A Totally Software Approach to GPS Signal Processing Ailbws the Same Hardware to Perform Satcomms or GLONASS

Philip Mattos, Mark Beach

Abstract:	The advent of the transputer, with 50 nanosecond instructions and 20Mbit/sec links operating with 50nS internal memory allows many tasks previously performed in hardware to be performed in soft- ware. Two such tasks are 1) the correlation and frequency tracking tasks needed to receive GPS and GLONASS, and 2) the demodulation, convolving, de-interleaving, de-scrambling, Viterbi decoding needed for the INMARSAT-C satellite data sys- tem for ships and trucks. Additionally, moving all these tasks to software, allows the same equipment to perform both tasks on a time multiplexed basis. The main task in GPSGLONASS is the correlation of the incom- ing signal with a locally generated code. For a portable set, a major parameter is the acquisition time. Whilst the best traditional hardware implementations take around 3 minutes to first fix from power-up, the software solution takes around five seconds !!!!. Of course, if it has not been used for a while, it will need to download the ephemeris data, which will usually take another 40 sets. A 32 bit transputer is needed to handle 4 GPS satellites in par- allel ( ie not sequencing/multiplexing them, but processing all the incoming signal with respect to each satellite. For GLONASS, a sequential approach is necessary due to the need to tune the ra- dio to different channels, and thus a 16 bit transputer ( IMS T225) is adequate for position fixing, but if a large amount of memory is needed for waypoints etc, the lowcost T400 32 bit transputer is recommended. A portable, handheld GPS has been built taking advantage of this approach, and is approximately 11 x 3.5 x 2 inches, including patch antenna and radio section. Preliminary designs have been done for a car-based black-box GPS, where neither keyboard, display or batteries are required, and after the analogue radio sections, the entire digital processor section fits on a board 2.25 x 1.75 inches. The GPS set has also been incorporated as an addon to a INMARSAT-C sat-comms set, where its small size allow it to be packaged in the same chassis. Such a combination is very de- sirable for position reporting in ships, trucks etc, and also allows geographic addressing of incoming messages.. ie all ships in area latllongl to lat2long2. Work continues into implementing both the GPS and the sat- comms on the same processor, rather than as an add-on board, and using the same radio for the two tasks in time multiplexed mode, rather than simply sharing antenna and radio front end. This work is sponsored by INMARSAT under contract INM/88- 363/HK. The high performance of the transputer has allowed a software ap- proach to GPS spread spectrum communications, and thus flexi- bility to handle GLONASS and satcomms on the same hardware, besides saving all the initial costs and inflexibility associated with custom ASIC designs.
Published in:	Proceedings of the 3rd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1990) September 19 - 21, 1990 The Broadmoor Hotel Colorado Spring, CO
Pages:	25 - 30
Cite this article:	Mattos, Philip, Beach, Mark, "A Totally Software Approach to GPS Signal Processing Ailbws the Same Hardware to Perform Satcomms or GLONASS," Proceedings of the 3rd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1990), Colorado Spring, CO, September 1990, pp. 25-30.
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