DBS bistatic altimetry. Scattering process and surface parameters estimation

For pulse-limited (PL) altimeters the spatial resolution is limited by the pulse width: the same concept is applicable to the bistatic altimeter. While the spacecraft (S/C) passes over the PL zone hundreds of looks can be acquired and non-coherently averaged to reduce the speckle noise. Typical PL altimeters average hundreds of looks obtaining height accuracy of few centimeters. PL altimeters are very well suited to open ocean applications, where wide stationary regions exist and the surface slopes on a large scale are nearly absent. As opposed, when encountering rougher surfaces, with prominent features and non zero slopes, such as land surfaces, coastal regions, ice sheets, etc. the poor spatial resolution inherent in the pulse limited approach becomes inadequate to the observed surface and the altimeter performance degrades severely. In order to overcome these problems the spatial resolution can be improved at least in the along track direction, by low-pass filtering the received echoes in the Doppler frequency domain. Instruments based on this concept have been called Doppler beam sharpened (DBS) altimeters and generally achieve this improvement in resolution at the expense of a reduction of the illuminated footprint size and thus of the non-coherent integration capability. Both these shortcomings can be overcome implementing a bank of parallel Doppler filters that span the whole illuminated footprint and using all the Doppler filters to observe each along track location. This is easily obtained in the Doppler radar altimeter (DRA)