Maximum-Likelihood-Based Acquisition and Tracking Scheme for Downlink Optical Telemetry

Free-space laser communication is a promising technology to meet the demand for cost-effective wideband support for future NASA missions. JPL has begun construction at the Table Mountain Facility in California of a 1-meter telescope system, known as the Optical Communication Telescope Laboratory (OCTL) and capable of tracking spacecraft from low Earth orbit to deep space, for development and validation of vital optical communication technologies. One of them is accurate pointing for both up (ground-to-space) and down (space-to-ground) links. This article describes an acquisition and tracking scheme for downlink optical telemetry that is based on a correlation-type technique developed for extended sources covering several elements of the detector array. The purpose of developing such a scheme is to enable simultaneous beam tracking and data detection using a small number of optical sensors. The open-loop acquisition derived from the maximum-likelihood criterion involves a transform-domain correlation between the received laser image and the reference image derived from the known intensity profile of the transmitting laser. The optimal acquisition algorithm requires solving two nonlinear equations, or iteratively solving their linearized variants, to estimate the coordinate of the transmitting laser when a rotation-invariant movement is considered. A maximumlikelihood-based closed-loop tracking algorithm is developed as well, in which the loop feedback signals are formulated as weighted transform-domain correlations between the received laser image and the previously estimated reference image. This scheme is expected to be able to achieve sub-pixel resolutions in a high-disturbance environment.