Phase Sequence Estimation for Laser Line-Scan Imagery in the Presence of Rayleigh Fading.

An airborne laser line-scan imaging system can determine the relative range to a target by power modulation the illuminating laser and detecting the relative phase of the reflected signal with respect to the modulation phase reference. The amplitude of the reflected signal may fluctuate greatly due to such factors as speckle noise and the varying reflectivity of the target, resulting in the severe degradation of performance of commonly used phase detection circuitry. In this report, a maximum-a posteriori MAP phase sequence estimator is developed based upon the Viterbi algorithm. The signal is observed in additive, zero mean, white Gaussian noise and the statistics of the amplitude are assumed to be Rayleigh. Perfect knowledge of each realization of the signal amplitude is assumed and the sensitivity to errors in the amplitude estimate is investigated. Expressions for approximate mean-squared error and a lower bound on mean-squared error are developed and verified by a Monte Carlo simulation of the estimator.