Monitoring global land surfaces using ERS-1 wind scatterometer data

The ERS-1 wind scatterometer (WSC) operates at 5.3 GHz with VV polarization. Three antennae provide a measurement of the radar backscattering coefficient for incidence angles ranging from 18/spl deg/ to 59/spl deg/, each with its own viewing direction. The spatial resolution is coarse (about 50 km) but its temporal repetitivity of a few days allows terrestrial vegetation to be monitored at a temporal scale compatible with processes related to vegetation dynamics. Past studies using the Seasat scatterometer had already highlighted the interest of such instruments for global observation of terrestrial surfaces. These results were recently confirmed with ERS-1 WSC data showing the suitability of a C-band system for land applications due to its sensitivity to surface parameters such as soil moisture content, soil roughness and vegetation cover. The potentialities of multi incidence acquisitions are pointed out. At low incidence angle, WSC data, mainly sensitive to soil moisture content, can be related to rainy events. At large incidence angle a comparison between WSC data and Global Vegetation Index (GVI) data over main vegetation types of Africa showed a good correlation between both data sets indicating the sensitivity of WSC data to vegetation parameters. Moreover, the WSC appeared to be better suited for vegetation monitoring over sparse vegetation canopies because of the saturation quickly reached by a C-band system over densely vegetated areas. In effect, a yearly amplitude around 5 dB of the radar signal was observed over Sahelian exhibited by tree savannas illustrate the spatial and temporal amplitudes of the backscattering coefficients throughout the year.