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In order to realize a compact L-band transponder design for the calibration of spaceborne synthetic aperture radar (SAR) systems, a novel antenna was developed by DLR. As with previous designs for different frequency bands, the future transponder is based on a two-antenna concept. This paper addresses the issue of antenna coupling between corrugated L-band horn antennas, which are operated in close proximity. The antenna coupling is analyzed via simulations and measurements by utilizing specifically defined coupling parameters. Additionally, improvements to further lower the mutual antenna coupling have been designed, tested, and are described in this paper.

Spaceborne synthetic aperture radar (SAR) systems are often used for earth observation capable for acquiring accurate high-resolution data. In order to ensure the quality of these SAR data, the SAR system has to be calibrated first. For this purpose active targets with well-known backscatter properties, called transponders, serve as an external reference. The enhancement of the operational bandwidth up to 1.2 GHz of future civil SAR systems requires the development of appropriate broadband transponders and their accurate calibration. In order to be well prepared for these missions, DLR has been developed a broadband X-Band transponder and an innovative technique for the frequency-dependent determination of the transponder’s radar cross section (RCS) which promises an accurate measurement over the full transponder bandwidth. In this paper the calibration of a broadband transponder according to this new approach is described including the analysis of corresponding measurements. The derived results are verified with a second independent calibration method and finally evaluated.

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.