Passive ranging accuracy using arrays deteriorates at long ranges because of the vanishing wavefront curvature observable by an array. The idea of including a priori knowledge of propagation conditions in ranging procedures for the purpose of improving the range estimate has received considerable attention in recent years. The interest is based on an observation that a large propagation delay measured between two coherent arrivals at a single receiver is equivalent geometrically to a delay generated by a single arrival in propagating to two widely spaced receivers. If the equivalent sensor spacing is on the order of or larger than the dimensions of an actual array, multipath ranging appears to be an attractive alternative to conventional curvature measurement approaches. This paper considers the potential performance improvements attainable in the process of exploiting a second path in long range ranging. The performance of line arrays is pursued in detail for a problem setting in which all of the parameters describing the signal and channel parameters are known a priori.
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