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Thedevelopmentofa high-data-rate communications electronics subsystem for future application ul free-space, directdetection laser communications is described. The dualchannel subsystem uses quaternary pulse position modulation (QPPM)* and operates at a throughput of 650 megabits per second. Transmitting functions described include source data multiplexing, channel data demultiplexing, and QPPM symbol encoding. Receiving functions include analog signal conditioning, slot and symbol clock recovery, QPPM data recovery and decoding, channel data multiplexing, and destination data dernultiplexhig. Implementation of a prototype version in discrete gallium arsenide logic, radiofrequency components, and microstrip circuitry is presented. The approach to developing an integrated package for potential spaceflight demonstration is described. Link effects simulation, full-duplex data and video communications demonstration, and computer control and monitoring approaches are discussed. Packaging of the communications electronics subsystem and plans for future development conclude the paper.

Operating three unique software defined radios (SDRs) in a space environment aboard the Space Communications and Navigation (SCaN) Testbed for over one year has provided an opportunity to gather knowledge useful for future missions considering using software defined radios. This paper provides recommendations for the development and use of SDRs, and it considers the details of each SDR's approach to software upgrades and operation. After one year, the SCaN Testbed SDRs have operated for over 1000 hours. During this time, the waveforms launched with the SDR were tested on-orbit to assure that they operated in space at the same performance level as on the ground prior to launch to obtain an initial on-orbit performance baseline. A new waveform for each SDR has been developed, implemented, uploaded to the flight system, and tested in the flight environment. Recommendations for SDR-based missions have been gathered from early development through operations. These recommendations will aid future missions to reduce the cost, schedule, and risk of operating SDRs in a space environment. This paper considers the lessons learned as they apply to SDR pre-launch checkout, purchasing space-rated hardware, flexibility in command and telemetry methods, on-orbit diagnostics, use of engineering models to aid future development, and third-party software. Each SDR implements the SCaN Testbed flight computer command and telemetry interface uniquely, allowing comparisons to be drawn. The paper discusses the lessons learned from these three unique implementations, with suggestions on the preferred approach. Also, results are presented showing that it is important to have full system performance knowledge prior to launch to establish better performance baselines in space, requiring additional test applications to be developed pre-launch. Finally, the paper presents the issues encountered with the operation and implementation of new waveforms on each SDR and proposes recommendations to avoid these issues.

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