SUB NANOSECOND CALIBRATION OF REGIONAL TIME LINKS USING CONTINUOUS GNSS CARRIER PHASE OBSERVATIONS

摘要

The calibration of GPS or TWSTFT links is an important requirement for accurate time transfers. A simple method commonly used is to transport a clock between the two ends of the link and perform time interval measurements between the carried clock and the respective laboratory clock. As the main disadvantage one can see the difficulty to model the behavior of the carried clock during the transport. An accurate prediction usually requires highly stable clocks to be transported with all the risks and costs involved. One solution to this is to estimate the carried clock's drift during transportation relative to a fixed clock using precise GNSS observations. This paper describes the method of a GPS aided clock transport experiment with the aim of calibrating the GPS-P3 time link between SP Swedish National Testing and Research Institute and Onsala Space Observatory, which are separated by a distance of about 70 km. In a first attempt data from a local round trip transport from and to SP were analyzed in order to test and verify the method. A moderate rubidium oscillator was connected to and transported together with a geodetic GNSS receiver system. The collected data were post-processed with a common view GNSS code and carrier phase filter yielding precise differential time series between the rubidium clock and the reference clock at SP. These results were used to link the two time interval measurements taken between the clocks at the beginning and the end of each transport together. An achievable sub nanosecond precision of the link calibration is of value for applications requiring highly accurate time synchronization such as distributed time scales.