Use of GPS carrier phase for high precision frequency (time) comparison

Quality in time metrology depends on the performance of the atomic clocks and the means for time-frequency comparison. New generation of caesium standards have accuracy below 2x10-15 and the classical common-view method is no longer satisfactory. Use of GPS carrier phase combined with code measurements is a promising technique for the accurate comparison of remote clocks. This paper discusses some error sources disturbing the uncertainty of the comparison using the code-phase method, mainly on the equipment capability and the data processing strategy. In fact, the receiver system, comprised with the main unit, the antenna and cables, is very sensitive to the environmental temperature and this may seriously influence the comparison result. Due to problems with the raw data or some defeats in the data processing, there often exist phase discontinuities in comparison results, ranging from several hundred ps up to several ns, which may be mistaken as drifts produced by the compared clocks and increase the uncertainty of the carrier-phase method so as to limit its application for highly accurate frequency transfer. A number of equipment setting up conditions and a multi-day data processing strategy are proposed in which the temperature effect and the discontinuities are significantly reduced and the frequency stability is improved. An uncertainty of 1X10-15 for an averaging time of 1 day is expected.