ARCTIC EXPERIMENTS TO SUPPORT CRYOSAT

Cullen, R.,1

Davidson, M. W. J.1, Francis, C. R. and Wingham, D. J.

2

1European Space Agency, ESTEC, Keplerlaan 1, Postbus 299, 2200 AG Noordwijk, The Netherlands. 2Department of Earth Sciences, University College London, Gower Street, London WC1E 6BT, UK.

Abstract

The primary goals of CryoSat-2 are to derive improved

rates of change of elevation and thickness estimates of

the Earth’s land and marine ice fields. Validating such

retrievals obtained from a nadir polar observing

Synthetic Interferometric Radar Altimeter (SIRAL), the

primary payload, is not a simple one. A key problem is

to fully understand the uncertainties and acquire many

different types of in-situ measurements in highly

inhospitable regions of the cryosphere and at particular

times of the year to allow detection of inter-annual

cycles. In order to correlate retrievals from CryoSat

with the local in-situ data it was decided early in the

CryoSat development that an aircraft borne radar

altimeter with similar functionality to SIRAL would

provide the necessary link between local and regional

spatial scales and provide pre-launch incite into

expected performances and issues.

In 2001 ESA commenced the development of a

prototype radar altimeter that mimics the functionality

of SIRAL-2 with subtle functional differences. The

airborne SAR/Interferometric Radar Altimeter System

(ASIRAS) has now been the centrepiece instrument for

a number of large scale land and sea ice field campaigns

in the Arctic during spring and autumn 2004 and 2007.

This paper describes the different types of airborne and

in-situ measurements acquired, key science results and

the importance of past campaigns and those planned in

the run up to a CryoSat-2 launch.

1. INTRODUCTION

CryoSat was the first of the ESA Earth Explorer

opportunity missions, a mission proposed by the

scientific community (Wingham et al, 1998). The

proposal was underpinned by an urgency to reduce

uncertainties in elevation measurements over land and

sea ice fields not available previously using pulse-width

limited radar altimetry. CryoSat was to be launched into

orbit on 8th

October 2005; however, a catastrophic

failure of the Eurokot launcher occurred a few minutes

into the launch sequence resulting in the loss of the

mission. Since the improvement in the understanding of

key climate issues that instigated the original CryoSat

proposal are more urgent than ever (see §7.5, IPCC,

2001 and 2007) it was recognised on an international

scale that there was an overriding need to recover the

mission. In Feb 2006 the ESA Programme Board for

Earth Observation approved a recovery mission and

ESA are procuring CryoSat-2 for a launch in 2009.

One of the many major challenges with obtaining

mission success is with the verification and validation of

the CryoSat retrievals. CryoSat is primarily a mission to

measure ice sheet elevation, sea-ice freeboard and

thickness and their rates of change (Wingham, et al,

1999). The key uncertainties for sea ice thickness and

freeboard measurements are the knowledge of snow/ice

density, snow depth/loading and floe sampling and

omission. For land ice elevation determination the

temporal and spatial changes in snowfall and near

surface density are the main uncertainties.

It is necessary to design and test a validation

infrastructure combining airborne and field campaigns,

the CryoSat Validation Experiment (CryoVEx), at an

early stage that could cope with both the logistical

difficulties of operating in some of the most hostile

environments of the Earth whilst being effective both in

terms of cost and science return. These activities are

undertaken by the CryoSat Validation and Retrieval

Team (CVRT).

2. THE DESIGN AND OBJECTIVES OF

CRYOSAT

2.1. The Mission Objectives

The CryoSat proposal (Wingham et al, 1998) and

formalised mission requirements (Wingham, 1999)

highlighted key science concerns that translate into two

primary mission objectives to reduce uncertainties in the

knowledge of sea ice thickness and its rate of change

(Laxon et al, 2003), its role in radiative balance (§7.5.3

of IPCC, 2001) and improve understanding of mass

balance and its rate of the major land ice fields (see

§11.2.3 and §11.6.4 of IPCC, 2001). The key mission

requirements are provided in Table 2-1 were computed

based on estimates of known uncertainties for a mission

exploitation phase of 3 years using a new type of pulse-

width limited phase coherent radar altimeter with

interferometric capability.

Surface Area

(Km2)

Measurement requirement

(cm/year)

Arctic Sea Ice 105 1.6

Ice Sheets 104 3.3

Ice Sheets 1.3 x 106 0.7

Table 2-1 Mission requirements taken from Wingham 1999.

