We report a free space laser communication experiment from the satellite laser ranging (SLR) station at NASA Goddard
Space Flight Center (GSFC) to the Lunar Reconnaissance Orbiter (LRO) in lunar orbit through the on board one-way Laser Ranging (LR) receiver. Pseudo random data and sample image files were transmitted to LRO using a 4096-ary pulse position modulation (PPM) signal format. Reed-Solomon forward error correction codes were used to achieve error free data transmission at a moderate coding overhead rate. The signal fading due to the atmosphere effect was measured and the coding gain could be estimated.
NASA Goddard Space Flight Center is developing a direct-detection free-space laser communications transceiver test bed. The laser transmitter is a master-oscillator power amplifier (MOPA) configuration using a 1060 nm wavelength laser-diode with a two-stage multi-watt Ytterbium fiber amplifier. Dual Mach-Zehnder electro-optic modulators provide an extinction ratio greater than 40 dB. The MOPA design delivered 10-W average power with low-duty-cycle PPM waveforms and achieved 1.7 kW peak power. We use pulse-position modulation format with a pseudo-noise code header to assist clock recovery and frame boundary identification. We are examining the use of low-density-parity-check (LDPC) codes for forward error correction. Our receiver uses an InGaAsP 1 mm diameter photocathode hybrid photomultiplier tube (HPMT) cooled with a thermo-electric cooler. The HPMT has 25% single-photon detection efficiency at 1064 nm wavelength with a dark count rate of 60,000/s at -22 degrees Celsius and a single-photon impulse response of 0.9 ns. We report on progress toward demonstrating a combined laser communications and ranging field experiment.
At NASA's Goddard Space Flight Center (GSFC), space qualified integrated circuits for several key elements in space communication systems have been in development to increase data return in bandwidth constrained channels for future missions. Particularly in the area of digital communication, the development includes data compression, channel coding and modulation. In on-board data compression area, development focuses on a high-speed compression scheme that serves both push-broom and frame sensors. The compression ratio can be easily adjusted for different applications from lossless to visually lossless. The algorithm conforms to the Consultative Committee on Space Data Systems (CCSDS) new compression recommendation to be released 2005. The radiation-tolerant (RT) hardware will afford 20 Msamples/sec processing on sensor data. For bandwidth efficient channel coding, newly developed low density paritycheck codes (LDPCC) will double channel utilization as compared to previously used concatenated convolutional/Reed- Solomon (CC/RS) coding scheme. An RT implementation of the encoder is expected to work up to 1 Gbps serving both low-rate and high-rate missions. In modulation, a versatile multi-function base-band modulator allows missions the flexibility to choose from 2 bits/symbol/Hertz quadrature phase shift keying (QPSK)-type schemes, to 2.0, 2.25, 2.5, and 2.75 bits/symbol/Hertz 8 phase shift keying trellis-coded modulation (8-PSK TCM) schemes--all CCSDS recommendations. Along with 8PSK, 16-quadrature amplitude modulation (16-QAM), 16-ampliture phase shift keying (16-APSK), all modulations are implemented in a single RT chip with expected throughput of over 300 Mbps. This paper describes the development of these three technology areas and gives an update on their availability for space missions.