gravity field and steady-state ocean circulation explorer...

GGravity field and steady-state OOcean

CCirculation EExplorer

Mark Drinkwater,Mark Drinkwater,

Roger Haagmans &Roger Haagmans &

Michael KernMichael Kern

Mission Science DivisionMission Science Division

www.esa.int/livingplanet/goce

Mission Objectives

and Requirements

Mark Drinkwater, R. Haagmans & M. Kern – November

2006

GOCE

2GOCE Origin

n GOCE owes its origin to:

– Solid-Earth Science and Applications Mission for Europe (SESAME) inthe Early 1980’s (ESA, SP-1080)

– 1987 ESA-NASA Workshop on Joint Solid Earth Programme (SP-1094)

– Former ESA Aristoteles mission (ESA, 1991)

n The first Core missions originated via Consultations with the Usercommunity in 1991 and 1994 (ref: SP-1143; SP-1186)

– Gravity mission recommended as a priority for study

n Early studies by Consortium Involving Gravity Advanced Research:

– CIGAR I and II

• Study of precise gravity field determination and mission requirements.Phase 1 ended Jan. ’89 and Phase 2 in Mar. ‘90.

– CIGAR III and IV

• Study on gravity field determination using gradiometry and GPS.Phase III ended May ‘93 and Phase IV ended in Jan. 1995.

– CIGAR V

• Study of advanced reduction methods for spaceborne gravimetry data andcombination with geophysical quantities. Ended Aug. ‘96.


2006

GOCE

3Scientific Context of Mission

and from the above, improved rate

estimates of Sea Level Rise

Solid Earth Physicsanomalous density structureof lithosphere and uppermantle, and betterconstraints for modelling ofEarth’s interior

Oceanographydetermination of dynamic

ocean topography, absolute

ocean circulation, and mass

and heat transfer

Geodesyunified height systems,

“levelling by GPS” (i.e.

orthometric heights)

Ice Sheetsimproved knowledge of

ice sheet mass balance


2006

GOCE

4Scientific Objectives – Part 1

n Determination of absolute ocean

circulation requires knowledge of

the static geoid, or mean sea-

surface (representing the ocean

at rest)

– Ocean ‘dynamic topography’ is

the difference between the

altimeter-measured ocean

surface and the geoid.

– 1 mGal gravity anomalycorresponds to ~1mrad slope in

ocean surface, or a geostrophic

surface current velocity of ~0.1

m/s

Ocean ‘dynamic

topography’


2006

GOCE


n To provide high spatialresolution information ongravity anomalies in relationto density structure of thelithosphere and uppermantle

n To deliver information ofrelevance to the study ofEarthquakes, Volcanoesand other natural hazards

n To provide information onhigh spatial harmonics inpost-glacial rebound andimprovements in ice sheetmass balance estimates


2006

GOCE

6

+

GOCE geoid

Local

measurements

=

Detailed

regional geoid

n To provide a unified global heightreference surface from which ‘pseudo-levelled or ‘orthometric’ heights canbe derived.

Scientific Objectives – Part 3


2006

GOCE


n Unification of tide gauge records

achieved with knowledge about

post-glacial rebound and by GPS

levelling, (to avoid spurious sea-

level rise/fall estimates).

NL,

1953


2006

GOCE

8Mission Requirements

n To determine the Earth’s gravity field with an accuracyof 1 mGal (1 mGal = 10-5 m/s2 )– 1 “milli-Galileo” is ~ 10-6 of the acceleration of gravity, g

– Requires determining the 3d rate of change of gravitybetween pairs of accelerometers with a desired sensitivity of~4 mE (milli-Eötvös) in each axis

Where 1 E = 1 mGal / 10 km ! 10-9 s-2 (i.e. m/s2 /m)

– Noise specification for single accelerometer in MBW:

2 x 10-12 m/s2 /Hz0.5 (c.f. CHAMP ~10-9; GRACE ~10-10)

n To determine the geoid (= equipotential surface for ahypothetical ocean at rest) with 1-2 cm accuracy

n achieve this at a resolution or half wavelength scale of100 km (approx. degree and order 200)


2006

GOCE

9

0.2 gram

~2E-03 N

1 000 000 tonne

Downforce

Super-tanker acceleration:

29

3

s

m 102

kg 101

N 102 12!!

"#"

"

… get a feeling for the numbers


2006

GOCE

10Comparison of Gravity Missions

CHAMP

• Satellite-satellite

tracking in high-low

(SST-hl) mode

• cm orbit determination

• no drag-free control

• STAR accelerometer

* mission has developed

low resolution geoid

GRACE

• Satellite-satellite tracking

in high-low and low-low

modes


• two platforms with 3d

accelerometers

• intersatellite link

measures precise platform

separation

• no drag-free control

•SuperSTAR

accelerometers

GOCE

• Satellite-satellite tracking

in high-low mode


• Electrostatic gravity gradiometer

comprising 6 accelerometers

• drag-free control to maintain

“free-fall” around proof masses.

