This paper reports the Phase A study results of the interferometric extension of the High-Resolution Wide-Swath (HRWS) mission with three MirrorSAR satellites. According to the MirrorSAR concept, small, low cost, transponder-like receiveonly satellites without radar signal demodulation, digitization, memory storage, downlink, and synchronization are added to the planned German X-band HRWS mission. The MirrorSAR satellites fly a triple helix orbit in close formation around the HRWS orbit and span multiple single-pass interferometric baselines. A comprehensive system engineering and performance analysis is provided that includes orbit formation, MirrorLink, Doppler steering, antenna pattern and swath design, multi-static echo window timing, SAR performance, height performance and coverage analysis. The overall interferometric system design analysis of Phase A is presented. The predicted performance of the global Digital Elevation Model (DEM) is improved by one order of magnitude compared to presently available global DEM products like the TanDEM-X DEM.

TanDEM-X is a formation flying interferometer consisting of two SAR satellites. Together they acquire bistatic SAR data of the Earth in order to generate a global Digital Elevation Model as well as individual scientific interferometric data sets on user request. This paper gives an overview about ten years of acquisition and formation planning for the TanDEM-X mission. This planning on the one hand includes the derivation of the formation, i.e. the 3D distance between both satellites in orbit. On the other hand it needs to consider the regions of interest and the mission, satellite, and instrument constraints in order to effectively plan the bistatic acquisitions. During the ten years in operation TanDEM-X successfully completed various mission goals during dedicated mission phases. All these phases, their characteristics as well as the scientific and commercial outcome are described in detail.

The TanDEM-X mission, formed by the TanDEM-X satellite (TDX) in cooperation with its almost identical twin TerraSAR-X (TSX), has mainly been designed to acquire bistatic Synthetic Aperture Radar (SAR) images of the Earth. Initiated in 2010, the primary mission objectives were to generate a global Digital Elevation Model (DEM), to perform scientific measurements and to explore novel SAR techniques. Up to the present day all these objectives have been completed with outstanding results. In addition, the satellites maintained a stable image quality from the start throughout their entire lifetime. The ground segment, which was tailored to the capabilities of the space segment, as well as an excellent design and manufacturing of the satellites were the key elements to achieve such a performance. In addition, the elaborate calibration concept and a careful treatment of the on-board resources enabled full operability up to the present day and allowed additional mission objectives such as the generation of a Change-DEM layer as an update to the global DEM. The stable bus and SAR payload performance as revealed by long-term system monitoring provide the basis for further extension of the TanDEM-X mission for several more years. In the following paper these long-term system monitoring results are presented to provide an overview of the evolution of the radar system. The results and experience gained with more than a decade of TanDEM-X mission operation are a great value for both the scientific and commercial community and serve as a forerunner for the development of future SAR missions.

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