Bearing Estimation In Beamspace Employing Frequency Diversity
M.D. Zoltowski, Ta-Sung Lee · Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE · 1989
We here develop a beamspace domain based Maximum Likelihood (BDML) estimation scheme for low-angle radar tracking. In the low-angle radar tracking scenario, echoes return from the target via a specular path, as well as by a direct path. The angular separation between the direct and specular arrivals is a fraction of a beamwidth, negating the use of classical monopulse bearing estimation based on sum and difference beams. In the new scheme, the element space snapshot vectors are first operated on by a Butler matrix composed of three orthogonal beamforming vectors. The conversion to 3-D beamspace domain via the Butler matrix beam former is shown to facilitate a simple, closed-form BDML estimator for the direct and specular path angles. To avoid track breaking in cases where the two signals arrive at the center of the array either perfectly in-phase or 180° out-of-phase, the use of frequency diversity is incorporated. The coherent signal subspace concept of Wang and Kaveh is invoked as a means for retaining the computational simplicity of single frequency operation. With proper selection of the auxiliary frequencies, it is shown that perfect focusing at the reference frequency may be simply achieved at the outset, i. e., without iterating. Simulations are presented demonstrating the performance of the BDML estimation scheme employing frequency diversity in a low-angle radar tracking environment.