GPS Time Differentials at Widely Separated Sites

John A. Kusters, David A. Allan, Robin Giffard

Abstract:	Wide-area communication systems based on low-earth orbit satellites require time synchronization to levels significantly tighter than GPS currently provides through the Standard Positioning Service (SPS). The accuracy of the SPS is currently dominated by Selective Availability (SA). Time synchronization between ground receiver sites separated by continental distances is desired by our customers to be within 50 nanoseconds. GPS provides a vehicle to ensure this using GPS Common-view techniques, but the system desire is to be able to operate autonomously and independently, and still maintain 50 nanosecond synchronicity. A series of extended experiments involving multiple Hewlett-Packard sites, the National Institute of Standards and Technology in Boulder, CO, and the US Naval Observatory in Washington DC has provided a set of correlation coefficients relating GPS satellite common visibility to distances between sites. In all cases, the same GPS multi-channel engine was used to gather the data. The data gathered at various sites has now been examined determine the level of correlation. As the data collection period exceeded one month, the correlation coefficients include all of the normal effects of SA and ionospheric and tropospheric effects. HP's SmartClock technology provides an estimate of time that has a standard deviation (or RMS stability) of about nanoseconds for fixed site operation. This provides a peak-to-peak time difference at a single site of about 80 nanoseconds. The combination of SmartClock technology and high GPS commonality permits calculation of the expected peak-to-peak time differential between sites separated by continental distances. In this case, the peak-to-peak differential calculates to be better than the 50-nanosecond requirement at the 95 th percentile.
Published in:	Proceedings of the 10th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1997) September 16 - 19, 1997 Kansas City, MO
Pages:	1093 - 1097
Cite this article:	Kusters, John A., Allan, David A., Giffard, Robin, "GPS Time Differentials at Widely Separated Sites," Proceedings of the 10th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1997), Kansas City, MO, September 1997, pp. 1093-1097.
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