Pulse position modulated (PPM) ground receiver design for optical communications from deep space

Pulse position modulation (PPM) provides a means of using high peak power lasers for transmitting communications signals from planetary spacecraft to earth-based receiving stations. Large aperture (similar to10 m diameter) telescopes will be used to collect and focus the laser communications signal originating from a deep space transmitter on to a PPM receiver. Large area (1-3 mm diameter) sensitive detectors, preceded by appropriate narrow (0.1-0.2 nm) optical band-pass filters and followed by low-noise, high-gain, amplifiers will serve as the PPM receiver front end. A digital assembly will form the backbone of the receiver. The PPM receiver will achieve and maintain slot synchronization based on sub slot sums generated by a field programmable-gated array (FPGA). Spacecraft dynamics and timing issues between the ground-based receiver and the transmitter on board the spacecraft must be taken into account. In the present report, requirements and design of a prototype PPM receiver being developed over the next year will be elaborated. The design is driven by the need to demonstrate and validate PPM reception using a variety of detectors under simulated conditions representative of those to be encountered in a deep space optical communications link.