Calibrating GPS with TWSTFT for Accurate Time Transfer

Z. Jiang and A. Niessner

Abstract:	The uncertainty of the UTC generation is based on accurate metrological calibration. Organizing and maintaining the calibration of the time transfer facilities contributing to UTC is among the responsibilities of the BIPM. At present, the primary time transfer techniques are GPS and TWSTFT (Two-Way Satellite Time and Frequency Transfer, TW for short). 83% of UTC time links are obtained via GPS observations. The uB (calibration uncertainty) is 1 ns for TW, 5 ns for GPS P3 code, and 5 ~ 7 ns for GPS C/A code equipment. The uB of the time links dominates the total uncertainty of [UTC – UTC (k)]. For a UTC participating laboratory linked by GPS P3, 98% of the uncertainty budget comes from the calibration. Reducing the calibration uncertainty in a GPS time link results is a direct improvement in the related [UTC – UTC (k)]. The traditional GPS equipment calibration by the BIPM consists in circulating a master receiver among the UTC laboratories to carry out a side-by-side differential calibration with the local receivers. All the TW laboratories are backed up with GPS facilities; however, the calibrations of TW and GPS equipment are performed independently. This paper proposes a new calibration procedure. Instead of travelling the BIPM master receiver to every laboratory worldwide, we simply transfer the TW calibration to GPS equipment at the laboratories that operate both TW and GPS. The advantages are: a) unifying the TW and GPS equipment calibration; b) reducing the GPS calibration uncertainty; c) lightening the organization of the calibration work; d) monitoring the long-term variation of the time link calibrations, which is one of the major uncertain factors for the instability of UTC. To test the method, we compute the calibrations for a dozen of GPS PPP equipment. Further study in unifying the UTC time transfer calibration is next step of this study.
Published in:	Proceedings of the 40th Annual Precise Time and Time Interval Systems and Applications Meeting December 1 - 4, 2008 Hyatt Regency Reston Town Center Reston, Virginia
Pages:	577 - 586
Cite this article:	Jiang, Z., Niessner, A., "Calibrating GPS with TWSTFT for Accurate Time Transfer," Proceedings of the 40th Annual Precise Time and Time Interval Systems and Applications Meeting, Reston, Virginia, December 2008, pp. 577-586.
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