From TOPEX-POSEIDON to JASONScience Working Team Meeting

GRACE Mission Status

Arles, FranceNovember 18-21, 2003

Byron D. Tapley(Principal Investigator)

Center for Space ResearchUniversity of Texas at Austin

Ch. Reigber(Co-Principal Investigator)GeoForschungsZentrum

Potsdam

• Spacecraft & System– Launched 09:21 UTC, March 17, 2002– Commissioned on May 14, 2003

• CoM adjustment completed (~30 microns)• Successful K-Band Bore-sight calibration

– Loss of some redundancy on GRACE-1– Satellites currently in Validation Phase and collecting

excellent science data

• Mission Operations– GSOC successfully operating twin satellites in a multi-

mission environment– Over 99% science data recovered

GRACE Project Status

• Science Data System (CSR,JPL,GFZ)– Initial gravity model determinations

• Time variable gravity effects detected– On-going assessment of the flight segment and processing procedures

• Improvements to on-board software and Level-1 processing has resulted in further improvements over data used for previous gravity solutions

Mission Life Forecast

Item

• Battery Cycles

• Orbit Decay • Cold Gas Fuel

• Thruster Actuations

Critical Units

MWA, USO*, ICU SUSU

Life Prognosis

> 13 years

> 10 yrs (P>50%)

> 10 years

8 to 16 years

Number

41

Satellite

GR-2

Both

GR-1

GR-1

GR-1GR-2

(Stable since May 2002)* same class USO as T/P

Preliminary GRACE Solutions

• GGM01S

– Estimate 120x120 using only data from GRACE

– 111 days of GPS, KBR, ACC and SCA data used

– No ‘Kaula’ constraint, no other satellite information, no surface gravity information and no other a prior conditioning

• GGM01C

– Combine GGM01S with surface information to 200x200

• EIGEN-GRACE01S

– Weak ‘Kaula’ conditioning applied

The geoid is the level (equipotential) surface that best coincides with mean sea level

The geoid height varies by ~200 m, but oceanographic applications need this to be determined to cm accuracy

Geoid height ( m )

Gravity Errors Predicted by Full Covariance

Predicted geoid height errors for EGM96*

Predicted geoid height errors for GGM01S*

Errors as large as 38 cm

Errors less than 2 cm

* at ~300 km resolution (degree/order 70)

Predicted gravity anomaly errors for EGM96*

Errors as large as 3.7 mgal

Errors less than0.2 mgal

Geoid errors from GRACE are much more uniform, without land/sea discrimination

Predicted gravity anomaly errors for GGM01S*

Progress in GRACE Gravity Solutions

120110100908070605040302010010 -1

10 0

10 1

10 2

10 3

Geoid (EGM96 Degree Variance)

EGM96 Estimated ErrorsNCEP Hydrology (Aug-May)GRACE05 Estimated ErrorsGRACE19 Estimated ErrorsGGM01S Estimated ErrorsCurrent Estimated Errors

Spherical Harmonic Degree

Geo

id H

eig

ht

(mm

)

• As Level-1 and Level-2 processing techniques have improved, the estimated error has improved.

• Low degree error estimates for GGM01S, based on subset solutions, was probably reflecting real signal, not error, and thus may have been pessimistic at the low degrees.

• Newest error estimate was based on independent solutions for the same month of data.

20 cm changes around western boundary currents and equatorial currents

Scale is +/- 0.5 m.

Marine Geoid Differences between GRACE Model and EGM96

meters

Zonal Geostrophic CurrentsDetermined from rel to 3000-4000m

(rel calculated from WOA by V. Zlotnicki) CSRMSS98 - EGM96

CSRMSS98-GGM 01

+ eastward

Geostrophic Currents TestComparison of zonal and meridional ocean currents implied by mean sea surface (CSRMSS98) minus various geoid models

The zonal tests appear to have run into the limitations of the test data (MSS or Levitus)

The meridional tests are sensitive to the quality of the ‘near sectorials’ and continue to be a useful probe into the quality of the gravity solutions

Model Zonal Merid.

