Performance Prediction and Verification for Bistatic SAR Synchronization Link

Abstract

Bistatic SAR systems have a high potential for scientific, commercial and security applications. One of the benefits is the possibility to generate highly accurate digital elevation models using bistatic interferometry. Examples for proposed bi- and multi-static satellite missions with interferometric capabilities are TanDEM-X and Cartwheel. Both are based on radar instruments placed on different spacecrafts, which gives rise to several technical challenges for the system realization. A factor which may severely degrade the performance of a bistatic SAR is the phase instability of the two oscillators involved. Investigations have shown, that, unless highly stable oscillators are used, the oscillators phase noise has to be compensated; this is possible through the processed SAR data in combination with ground control points, or by establishing a synchronization link to directly exchange signals providing information on the oscillator phase noise. The later method is based on recording the received demodulated phases, which are then used to derive a compensation signal to correct the SAR data. The paper describes a possible configuration for the synchronization link, sharing hardware components with the SAR instrument. Such a system is currently intended for the TanDEM-X mission. The resulting restrictions on the synchronization scheme, timing, and accuracy are investigated. The statistical properties of the compensation signal are used to derive a figure-of-merit for synchronization performance. The focus is on the influence of the synchronization link RF hardware on the quality of the derived compensation signal. Therefore a hardware setup is realized and characterized to obtain realistic data. Further, a simulation tool is implemented to predict the performance. The simulation tool is based on a synchronization link hardware model which can use measurement data as an input. Finally, the complete synchronization process is verified by measurements using the test hardware.