Validating the Airborne and Ground Based Components of a Differential GPS System

F. G. Edwards, D. M. Hegarty, R. N. Turner, F. van Graas, S. Sharma

Abstract:	A differential global positioning system (DGPS) mechanization of an external data-link concept has been under development by NASA Ames Research Center for several years. The system was developed to evaluate the use of DGPS to support helicopter approach operations. The system consists of a ground based component, operating at a surveyed site and an airborne user component operating in the local area. A real-time telemetry link between the ground system component and the user aircraft system component transmits differential correction information to the user aircraft system component. The system underwent extensive validation testing to assure correct implementation of the hardware and software as well as consistency between the aircraft and the ground-system navigation solutions. The validation tests were conducted with the user aircraft on the ground and located adjacent to the ground system. The GPS data recorded simultaneously from the aircraft and ground-GPS receiver sets were analyzed to insure that receiver unique errors were separated from common system errors caused by the satellite and transmission medium. This separation is essential because the common system errors are the differential corrections, which are the key elements of the DGPS concept. The ground system tracked as many as six satellites during the validation sessions. The differential corrections for each satellite were computed and transmitted to the user aircraft system. The aircraft system chooses from the set of transmitted corrections those which correspond to the four satellites being tracked by the aircraft receiver. The corrections ere integrated into the real-time navigation solution to provide the user with an onboard differential solution. Efforts are continuing in preparation for the flight test evaluation of the system. This paper will describe the DGPS system, the methods used for its validation, the results of the static ground testing, and the preliminary results from the in-flight test program.
Published in:	Proceedings of the 1988 National Technical Meeting of The Institute of Navigation January 26 - 29, 1988 Santa Barbara, CA
Pages:	46 - 56
Cite this article:	Edwards, F. G., Hegarty, D. M., Turner, R. N., van Graas, F., Sharma, S., "Validating the Airborne and Ground Based Components of a Differential GPS System," Proceedings of the 1988 National Technical Meeting of The Institute of Navigation, Santa Barbara, CA, January 1988, pp. 46-56.
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