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The effect of photon counting detector blocking on centroiding for deep space optical communications

Deep-space optical communications systems often utilize an uplink optical beacon in order to provide a reference for optical flight terminal stabilization and downlink pointing. The single-photon-counting detector arrays that are used to sense this beacon under photon-starved conditions may be count-rate limited due to blocking from detector quench times and other readout circuit constraints. This blocking effect reduces the effective detection efficiency in a non-uniform manner over the array, changing the photon counting channel model and degrading the performance of uplink centroiding algorithms. In this paper, we discuss an uplink beacon centroiding algorithm and the impact of detector blocking on this algorithm. Statistical models of detector output data and centroiding observables are presented based upon analysis, Monte Carlo simulations, and measured laboratory data, incorporating realistic effects such as beacon signal parameters and Earth background. A simple blocking compensation algorithm is presented and shown to mitigate the effect of blocking, allowing one to maintain sub-microradian centroiding errors under channel conditions representative of deep space optical links.