| Over-the-horizon (OTH) radar uses the refractive properties of the ionosphere for wide-area surveillance of targets at long ranges. Currently, OTH radars can localize targets in latitude and longitude, but have difficulty estimating target altitude, which is important for classification purposes. Methods which have been proposed for aircraft altitude estimation using OTH radar take advantage of micro-multipath returns due to ground reflections local to the aircraft and are typically limited in performance by radar bandwidth and observation time.; In this work, electromagnetic matched-field processing is proposed for estimating aircraft altitude by exploiting the altitude dependence of unresolved multipath returns in complex range-Doppler space. Methods using both a single radar dwell and multiple dwells are presented. Simulations suggest that the performance of the single-dwell method suffers in situations where the coherent integration time (CIT) and bandwidth of the radar waveform are short. To overcome this limitation, a matched-field estimation approach is presented which additionally exploits the altitude dependence of dwell-to-dwell shape changes in the complex range-Doppler multipath return as the basis for multi-dwell maximum likelihood altitude estimation. Monte Carlo simulation results indicate that using a short CIT, moderate signal bandwidth, and a 30 second revisit interval, multi-dwell matched-field altitude estimation can achieve accuracy to within 2500 feet after as few as four radar dwells. The results of processing actual radar data for both high flying commercial aircrafts and a low flying twin engine aircraft are also presented and validated against aircraft altitude ground truth.; The multi-dwell matched-field altitude estimation algorithm is extended to jointly estimate altitude and altitude rate under the assumption that the altitude rate is constant. Simulation results show that the joint altitude and altitude rate estimation algorithm is able to classify an aircraft altitude track as ascending, descending, or level with only a slight performance loss in the altitude estimate. |
