Using the Canadian Active Control System (CACS) for Real-Time Monitoring of GPS Receiver External Frequency Standards

Francois Lahaye, Paul Collins, Pierre Heroux, Mike Daniels and Josef Popelar

Abstract:	The Canadian Active Control System is a national network of twelve continuously operating geodetic quality GPS receivers equipped with external atomic frequency standards. Each GPS tracking station provides dualfrequency pseudorange and carrier phase measurements in real-time at 1 Hz to a central computer where wide area GPS corrections are generated using filtered ionospherefree combined pseudoranges and predicted ultra-rapid GPS orbits. In contrast to the technique of fixing one of the station clocks, the central process estimates satellite and station clock phase offsets with respect to a Virtual Reference Clock (VRC) that is maintained as a weighted mean of all estimated receiver clocks. The VRC is related to the smoothed mean GPS system time as provided by the broadcast clock parameters of tracked satellites, and its alignment is maintained by regular steering operations. This approach mitigates the effects of instabilities of individual station frequency standards and provides continuity of the time reference. This paper discusses the use of this system to monitor GPS satellite and receiver clocks. First, the central computing system provides continuous synchronisation of the network of GPS receiver clocks and GPS satellite clocks at the ns level. Second, using the wide area satellite orbit and clock corrections, users can synchronise dual-frequency GPS receivers in real time at a precision consistent with the satellite clock estimates.
Published in:	Proceedings of the 14th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2001) September 11 - 14, 2001 Salt Palace Convention Center Salt Lake City, UT
Pages:	2220 - 2228
Cite this article:	Lahaye, Francois, Collins, Paul, Heroux, Pierre, Daniels, Mike, Popelar, Josef, "Using the Canadian Active Control System (CACS) for Real-Time Monitoring of GPS Receiver External Frequency Standards," Proceedings of the 14th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2001), Salt Lake City, UT, September 2001, pp. 2220-2228.
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