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Multipath Signal Processing for Over-the-Horizon Skywave Radar Target Localization

Jeffrey Krolik and Richard Anderson
Department of Electrical and Computer Engineering
Duke University
P.O. Box 90291/184 Engineering Bldg.
Durham, NC 27708-0291
tel: (919) 660-5274
email: jk@ee.duke.edu

Michael Papazoglou
Naval Research Laboratory

Abstract This paper concerns methods for improving the performance of high frequency skywave over-the-horizon radar by exploiting realistically attainable knowledge of complex multipath propagation conditions. Two different problems are addressed here: (1) matched-field estimation of aircraft altitude, and (2) coordinate registration and mode linking for multi-target latitude and longitude estimation. Multi-dwell matched-field altitude estimation (MFAE) works by modeling the changes in the coherent, but unresolved, direct and surface reflected target returns from dwell-to-dwell. In particular, the pattern of rapid shape changes due to aircraft motion, seen as fading of the complex radar return in delay-Doppler space, are strongly dependent on aircraft altitude. Using a raytrace multipath propagation model to predict rapid fading, and first-order Markov modeling to handle slow fluctuations due to target aspect changes and medium fluctuations, a maximum likelihood estimate (MLE) of aircraft altitude is developed. Real and simulated radar data indicates an altitude accuracy of better than 2500 ft at ranges out to 2300 km can be achieved. The second problem addressed here concerns associating resolved multipath arrivals from different layers of the ionosphere with the correct target and raymode type. This is critical to converting multipath time delays measured by the radar with the target's ground range. The process of associating slant tracks with raymode types and targets is commonly referred to as "mode linking." Mode linker performance depends on both the accuracy of the ionospheric model as well as the radar's slant tracker. The approach taken here is to use a statistical model for ionospheric uncertainty in the mode linking/coordinate registration algorithm in order to improve its robustness to environmental mismatch and hence minimize the probability of incorrect mode linking decisions. Results for real and simulated radar data are presented which indicate that the new technique can achieve better than a 2:1 improvement in target localization over conventional coordinate registration/mode linking methods.

Presentation (pdf format)



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