Assisted GPS Without Network Cooperation Using GPRS and the Internet

P.G. Mattos

Abstract:	Assisted GPS systems for mobile phones have been slow to appear largely due to the network infrastructure needed to support them. This has particularly hampered experimental, scientific and demonstration systems that must deploy long before commercial networks are prepared. The system described here evolved from a European Space Agency EGNOS project called SISNET, delivering integrity and corrections over the Internet. The SISNET server also offers the GPS broadcast ephemerides in quasi-real time, so the GPS software was extended to read its ephemeredes from this source, and the principle of the system was born. To make it a mobile, always-on system modem was exchanged for a GSM/GPRS version, offering quasi-continuously available Internet. The ability of the GPS processor to run the software required for the TCP/IP stack of the GPRS modem, as well as the PPP/SLIP protocols to set up the GPRS connection, means that no additional processor is required. Thus a complete assisted-GPS phone can be assembled from the GSM-GPRS module, 2 chip GPS, plus conventional screen/keyboard/headset/battery. Work done on the TCP/IP software as a client for the SISNET system showed that it was equally simple to write a server, so a local server was written that could deliver the ephemeris data in a much condensed form, requiring less messages and achieving a more rapid response time. This could also use a local GPS frontend, as the EGNOS correction data was not required. In addition to simple provision of ephemerides, further assistance can be provided. Timing can be achieved to a few tens of milliseconds by measuring the loop delay from mobile to server and back or vice versa, and the base-station can also take on more of the mathematical work, meaning a full GPS receiver is not required in the phone, merely a host-based, or signal-tracker-only version.
Published in:	Proceedings of the 16th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS/GNSS 2003) September 9 - 12, 2003 Oregon Convention Center Portland, OR
Pages:	1642 - 1649
Cite this article:	Mattos, P.G., "Assisted GPS Without Network Cooperation Using GPRS and the Internet," Proceedings of the 16th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS/GNSS 2003), Portland, OR, September 2003, pp. 1642-1649.
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