A Novel MirrorSAR Concept for Augmenting the Next German Synthetic Aperture Radar Mission HRWS with Single-Pass Interferometry

Abstract

The upcoming Synthetic Aperture Radar (SAR) mission High Resolution Wide Swath (HRWS) will continue the successful program of German X-band SAR missions, which started with the Microwave Remote Sensing Experiment (MRSE) onboard the US Shuttle SpaceLab (1983, Germany), followed by two Shuttle Imaging Radar SIR-C/X-SAR missions (1994, Germany, Italy, USA), the Shuttle Radar Topography Mission SRTM (2000, Germany, Italy, USA), and more recently by the radar satellites TerraSAR-X (since 2007, Germany), and TanDEM-X (since 2010, Germany). In December 2020, the HRWS mission has been approved for realization. This mission consists of the main HRWS satellite which is equipped with a high transmit power radar system with digital beam-forming and is able to work as a stand-alone transmit/receive SAR satellite, producing images with a spatial resolution down to 25 cm. During the Phase 0/A of the HRWS mission, three small companion satellites were added, following the DLR-HR developed MirrorSAR concept for multistatic interferome-try. MirrorSAR allows for interferometric single-pass capability with small radar receive-only satel-lites. This follows the general NewSpace development in satellite missions that goes in the direction of several, but smaller, cost-effective satellites with a fractionated architecture. By means of the MirrorSAR concept, the generation of high-quality Digital Elevation Models (DEM) has been added to the HRWS mission in order to allow a continuation of the DEM generation that has been established since SRTM in the program line of German X-band SAR missions. The objective is a DEM quality that is enhanced by an order of magnitude compared to the TanDEM-X mission. The paper reports the mission analysis results from the Phase 0/A with focus to the DEM generation based on the X-band HRWS satellite as transmitter and three MirrorSAR satellites. This formation provides single-pass baselines of different lengths by flying interlaced helix orbits. After providing an introduction into the general MirrorSAR concept, several aspects are discussed that need to be considered in the mission engineering. The baseline concept and its safe realization with helix orbits is described. A novel MirrorLink concept for phase preserving forwarding of the ground reflected radar signals from the MirrorSAR satellites to the HRWS satellite is presented. An analysis on multi-static echo window timing is provided that includes the along-track separation between the HRWS transmitter and the MirrorSAR receivers as well as transmit and nadir events. An approach for multi-static zero Doppler steering is discussed as well as the interaction between orbits, baselines, Dop-pler steering, and MirrorLink. The expected DEM performance is estimated and compared to the desired height error of maximum 2 meters (90 percentile) at a horizontal spacing of 4 meters x 4 meters. Be-sides the generation of a high-resolution DEM on demand for scientific, commercial, institutional and security related applications, the MirrorSAR concept will allow the generation of a new class of DEM on a global scale.