_____________________________________________________

Proc. ‘Envisat Symposium 2007’, Montreux, Switzerland 23–27 April 2007 (ESA SP-636, July 2007)

2.2. The Platform, Payload and Ground Segment

In order to achieve the scientific requirements for the

mission requires the development of a specific platform

and payload. The platform incorporates the primary

radar altimeter payload with supporting instrumentation

and is to be placed into a near polar circular orbit of

~92° inclination (see Ratier et al, 2005). The re-build of

CryoSat as shown in Figure 2-1 incorporates some

improvements based on lessons learned from the

original design and is described in Francis, 2007.

Figure 2-1 An internal view of the CryoSat-2 system with SIRAL-2

radar in the front (right hand) portion of the system. Some

improvements have been implemented to improve reliability.

The payload consists of:

13.65 GHz Synthetic Interferometric Radar

Altimeter (SIRAL) with 3 science modes of

operation:

• Low rate pulse limited mode (LRM) for use

over land ice sheet interior – typical of an

ocean altimeter (McGoogan, 1975).

• Synthetic aperture radar (SAR) mode for use

over sea ice.

• Synthetic aperture & interferometer (SARIn)

mode for use over the ice sheet margins.

• Several internal calibration modes.

Use of the DORIS sub-system and beacon

network for precise orbit determination (POD).

Laser Retro Reflector to assist with POD.

Three star trackers for precise SIRAL

interferometer baseline orientation

determination.

Ionospheric and tropospheric effects have low spatial

and temporal variability over the poles and so CryoSat

is able to meet all its science objectives (Wingham,

1999) without the need for additional microwave

radiometer or dual-channel altimeter payloads.

The ground segment converts raw science and

supporting data into user products and is described in

Francis et al, 2005. It is the purpose of this paper to

explain some of the level 2 product validation methods

and experiments already conducted with those that are

in preparation pre launch and post launch during the

commissioning and exploitation phases. Figure 2-2

shows the geographic mode mask developed for

CryoSat-1 and indicates which science or cal/val mode

SIRAL will be operated in for any given ground-track

location. The mask will be updated to take into account

new cal/val zones prior to CryoSat-2 launch.

Figure 2-2 CryoSat Cal/Val measurement masks and AO zones which

will updated for CryoSat-2

Other CryoSat related documentation is available for

download on the ESA living planet web site

(http://www.esa.int/esaLP/LPcryosat.html).

3. VALIDATION OF RETRIEVALS

It is worth noting the differences between calibration

and validation as identified for the CryoSat mission.

Calibration is required as a result of imperfections

within the CryoSat measuring system that will lead to

errors within level 1b data sets if not corrected. To give

an example, the transfer and impulse response functions

of the SIRAL radar are subject to imperfections (in

phase & amplitude) within its electronic sub-systems

that degrade the performance of the instrument. Level

1b products, by definition, are calibrated to reduce, as

far as possible and within requirement budgets, residual

measurement errors.

The understanding of the geophysical errors and

remaining measurement errors that allows one to

determine to what accuracy one can convert level 1b

(echo waveforms) to level 2 products such as estimates

of land ice elevation and sea-ice thickness and

freeboard, for example, is the objective of validation. In

order to verify that mission requirements are met it is

necessary to determine the co-variances of level 1b and

level 2 errors (CSAG, 2001). For CryoSat, an

independent validation strategy via the use of

coordinated ground based in-situ and airborne

campaigns is required in order to allow the bridging of

the spatial scales between in-situ and CryoSat

measurements. Furthermore, since radar energy

interacts differently with snow and ice as a function of

annual and seasonal cycles it is necessary that time-

variant errors can be analysed following repeat

acquisitions; this results in the constraint as to what

periods of the year campaigns are planned to allow the

optimal extraction of seasonal/annual information.

Unfortunately airborne and in-situ acquisitions are

subjected to the uncertainty of the local meteorological

conditions at any given time of a campaign.

Land ice uncertainties

Uncertainty Proposed CAL/VAL

activity

Error in spatially averaged ice sheet

imbalance (Containing 4 elements)

1. Covariance of snowfall fluctuation New ice core observations

etc.

2. Covariance of near surface density Improved modelling, etc.

3. Covariance of elevation trend error Defined sub-uncertainties

4. Post glacial rebound error Model inter comparison

Sea ice uncertainties

Error in spatially averaged sea ice

thickness (Containing 4 elements)

Moored and submarine

measurements

EM bird thickness measures

Borehole measures

1. Covariance of uncertain snow

loading

Re-appraisal of snow depth

records

2. Covariance of ice density EM thicknesses and freeboard

sounding.

