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Satellite Time and Frequency Transfer and Dissemination

Handbook

image of Satellite Time and Frequency Transfer and Dissemination

This is the first ITU Handbook to provide detailed information on the applied methods, technologies, algorithms, data structure and practical use of frequency and timing signals provided by satellite systems.

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Geodetic techniques using GPS phase and code measurements

GPS methods have been the basis for most high-accuracy time and frequency transfers for more than two decades. The usual approach for maintaining Coordinated Universal Time (UTC) has relied primarily on single-frequency pseudorange (C/A-code) data and simple common-view (CV) data analyses that assume cancellation of most systematic errors [Allan and Weiss, 1980]. With improved data yields thanks to widespread replacement of the earlier single-channel receivers by multi-channel units, intercontinental CV comparisons have achieved uncertainties of a few ns averaged over five-day intervals [Lewandowski et al., 1997]. In contrast, the parallel development of high-accuracy geodetic methods using dual-frequency GPS carrier-phase observables has demonstrated positioning repeatabilities at the cm level for one-day integrations [Zumberge et al., 1997]. Assuming such positioning results can also be realized as equivalent light travel times (~33 ps), the potential for GPS carrier phase-based geodetic techniques to permit sub-ns global time comparisons is evident, as widely recognized by the 1990s. In fact, the method has been shown to have a precision approaching ~100 ps at each epoch in favorable cases for one-day analysis arcs [Ray and Senior, 2003]. The absolute time transfer capability remains limited to >1 ns, however, due to instrumental calibration uncertainties [Petit et al., 2001]. In addition to higher precision (equivalent to frequency stability), the geodetic approach easily lends itself to global time and frequency dissemination. This is consistent with the basic GPS operational design (albeit with replacement of the GPS broadcast message with more accurate information), unlike the point-to-point nature of CV, which furthermore degrades as baseline distances increase..:

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