Calibrating GPS With TWSTFT For Accurate Time Transfer
BUREAU INTERNATIONAL DES POIDS ET MESURES SEVRES (FRANCE)
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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 u sub B calibration uncertainty is 1 ns for TW, 5 ns for GPS P3 code, and 5 to approximately 7 ns for GPS CA code equipment. The u sub B 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.
- Test Facilities, Equipment and Methods