Nadir Echo Properties and Suppression in Spaceborne Synthetic Aperture Radar Systems - A Study Based on TerraSAR-X Data

Abstract

In images obtained by spaceborne Synthetic Aperture Radar (SAR) systems the Nadir echo, i.e. the signal received from the area surrounding the point with the closest distance to the satellite, is an unwanted effect. For a known altitude and ground geometry, it is possible to avoid Nadir echo by carefully choosing the pulse repetition frequency. This constraint reduces the degrees of freedom and represents a significant drawback especially for the forthcoming generation SAR systems, conceived for high resolution and wide swath imaging. This has recently motivated on one side the development of digital filter techniques for Nadir echo suppression and on the other side the studies on Nadir echo features. This work investigates the properties of the Nadir echo. The analysis is based on a selected spaceborne SAR data sets, obtained with the TerraSAR-X system and provided by the German Aerospace Center (DLR). The data sets were commanded to contain strong Nadir echo and acquired over surfaces with homogeneous backscattering and topographic characteristics both in the swath of interest and in the Nadir region. The Nadir features in terms of duration, amplitude, phase, frequency and its behaviors, as point or distributed target, are studied and examined based on raw, range compressed and image data analysis.