HR: 0800h
AN: G51C-0100    [Abstracts]
TI: Improved Global Marine Gravity Field From Reprocessing of Geosat and ERS-1 Radar Altimeter Waveforms
AU: * Smith, W H
EM: Walter.HF.Smith@noaa.gov
AF: NOAA Lab for Satellite Altimetry code E/RA-31, 1335 East-West Hwy, Silver Spring, MD 20910 United States
AU: Sandwell, D T
EM: dsandwell@ucsd.edu
AF: University of California San Diego code 0225, 9500 Gilman Dr, La Jolla, CA 92093-0225 United States
AB: The Geosat and ERS-1 radar altimeter satellites have mapped the global marine gravity field down to 20 to 30 km full wavelengths with errors of 3 to 8 milliGals (mGal). This mapping is adequate to reveal the main features of the ocean floor but we would like to resolve more detail. At shorter wavelengths the altimeter error budget is dominated by random errors caused by ocean waves. We have reprocessed nearly one billion radar waveforms returned to the Geosat and ERS1 spacecraft from the ocean surface, using an algorithm designed to minimize the sea surface slope error and decouple it from significant wave height (SWH) error. Standard waveform retracking algorithms include 5 parameters - sea surface height, SWH, return amplitude, pre-return noise, and decay caused by attitude excursion. We model Geosat waveforms using all 5 parameters while ERS-1 needs only a 3-parameter model because pre-return noise is truncated and nadir pointing is accurately maintained. In both cases, the least-squares objective function is non-linear in sea surface height and SWH, and we find maximum-likelihood weighting increases the correlation of errors in these two parameters. To minimize the noise caused by ocean waves, we fit a multi-parameter model to each waveform, smooth the parameters along track, and then refit the height parameter only while fixing the other parameters to smoothed values. The result yields sea surface slope errors 40% smaller than previously obtained, and relatively insensitive to SWH. Sandwell and Smith [this meeting] show the improved resolution of seamounts. While our method is optimized for gravity field recovery, it may also improve the resolution of sea surface height signals of interest in physical oceanography and global sea level rise [Lillibridge et al, this meeting]. Our dataset also eliminates the along-track phase shift inherent in the on-board tracker.
DE: 4275 Remote sensing and electromagnetic processes (0689)
DE: 3010 Gravity
DE: 3094 Instruments and techniques
DE: 1294 Instruments and techniques
SC: Geodesy [G]
MN: 2004 AGU Fall Meeting