In this presentation, we discuss the first demonstration of a lasercom downlink from a LEO 1.5U 2.3 kg CubeSat to our optical ground station at The Aerospace Corporation in El Segundo, CA. Two vehicles, AC7-B&C, built under NASA's Optical Communications and Sensors Demonstration (OCSD) program and described in previous presentations, were launched in November 2017 and placed in a 450-km circular orbit. Following on-orbit checkouts and preliminary pointing calibration utilizing on-board star trackers, we have demonstrated (at the time of this manuscript submission) communications links up to 100 Mbps with bit error rates near 10-6 without any forward error correction. Further optimization of the vehicle pointing and detection electronics and operating the transmitter at its full power capacity should enable performance improvements and potential for higher data rates.

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Software defined radio is a widely accepted paradigm for design of reconfigurable modems. The continuing march of Moore's law makes real-time signal processing on general purpose processors feasible for a large set of waveforms. Data rates in the low Mbps can be processed on low-power ARM processors, and much higher data rates can be supported on large x86 processors.The advantages of all-software development (vs. FPGA/DSP/GPU) are compellingmuch wider pool of talent, lower development time and cost, and easier maintenance and porting. However, very high-rate systems (above 100 Mbps) are still firmly in the domain of custom and semi-custom hardware (mostly FPGAs). In this paper we describe an architecture and testbed for an SDR that can be easily scaled to support over 3 GHz of bandwidth and data rate up to 10 Gbps. The paper covers a novel technique to parallelize typically serial algorithms for phase and symbol tracking, followed by a discussion of data distribution for a massively parallel architecture. We provide a brief description of a mixed-signal front end and conclude with measurement results. To the best of the author's knowledge, the system described in this paper is an order of magnitude faster than any prior published result.

Signal generation in the GPS III satellites employs weighted voting to combine the baseband P(Y) signal with both components of the baseband L1C signal on the in-phase part of the L1 carrier. Weighted voting employs majority voting with pseudorandom time multiplexing of pure signals, producing a constantenvelope real-valued combination of the three biphase inputs with different useful received powers. Weighted voting introduces jitter into receivers' correlation functions, adding to jitter from noise and interference. This paper quantifies the effect of weighted voting on receiver input signal-to-noise ratio (SNR), then predicts the effect of weighted voting on carrier tracking by conventional, codeless, and semicodeless P(Y) receivers. Analysis and computer simulation results are supplemented by receiver measurements, providing conclusive evidence that degradation by weighted voting is evident only at high SNR, having an insignificant effect on receiver performance.

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