Performance Prediction and Verification for the Synchronization Link of TanDEM-X

Abstract

Bistatic Synthetic Aperture Radar (SAR) systems have a high potential for scientific, commercial and security applications. One of the benefits is the possibility to generate digital elevation models using bistatic interferometry. A bistatic satellite mission with interferometric capabilities is TanDEM-X, which consists of two nearly identical SAR satellites. The first, TerraSAR-X, will be launched in February 2007, while the second is currently in Phase-C. The fact that both radar instruments are placed on different spacecrafts, 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. Previous studies have shown, that the oscillators phase noise has to be compensated; this is achieved by establishing a synchronization link to directly exchange signals at RF frequency between the two satellites providing information on the oscillator phase noise. The 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 the configuration for the synchronization link of TerraSAR-X and TanDEM-X, sharing hardware components with the SAR instrument. The resulting restrictions on the synchronization scheme, timing, and accuracy are stated. A signal model is presented which includes the contributions of the various stages of the synchronization link. Using the model an analytical expression for the phase compensation signal is obtained, which includes the possible error sources such as sampling, aliasing, receiver noise and timing errors. The statistical properties of the compensation signal are used to derive a figure-of-merit for synchronization performance, which is based on a calculation of the residual phase error after range and azimuth compression. The influence of the synchronization link RF hardware on the quality of the derived compensation signal is crucial for the performance. Therefore the synchronization link RF hardware of the TerraSAR-X satellite is characterized to obtain realistic measurement data. These measurements will be used for the calibration of the synchronization link, i.e. specifically to remove systematic errors due to temperature drifts of the instrument. In addition, synchronization measurements using the flight and the engineering hardware were performed on ground to verify the synchronization process. 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. The paper states the performance of the synchronization link based on the measurements and interprets the results based on the theoretical analysis.