3. Error covariance due to sampling of

thick floes

Joint PDF of floes area thick

ness, etc

4. Ice freeboard error covariance Defined sub-uncertainties

Table 3-1 Land/Sea ice high-level uncertainties taken from (CSAG,

2001). Top level uncertainties are indicated in red and uncertainties

in blue indicate they are broken up in sub-categories in the planning.

3.1. The role of the CVRT

Following the release of the CryoSat calibration and

validation concept (C-SAG, 2001) an ESA AO was

released in 2004 with the aim of developing

experiments and associated plans to tackle, where

possible, each of the uncertainties described in Table

3-1 and shown pictorially in Figure 3-1. The CVRT has

an international membership and has recently been

complemented with the selection of PIs from the

CryoSat-2 Cal/Val AO released in early 2007. As will

be described later in §4 and §5 the CVRT have already

made many breakthroughs in terms of experiments.

CVRT operates under a strict ESA campaign budget,

are further funded by national funding agencies and

generally meet a few times per year with the intention of

developing, planning and scheduling experiments in the

logistically difficult environmental regions of the

cryosphere. Given the added constraint of repeating

measurements to pick up seasonal and annual trends

adds to the logistical complexity. CVRT and ESA have

developed methods of communication and coordination

whilst actively implementing campaigns and lessons are

continually learned based on experiences in the field.

Since there have already been a number of campaigns

over the past few years there have been opportunities to

fine tune methods and to identify new areas of research

based on steadily improving knowledge of sea/land ice

properties and changes in uncertainty levels. In view of

this CVRT continues to function and prepare with pre-

launch campaigns in anticipation of the CryoSat-2

launch expected in 2009.

Figure 3-1 Land and sea ice errors in pictorial form.

3.2. Airborne and In-Situ Instrumentation

There are a number of instruments mounted on-board

aircraft and used in-situ for land and sea ice

measurements. Here a non-exhaustive number of them

are described.

ASIRAS - The 13.65 GHz radar altimeter

payload on-board CryoSat-2 is a unique space borne

Earth observation instrument and the validating of its

retrievals will not be simple. Ideally one wants to use

the more accurate in-situ results to perform this however

the larger spatial scales of acquisition cannot be

achieved. An alternative is to use a similar prototype

instrument mounted on-board an aircraft. Although the

geometry is different to a satellite platform (instrument

speeds of ~70m/s opposed to ~7500m/s and elevations

of ~1km opposed to ~720km) one has an opportunity to

cheaply acquire data sets of a similar nature to SIRAL

and learn about the interactions of radar energy with

subtly differing snow and ice surfaces (radar penetration

characteristics) with phase coherent azimuth processed

data.

The D2P (Raney and Jensen, 2000) proved the concept

for an airborne phase coherent altimeter. However, prior

to the 2000 D2P test it was considered by ESA that an

instrument developed under its control could more

easily upgraded to suit the needs of CryoSat validation

and be maintained and tested close to ESA. In view of

this and the D2P results it was decided that the

implementation of ASIRAS in 2001 (Mavrocordatos et

al, 2004) would at a carrier frequency of 13.65 GHz but

with a higher range resolution (~10 cm opposed to

~50cm) to SIRAL/D2P and would allow any internal

layering over land ice to be more easily resolved. As

shown in §5 has already been proven to be the case.

ASIRAS has undergone a number of hardware upgrades

on an as need basis to improve acquisition methods and

it always provides interesting results. Verification of the

ASIRAS L1b data is provided by comparison to laser

scanner derived DEMs and other in-situ results. There

are two science modes of operation for ASIRAS. The

first is the HAM-SARIn mode which allows SAR

interferometric operation between aircraft elevations of

~1 km to 7 km. The LAM-SAR allows SAR operation

between elevations of 300 m and 1 km. Precise location

of instrumentation is provided from DGPS (2 GPS are

generally placed on an aircraft) and orientation provided

from inertial navigation system (INS).

Airborne Laser Altimeter - It is understood that

radar altimeter pulse energy at frequencies of ~ 13 GHz

penetrates through snow to depths which are a function

of the snow properties (density, for example). It is also

understood that laser echo returns from the air-snow

interface and thus placing a laser scanner on-board the

same platform as ASIRAS allows a comparison to take

place between the two measures and an understanding

of ASIRAS pulse penetration within snow to be gained.

