Free space optical communications links from deep space are projected to fulfill future NASA communication requirements for 2020 and beyond. Accurate laser-beam pointing is required to achieve high data rates at low power levels. For the highest pointing accuracy, a laser beacon transmitted from near the Earth receiver location is acquired and tracked by the space transceiver to obtain accurate knowledge of the Earth receiver position in the pitch and yaw degrees of freedom. This pointing knowledge is generated by forming estimates of the beacon transmitter location by centroiding the position of a focused spot on a focal plane detector array in the space transceiver, perhaps a two-by-two pixel array (a quad detector), but often on a larger array to ease initial spatial acquisition. The accuracy of those estimates, and, therefore, the accuracy of the space transceiver pointing, is a function of the received optical signal power, accepted optical background power, and detector readout noise. The centroiding performance of a typical focal plane array can be 10 to 100 times poorer than the shot noise limit due to readout noise. A focal plane array of single-photon detectors can fully close this gap, and thereby require 10 to 100 times less beacon transmit power, but specialized per-pixel processing circuitry is required.

This innovation is a per-pixel processing scheme using a pair of three-state digital counters to implement acquisition and tracking of a dim laser beacon transmitted from Earth for pointing control of an interplanetary optical communications system using a focal plane array of single sensitive detectors. It shows how to implement dim beacon acquisition and tracking for an interplanetary optical transceiver with a method that is suitable for both achieving theoretical performance, as well as supporting additional functions of high data rate forward links and precision spacecraft ranging.

Spatial acquisition and tracking on the uplink laser beacon from Earth can be achieved on the space transceiver focal plane array by connecting two counters to every array pixel. This scheme provides a low-complexity method to monitor all pixels in the detector array until a beacon signal is detected. Temporal acquisition of the uplink laser beacon square wave signal is performed using outputs from a pair of phase-offset counters. The counters alternate among three states denoted by “up,” “down,” and “idle.” In the up state, a counter increments its value when its pixel registers a photon arrival. In the down state, the counter decrements its value when a photon arrival is detected. The counter maintains its value in the idle state. For an outer modulation signal of 2 PPM + two inter-symbol guard time slots with slot widths Tslot, the counters cycle through the three states with period of 4Tslot. The counters can be seen as approximations to a maximum-likelihood timing estimation with a modified pulse shape. Post-processing in software allows the outputs of the counters to be integrated in time.

This work was done by William H. Farr, Kevin M. Birnbaum, Kevin J. Quirk, Suzana E. Sburlan, and Adit Sahasrabudhe of Caltech for NASA’s Jet Propulsion Laboratory. NPO-48153

This Brief includes a Technical Support Package (TSP).
Per-Pixel, Dual-Counter Scheme for Optical Communications

(reference NPO-48153) is currently available for download from the TSP library.

Don't have an account? Sign up here.