GPS Monitor Station Upgrade Program at the Naval Research Laboratory

Ivan J. Galysh, Dwin M. Craig

Abstract:	One of the measurements made by the GPS Monitor Stations is to measure the continuous pseudorange of all the passing GPS satellites. The pseudo-range contains GPS satellite and Monitor Station clock errors as well as GPS satellite navigation errors. Currently the time at the GPS Monitor Station obtained from the GPS constellation and has an inherent inaccuracy as a result. Improved timing accuracy at the GPS Monitor Stations will improve GPS performance. The U.S. Naval Research Labortory (NRL) is developing hardware and software for the GPS Monitor Station Upgrade program to improve the Monitor Station clock accuracy. This upgrade will allow a &hod independent of he GPS satellite constellation of measuring and correcting Monitor Station time to US. Naval Observatory (USNO). The hardware consists of a high-performance atomic cesium frequency standard (CFS) and a computer which is used to ensemble this CFS with the two CFSs currently located at the Monitor Station by use of a dual-mixer system. The dual-mixer system achieves phase measurements between the high-performance CFS and the existing Monitor Station CFSs to within 400 femtoseconds. Time transfer between USNO and a given Monitor Station is achieved via a Two-way satellite time-transfer modem. The computer at the Monitor Station disciplines the CFS based on a comparison of one pulse per second sent from the master site at USNO. The Monitor Station computer is also used to perform housekeeping functions, as well as recording the health status of all three CFSs. This information is sent to USNO through the time-transfer modem. Laboratory time synchronization results in the sub-nanosecond range have been observed and: the ability to maintain the Monitor Station CFS frequency to within 3.0x10^-14 of the master site at USNO.
Published in:	Proceedings of the 27th Annual Precise Time and Time Interval Systems and Applications Meeting November 29 - 1, 1995 The Doubletree Hotel at Horton Plaza San Diego, California
Pages:	35 - 50
Cite this article:	Galysh, Ivan J., Craig, Dwin M., "GPS Monitor Station Upgrade Program at the Naval Research Laboratory," Proceedings of the 27th Annual Precise Time and Time Interval Systems and Applications Meeting, San Diego, California, November 1995, pp. 35-50.
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