Cost-effective calibration transponders for future synthetic aperture radars

This paper summarises some results of a study funded by the Technology Research Programme (TRP) of the European Space Agency (ESA). The study was undertaken by Systems Engineering and Assessment Ltd (SEA) supported by the Defence Evaluation and Research Agency (DERA). The contract addressed concepts and architectures for cost-effective calibration transponders for future SAR systems. Coding, i.e. deliberately modifying the transponded signal in some way to distinguish it from normal echoes, is of interest for future SAR calibration transponders. Therefore, a number of coded transponder trials were performed and analysed, with the results feeding into the study aspects. The coded transponder trials were performed during 1998, using a transponder supplied by DERA and the Digital SAR Experimental Modulator (DSEM) developed by SEA. The DSEM is a programmable multi-channel digital processor which allows the generation of many different coded signals. The equipment was deployed at Lasham Airfield in England and operated successfully on several ERS-2 passes. A variety of coding schemes was tried, including both azimuth and pulse coding, usually with a number of different coded signals on a single pass. The trials data were processed and analysed by SEA using a modified version of the SEA SAR Processor with decoding functions included. An Advanced Calibration Transponder is proposed which is particularly suited for ASAR and future SAR systems where many more measurements will be required.