 year planned lifetime.

• Geosat Follow-On – Single frequency Ku band altimeter launched in 1998, 17 day repeat, 108° inclination orbit. Initial problems with satellite resets now overcome.

• TOPEX/Poseidon - Accurate, but 10 yearold, sea surface topography altimeter.Dual frequency (Ku/C band) altimeter withradiometer and accurate orbit positioning.10-day repeat orbit. Now in an orbit withinterleaved tracks with Jason-1.

• ERS-2 - Single frequency Ku bandaltimeter (launched 1995). On 35 dayrepeat orbit. Has experienced someproblems maintaining accurate attitudedue to failed gyroscopes. However thisproblem is now largely overcome, throughthe application of an updated on boardalgorithm. Subsequently, in 2003, the on-board tape recorder has failed andERS-2 measurements are now onlyavailable for locations when the satelliteis in direct view of ESA ground stations.

3.1 Capabilities of Available andNew Altimeter Technology(For full report see: http://www.altimetrie.net/docs/GAMBLE_mtr_d8.pdf)

3

4

5

6

7

8

9

10

11

12

13

14

15

2

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

473 Future Strategy

Planned Missions:• Jason-2/OSTM – Will operate Poseidon-

2 dual frequency solid-state altimeter asmain instrument, and will continueTOPEX/Jason ocean monitoring mission.May carry wide swath altimeter asdemonstration instrument. Scheduled forlaunch in 2007.

• NPOESS – Dual frequency (Ku/ S band)altimeter to be operated as part of the USNational Polar-Orbiting OperationalEnvironmental Satellites System. Firstlaunch expected 2013 at the earliest.

• CRYOSAT - The SIRAL altimeter (Syntheticaperture interferometric altimeter)designed for the Cryosat mission (plannedlaunch 2005) should also be mentioned,although this is an ice-monitoring mission.This makes use of interferometrictechniques and uses the reflected Dopplerspectrum to sub-sample the echo, andcan provide measurements at along trackresolution of better than 1 km.

3.1.2 Altimeter CapabilitiesTable 3.1 reviews the capabilities of variousexisting and proposed altimeter missions.Two spatial resolutions that can be achievedare given, the first is the effective trackseparation over a period of 1 day, the secondthe best resolution for a climatologyaveraged over a period of 1 month. Seasurface height accuracy is for single pass1 Hz data (including errors due to orbit,troposphere, ionosphere and EM biascorrections, and range noise).

3.1.3 Issues and TechnicalSolutionsIssues to be addressed include:• Range Accuracy

Corrections (ionosphere, troposphere,sea-state bias).

• Orbits and TrackingAccurate knowledge of orbits - particularly for micro-satellites.Selecting best orbits to meet mission goals.

• Sea StateCan the requirements for new wavemeasurements (period, directional spectra)be satisfied by developments to“standard” altimeters, or do they requirespecific instrumentation?

Improve accuracy and reliability ofmeasurements under severe conditions.

• Sampling RegimesWhich sampling regimes are preferred fordifferent applications?

3.1.4 Sea StateWave PeriodNew wave period algorithms are beingdeveloped. They promise an accuracy of~0.5 s, though further testing is required.

Directional Wave Spectra / Separate windsea /swell informationDirectional wave spectra (and separate windsea swell information) can be provided onlyby more sophisticated instrumentation,capable of taking multiple looks at the samepatch of ocean from a full range of angles.This can only be achieved realisticallythrough synthetic or real aperture radaroperations.

Severe ConditionsFurther studies are required to developimproved algorithms and verify theaccuracy of altimeter wave and windmeasurements in extreme conditions. Thismay involve case studies of individual well-monitored events.

3

4

5

6

7

8

9

10

11

12

13

14

15

2

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47Parameter Accuracy Range of valid data Spatial Res 1 day / Comments

monthly climatol.Jason GDR1 SSH 4 cm

Hs 0.25 m 0-15m U10 1.5 ms-1 0-15ms-1 1350 km/ 2° x 2°Tz 0.5s 4-10 s Dirn spectra, Not Availableseparate wind sea/swell∆σ0

2 (various applications) 0.1 dB 1350 km/ 2° x 2°

ENVISAT SSH 5.0 cm as Jason 1350km / 2° x 2° 2nd frequency S bandHs, U10, Tz, ∆σ0) as Jason

Jason & SSH 4 – 6.5 cmTOPEX & Hs, U10, Tz (also ∆σ0 43 satellite samplingENVISAT / for JAS, TOP, & as Jason as Jason ~350km4 / 1° x 1° available now (fromERS-2 & GFO ENV) Sept. 2002 onwards)

SIRAL SSH, Hs, U10, Tz & as Jason Possibility to derive other∆σ0 as Jason as Jason High resolution along sea state parameters

track ( < 1km)

Swath SSH 4-10 cm? 20km in 10 days 100km dual sided swathaltimetry (variable across

