Geosat Follow-On Waveforms: Retracking for Hydrology Applications
John Lillibridge(1) and Stephane Calmant(2)
Laboratory for Satellite Altimetry,
Silver Spring, MD 20910,
(2) CNES, Lab. d'Etudes Geophysique et Oceanographie Spatial, 31401 Toulouse Cedex 9, France
Geosat Follow-On is a Navy altimetry satellite whose primary mission is to monitor sea level, wind speed, and significant wave height for fleet operations. Additionally, for measurements of continental ice sheets, altimetry waveform data are collected for five passes per day that cross Greenland and portions of Antarctica. Fortuitously, these passes provide near total coverage of the African and N. American continents (ascending and descending portions, respectively) as well as parts of Asia and Europe. However, the waveform data are rather noisy, in particular because the onboard tracker used to fit the leading edge of the waveform to determine range is not suited to radar echoes reflected back by continental waters (rivers and lakes). We have processed all the available GFO waveform data using different trackers in order to improve the height values over continental waters. We present examples of hydrological time series so obtained, and an absolute comparison of retracked altimetric heights vs. GPS in-situ measurements over the Issyk-kul Lake (Kirghizstan).