Processed laser scanner data can be verified by the use

of acquisitions over known targets such as hangar

buildings and runways. Runways can very easily be

characterised using GPS profiling. Laser

altimeter/scanners suffer from contamination from

clouds and thus the use of this instrument is highly

dependent on the local conditions. A review of Lidar

usage for validation purposes can be found in Forsberg

et al, 2002.

Helicopter Electromagnetic Induction (HEM-

Bird) - Used for sea ice thickness determination, the

HEM-Bird is a piece of equipment consisting of an

electromagnetic sensor for which an electromagnetic

induction sounding technique is used to obtain

thickness. The HEM-Bird is suspended some few tens

of meters above the identified surface. Typically, a laser

scanner will be placed on the ‘bird’ to measure the

distance to the air-snow surface and improve inter-

comparison of results.

In-situ experiments for land ice cover activities to

determine surface height including characterisation of

near surface snow and ice properties in order to address

retrieval errors due mainly to spatial and temporal

snowfall and near surface density. Instrumentation

includes Ground Penetrating Radar (GPR) at several

frequencies (200-1000MHz), gamma profiling,

extraction of deep and shallow ice cores, neutron probe

density profiling, nested stake locations, DGPS

profiling, surface roughness characterisation via

photography at varying distances and coffee can

densification monitoring. Near surface characteristics

can be determined by the use of snow pits however this

activity disturbs the surface and therefore must be

conducted away from airborne ground-tracks that may

be repeated as a function of season. Corner reflectors

allow analysis of radar energy penetration.

Sea ice in-situ experiments allow distributions of local

and regional sea-ice thickness’ to be derived and

compared from those from CryoSat. Instrumentation

includes drilling to determine thickness and freeboard

height. In some regions upward looking sonar (ULS)

and autonomous underwater vehicles (AUV) will be

deployed. Buoys can also be deployed allowing the

recording of ice thickness. Determination of ice and

snow bulk density can be estimated by a means of

comparing sample cases or via the use of dielectric

resonators. Salinity, wetness and grain size are also

derived. Snow thickness can be determined by the use

of ruler or drill holes. Deployed corner reflectors allow

analysis of the airborne data and the penetration of radar

energy into the snow.

Other equipment used typically consists of Automatic

Weather Stations (AWS), hand-held GPS, bulk density

and electromagnetic measurements and laser levelling

devices.

4. EXPERIMENTS FOR CRYOSAT

VALIDATION (CRYOVEX)

4.1. Pre 2003 experiments

The Greenland testing of the Johns Hopkins University

prototype Delay-Doppler radar (D2P) during June 2000

(Raney and Jensen, 2000) is what one can consider as

the starting point for the airborne aspects of the CVRT

implementation. The unique results acquired from the

test showed the usefulness of an airborne phase coherent

radar altimeter. Results from azimuth processing

provide improved SNR and along-track sampling over

snow covered surfaces of differing properties and show

evidence of radar penetration in the upper surface

layers. The results spurred ESA to join with NASA to

fund the Laser and Radar Experiment (LaRa) in 2002

covering acquisitions over both land and sea ice regions

in the Arctic on a NASA P-3 aircraft (Raney et al,

2003a and 2003b). Further evidence of the effects of

penetration over the Greenland ice sheet in a dry snow

zone was provided and the wide variation in radar

returns between first year and multi-year sea-ice echoes.

Comparing the acquired Laser (ATM3) with D2P

waveforms showed what is believed to be the return of

the laser from the air-snow interface with effects from

the radar waveforms. HEM-Bird was also used in the

campaign.

4.2. ESA Funded Campaigns 2003-2007 and

beyond

In many ways it is impossible to summarise in a paper

all the achievements of conducting campaigns over the

past few years under the cost and logistical constraints.

Here we list some of the high level details concerning

campaigns. It is worth noting that all airborne ASIRAS

and laser data acquired by AWI and DNSC have been

processed to level 1b and, with the exception of the

recently acquired 2007 data, are available for use by the

existing CVRT. ASIRAS data processing is conducted

at AWI using a processor based on the CryoSat level 1b

processing. Hence the CryoSat level 1b SAR/SARIn

processing method had already been verified prior to the

CryoSat-1 launch failure (Cullen et al, 2007).

