High Resolution Imaging of the Ocean Surface Backscatter by Inversion of Altimeter Waveforms

All the satellite altimeters use pulse-limited-geometry and full-deramp technique to measure the power backscatter by the ocean surface as a function of time, i.e. the altimeter pulse echo waveform, see for example [1] for explanation on pulse limited altimeters). Assuming that the distribution of the sea surface roughness and elevation are homogeneous over the altimeter footprint, the backscatter coef cient of the echo waveform can be expressed as a double convolution product of the radar point target response, the at sea surface response and the joint probability density function of slope and elevation of the sea surface [2, 3]. Further assuming a Gaussian shape for the antenna beam pattern, the compressed altimeter pulse P and the joint probability of sea surface slope and elevation, the cross section as a function of time follows the classical Brown (1977) model. However, previous studies such as [4, 5] have shown that the basic assumption of the Brown model, the homogeneity of the sea surface roughness distribution is not always valid. In particular in presence of rain, sea ice,small island or reef, surface slick or under very low wind conditions. In such cases, an altimeter can be seen as an imager of the sea surface backscatter whose imaging process is more complex than a classical one in the sense that pixels are not rectangular but annular. We present a method to invert the measured waveforms in terms of surface backscatter at a resolution of the order of the along track 20 Hz resolution (290 m for JASON). The method is then validated using simulated waveforms and is shown to be unbiased and to have an rms of the order of 0.25 dB. Several applications concerning surface slicks, small islands and submerged reefs, interaction between current and backscatter are presented.