Hs, U10, Tz & ∆σ0 swath) as Jason as Jason Sea state measurements(only at nadir) (only at nadir) 1350 km/ 2° x 2° only given at nadir

GANDER - SSH Alt requires 3 sats ~350 km /“Basic” Ku modification 1° x 1.3° Single frequency, soaltimeter for range no ∆σ0 parameters.Constellation Hs, U10, Tz (not ∆σ0) measurements as Jason(no radiometer) Original design 6 sats ~175 km /

specification 0.75° x 1°0.5 m, 2 ms-1

AltiKa SSH 5.5 cm4 3 sats: Single frequency, so noConstellation Hs, U10, Tz (not ∆σ0) Original design ~350 km / 1° x 1.3° Ds0 parameters.

specification as Jason 6 sats: Ka less attenuated by0.5 m, 2 ms-1 ~175 km / 0.75° x 1° ionosphere than Ku

~5 km footprint (but more attenuated by rain)

SWIMSAT SSH 6 cm Nadir – as Jason. It A single frequency Ku bandHs, U10, Tz (not ∆σ0) Nadir – as Jason Nadir – as Jason may be possible to nadir radar plus a rotating

estimate Hs, U10, Tz wave measuring real apertureoff-Nadir. radar. 100 km dual sided swath

Directional spectra 15° direction 50-90km cells,10% wavelength 100km res. after 7 days

Separate wind sea/swell, Possiblelong wave steepness

WITTEX5 SSH 4 cm Various Tracks may not be evenlyhigh res. along track separated.

Hs, U10, Tz (not ∆σ0) as Jason as Jason ( < 1 km)

GPS SSH 6cm after 10 days6 estimated average Feasibility under study, notreflectometry track separation of available until 2010+.

Hs, U10, Tz To be determined 75 km over 1 day. No ∆σ0 parameters.

Table 3.1 Measurement capabilities of existing and proposed missions.

1 The near real time data set (OSDR) will have lower accuracies.2 Various parameters derived from ∆σ0 , mostly experimental, and

accuracy yet to be verified.3 ERS-2 and ENVISAT effectively sample the same ocean signal (on the

same ground track 30 minutes apart).4 Assumes a 5 cm orbit accuracy and 0.8 cm range noise.

5 The Wittex proposal includes a number of possible orbit configurations,

including tracks separated by ~30km to provide across track slope

and hence full 2D geostrophic velocities. Such a configuration would

provide highly correlated sea state information however, and would be

of limited interest in sea state applications.6 Averages of a number of measurements over a period of days may be

required to bring down the errors.

3.1.5 Sampling RegimesStudies investigating the effectiveness ofdifferent sampling strategies for monitoringmeso-scale sea surface height features haveindicated that the sampling provided byevenly spaced ground tracks (achievedthrough “interleaving” orbits of differentmissions) is to be preferred to that frommore closely spaced parallel tracks (e.g.0.5° proposed to provide continuous acrosstrack slope and so 2-D geostrophic velocity).

Intuitively the same result may be expectedto hold for sea state applications. Two datasets on parallel tracks separated by 50kmcan be expected to contain highly correlatedinformation, and so would provide lessuncorrelated data (and so a smaller volumeof independent sea state information) thanwould tracks separated by (say) 200km.

However, further simulations are requiredto provide a rigorous assessment of therelative benefits of different samplingscenarios offered by e.g. swath altimetryand micro-satellite constellations, beforeone is adopted in preference to the other.

3.1.6 SummaryTechnical solutions exist to address manyof the issues raised. Some solutions arerelatively straightforward and could be easilyapplied, for instance the use of dual satellitecrossovers to improve orbit accuracy. Othershave significant associated costs; indeedto satisfy some requirements would requirespecific missions.• Range corrections – Further studies are

required to investigate errors associatedwith using modelled wet tropospherecorrections. The use of modelledionospheric corrections has proved asatisfactory solution for single frequencyaltimeters, especially when using Ka band.

• Orbit tracking – Dual satellite crossovers can be used to generatesatisfactorily precise orbits (5 cm radial accuracy). DORIS offers an option to provide near-real-time orbits.

• Orbit Choice – There are conflictingrequirements between higher altitudegiving more accurate orbits, but alsorequiring more power from the altimeter.

• Wave direction, Wave spectra, Separatewind sea/swell measurements –Requires specific wave measuringinstrumentation – probably on a specific mission.

• Wave period, Wave/windmeasurements under severe conditions– Improved algorithms should bedeveloped, as well as appropriatetechniques for merging different sets of data into models which will improveour capacity of detecting and forecastingsuch events.

• Improving Sampling – What are relative benefits of various techniqueswhich can be used to improve sampling(swath measurements, constellations ofmicro-satellites)? Further simulations arerequired to find best sampling regimesfor different applications.