CryoVEx 2003 (Land/Sea Ice)

Prior to the availability of the ASIRAS instrument ESA

funded a medium scale Arctic campaign that took place

in April 2003 using the D2P and laser scanner and

resulted. This campaign made significant acquisitions

over land ice combining radar, laser and additionally

with (EM-Bird) over sea ice. Flight tracks are provided

in Figure 4-1.

Figure 4-1 CryoVEx 2003 Flights. (Taken from Keller et al 2003)

CryoVEx 2004 (Spring and Autumn) Arctic grand tour

In 2004 two large airborne (under AWI coordination)

and in-situ campaigns were carried over land ice in

Greenland, Devon island and Svalbard. The two

campaigns were carried out in April/May and then also

September for which it should be possible to extract

seasonal effects of snow melt. This appears to be the

case and if one analyses the ASIRAS waveforms in

some regions power levels do with respect to layering

appear to vary.

CryoVEx 2005 (Bay of Bothnia) – One problem noted

in previous campaigns was the combined use of laser

and radar airborne equipment was not optimal and could

be improved by the development of the LAM-SAR

mode within ASIRAS allowing both radar and laser to

be operated at aircraft elevations below 1000m. This

small campaign lasting a few days and allowed the

testing of the new mode with a range of ground based

measurements. Data and results of these data are in

circulation within the CVRT.

CryoVEx 2006 Arctic grand tour – Following

certification of the ASIRAS on Air Greenland aircraft

(under DNSC coordination) an extensive campaign took

place in April/May 2006 combining both land and sea-

ice (north of Alert) acquisitions with an equally

extensive series of land/sea-ice based experiments.

Results of this campaign are being analysed.

Table 4-1 CryoVEx 2006 aircraft ground track. The aircraft component

of the campaign was conducted by DNSC. (Courtesy DNSC)

CryoVEx 2007 Land ice (Austfonna ice cap)

In early 2007 what is likely to be the last of the major

hardware upgrade to the ASIRAS was performed. This

was due to an unfortunate side effect of the LAM-SAR

mode resulting in very high data volumes. A land ice

campaign conducted on-board DLR D-Code do-228

took place in Svalbard during mid-late April with the

science objective of acquiring airborne laser and radar

altimeter data over Austfonna along ground based

profiles for which GPR, Neutron scattering probe, etc.

were also acquired. In addition many airborne

acquisitions were made along the Kongsvegen glacier

for testing of the new reduced data volume LAM-A-

SAR mode and to conduct some experiments to

understand the effects of interferometric coherence for

the HAM-SARIn mode as a function of height. Finally,

on April 21st an opportunity arose to fly along an

EnviSat ground track for which RA-2 data had already

been acquired on 15th

April as shown in Figure 4-2.

Data from this EnviSat acquisition has already been

made available to CVRT. Some other airborne

acquisitions were made to the east of Svalbard over sea

ice.

Figure 4-2 Three colour coded ASIRAS acquisitions (21st April 2007)

covering EnviSat RA-2 ground-track pass 26787 (15th April). The

yellow ASIRAS track traverses Austfonna ice sheet.

Future campaigns leading to launch and beyond

The CVRT is now planning the next activities for pre-

launch that include a period of analysis of existing data

sets with future campaigns covering both land and sea-

ice acquisitions not covered in previously. Following

CryoSat-2 launch there will be a verification campaign

to allow initial analyses of the CryoVEx/CryoSat

retrievals to take place before the actual post-launch

validation campaigns commence.

5. KEY RESULTS

One of the preparation activities for developing and

implementing a validation plan for CryoSat was to build

an operational level 1b processor for the ASIRAS

retrievals. Furthermore, there was a need to install it in a

suitable establishment, ideally close to one of the bodies

conducting the airborne elements of the CryoVEx

campaigns and to allow routine processing of campaign

data. The ASIRAS level 1b processor was built at

ESTEC, ESA and more or less contains the same

science algorithms as those within the CryoSat

SAR/SARIn level 1b processors (Wingham et al, 2006),

though data interfaces are quite different. Results to date

provide an independent proof of verification (see Cullen

et al, 2007) for the CryoSat level 1b processing method.

Since one of the early activities following development

of the ASIRAS instrument was for AWI to show

ASIRAS could be installed, certified and operated

successfully on aircraft it was decided the processors

should also be installed at AWI to prove the pre-launch

end to end concept. The ASIRAS processors have been

installed since 2004 and to date AWI have processed

five large airborne data sets (CryoVEx 2004 spring to

CryoVEx 2007). They are close to delivering the last of

these data sets (2007) to CVRT. The total data

acquisition stands at some ~20Tb of raw data.

