Use of Topex-Poseidon and Envisat Dual-Frequency Radar Altimeters Data over Continental Surfaces
Fabrice Papa(1) , Benoit Legresy(2) , and Frederique Remy(2)
10025 New York NY,
(2) CNRS-LEGOS, 18, Avenue Edouard Belin, 31400 Toulouse, France
Initially developed to operate over ocean surfaces and make precise measurements of the sea surfaces topography, the satellite-altimeter radars, which are nadir-pointing active microwave sensors, rapidly exhibited on strong capabilities to provide information over continental surfaces.
At this time, radar altimetry mainly shows its capabilities over ice caps with the construction of precise topography or the characterisation of the snow surface roughness and snowpack structure. In parallel, the development of altimetry to monitor continental water surfaces and measure their stage elevation makes this sensor a powerful tool to study regional hydrological systems.
Many other particularities of radar altimeter over continents are newly exploited, like the opportunities to use all retracked waveform parameters, to use the dual-frequency measurements or to develop a synergy with passive microwave data obtained by the radiometer that operated simultaneously on altimeter platform.
The capability of Topex-Poseidon and Envisat radar altimeter data for land surface studies at regional or global scale are so investigated. The analyzed data, available since mid-1992 for Topex-Poseidon consist of dual-frequency backscattering coefficients measurements (Ku and C band) estimated along the satellite tracks. The processing of Envisat data available since 2002 (Ku and S band) using the ICE-2 algorithm enables now the study over continents with all the wave forms parameters.
It first shows, at a global scale, the capabilities of dual-frequency altimeters to monitor forests, deserts, boreal regions… and their seasonal variations.
At the regional scale, radar altimeter backscatters was successfully used to recover snow depth evolution through winter over the snow covered areas of the Northern Hemisphere. It also enables to determine the duration of the snow period, within the beginning and the end of the winter snow pack.
Topex-Poseidon data are also used to develop a synergetic method using active and passive microwave measurements to study the sea ice variability of the Caspian and Aral Sea. This work was also enlarged to Antarctic Ocean to recover snow depths over sea ice. Recently, Topex-Poseidon 10 year-span dataset were successfully used to recover wetlands extent over the Boreal Regions.
These works were used for the calibration/validation phase of the dual-frequency radar altimeter on board ENVISAT over land surfaces and ice sheets. ENVISAT dual frequency radar altimeter offers now for the first time to the scientific community a dataset processed with an algorithm entirely devoted to continental surfaces studies and offers new capabilities to retrieve parameters, such as inundated areas extent on the global scale or accumulation rates over the ice sheets.