Abstract-The laser beacon power required by a communication terminal for acquisition and tracking in deep space optical link scenarios can be reduced by a factor of 10 to 100 by replacing an integrating array, such as a CCD, with an array of single photon detectors. An additional benefit of the single photon detector array is that each pixel can have MHz bandwidths, allowing simultaneous recovery of photon time-ofarrival information that can be used for uplink data recovery or range measurements.

We propose a solution for pointing and tracking an optical terminal using one or more beacons and a slowly varying background image. The primary application is a deep space optical communication terminal, where multiple source tracking provides robustness against beacon outage. Our solution uses optical orthogonal codes modulated on each beacon to separate the signal from each source for centroiding. This technique allows calculation of the transmit pointing vector from each beacon location as well as from the background image. The latter can be used to track during beacon outages. We present a simple algorithm for performing this separation, and apply it to experimental data from a photon-counting detector illuminated by two beacons and one constant source. Our results show that the photon flux from each source can be accurately estimated even in the low signal, high background regime.