Development and operation of an airborne VHF SAR system-lessons learned

The interest in using VHF/UHF ultra-wideband SAR for the purpose of target detection and recognition has grown in the past few years. Low operating frequencies are required to penetrate an obscuring canopy or soil. Several crucial design issues can be identified for a system to be operated in the lower VHF-band (20-90 MHz), in particular the antenna system, the receiver chain and the waveform generator. The low directivity of the antenna implies that the synthetic aperture will be long, and this sets requirements on the accuracy and temporal stability of the navigation system to be able to apply the motion compensation properly. The large amount of data will also have an impact on the SAR processing schemes to be used, trading off processing time versus some image quality parameter. A lot of experience has been gained from the realization and operation of the airborne CARABAS I system and has now been invested in the upgraded system, CARABAS II, operational since 1996, and with considerably improved performance figures. CARABAS II has so far acquired data appropriate for studies of applications such as foliage penetration, biomass estimation and SAR interferometry.