• Maturity of Technologies – On what time scale will GPS reflectometry providea practical measuring capability?

3.2 Recommendationsfor Altimeter MissionsFor full report see: http://www.altimetrie.net/docs/GAMBLE_WP8v1.pdf

3.2.1 Objectives and ApproachThe aim of this final aspect of GAMBLE wasin many ways the most important. Takinginto account:• Technical capabilities of available (and

developing) technology,• Characteristics of existing and anticipated

altimeter missions,• State of existing and emerging

technologies (for platforms andinstrumentation),

• Estimated time to develop and bring thesenew technologies into operation,

• The need to optimise costs,

- we wished to identify realistic options forsatellite missions which would satisfy thekey requirements from the operational,climate monitoring and researchcommunities.

Based on the input of the GAMBLE reportsfrom Themes 1-4 CNES compiled a draftset of recommendations. These wereexpressed for 3 time frames:• Short-term (2004-2007)• Mid-term (2008-2011)• Long-term (> 2011)

These recommendations were reviewed bythe whole GAMBLE team, and at a workshopheld in Toulouse in September 2003. Thefinal recommendations outlined below werealso presented to the wider community(including representatives from the USA) atthe final GAMBLE workshop held in Arlesin December 2003.

3.2.2 Summary of RequirementsSea Surface Height ApplicationsThe minimum requirement will be to continueflying a two satellite configuration with onelong-term precise mission (T/P/Jason series).This can be very significantly improved withan optimized three satellite configuration(reduction of sea level and velocity mappingerrors by a factor of about 2 to 3). To resolvethe high frequency and high wavenumbersignals, a constellation of more than 6satellites and/or use of wide swathtechniques would be required.

Sea State ApplicationsPriorities are improved sampling in time and space (10-30 km/1h-6h) by satelliteconstellation and/or wide swath altimeters(e.g. SWIMSAT), availability of wavedirectional and wave period information by SAR and/or new concepts SWIMSAT),improved performances at coast. Near-realtime availability of data (1-3 hours) isimportant for forecasting and monitoring ofsevere conditions.

Offshore OperationsFor offshore operations, a priority is to gainaccurate knowledge of ocean currents, withimproved spatial resolution. Wave heightremains the most important parameter foroperations and design, but improved space-time resolution is required. Wave directionand period, swell and extreme waves arealso important.

3 N.B. ERS-2 is no longer

providing global coverage.

3

4

5

6

7

8

9

10

11

12

13

14

15

2

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

Box 3.1 Recommendations for altimeter missions for the short term –2004-2007

Recommendations for 2004-2007• Maintain Existing Missions

• As long as they are producing exploitable data,• Even in a degraded configuration (due for instance to deficiencies of

one frequency of dual-frequency altimeters, radiometer, POD systems...),- Assuming the impact is not so severe so as to be incompatible with

basic requirements.- Using dedicated tools and/or models to compensate (tropo/iono

model, cross-over minimization, accurate mean sea surfaces...).

• Continue T/P-Jason tandem mission• as long as possible because of its proven usefulness.

• Continue to provide near-real time capabilities• with the shortest possible delay -

- < 2 days for SSH,- < 5 hours for wind/wave

• - into the operational systems on time, e.g.- DUACS for SSH into Mercator, FOAM, Topaz, MFS.- GTS for wind/wave into ECMWF and national Met Agencies.

• Maintain and support satellite laser tracking,• because of its significant contribution in Precise Orbit Determination.• to provide a fail-safe backup capability.

• Prepare CRYOSAT altimeter data for Oceans applications by:• Ensuring IGDR/GDR data production and access to the ocean users.• Developing appropriate techniques for processing the data.

- Using the repeat sub-cycle (30 days), after constructing, from multiplealtimetric time series, accurate reference mean sea level profilesalong CRYOSAT tracks.

- Developing GIM and Doris dual-frequency ionosphere models.- Improving wet troposphere models, based on other in-flight

radiometers.- Studying the capability of near-real time products for ocean

applications.

3.2.3 Mission Recommendationsfor the Short Term 2004-2007Over this time period, Jason-1. ENVISAT(and GFO) are expected to operate. ERS-23 and TOPEX may continue, although they are now operating beyondtheir planned lifetime. The recommendationsfrom GAMBLE for this period are given inBox 3.1.

3.2.4 Mission Recommendationsfor the Medium Term 2008-2011The GAMBLE team has strong concernsregarding the situation that will exist during2008-11. For this period only one satellitemission, Jason-2 OSTM, is presentlyscheduled. As indicated in earlier sectionsof this report, one single operational missionis not acceptable with regard to SSH, seastate, or operational requirements (e.g.DUACS error mapping higher than 20% withonly one satellite, a limited 13% impact ofaltimetric SWH data assimilation into WAMsea-state model).

