Low-angle tracking radars and non-linear time-delay estimation

A new approach, based on non-linear time-delay estimation, is described for estimating the elevation angle in the combined presence of specular multipath and receiver noise—a situation that is encountered in the low-angle tracking radar environment. Estimation of the elevation angle is accomplished using the time delay between the signals impinging on the antenna array elements. Thus, the accuracy in the elevation angle estimation is dependent on the accuracy of the time-delay estimation algorithm. The formulation of the problem enables one to find the desired estimates independently of the reflection coefficient, which is a significant result. In the proposed approach, a newly derived non-linear filter due to the author (1986) is used. The filter structure is estimated using the Pontryagin minimum principle and the method of invariant imbedding, and is used to find an estimate for the delayed and undelayed signal. Subsequently, the time delay and the elevation angle are estimated.