Multimode Tracking for Next Generation Over the Horizon Radar (NG OTHR)

ABSTRACT: Coordinate Registration (CR) of OTHR targets requires accurate real-time ionospheric modeling coupled with HF propagation calculations to convert radar-measured target signal delays and beam steers to geographical position. Our team is the developer of the most sophisticated CR capability currently in existence, CREDO (Coordinate Registration Enhancement by Dynamic Optimization). CREDO uses OTHR vertical and oblique backscatter soundings to model the ionosphere using Tikhonov Regularization, a constraint that produces the smoothest ionosphere that agrees with all input data to within measurement error. Recently we extended CREDO to utilize Known Reference Points (OTHR returns from known geographical coordinates, including FAA ground tracks). When multimode returns from the same target can be identified, an additional strong constraint on the ionospheric model is imposed; skywave propagation must connect all modes to the same geographical location. In Phase I we incorporated this multimode constraint in CREDO"s optimization algorithm and demonstrate resulting CR accuracy improvement. In Phase II we will use this improved ionospheric modeling to develop a multimode tracker using NWRA"s SIFTER extension of track-before-detect, with tracking performed in geographical coordinates instead of radar coordinates. This approach removes the requirement for explicit mode linking, eliminating the longstanding problem of misidentified modes. BENEFIT: The incorporation of additional ionospheric data beyond conventional OTHR vertical and oblique backscatter soundings improves the fidelity of real-time ionosphere models, resulting in improved OTHR Coordinate Registration accuracy. Skywave representation of Known Reference Points such as FAA ground truth targets is one example of such extra ionospherically-related data. Identification of skywave modes that belong to the same target (called"family members"in ROTHR terminology) allows the incorporation of an additional strong constraint on the derived ionosphere model; skywave propagation in the model must properly co-locate family-member modes geographically. This was addressed in Phase I. The Phase II development of a geographic-space tracker eliminates mode-linking errors (the cause of the largest CR errors in OTHR) and provides enhanced sensitivity to low RCS targets (e.g., small boats or cruise missiles), extending the applicability of NGOTHR to important Homeland Security issues. Improved CR accuracy will enhance the applicability of OTHR as a wide area surveillance asset for Air Force and Homeland Security applications with dramatic cost savings over alternative microwave radar solutions.