Moreover, because of the launch ofJason2/OSTM now scheduled end of 2007at the earliest, there is a high probability ofhaving a very damaging gap between theJason-1 and Jason2/OSTM missions, andso a gap in the time series.

In case of an unexpected nightmarescenario, i.e. a failure of Jason-2/OSTM,and no other altimeters flying, there is nobackup contingency.

Such a situation is incompatible with highpriority issues related to global oceanmonitoring, coastal area survey and waterresources management, as indicated inthe European GMES programme, nor is itsufficient to support operational oceanmonitoring programmes like MERSEA andMFS in Europe, or Mercator and FOAM atthe national level, which are part of theInternational GODAE initiative.

Bearing these strong concerns in mind, theGAMBLE Team recommendations for 2008-2011 are given in Box 3.2.

Options for a complementaryaltimeter mission, 2008-2011Considering the absolute need and the highpriority urgency for at least one conventionalhigh inclination mission, complementary toJason2/OSTM, the GAMBLE teamrecommends a strong endorsement by theEC, with support of space and operationalagencies, to support a decision before mid-2004, between the two options which arerealistically feasible with such a limited leadtime, a “Proven Technology” option (takingthe essential basic components from existing

technology) – Box 3.3, and a “NewTechnology” (partly developed and notcompletely proven) option – Box 3.4.

On the next page we compare the relativemerits of the “Proven” and “New” technologyoptions for the recommended 2nd AltimeterMission to be flown in the 2008-2011 timeframe.

Box 3.2 Recommendations for altimeter missions for medium term –2008-2011

GAMBLE Recommendations for 2008-2011• Maintain current missions, and their real-time capabilities, even in a

degraded configuration, for as long as exploitable data are provided.

• Ensure that Jason2/OSTM is launched on schedule, without further delaybeyond end of 2007, in order to avoid any potential gap in data coveragefrom the Jason-1 mission.

• Activate a Jason1/Jason2 tandem mission.

• Encourage the in-flight demonstration of new techniques,• wide swath radars (WSOA, SWIMSAT),• micro-satellite altimeters (Gander, AltiKa, Doppler, laser altimeters),• over-ocean GPS reflections.

The objective is to assess these systems with regard to requirements, forpossible deployment in future missions (> 2011).

• Demonstrate and assess WSOA on-board Jason2/OSTM, involve PIs/CoIs.

• To consider and encourage public/private partnership to support futuremulti-satellite constellations adapted to sea-state operational requirementsand consequently to SSH high resolution requirements.

• Before mid-2004, to decide upon and initiate plans for at least onecomplementary mission in addition to Jason-2/OSTM to be operationalin 2008.- Such a two-satellite constellation is the minimum configuration, though

not optimum, for dynamic topography needs, but does not meet theSea-State operational requirements.

- Two practical options available: the “proven” or “new” technologyoptions.

Proven Technology(Poseidon 3B)

Less expensive – Poseidon 3Bapproximately half the price ofPoseidon 3A if built at same time

Satellite and payload developmentfeasible in time-frame (< 4 years) ifdecision is made before mid 2004.

Jason 2B (Jason 2 replica) providesopportunity for proven low-costcomplete altimetric system, with 1cmclass POD and radiometerPoseidon 3B on micro-satellite: noradiometer, no PODDual Frequency altimeter –ionospheric corrections, supportsdual frequency applications

New Technology (AltiKa)

Cost higher because of need todevelop new technology

Time-frame may be short fordevelopment required

New class of compact altimeter moresuited to specific applications:coastal, ice, inland.

Opportunity to demonstrate newtechnology which could be appliedin constellations. Possible researchsupport from space agencies?

AltiKa on micro-satellite – built in Karadiometer, DORIS and GPS anoption

Single frequency applications only –but Ka band not sensitive toionospheric delay

Box 3.4 Options using “New” Technology for a 2007altimeter mission.

“New” Technology Option for 2007Altimeter Mission• Altimeter:

AltiKa single-frequency altimeter and integratedradiometer (phase B completed).

• Platform Option 1A micro-satellite platform equipped with• laser retro-reflector for orbit determination (10 cm class

orbit capability)• appropriate attitude control/knowledge (0.1°)• DORIS POD system and/or GPS receiver (< 4cm class

orbit).• Near real time capability.

Repeat orbit to take advantage of past reference meanpasses35 days (ERS type) or 17.5 days (GFO type)

Altitude: 800 km “class”• compatible with micro-satellite operation• better for assimilation• improved low altitude earth gravity field expected from

GRACE and GOCE

• Platform Option 2Passenger on-board an opportunity satellite (no orbitchoice).

Launch opportunity to be found

Box 3.3 Options using “Proven” Technology for a 2007altimeter mission.

“Proven” Technology Option for 2007Altimeter Mission• Altimeter:

Poseidon 3 B altimeter (replica of the Jason2/OSTMaltimeter).

