1. From TOPEX-POSEIDON to JASON Science Working Team Meeting GRACE Mission Status Arles, France November 18-21, 2003 Byron D. Tapley (Principal Investigator) Center for Space Research University of Texas at Austin Ch. Reigber (Co-Principal Investigator) GeoForschungsZentrum Potsdam

2. 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

3. Mission Life Forecast Item Battery Cycles Orbit Decay Cold Gas Fuel Thruster Actuations Critical Units MWA, USO*, ICU SU SU Life Prognosis > 13 years > 10 yrs (P>50%) > 10 years 8 to 16 years Number 4 1 Satellite GR-2 Both GR-1 GR-2 (Stable since May 2002) * same class USO as T/P

4. 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 )

5. 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 than 0.2 mgal Geoid errors from GRACE are much more uniform, without land/sea discrimination Predicted gravity anomaly errors for GGM01S*

6. Progress in GRACE Gravity Solutions 1201101009080706050403020100 10 0 1 2 3 Geoid (EGM96 Degree Variance) EGM96 Estimated Errors NCEP Hydrology (Aug-May) GRACE05 Estimated Errors GRACE19 Estimated Errors GGM01S Estimated Errors Current Estimated Errors Spherical Harmonic Degree Geoid Height (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.

7. 20 cm changes around western boundary currents and equatorial currents Scale is +/- 0.5 m. Marine Geoid Differences between GRACE Model and EGM96 meters

8. Zonal Geostrophic Currents Determined from  rel to 3000-4000m (  rel calculated from WOA by V. Zlotnicki) CSRMSS98 - EGM96 CSRMSS98-GGM 01 + eastward

9. Geostrophic Currents Test Comparison 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 Standard Deviation wrt Levitus * (cm/s) 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 kind GGM01C included terrestrial information from TEG4 GFZG1S (= EIGEN-GRACE01S) used weak ‘Kaula constraint’ New monthly solutions using Version 0 of Level-1b data

10. Note the high correlation coefficient = 0.987 Arctic Gravity from GRACE McAdoo et al., 2003 (Low-pass Filtered with 2.5-degree Gaussian)

11. Satellite Orbit Comparisons 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

12. 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

13. 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

14. 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