The calibration of radar satellites is a key research area at the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR). "In the last few years, we have earned the undisputed status of an international calibration centre for radar satellites," says Alberto Moreira, Director of the DLR Microwaves and Radar Institute. The innovative methods used for the TerraSAR-X and TanDEM-X radar satellites yielded outstanding radiometric and geometric precision. These methods were also adopted by the European Copernicus Programme for the Sentinel-1 mission and implemented by DLR on behalf of the European Space Agency (ESA).
The supply of transponders to the Radarsat Constellation Mission is the latest highlight in this success story. This involved the construction of two transponders with remote control capability for CSA, based on previously developed calibration targets. "Based on innovative designs we have set new benchmarks by achieving precedented precision in the development of calibration instruments in this area," explains Marco Schwerdt, Head of the Calibration Group at the Microwaves and Radar Institute. "We determine the backscatter properties of our transponders – meaning their ability to return the signal transmitted by radar satellites – to an accuracy of 0.2 decibel range and are hence on a par with highly accurate laboratory devices."
Extensive modifications were necessary to satisfy the specific requirements defined on the Canadian side and to withstand the local climatic conditions. All the electronic components, including the antennas, are located in a temperature-controlled housing in order to achieve high accuracy and reliability. The order had a value of 2.3 million euro. CSA needs the devices for its future Radarsat Constellation Mission – a set of three SAR satellites scheduled to launch in 2019.
Precise calibration of space radar systems is necessary in order to derive geophysical information products from their data. This includes extensive measures that begin early in the development phase. Active transponders are used most frequently as calibration references due to their high backscatter cross-sections, combined with their comparatively compact structure. A radar transponder works according to the following principle – it receives the signals transmitted by the satellite, amplifies them to a highly accurate, predefined level and sends them back to the spacecraft. The transponders are shown as bright crosshairs in the radar images; the scientists use this information to calibrate the entire radar system.
DLR researchers are already working on the next generation of transponders. They will primarily focus on future long-wave missions, in particular the environmental and climate mission Tandem-L.