• Platform Option 1A micro-satellite platform equipped with• laser retro-reflector for orbit determination (10 cm class

orbit capability)• appropriate attitude control/knowledge (0.1°)• near real time capability.Repeat orbit to take advantage of past reference meanpasses

35 days (ERS type) or 17.5 days (GFO type)Altitude: 800 km “class”

• compatible with micro-satellite operation• better for assimilation• improved low altitude earth gravity field expected

from GRACE and GOCE.

• Platform Option 2Passenger on-board an opportunity satellite (no orbitchoice)

• Platform Option 3Jason 2B clone (same satellite and same payload asJason 2/OSTM, including 1 cm class POD and radiometer)

Launch opportunity to be found

3

4

5

6

7

8

9

10

11

12

13

14

15

2

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

3.2.5 Mission Recommendationsfor the Long Term - Post 2011The GAMBLE recommendations for the“long-term”, i.e. beyond 2011 are given inBox 3.5 Recommendations for threepossible missions, in addition to the plannedNPOESS / Jason pair are given:• A multi-satellite constellation based on

new design altimeters which can behosted on micro-satellites.

• Operational use of wide swath sea surfaceheight measuring techniques.

• SWIMSAT wave measuring radar –theonly option which addresses the userrequirement for directional wave spectra.

A further recommendation suggests thatresearch continues into the technique ofusing reflection of GNSS signals from thesea surface.

It is possible to conceive a combination ofswath altimetry and micro-satelliteconstellations, the first to provide fineresolution (15km) of features in sea surfaceheight, the second to provide frequentrevisits of ocean regions and to improve seastate sampling.

Right we compare the capabilities of thetwo techniques.

Wide Swath Altimetry

New technology, with challengingdevelopments required. Unlikely tobe ready for operational applicationuntil > 2011.

No sea state measurements, exceptat nadir.

Can satisfy operationaloceanography requirements for seasurface height, but not sea state.

Can provide global mapping of seasurface height at 15km resolution in5 days. Allows “imaging” of seasurface height features.

Single satellite – severeconsequences of failure, if this werethe only altimeter mission.

Constellation of Micro-satellites

More conventional technology, lessdevelopment required. Can be readyfor operation in 2007, if decision istaken in early 2004.

Sea surface height and sea statemeasurements.

Can satisfy operationaloceanography requirements for seasurface height and sea state.

Will provide lower spatial resolutionin sea surface height than swathaltimetry.

Multi-satellite constellation is robustto single satellite failure. May require> 1 launch, more complex groundsegment?

GAMBLE Recommendations for Post 2011Because, as recommended in European environmental monitoring programmes (e.g.GMES), there is a need to sustain an optimum altimetric service compatible withoperational requirements and to support important new research in the ocean (meso-scale, coastal, tides, sea-state), over ice and inland waters, the GAMBLE teamrecommends that the EC, space and operational agencies:

• Maintain the continuity of the T/P-Jason series over the long termto keep alive this unique reference time series which started in 1992 and which isessential in many applications because of its high accuracy, consistency androbustness.

• Develop and provide operational altimetric systemsProvide an operational altimetric system for systematic and continuous high-resolution sampling of the ocean by:

1.Building and supplying a series of multi-purpose Gander/AltiKa/Ku redundantaltimeters in order to produce low-cost (effect of mass production) altimeterswith integrated radiometers, which can provide the required < 3 cm range accuracyand be accommodated on board a micro-satellite. They could be used for micro-satellite constellation, but also as nadir altimeters for other missions (e.g. WSOAmissions, NPOESS, Ocean Watch...).

2.Launching (in 2011), a multi-satellite constellation (a minimum of 3 satellitescompleted by the NPOESS altimetric mission), based on low-cost, mass-producedmicro-satellites and simultaneous launches. Preferential orbits are 800 km class-orbit, high inclination, sun-synchronous ERS 35 days repeat orbit or non sun-synchronous GFO 17.5 days repeat orbit, but other orbits may be acceptable. < 10 cm class POD based on laser and low-class GPS and altimeter cross-overminimization. Laser, Doris and/or geodetic GPS tracking are optional for moreaccurate, independent and robust POD. Real-time capability. Radiometer on-board preferable but not absolutely needed. Altimeter measurement noise at alevel of < 3 cm.

3.Launching an operational Wide Swath radar altimeter mission, dependant onexperience from in-flight demonstration (WSOA/Jason2), preferably on a highinclination sun-synchronous 10-20 days repeat orbit (non sun-synchronous orbitmay be acceptable). Nadir altimeter needed. Very precise attitude knowledge isneeded. POD preferable but not absolutely required. On board radiometerpreferable.

