High resolution processing with an active phased array SAR

The Dutch PHARUS system is a polarimetric active phased array SAR which is capable of performing advanced SAR modes such as spotlight SAR, MTI and Scan SAR. High resolution can be obtained using the spotlight mode. Although in general the processing for such a mode is well-known for conventional beam steering, electronic beam steering with an active phased array SAR presents particular difficulties. In order to successfully implement the required coherent integration of a long synthetic aperture, the phase behaviour of the antenna as a function of beam steering angle and frequency must be well understood. Another technique, implemented for range resolution improvement, is the use of stepped frequency pulses. Because of the higher total bandwidth involved, the antenna is pushed to its limits in terms of frequency range. The frequency dependence of the antenna has to be well known and compensated in this case. Finally, PHARUS will be used for repeat pass interferometric experiments. In order to obtain coherence between the two passes, the azimuth aspect angle of the two passes must be nearly the same. This necessitates active, real time beam steering, to compensate for aircraft attitude. Again, knowing the antenna phase behaviour is essential in this case. This paper describes the current slate of investigations into these matters. The techniques used to investigate the active antenna phase behaviour are Near-Field antenna measurement, in which the antenna far field pattern is calculated from measurements of the near field, and specialised autofocus techniques, which aim to extract phase behaviour over a coherent integration interval, and over a wide frequency band