Jason A. Soloff1, Bernard L. Edwards1 and Scott D. Hoy2
1 NASA Goddard Space Flight Center
2 Lockheed MartinIntroduction
Advanced mission concepts in NASA’s earth and space sciences roadmap require mature intersatellite crosslink communications capability to support formation flight and autonomous navigation functions. In order to develop this capability, NASA’s Goddard Space Flight Center is developing an intersatellite crosslink testbed. The testbed will support ongoing evaluation and development of crosslink signaling and modulation methods, as well as advanced networking concepts suitable for large scale constellation missions.
To accomplish the goals of the testbed, a software radio based architecture was selected. COTS systems based on Xilinx FPGAs and PowerPC processors are being used to develop and test communications and signaling methods in a real-time, hardware-in-the-loop environment. The flexibility of a software radio architecture allows the testbed to quickly adapt to test new protocols or simulate constellation spacecraft when evaluating a commercial crosslink capable transceiver for use in flight.
Work to Date
Goddard’s Microwave and Communication Systems Branch has established an intersatellite crosslink testbed laboratory consisting of three COTS software radio processors. The processors are Spectrum Signal Processing’s SDR-3000 architecture and consist of four Xilinx Virtex II FPGAs and two Motoroloa MPC7410 PowerPC processors. The SDR-3000 system integrates the FPGAs, PowerPCs, and ADC/DAC hardware into a single CompactPCI chassis. The SDR-3000s use VxWorks as a real time operating system.
The development team, consisting of government and contractor personnel, has implemented several digital modulators and demodulators, including BPSK and QPSK systems at rates up to 3 MBPS. Network protocols and signaling methods have also been studied. This work has formed the foundation for integration into software radio based transceivers capable of simulating the crosslink spectra and protocols used by commercial crosslink transceivers. Once integrated, each SDR-3000 will simulate the presence of either a commercial crosslink transceiver or of a new or custom design for evaluation.
Elements of the Paper / Presentation
The proposed paper will discuss the selection of COTS hardware for NASA’s testbed, as well as the development of communications algorithms for implementation in the hardware FPGAs and PowerPC processors. An emphasis will be placed on the flexible nature of using software radio based systems, and its specific benefits to communications testbed development. Finally, results of the signaling and networking protocols developed as part of the intersatellite crosslink testbed will be presented.