DLR’s Innovative Point Targets for SAR System Calibration and their Application to Sentinel-1

Abstract

Ongoing and future spaceborne Synthetic Aperture Radar (SAR) missions use complex imaging modes, a multitude of frequency bands and further push the limits towards better image quality. The increased complexity of these SAR systems requires advanced strategies and innovative ground equipment to provide an accurate in-orbit calibration. The German Aerospace Center (DLR) meets these challenges by continuously developing highly accurate active and passive calibration targets. In a recent effort, DLR developed, built and deployed six innovative multi-mission SAR calibration point targets: three C-band transponders and three 2.8 m trihedral corner reflectors, all of which are permanently installed across a 120 km wide strip parallel to latitude N48 in South Germany. They were first used operationally for DLR’s independent system calibration of ESA’s Sentinel-1A and Sentinel-1B satellites in the framework of the European Copernicus program. Their remote-controlled operation permits regular short-interval calibration measurements as currently performed within the scope of routine operation support of Sentinel-1. In their latest stage of development, two C-band transponders build by DLR will be installed at sites at the Canadian Space Agency (CSA) to support the Radarsat Constellation Mission (RCM). Starting with an overview of DLR’s calibration activities, this paper focuses on the development and performance of the highly accurate transponder systems and presents in brief the remote controlled corner reflectors. These reference targets allow for radiometric, polarimetric and geometric calibration of spaceborne SAR systems. The transponder design will be introduced, covering the RF analog electronics, the digital subsystem and mechanical aspects. In order to achieve a high radiometric accuracy, the transponders have to be calibrated themselves. An internal calibration loop monitors and compensates the time variation of the Radar Cross Section (RCS) ensuring stable operation. Based on DLR’s Sentinel-1 calibration activities, such as radiometric calibration or antenna pattern verification, the application and performance of the novel calibration targets are discussed. In conclusion, an outlook towards DLR’s future developments of calibration systems dedicated to upcoming SAR missions is given.