1. Improved Marine Gravity from CryoSat and Jason-1 David T. Sandwell, Emmanuel Garcia, and Walter H. F. Smith (April 25, 2012) gravity anomalies from satellite altimetry new track coverage from CryoSat and Jason-1 CryoSat results (1 year of data) Expected better E-W resolution with Jason-1 (Funding from ConocoPhillips, NSF, and ONR.)

2. radar altimetry Our objective of 1 microradian slope accuracy requires 1 cm range precision over 10 km distance. Ocean surface waves are the main limiting factor on range precision.

3. available altimeter data before CryoSat 1985 1995 Note - The Topex track spacing is > 200 km so it provides little new information. GRACE and GOCE cannot resolve features smaller than 200 km.

4. Achieving 1 mGal Gravity Accuracy Improved range precision -- A factor of 2 or more improvement in altimeter range precision, with respect to Geosat and ERS-1, is needed to reduce the noise due to ocean waves. Fine cross-track spacing and long mission duration -- A ground track spacing of 6 km or less is required. Moderate inclination -- Current non-repeat-orbit altimeter data have high inclination and thus poor accuracy of the E-W slope at the equator. Near-shore tracking -- For applications near coastlines, the ability to track the ocean surface close to shore is desirable.

5. CryoSat Launched by ESA in February, 2010. Primary mission to measure sea ice thickness and ice cap volume. Also operates over all ocean areas to 88˚ latitude. First new altimeter mission with long repeat cycle (369 days) since ERS-1 in 1995. Operates in 3 modes: –LRM –- conventional mode used by all previous altimeters. (excellent data quality) –SAR –- synthetic aperture radar mode may provide 2-4 times better range precision. –SARIN -– uses two receiving antennas to also measure cross-track slope. (poor data quality) source: ESA

6. Jason-1 Launched by NASA and CNES in 2001. Primary mission to monitor global ocean circulation in 10-day repeat cycle at 66˚ inclination to continue the Topex time series. 300 km track spacing not useful for marine gravity recovery. Replaced by Jason-2 in June 2008. Engineers concerned that when Jason-1 dies, it will collide with Jason-2 and they recommend moving the satellite to a different orbit. “End of life” committee suggest a 419- day geodetic orbit. Jason-1 goes into safe hold in March 3, 2012. Maneuvers to new orbit began April 24, 2012. New operations begin May 4, 2012.

7. 419-day coverage begins May 4, 2012 Caspian Sea altimeter tracks old tracks new tracks planned tracks

8. 3 years of CryoSat and 419 days of Jason predicted gravity improvement Error in north and east components was averaged.

9. CryoSat Data Acquisition over 13 Months [CryoSat L1b data provided by ESA ESRIN] LRM SAR SARIN

10. Comparisons in the Gulf of Mexico ship gravity satellite gravity with CryoSat LRM 5 mGal contour interval source: EDCON

11. satellite gravity with CryoSat LRM vs. ship gravity rms < 2 mGal Comparisons in the Gulf of Mexico rms = 1.86

12. orbit inclination controls error anisotropy  Error propagation  - local inclination of track  - error in along-track slope  x - error in east slope  y - error in north slope  Orthogonal tracks are optimal north slope   east slope

13. Without Jason, the error in the east slope component is large so N-S features such as the East Pacific Rise will be poorly resolved. One 419-day cycle provides about 25% improvement in east slope and two cycles provides about a 33% improvement. Most of the area of the earth is at latitude less than 60 degrees where Jason will make the largest improvement. contributions from Jason-1