Robust Wideband Aircraft Altitude Estimation for Over-the-Horizon HF Radar

Robust altitude estimation for over-the-horizon radar (OTHR) requires a combination of advanced signal processing and high fidelity ionospheric propagation modeling. Since aircraft height is manifested in closely-spaced direct and ground-bounce reflections off the target, during Phase I, we have developed two new approaches for discriminating small delay differences from a sequence of radar dwells. Both methods exploit wideband frequency-hopped illumination. The first performs maximum a posteriori probability altitude estimation by modeling the frequency-selective fading of the target amplitude-range-Doppler peak. In the second approach, multiband chirp synthesis is used to stitch together dwells so as to resolve small multipath delay differences using a larger effective bandwidth signal. A robust maximum likelihood approach is then applied to map these delay differences to aircraft altitude. In Phase II of this project, we propose to evaluate these methods using operationally-relevant performance metrics and high fidelity ionospheric propagation models which include both ordinary and extraordinary raypaths. In order to study the robustness of our methods, we will also perform Monte Carlo analyses under realistic model mismatch conditions. Finally, existing real OTHR data sets will be identified and a detailed experimental plan will be generated for subsequent full-scale evaluation using a dedicated aircraft. BENEFIT: The development of robust altitude estimation for over-the-horizon (OTH) HF radar would represent a major breakthrough in wide area air surveillance. In addition, we foresee scaling of these techniques to several small-system commercial scenarios. An example is a family of hardware and software modules for a Wide-Area Wi-Fi Radar Intruder Detection System.