• Continue the development of new techniques by conducting:

1.A demonstration in-flight SWIMSAT mission. An instrument for measuringdirectional spectra, wind speed, significant wave height, dominant wavelengths,wave slope statistics, providing a unique contribution for constraining much bettersea-state models and improving forecasts. A low altitude orbit (~ 500 km), high inclination (97-98°), is required with a repeat cycle between 8 and 35 days.Additional Sea Surface Topography measurements at the nadir, are exploitableassuming Orbit Determination accurate enough (10 cm class-orbit).

2. Investigate the potential of measurements, by low altitude satellites, of GPSreflections over ocean to measure sea state and sea surface height.

Box 3.5 Recommendations for future altimeter missions in the post 2011 time-frame.

3

4

5

6

7

8

9

10

11

12

13

14

15

2

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

4 Conclusions4.1 GeneralThrough an extensive consultation exercisethe GAMBLE team has carried out a detailedreview of requirements for data productsderived from altimeter measurements of seastate and sea surface, for operational andscientific use (including climate monitoring).The GAMBLE team makes the followinggeneral recommendations:• Encourage space and operational

agencies (CNES, ESA, Eumetsat, NASA,NOAA...) to define a common long-termstrategy, based on the recommendationsin section 3.2.

• Point out the vital link of theserecommendations with the running ofoperational European environmentalsurvey programs, like GMES andMERSEA, which rely on an uninterrupted,adapted data production service. Thebenefits encompass many aims centralto GMES, including the monitoring of longterm climate change (sea level), improvedunderstanding and prediction of inter-annual and seasonal climate variability (El Niño, NAO), and enhancing securityand safety of offshore operations(including shipping, offshore energyexploitation and defence).

4.2 Links to The EuropeanGMES programmeOver the last two years, the joint EC/ESAGMES programme has been taking shapeand the initial period of the GMES actionplan has been completed. GMES nowencapsulates many of the EC priorities inthe realm of environmental policy, anddevelopment of strategic EC capacity inmonitoring the environment.

Ocean Monitoring is one of the prioritythemes under GMES. The stated goal ofthis theme is to expand the Europeancapacity to produce global oceaninformation systems based on existingmonitoring capabilities, in support ofseasonal weather prediction, global changeresearch, commercial oceanography anddefence.

GAMBLE has directly addressed thesetargets and in many ways can be seen tohave pre-empted these recommendations.It has consulted with users frommeteorological agencies, those engaged inglobal change research, in commercialoceanography, defence, and in developingintegrated “operational oceanography”systems. One of the key issues to recognisehere is that, under present plans, “existingmonitoring capabilities”, in regard toprovision of satellite altimeter data, areexpected to degrade significantly in themedium term. At present we are in thefortunate position to be receiving data from5 satellite altimeters. After 2007, unlessaction is taken, we can expect to receiveinformation from only one altimeter. This willhave severe implications for the capabilityof operational systems being developed,such as MERCATOR, MERSEA, MFSTEP,FOAM, and TOPAZ – all of which requireregular assimilation of reliable altimeter datato control the dynamics of their oceancirculation models. In addition, a failure ofthe one altimeter mission planned for theperiod 2008-2011 would break thecontinuous chain of precise altimeter globalsea level measurements seen as an essentialcomponent of the international climatechange monitoring programmes. Early actionis required to avoid this potentially disastroussituation.

4.3 ResearchConstellations of altimeters will play a vitalrole in future scientific research.

Areas where altimeter constellations willhave a significant impact include: -• Sea surface height: Ocean mesoscale,

barotropic signals; coastal oceanography.• Sea State: Mapping the wave climate,

better extreme waves for design, freakwave detection.

• Other applications: Air-sea gas transfer(Kyoto monitoring), land hydrology, landand sea ice monitoring.

Areas that need investigation with regard toconstellations of altimeters include: -• Cost savings for batch production,

constellation optimisation, redundancyand robustness.

• Should we mix wide swath and wavespectrum (SWIMSAT) altimeters in theconstellation? How?

4.4 Future Missions toIncrease SamplingThe solutions proposed by the GAMBLEteam to reduce the problem of inadequatesampling fall into 2 distinct categories:i) Develop and fly new sensors that increase

the swath width.ii) Employ constellations of small satellites

designed to carry special-purposealtimeters developed from existing nadir-pointing models.

Such was the seriousness of the samplingproblem that the GAMBLE team believedthese two approaches should be seen ascomplementary, not alternatives.Recommendations for new activities andmissions included:

2004-2007• Encourage operation of CRYOSAT to

provide ocean data.• Support continuation of laser tracking.

2007-2011• To decide, within the next months, at least

one complementary mission for flying in2008

• To encourage demonstration of newtechniques, including altimeters for micro-satellites, wide swath altimeters, and newwave radars.

• To demonstrate the “Wide Swath OceanAltimeter” on-board Jason2/OSTM

2011 onwards• Provide an operational altimetric system

for systematic high-resolution samplingof the Ocean by:• Buildinga series of multi-purpose low

cost redundant altimeters.• Launching a (minimum 3) multi-satellite

constellation based on low-cost micro-satellites and altimeters andsimultaneous launches (in 2011).

