Passive ranging in a multipath dominated environment

At long ranges the Cramer-Rao lower bound on passive ranging error is proportional to the fourth power of the range-to-array-baseline ratio. This behavior results from the diminishing wavefront curvature observed by an array at long ranges, and leads to unacceptable performance in many applications of interest. If energy arrives over two different paths one can measure the differential delay between the arrivals and use it to improve the range estimate. This paper demonstrates that the resulting improvement in range accuracy can be very substantial when the propagation conditions encountered by the two arrivals are known precisely. Even modest uncertainties concerning propagation conditions can seriously limit that improvement. With large uncertainties one generally cannot achieve significant improvements over single path measurements.