• new accelerometers

* GOCE mission designed to resolve high

spatial resolution gravity field

* GRACE mission

designed to resolve time-

varying gravity field at

long wavelengths > 300

km


2006

GOCE

11Measurement Approach

• Electrostatic Gravity Gradiometer(EGG):

Measures the components of thegravity gradient tensor in thegradiometer reference frame within abandwidth of 5-100 mHz

• Satellite-to-Satellite TrackingInstrument (SSTI):

Geodetic-quality GPS receiver allowsorbit reconstitution with an accuracy of~1 cm in all directions and recovery oflower order harmonics

GOCE combines satellite gradiometry

and high-low satellite-to-satellite tracking

in a low Earth orbit of ± 250km altitude,

with unique continuous operation of Drag-

Free Attitude Control to combat the

effects of air drag

EGM96 gradients; Mean value removed


2006

GOCE

12Gradiometer Principle


2006

GOCE

13Gradiometer L1b Data Product

n Gravity gradients: 2nd spatial derivatives of gravitational

potential V

– Gravity gradient tensor (GGT) consists of 9 elements, of which

5 (shown below) are independent due to symmetry and

traceless condition

MBW = Hz1010513 !!

!"

s2005!

km160040!


2006

GOCE

14

Gravity Field Requirements for Science (see SP-1233(1), July 1999)

Gravity Field Requirements Table


2006

GOCE

15Requirements: Solid-Earth Physics

GRACE

gravity field


2006

GOCE

16Requirements: Oceanography

GRACE geoid


2006

GOCE

17Requirements: Geodesy

GRACE geoid


2006

GOCE

18Science and Applications

solid earth sea levelgeodesyiceocean

gravityanomalies

geoidgravityanomalies

geoid

tide gaugesaltimetry

positioning (GPS)

icetopography

oceanaltimetry

post glacialRebound

mean oceancirculation

ice massbalance

orbits

unified heightsystems

levelled heights

unifiedheight system

bedrocktopography

mean oceancirculation

seismictomograpy

topography

deformations

laboratory

massbalance ofice sheets

constraintson

mass&

heat transport

anomalousdensity

structure

gravityanomalies

INS + orbits

+ + ++


2006

GOCE

19

OongoingReleased Oct. 06 – closing date Dec. 06Data A.O.

!Arctic gap

filled

AO closed

ESAG airborne campaign and ArcGP

project

Polar Gap,

Calibration and Validation !Level 1b Cal/Val plan available

OongoingOcean shelf study, sea-ice (ArcGICE)

See R. Forsberg presentation this

morning & Poster by E. Jeansou

Data combination and

altimetry data

exploitation/assimilation

Baseline for

methods &

products

Baseline

concept

confirmed

Baseline

gradiometer

and DFAC

Result

!

!

!

!

3 studies on algorithms, products,

calibration/validation, quick look tools

Level 1 and 2

Tectonics, GIA and Sea level

Ocean circulation retrievalMission impact

GOCE End to End Performance Analysis

and Observation Requirements Studies

Mission analysis

ActivitiesIssue

GOCE Scientific Issues


2006

GOCE

20Level 2 Data Products

n Externally calibrated and corrected gravity gradients

n Global Earth gravity potential modelled as sphericalharmonic series up to deg/order 200 –corresponding to 100km spatial res. (incl.coefficients and error estimates)

n Global ground-referenced gridded values of:– geoid heights (Earth geoid map)

– gravity anomalies (Earth gravity map)

– geoid slopes

n Variance-covariance matrix of final GOCE Earthgravity field model


2006

GOCE

21GOCE Level 2 Products

! Final GOCE Earth gravity field model as spherical harmonic series including error estimates.

Target: 1-2 cm / 1 mGal up to degree and order 200 corresponding to 100 km spatial

resolution.

! Grids of geoid heights, gravity anomalies and geoid slopes computed from final GOCE Earth

gravity field model including propagated error estimates

! Quality report for final GOCE gravity field model

EGM_GOC_2

! Variance-covariance matrix of final GOCE Earth gravity field modelEGM_GVC_2

! GOCE precise science orbits final product

! Quality report for precise orbits

SST_PSO_2

! Externally calibrated gravity gradients in Earth fixed reference frame including error estimates

for transformed gradients

! Transformation parameters to Earth fixed reference frame

EGG_TRF_2

! Externally calibrated and corrected gravity gradients in GRF (2 weeks latency)

! Corrections to gravity gradients for temporal gravity variations

! Flags for outliers, fill-in gravity gradients for data gaps with flags

! Statistical information

EGG_NOM_2

DescriptionIdentifier


2006

GOCE

22Uniqueness and Relevance

n Only mission with satellite gradiometry (3D) anddrag-free control in low orbit (250km)

n GOCE will provide global static gravity field withhomogeneous quality of unprecedented accuracyand resolution

n Key step in improving ocean, solid Earth and sea-level modelling

n Large impact on national height systems andsurveying applications on land and sea

n Essential benchmark technique forunderstanding mass distribution and change


2006

GOCE

23Future Outlook

nn GOCE Scientific Preparations almost completeGOCE Scientific Preparations almost complete

nn GOCE Ground Segment development on track (including L2GOCE Ground Segment development on track (including L2scientific data processing)scientific data processing)

nn Planned GOCE Data AO Release Planned GOCE Data AO Release –– Oct 2006 Oct 2006

nn Data AO Deadline Data AO Deadline –– 8 December 2006 8 December 2006

nn GOCE Science Data Users have GOCE Science Data Users have ““cut their teethcut their teeth”” on CHAMP on CHAMPand GRACE dataand GRACE data

nn Main technical challenge is completion of flight modelMain technical challenge is completion of flight modelprogrammeprogramme

nn GOCE primary Mission Objectives can still be met with a launchGOCE primary Mission Objectives can still be met with a launchin late 2007in late 2007

n GOCE launch scheduled for Sept. 2007

gravity field and steady-state ocean circulation explorer...

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