Multi-Target Tracking via Nonlinear Least Squares Using Doppler Measurements from a Passive Radar System
Joshi, Sujay S.
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A passive radar systems opportunistic ability to exploit ambient radio signal reflections makes it ideal for covert target tracking. This strategy, referred to as passive covert radar (PCR) or passive coherent location (PCL), typically exploits FM radio or television signals from powerful local transmitters. In addition to covertness, the absence of a dedicated transmitter helps reduce costs and overall system complexity. While a variety of measurements can be used to estimate a targets position and velocity, such as time difference of arrival (TDOA) and direction of arrival (DOA), this thesis focuses on using only Doppler shift measurements to estimate a targets state. The work presented in this thesis examines the use of Doppler shift measurements from multiple receivers to solve the target tracking and association problem. A nonlinear least squares error (NLSE) estimation technique, called the Levenberg-Marquardt (L-M) algorithm, is used to determine a targets state (position, velocity) from these Doppler shift measurements. More than one target state can potentially produce identical Doppler shift profiles. In a single-receiver, single-target scenario, it is shown that three additional ghost targets caused by symmetry produce the same Doppler shift response. These ghosts may make state estimation impossible if receive antennas are not physically positioned to block out ghost targets. While the NLSE technique tends to give an accurate solution in one quadrant, three other solutions will symmetrically exist in each of the remaining three quadrants. The addition of either another receiver or another measurement (such as DOA) is needed to break this quadrant ambiguity. This thesis considers adding multiple receivers to accurately associate and track multiple targets. Two target association methods (sequential and simultaneous) are developed, and their computational requirements and accuracy are compared. A grid-aided L-M search technique is investigated in an attempt to provide a better initial target state guess to these association and tracking algorithms. The analysis and simulation results suggest it is feasible to perform multi-target association and tracking using Doppler shift as the sole measurement. Both of the proposed methods gave optimal target association and converged to reasonably accurate state estimates in most of the Monte Carlo runs.