Calibration of TWSTFT Links Through the Triangle Closure Condition
BUREAU INTERNATIONAL DES POIDS ET MESURES SEVRES (FRANCE)
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Nanosecond uncertainty TWSTFT Two-Way Satellite Time and Frequency Transfer, TW for short is, together with GPS time transfer, the primary technique used for UTC generation. The high accuracy of TW with a potential sub-nanosecond uncertainty is based on a metrological calibration which is, however, not only labor- and cost-intensive, but also limited by the complexity of the organization and the availability of equipment. TW is operated in a network of which every link is measured independently. For an N-point network, there are NN-12 independent links, of which N-1 are UTC links and N sq-3N22 redundant links. In this network, only a limited number of links were calibrated to enable true time transfer for UTC or other scientific applications. In this paper, we present a strategy to calibrate the whole TW network by transferring the existing TW calibrations to the uncalibrated links using the so called triangle-closure condition. The uncertainty of a TW calibration is typical uB1 ns. The uncertainty of the triangle calibration is increased by a factor of about square root of 2, corresponding to an effective uncertainty of about 2 ns for the redundant links, which is satisfactory at present for the metrological time transfer necessities. Applying the triangle-closure condition, we computed all the non-UTC link calibrations and the results have been implemented in the European-American TW network since MJD 54677 30 July 2008.
- Test Facilities, Equipment and Methods