Time-domain adaptive beamforming of HF backscatter radar signals

Experimental results relating to the use of time-domain, continuously adapting beamformers in an HF bistatic FM/CW backscatter radar system are presented. Data for the study were obtained using the Wide Aperture HF Radio Research Facility (WARF) which is located in the central valley of California and is operated by Stanford Research Institute, Menlo Park, CA. Eastward-looking transmissions were employed with an operating frequency chosen so as to provide single-hop ionospheric propagation on both the forward and backscatter paths. Digital real-time recordings were taken at eight received subarray outputs. These recordings were then processed off-line using a large general purpose computer. The beamforming methods studied, however, are computationally simple and may be readily implemented in real-time using a commercially available minicomputer. Two adaptive algorithms were studied and in both cases it was shown that signal-to-noise ratio improvements of 10 to 15 dB are readily achieved when adaptive beamforming is compared with conventional, Dolph taper beamforming methods using identical received data in an HF backscatter environment. It was also demonstrated that the time scale of coefficient variation in an adaptive processor operating in this environment is the order of 1 s. Successful tracking of the adaptive algorithm under these conditions was demonstrated. The use of moving target indication (MTI) clutter suppression filters at the subarray outputs, prior to adaptation, was investigated. No significant improvement was observed with the use of these filters on experimental data. Finally, it was shown that the presence of fading nulls can significantly affect the determination of optimal subarray location and spacing in an HF environment. In general, the adaptive beamformer performance was found to be less dependent upon array geometry than was the case for conventional processing.