Calibration of SAR systems is a traditional R&D field in the DLR Microwave and Radar Institute. During the last 25 years, not only the demand on high accuracy spaceborne SAR data products has increased but also the level of complexity in the SAR instrument architecture and calibration.
Since the late 1980s, see Table 2, we have been developing analysis tools and equipment required for calibration and have successfully used them for the missions ERS-1/2, SIR-C/X-SAR, SRTM, ENVISAT/ASAR, ALOS-1/2 and for the Institute's airborne SAR. While a passive slotted waveguide array was used for ERS-1/2 and X-SAR to illuminate the Earth surface with a single and fixed antenna beam, recent SAR systems like TerraSAR-X and Sentinel-1 are based on an active phased array antenna employing several hundred transmit/receive modules, offering electronic beam steering capabilities and, thus, a multitude of different SAR modes. DLR has continuously invested in this strategic field resulting in the development of more sophisticated calibration procedures to constantly increase the achievable calibration accuracies. These innovative procedures for high-precision calibration of TerraSAR-X and TanDEM-X set new standards and have paved the way for the Sentinel-1 calibration concept.
A further milestone – the development of a new generation of remotely controlled high-precision C-band calibration transponders – enabled new calibration services. These transponders and corner reflectors, deployed in our calibration field nearby Oberpfaffenhofen, are the primary calibration targets for the Sentinel-1A/B mission. They were used to calibrate Sentinel-1A/B during the commissioning phase and they are still being used for long-term monitoring.
In addition, the Calibration Group developed two C-band transponders for the Radarsat Constellation Mission (RCM) of the Canadian Space Agency (CSA). Both transponders were deployed in Canada in 2017 and have been fully operational since then. With our undisputed role as calibration center for Sentinel-1A/B, with the very successful transponder developments and with further support requests by the Korean Aerospace Research Institute (KARI) for their Kompsat-6 mission, today, the anticipated role as a DLR SAR Calibration Center is now a reality, supporting national and international SAR missions.
However, the Institute has the goal of further expanding its core competence on calibration for future SAR missions. In addition to longer wavelengths used by future SAR systems (Tandem-L, BIOMASS) or very large bandwidth in X band (HRWS), future SAR systems will also be characterized by innovative operating modes (e.g., TOPSAR, ScanSAR interferometry) or digital beamforming. Innovative methods and efficient concepts for calibrating these sensors and the entire data chain must be developed in order to meet the very high quality requirements of future data products. Development and implementation of such calibration concepts is the focus of these activities. This includes the definition of algorithms for deriving calibration parameters, as well as the specification and development of necessary reference targets.