EGM96 6.9 4.8GGM01S 2.6 3.0GGM01C 2.6 2.9GFZG1S 2.6 3.0Aug ‘02 2.6 3.1Apr ‘03 2.6 3.2May ‘03 2.6 3.0Aug ‘03 2.6 2.9

Standard Deviation wrt Levitus* (cm/s)

Geoid Zonal Merid.EGM96 0.45 0.36

GGM01S 0.93 0.52GGM01C 0.93 0.55GFZG1S 0.93 0.53Aug ‘02 0.93 0.50Apr ‘03 0.93 0.48May ‘03 0.93 0.51Aug ‘03 0.93 0.57

Correlation with Levitus*

* Topography map determined from World Ocean Atlas 2001 (WOA01) data relative to 4000 m (courtesy of V. Zlotnicki)

GGM01S used no conditioning of any kindGGM01C included terrestrial information from TEG4GFZG1S (= EIGEN-GRACE01S) used weak ‘Kaula

constraint’

New monthly solutions using Version 0 of Level-1b data

Note the high correlation coefficient = 0.987

Arctic Gravity from GRACE

McAdoo et al., 2003

(Low-pass Filtered with 2.5-degree Gaussian)

Satellite Orbit Comparisons

Gravity Model StarletteSLR(cm)

StellaSLR(cm)

Lageos-1SLR(cm)

Lageos-2SLR(cm)

ICESatGPS DD SLR * (cm) (cm)

JGM-3 4.3 6.4 0.96 1.01 1.74 5.5

EGM96 3.7 6.4 1.01 1.01 1.73 9.7

GGM01S 2.8 3.3 1.25 1.29 0.97 1.9

GGM01C 3.6 2.6 1.01 0.98 0.97 2.0

GFZG1S 2.9 3.9 0.90 0.95 0.97 1.9

CSR Aug 02 3.1 3.3 0.90 0.85 0.97 2.0

CSR Apr 03 2.9 3.2 0.90 0.84 0.97 1.8

CSR May 03 2.8 3.2 0.88 0.89 0.97 1.7

GRACE solutions have no other satellite information included yet perform better than models tuned with these satellites

New monthly solutions using Version 0 Level-1b data * not used in orbit solution

Contributions to POD for Altimeter Missions

While JGM-3 was a major improvement in gravity modeling for the T/P altimeter mission, there still remains significant geographically correlated orbit error due to gravity modeling errors.

For the SLR/DORIS “dynamic” orbits, the full amount of this error is retained. “Reduced-dynamic” approaches can substantially reduce the long wavelength part but not the shorter scale errors.

Using GGM01S, the SLR/DORIS dynamic orbits are much more consistent with GPS reduced-dynamic orbits.

Because longer wavelength errors have been reduced in GGM01S, we can infer that the shorter wavelength errors have been similarly reduced. (Beside covariance prediction, the crossover residuals for reduced-dynamics orbits are further reduced using GGM01S.)

Next GRACE gravity model should reduce gravity error contributions even further.

Estimate of geographically correlated orbit error due to JGM-3

Comparison of Jason-1 dynamic orbit based on SLR+DORIS orbit to “reduced-dynamic” orbit based on GPS (both using GGM01S)

Scale is +/- 15 mm

Data Release Plan

• Preliminary product release

– Initial mean fields : 7/15/03

– Selected monthly solutions from the first 14 months released to

Science Team for evaluation : 11/26/03

• Technical documents being finalized and data centers undergoing final

tests

• Science Team evaluation period : 11/26/03 - 5/14/04

• Operational data release: 5/14/04

Conclusions

• This current solutions provided a strong validation of the mission concept and the satellite/sensor on-orbit performance

– Significant improvement in mean field

• Essentially satisfied Minimum Mission requirement

• Significant Impact on Oceanographic Results

• Improved determination of High Latitude Geoid

– Time varying gravity signal has been detected

• Good correlation with expected hydrology signal