20 Years of Improvements to GEOSAT Altimetry
John Lillibridge(1) , Walter H.F. Smith(1) , David Sandwell(2) , Remko Scharroo(3) , Frank Lemoine(4) , and Nikita Zelensky(5)
Laboratory for Satellite Altimetry,
Silver Spring, MD 20910,
(2) University of California San Diego, 9500 Gilman Drive, La Jolla, CA 92093, United States
(3) Altimetrics, LLC, 330a South Parsonage Rd., Cornish, NH 03745, United States
(4) NASA, Space Geodesy Branch, Code 926, Greenbelt, MD 20771, United States
(5) SGT, Inc., 7701 Greenbelt Road Suite 400, Greenbelt, MD 20770, United States
The U.S. Navy GEOSAT mission provided the first long-term altimetric record for studies of ocean circulation, marine gravity/bathymetry and continental ice. The Geodetic Mission data (GM: 31-Mar-1985 to 30-Sep-1986) were declassified by the Navy in 1995 and released by NOAA together with the Exact Repeat Mission data (ERM: 8-Nov-1986 to 30-Dec-1989) in the 1997 JGM-3 Geophysical Data Records (GDRs). This first complete GEOSAT data set used improved tracking and orbit determination based on the JGM-3 gravity model, and was an update to the previous GDRs based on GEM-T2 orbits (ERM) and NSWC orbits (GM). The radial orbit errors were reduced from the decimeter to meter level down to about 7 cm rms.
We have now completed another major upgrade of the GM data by reprocessing the original Sensor Data Records (SDRs) and Waveform Data Records (WDRs) which were 'separated at birth' by the GEOSAT ground segment at Johns Hopkins/APL. The SDRs were recovered from NOAA archives and combined with the matching waveform data in the WDRs from the NASA/GSFC archive. This has allowed us to retrack all the over-ocean waveforms from the GM to improve measurements of sea surface height and its slope and thereby produce better models of marine gravity and predicted bathymetry. A unique two-pass retracking algorithm is used to further reduce the noise in along-track sea surface slope, and to reduce the correlation in errors between range and SWH from the waveform fitting. The first retracking pass is a traditional multi-parameter least-squares fit of a Brown model to the waveform, fitting pre-arrival noise, amplitude (AGC), half-power location (range track point), leading edge slope (SWH) and plateau decay (off-nadir attitude) at 10 Hz. The resulting values of noise, amplitude, SWH and attitude are then smoothed along track and a second waveform fitting is performed using these smoothed values with only the range travel time being re-estimated from a one-parameter fit.
The '20th Anniversary GEOSAT Geodetic Mission' data set also includes several improvements compared to the previous GDRs: range data with mm vs. cm precision; measurements of range, SWH, AGC, and all corrections at the full 10 Hz rate; a noise-free Doppler height correction based on orbit height rate and along track mean sea surface gradients; improved time-tagging and ultra-stable-oscillator height correction; and state of the art tide and sea state bias models. The precise orbit determination for the GM data uses the full set of 45 Doppler TRANET tracking stations and is based on gravity models tuned for Geosat Follow-On. The improved corrections and retracking are particularly beneficial to marine gravity and bathymetry, while the new precise orbits yield better estimates of sea level rise from the GEOSAT time period.