Return power estimation for targets spread in range

Abstract

Return power is perhaps the most fundamental measurement available to radar after round-trip time delay. For coherent lidars using aerosol targets that are spread in range, there are two approaches. In the more straightforward, an estimate is formed by squaring the heterodyne output of the photodetector and subtracting the mean noise level.1,2,3 This method is often analyzed in the context of heterodyne detection of static hard targets where no account need be taken of Doppler shift in the return. The alternative is, in effect, to reduce noise by filtering the return and using only data within the filter passband for power estimation. Estimation of the frequency shift is then in principle the first step, but it can be combined with estimation of return power by, e.g., forming maximum likelihood (ML) estimates of both parameters simultaneously. For Doppler radar, Zrnic4 analyzed this approach as applied to monochromatic returns and appropriately commented that the need for a filter to optimize the power estimate is 'intuitively satisfying'. The goal of this note is to compare the two approaches.