The calibration of spaceborne imaging radar systems is the main focus of the Calibration Group and has been an important research field in the Institute for over 25 years. An essential task of calibrating these Synthetic Aperture Radar (SAR) systems is to establish the relationship between radar measurements and geophysical parameters. This includes the geolocation of the SAR image, its backscattering characteristics (in amplitude and phase), and polarimetric information. Keeping up with the growing demand for accurate SAR data products on the one hand, and the growing complexity of innovative spaceborne SAR systems (with a multitude of different imaging beams and novel operation modes like TOPS (Terrain Observation by Progressive Scans), sliding spotlight, etc.) on the other, see Calibration Concepts, requires sophisticated concepts, precise algorithms and adequate facilities, see Transponder/Corner, in order to efficiently calibrate such complex spaceborne SAR systems.
Transponder/Corner
Calibration devices of the DLR SAR calibration center: remote-controlled transponder in the laboratory (left) and deployed in the calibration field (center), and remote-controlled corner reflector (right) at one of the permanent sites within our calibration field covering a total area of 120 km x 40 km.
Modern 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. Calibrating these modes by measuring each of the thousand beams separately (as done for ERS-1/2 or ENVISAT/ASAR), is not feasible. Thus, an efficient concept was developed, has been applied for calibrating TerraSAR X, TanDEM X and Sentinel-1, and is currently being expanded for low frequency SAR systems like Tandem-L. The goal of this concept is to reduce the measurement effort in space as much as possible by shifting most of the antenna characterization to pre-launch activities. In order to achieve this aim, two key elements should be available:
an accurate internal calibration facility allowing for drift compensation and for characterizing individual Transmit/Receiver Modules (TRMs) in-flight by the so called PN-gating method also known as PCC technique, and
a precise antenna model for providing the thousands of reference patterns and allowing accurate pointing determination. For this purpose we have developed and established our antenna model approach.
Applying these key elements, relative radiometric calibration of all SAR data products can be performed without any measurements against point targets. Furthermore, the expected gain and phase offset between different beam configurations can be derived from the antenna model. Therefore absolute radiometric calibration, i.e. the measurement of the whole SAR system against reference targets is not only independent of the target position within the swath, but also independent of the beam being operated. Consequently, only one absolute calibration factor has to be derived from deployed reference targets. This factor is valid for all operational conditions.
Establishing this strategy, only a small subset of suitable beams has to be effectively measured in-flight instead of thousands of possible antenna beams separately. Thus, most of the calibration effort is shifted from space to ground, i.e. the in-orbit calibration duration is minimized by more detailed pre-launch characterization on ground and modelling of the antenna. Hence, the tight schedule of commissioning a high performance spaceborne SAR system can be ensured and long term system monitoring can be effectively performed during the lifetime of the instrument.
