Improved Marine Gravity fromCryoSat 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.)

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.

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.

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.

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

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.

419-day coverage begins May 4, 2012

Caspian Seaaltimeter tracks

old tracks new tracks planned tracks

3 years of CryoSat and 419 days of Jason

predicted gravity improvement

Error in north and east components was averaged.

CryoSat Data Acquisition over 13 Months

[CryoSat L1b dataprovided by ESA ESRIN]

LRM

SAR

SARIN

Comparisons in the Gulf of Mexico

ship gravitysatellite gravity

with CryoSat LRM

5 mGal contour interval

source: EDCON

satellite gravity with CryoSat LRM vs. ship gravity

rms < 2 mGal

Comparisons in the Gulf of Mexico

rms = 1.86

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

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