NASA-Goddard Space Flight Center (GSFC) is designing and building a 650 Mbps laser communications testbedfor the development of ifight qualifiable hardware suitable for near-term operation on geosynchronous-to-geosynchronous ( GEO-GEO)crosslink missions. The primary purpose for the program is to develop and optimize laser communications unique subsystems. The design incorporates technologies that have previously been developed at GSFC, such as the quaternary pulse position modulation (QPPM) receiver, the GaAIAs diode laser beam combiner, and the integrated low- power digital gimbal controller. Requirements for the testbed experiments are to optimize the acquisition processes; to fully simulate the long range (up to 21,000 km) and the fine tracking characteristics of two narrow-beam laser communications terminals; and to fully test communications performance which will include average and burst bit errorrates (BERs), effects of laser diode co-alignment, degradation due to internal and external stray light, and the impactof drifts in the optical components. Transmit-to-receive alignment techniques will also be tested and optimized. Tradestudies and analyses have been performed to optimize the aperture, laser power, pointing requirements, modulation,

NASA's flight-technology development program: a 650-Mbit/s laser communications testbed

NASA-Goddard has undertaken the development of direct-detection optical communications for space applications at the Gbps data rate channel capability level. The primary challenges to optical communications designers lie in the development of reliable optical power sources, as well as of high performance pointing/acquisition/tracking systems required by the narrow widths of the transmitted optical beams. GaAlAs diodes and their arrays are currently the most attractive technology for optical transmitters; pioneering work has also been conducted on dichroic and grating techniques for combining the power of several laser diodes. Attention is given to the performance obtained for an optical link acquisition and submicroradian tracking/pointing system.

NASA/GSFC program in direct detection optical communications for intersatellite links

In this paper the NASA Goddard Space Flight Center (GSFC) laser communication pointing, acquisition, and tracking laboratory demonstration program is discussed. This program entails the construction of a flexible brass-board simulation tool, and a supporting computer simulation effort. The result of this program will be a viable test bed to support the design and specification of space-based laser communication terminals and, additionally, to address critical performance issues such as tracking robustness in the presence of background (e.g., stellar or earth) noise and/or internally generated thermal noise.

N. D. Fox

Papers

Performance And Modeling Of The Gsfc Pointing Acquisition And Tracking System For Laser Communications