Relative GPS Using DME/TACAN Data Link

James Waid and Dennis King

Abstract:	A local area Relative GPS (RGPS) landing system is described which has application to tactical or emergency situations where an accurate site survey is not possible. The system is also unique in its use of existing TACAN ground and airborne equipment to provide a high integrity data link. Data containing information on all satellites in view is up-linked to the aircraft, as well as enough siting data to support six simultaneous approaches. A relative solution using all common satellites is derived at the aircraft. Based on the calculated relative solution, aircraft guidance signals are generated which drive a CD1 display in real time. The up-link data is encoded onto ground-to-air DMlYlYACAN signals by pulse-position modulating a single data bit onto each existing DMlYlYACAN pulse pair. This data link has several advantages over Mode-S, VOR-band, and VHF alternatives: - Radiated signals meet existing ICAO specifications - DMEYTACAN band is internationally designated for navigational use - Existing ground equipment can be easily modified to encode the DGPS data during the transition to DGPS - As DME/TACAN is phased out, DGPS-specific ground equipment can be built at low cost l Simultaneous dual use of existing ground equipment - Encoding DGPS data on existing TACAN or DME has no effect on DMlYTACAN performance - Existing airborne TACAN and DME interrogators and antennas can receive the DGPS up-link data - DME/TACAN data rate has room for future expansion E-Systems has implemented a complete RGPS system with commercial-off-the-shelf GPS receivers and using a portable ANflRN-41 TACAN transponder and a low-cost King KX- 64 general aviation DME interrogator. A special data link error correction scheme was developed allowing full use of the DME independent of GPS data reception. Statistical analysis shows that the relative solution meets CAT-I accuracy requirements, and the data link meets CAT I integrity and continuity of service requirements in the presence of worst-case interfering in-band signals from other DMEs, ATCRBS and TCAS transmissions. Flight test results recorded in a Cessna 206 at the E-Systems DGPS test facility in Tooele, Utah, will be presented.
Published in:	Proceedings of the 7th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1994) September 20 - 23, 1994 Salt Palace Convention Center Salt Lake City, UT
Pages:	851 - 861
Cite this article:	Waid, James, King, Dennis, "Relative GPS Using DME/TACAN Data Link," Proceedings of the 7th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1994), Salt Lake City, UT, September 1994, pp. 851-861.
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