• Launching an operational Wide Swathradar altimeter if supported byJason2/OSTM.

• Continuing the development of newtechniques by conducting- A demonstration in-flight SWIMSAT

mission.- Investigating potential of measurements

of GPS reflections over ocean tomeasure sea-state.

4.5 The Last WordThe GAMBLE recommendations were formulated from a

thorough analysis of all aspects of satellite altimetry by a

representative group of European specialists drawn from

research, marine industries and service companies. They share

a common concern. Monitoring of the ocean surface by more

than one polar-orbiting, altimeter-carrying space platform is

vital to achieving their objectives, and the objectives of the

European Communities. There is little time left to implement

an effective system.

The mission recommendations from GAMBLE offer a

practical route to ensuring a back-up altimeter system is flying

in 2008-2011. This represents the most fundamental tool for

supporting the development of operational oceanography,

together with improved monitoring of severe ocean weather

conditions. We must also sound a warning note that, if the

EC/ESA wish to encourage the introduction of commercial

services on the back of these systems, businesses will be most

reluctant to commit capital unless they can be assured of long

term data supply. They will not risk investing in systems built

on satellites designed primarily for research use, whose

replacements cannot be guaranteed.

The opportunity now exists for Europe to take the lead in the

deployment of monitoring systems based on constellations of

robust, but technically advanced, micro-satellites. Such systems

have the benefit of built-in redundancy, and can be produced

very economically when compared to multi-instrumented

research satellites.

3

4

5

6

7

8

9

10

11

12

13

14

15

2

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

5 ReferencesBerry, P A M, 1995. Geodetic Research usingSatellite Altimetry. Proceedings, I.M.A.Conference on Image Processing, OxfordUniversity Press.

Berry, P A M, 1999; Global Digital ElevationModels – fact or fiction. Astronomy &Geophysics, June 1999. Vol. 40-3.

Challenor, P G and Srokosz, M A, 1989, Theextraction of geophysical parameters fromradar altimeter return from a non-linear seasurface. Pp 257-268 in Mathematics in remotesensing ed. Brooks, S.R., Institute ofMathematics and its Applications.

Chelton, D B, 2001. Report of the High-Resolution Ocean Topography ScienceWorking Group Meeting. College of Oceanicand Atmospheric Sciences, Oregon StateUniversity, Corvallis: University of MarylandConference Centre, Maryland.(http://www.oce.orst.edu/po/research/hot swg/)

Cotton, P. D., 1998, A Feasibility Study for aglobal satellite buoy inter-calibrationexperiment, Southampton OceanographyCentre Research and Consultancy ReportN0. 26, for the BNSC.

Elfouhaily, T, et al. 1998. Estimation of windstress using dual-frequency Topex data. J.Geophys. Res.,103, 25101-25108.

Gitay, H., Suarez, A., Watson, R.T., Dokken,D. J., 2002 Climate Change and Biodiversity,IPCC Technical Paper V, WMO – UNEP.

Glover D M, Frew, N M, McCue, S J, andBock, E J, 2002, A multi-year time series ofglobal gas transfer velocity from the TOPEXdual frequency, normalised radar backscatteralgorithm. Pp 325-332 in Gas Transfer atwater surfaces AGU Geophysical Monograph127 Ed. Donelan, M., Drennen, W M,Saltzman, E S and Wanninkhof, R., AGU.

Gommenginger, C P, Srokosz, M A andChallenor P G, 2002, An assessment of globalwind stress retrieval from nadir altimeters inthe presence of swell. Proc. 6th Pacific OceanRemote Sensing Conference (PORSEC 2002),Bali, Sept 2002.

Gommenginger, C P, Srokosz, M A; Challenor,P G; Cotton, P D, 2003, Measuring oceanwave period with satellite altimeters: A simpleempirical model. Geophys. Res. Lett., 30(22),2150, 0.1029/2003GL017743

Grant C., and C. Shaw, 2001, OperationalOceanographic Needs for the Offshore Oiland Gas Industry. GOOS Data products andServices Bulletin, Volume 1 (http://ioc.unesco.org/

gpsbulletin/GPS1&2/Vol1article.htm)

Hauser D., 2001, SWIMSAT - Surface WavesInvestigation and monitoring from Satellite,A proposal to the Earth Explorer OpportunityMissions of ESA.

Jolly, GW, and Allan, T.D, 2000, GANDERTechnical Feasibility Study, Report to theBritish National Space Centre, November2000.

Le Traon, P.-Y. and G. Dibarboure, 1999:Mesoscale mapping capabilities of multiple-satellite altimeter missions. J. Atmos. OceanicTechnol., 16, 1208-1223.

Le Traon, P.-Y. and G. Dibarboure, 2002:Velocity mapping capabilities of present andfuture altimeter missions: the role of high-frequency signals. J. Atmos. OceanicTechnol., 19, 2077-2088.

