The Use of the AOA TTR-4P GPS Receiver in Operation at the BIPM for Real-Time Restitution of GPS Time

Claudine Thomas

Abstract:	The Global Positioning System is an outstanding tool for the dissemination of time. Using mono-channel C/A-code GPS time receivers, the restitution of GPS time through the satellite constellation presents a peak-to-peak discrepancy of several tens of nanoseconds without SA but may be as high as several hundreds of nanoseconds with SA. As a consequence, civil users are more and more interested in implementing hardware and software methods for efficient restitution of GPS time, especially in the framework of the project of a real-time prediction of UTC, UTCp, which could be available in the form of time differences [UTCp - GPS time]. Previous work for improving the real-time restitution of GPS time with SA, to the level obtained without SA, focused on the implementation of a Kalman filtering based on past data and updated at each new observation. An alternative solution relies upon the statistical features of the noise brought about by SA: it has already been shown that the SA noise is efficiently reduced by averaging data from numerous satellites observed simultaneously over a sufficiently long time. This method has been successfully applied to data from a GPS time receiver, model AOA TTR-4P, connected to the caesium clock kept at the BIPM. This device, a multi-channel, dual-frequency, P-code GPS time receiver, is one of the first TTR-4P units in operation in a civil laboratory. Preliminary comparative studies of this new equipment with conventional GPS time receivers are described in this paper. The results of an experimental restitution of GPS time, obtained in June 1993, are also detailed: 3 to 6 satellites were observed simultaneously with a sample interval of 15 s, a sufficient smoothing of SA noise was realised by averaging data on all observed satellites over more than 1 hour. When the GPS system is complete in 1994, 8 satellites will be observable continuously from anywhere in the world and the same level of uncertainty will be obtained using a shorter averaging time.
Published in:	Proceedings of the 25th Annual Precise Time and Time Interval Systems and Applications Meeting November 29 - 2, 1993 Ritz-Carlton Hotel Marina Del Rey, California
Pages:	183 - 196
Cite this article:	Thomas, Claudine, "The Use of the AOA TTR-4P GPS Receiver in Operation at the BIPM for Real-Time Restitution of GPS Time," Proceedings of the 25th Annual Precise Time and Time Interval Systems and Applications Meeting, Marina Del Rey, California, November 1993, pp. 183-196.
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