TOPS Image Quality and Processor Verification Study: Doppler Centroid and High Azimuth Steering Angles

Abstract

The report presents the results from investigations on the variation of the Doppler centroid with the terrain height in the outer azimuth edges of a TOPS burst image, i.e. for targets illuminated at the beginning and the end of a TOPS raw data burst. Under the assumption that Total Zero Doppler Steering is applied a residual Doppler centroid is a consequence of the AOCS control accuracy. In TOPS imaging mode data, as in any kind of burst mode data, the Doppler centroid of targets depends on the individual azimuth position within the burst image. Since Total Zero Doppler Steering is applied w.r.t. the azimuth bandwidth of the complete scene, there will be a dependency of the Doppler centroid for targets at the outer azimuth regions of the burst images with the terrain height. A large range of azimuth antenna steering angles and terrain heights is included in the investigation of the variation of the Doppler centroid with the terrain height. The investigation results are furthermore quantified for Sentinel-1. The demonstration of the results is based on selected real TerraSAR-X data acquisitions. In order to investigate the variations of the Doppler centroid within real data w.r.t. fixed azimuth steering angles, squinted Stripmap acquisitions are used for the demonstration. As a conclusion the results are discussed w.r.t. TOPS SAR processing in order to evaluate the need of special consideration of Doppler centroid variation within the TOPS SAR processing. This report is the Technical Note 1 on “WP 210 Doppler Centroid and High Azimuth Steering Angles” and on “WP 211 Doppler Centroid Investigation/Demonstration with TOPS Processor” in the frame of the ESA/ESTEC Contract: “TOPS Image Quality and Processor Verification Study”. Both belong to the superordinate workpackage “WP 200 Sentinel-1 Image Quality”.