Le Traon, P.-Y. and R. A. Morrow, 2001: Oceancurrents and mesoscale eddies. SatelliteAltimetry and Earth Sciences. A Handbookof Techniques and Applications, AcademicPress ed. L.-L. Fu and A. Cazenave, Eds.,Academic Press, 171-215.

Quartly G D, 1998, Determination of oceanicrain rate and rain cell structure from altimeterwaveform data. Part I: Theory. J Atmos OceanTech, 15 (6),1361-1378.

Rosenthal W, Lehner S, Dankert H, GüntherH, Hessner K, Hortsmann J, Niedermeier A,Nieto-Borge J. C, Schulz-Stellenfleth J,Reichert K, 2003, Detection of Extreme SingleWaves and Wave Statistics, WP1 and WP3from Proceedings of MAXWAVE Final MeetingOctober 8-10, Geneva, Switzerland.

Srokosz M A, 1986, On The Probability OfWave Breaking In Deep-Water. J PhysOceanogr 16 (2), 382-385.

Verron, J., P. Bahurel, E. Caubet, B. Chapron,J.F. Cretaux, L. Eymard, C. le Provost, P.Y. leTraon, L. Phalippou, F. Remy, E. Thouvenot,L. Tournadre, and P. Vincent, 2001, A micro-satellite Ka-band altimeter mission, IAF01,October 2001, Toulouse, France.

Woolf, D K, and Challenor, P G, 2002,Statistical comparisons of satellite and modelwave climatologies. Pp 640-649 in OceanWave Measurement and Analysis Ed. Edge,B. L. and Hemsley, J.M, ASCE

AppendixLinks to GAMBLE Documents

Documentation Web Pagehttp://www.altimetrie.net/documents.shtml

Reports on New Altimeter conceptsAlcatel report: Radar state of the art and new concepts for GAMBLEhttp://www.altimetrie.net/docs/alcatel_altconcepts.pdf

ALTIKA: A micro-satellite Ka-band altimetry missionhttp://www.altimetrie.net/docs/altika.pdf

WITTEX: A multi-satellite delay doppler radar altimeter proposalhttp://www.altimetrie.net/docs/wittex.pdf

WSOA: The measurement capabilities of wide-swath ocean altimetershttp://www.altimetrie.net/docs/wsoa.pdf

High-resolution ocean topography from GPS reflectionshttp://www.altimetrie.net/docs/gps.pdf

SWIMSAT – A wave measuring satellite radarhttp://www.altimetrie.net/docs/GAMBLE_kickoff_swimsat.pdf

GANDER – A proposal for a constellation of wave measuring micro-satelliteshttp://www.altimetrie.net/docs/GAMBLE_delft_cotton_gander.pdf

GAMBLE “Theme” ReportsTheme 1 Final Report: Sea height error budgets, feature detectability:http://www.altimetrie.net/docs/GAMBLE_finalreport_theme1.pdf

Supplementary Theme 1 Report: The contribution of the TOPEX/Poseidon - Jason 1tandem mission to meso-scale variability studieshttp://www.altimetrie.net/docs/GAMBLE_tandemreport_theme1.pdf

Theme 2 Final Report: Sea state error budgets, feature detectability:http://www.altimetrie.net/docs/GAMBLE_finalreport_theme2_030126.pdf

Theme 3 Final Report: Orbit determination and satellite tracking:http://www.altimetrie.net/docs/GAMBLE_finalreport_theme3.pdf

Theme 4 Final Report: Future Priorities for Satellite Provision of Sea State and OceanCurrent Data - 2005 and beyond: Marine Operator’s Workshophttp://www.altimetrie.net/docs/GAMBLE_OWS_Frep.pdf

GAMBLE Mid-term Review: Technical Report on Error Budgets and Technical Solutionshttp://www.altimetrie.net/docs/GAMBLE_mtr_d8.pdf

Theme 5 Report: Framework for Recommended Research Programmehttp://www.altimetrie.net/docs/GAMBLE_WP7.pdf

Theme 6 Report: Recommendations for Future Altimeter Programmes (Satellites, payloadsand Orbits).http://www.altimetrie.net/docs/GAMBLE_WP8v1.pdf

3

4

5

6

7

8

9

10

11

12

13

14

15

2

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

Satellite Observing Systems

15 Church Street

Godalming

Surrey GU7 1EL

T: +44 (0) 1483 421213

F: +44 (0) 1483 428691

E: info@satobsys.co.uk

www.satobsys.co.uk

Satellite Observing Systems

Centre Spatiale De Toulouse (CNES)

18 Avenue Edouard Belin

Toulouse 31-401, CEDEX 4

FRANCE

T: 00 33 5 6127 4872

F: 00 33 5 6128 2595

www.cnes.fr

GAMBLE