There are now many examples showing the gains made

in understanding the uncertainties and complexities of

operating in the Arctic. Here we show two results from

the CryoVEx 2004 campaign in the published literature

regarding land ice acquisitions from ASIRAS. The first

is provided in Figure 5-1 (Hawley et al, 2006) who

showed a direct comparison between ASIRAS level 1b

waveforms and in-situ derived vertical density profiles

computed from neutron scattering probe data. Both data

sets show coherent sub-surface layering close to the T21

region of the dry-snow zone of the EGIG (l’Expedition

glacialogique internationale au Groenland) transect line

in Greenland. From this data it appears ASIRAS can

penetrate up to several meters in such regions.

The second key example provided in Figure 5-2 (Helm

et al, 2007) shows evidence in the processed ASIRAS

waveforms of a distinct sub-surface high density ice

layering from the previous years melt in a region of the

percolation zone of the EGIG line (T05) when

compared with laser scanner derived elevations. This

result is confirmed in an independent in-situ experiment

at the same location using GPR profiling (Scott et al,

2006) obtained during the same campaign and is in-line

with the understanding gained from previous non-

CryoVEx experiments in the EGIG percolation zone.

Figure 5-1 (Hawley et al., 2006). A Neutron scattering probe

measures snow density with depth. It was shown that regions of high

density from the probe compare well up to a few metres of depth with

layering seen in the ASIRAS level 1b data.

Examples of sea-ice analyses with respect to CryoVEx

are not yet available in the open peer reviewed literature

and so results are not presented here. However, there are

strong indications that one can compare ASIRAS with

HEM-Bird and laser scanner derived elevations. We

also see complexities with the interpretation of ASIRAS

returns typically from multi-year ice, pressure ridges

and rubble fields, for example. ASIRAS returns over

first year ice appear clean and relatively easier to

interpret.

Figure 5-2 (Helm, et al 2007) (top) Laser scanner derived DEM and

aircraft ground track computed from acquisitions during CryoVex 2004

grand tour of the Arctic. This profile is acquired from the percolation

zone at T05 on the EGIG line in Greenland. (bottom) Comparison

between laser scanner derived DEM interpolated at locations coincident

with ASIRAS level 1b locations. A specialised re-tracker was designed

to extract the snow air interface and layer from what is believed to be

the previous years melt.

6. CONCLUSIONS

Despite the loss of CryoSat-1 it has been shown that all

mechanisms were in place for a successful verification

and validation with CVRT being able to undertake and

test a large number of cost effective, logistical and

scientifically demanding validation challenges.

A compensation for the CryoSat interested science

community comes from 6 years of ESA led (joint with

NASA for LaRa in 2002) campaigns from which CVRT

has acquired and processed a large repository of laser

and radar altimeter airborne campaign data combined

with surface acquired in-situ measurements. ESA and

CVRT are in the process of interpreting this data.

ASIRAS related campaign activities re-commenced

after CryoSat launch failure in March 2006 with a brief

test campaign to check and obtain certification for the

ASIRAS installation on a Air Greenland Twin Otter

(under DNSC operation) with a number of additional

tests to improve understanding of LAM mode data

acquisitions.

Both a test and campaign of Svalbard land ice has taken

place in 2007 and campaigns are in the planning stages

prior to and following a CryoSat-2 launch.

With the combined efforts of ESA and the international

science community (stretching beyond the membership

of the CVRT) carrying out the coordination of CryoSat

campaigns, data acquisition and analysis, critical issues

relating to the exploitation of radar altimeter and laser

signals over land-ice, sea-ice and ocean continue to be

addressed. In terms of the cryosphere, analysis of issues

such as signal penetration (both spatially and

temporally), correlation of radar signals with known

snow & ice properties, density of near-surface snow

pack layers and snow loading on sea ice will contribute

to more accurate future ESA ocean, land and cryosphere

related radar altimeter missions, such as CryoSat-2 and

Sentinel-3, thus improving scientific exploitation of

their data.

Acknowledgements

The authors wish to thank ESA delegations for their

continuing commitment to CryoSat and respective

national funding programmes that funded a larger

number of studies with respect to CryoSat Cal/Val

activities not detailed in this paper. We thank all

members of the CVRT who provided material for the

paper.

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