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

Robust altitude estimation for over-the-horizon HF radar 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 multipath reflections off the target, we propose two new high resolution signal processing methods for discriminating small time delay differences from a sequence of radar dwells.  Both methods exploit frequency-hopped illumination over a wide band. The first exploits frequency-selective fading of the target peak over a sequence of amplitude range-Doppler surfaces. The second synthesizes an ultra-wideband coherent chirp by using a model to stitch together multipath returns across multiple radar dwells.  Since altitude estimation requires that returns be correctly associated with ionospheric raypaths (i.e. ordinary or extraordinary rays through the E, F1, or F2 layers), we propose using high fidelity propagation modeling to predict aircraft height from measured direct and ground bounce arrivals.  In order to study the robustness of our methods, we will perform a preliminary analysis of how ionospheric modeling errors affect altitude estimation accuracy. Finally, we will develop a plan for validating our analyses using existing OTH radar data and/or new altitude estimation trials. BENEFIT: The capability for reliable altitude estimation for OTH radar will benefit both existing and future OTH radar systems.  For example, robust altitude estimation could serve the national defense mission of a Next Generation OTH Radar by helping to distinguish a fast low-flying cruise missile from a high altitude Learjet-class aircraft of similar speed. Alternatively, altitude estimation could serve the counter-narcoterrorism mission of the Relocatable OTH Radar (ROTHR) system by providing a means of both identifying low-flying aircraft trying to evade ground-based radars, as well as vectoring intercept aircraft. We also forsee opportunities for adapted this technology to wireless urban and indoor multipath geolocation, namely situations where conventional GPS localization is compromised.