Abstract
Time and frequency transfer through global navigation satellite system (GNSS) precise point positioning (PPP) based on carrier-phase measurements has been widely used for clock comparisons in national timing laboratories. However, the time jumps up to one nanosecond at the day boundary epochs of adjacent daily batches lead to discontinuities in the time transfer results. Therefore, it is a major obstacle to achieve continuous carrier phase time transfer. The day-boundary discontinuities have been studied for many years, and they are believed to be caused by the long-term pseudorange noise during estimation of the clock offset in the daily batches and are nearly in accordance with a Gaussian curve. Several methods of eliminating the day-boundary discontinuity were proposed during the past fifteen years, such as shift and overlapping, longer batch processing, clock handover, and ambiguity stacking. Some errors and new noise limit the use of such methods in the long-term clock stability comparison. One of the effective methods is phase ambiguity fixing resolution in zero-differenced PPP, which is based on the precise products of wide-lane satellite bias (WSB) provided by the new international GNSS Service (IGS) Analysis Center of Centre National d’Etudes Spatiales (CNES) and Collecte Localisation Satellites (CLS). However, it is not suitable for new GNSS, such as the Beidou Satellite System (BDS), GALILEO, and QZSS. For overcoming the drawbacks above, Multi-GNSS Experiment (MGEX) observation data of 10 whole days from MJD 58624 to 58633have been network processed by batch least square resolution. These observations come from several ground receivers located in different national timing laboratories. Code and carrier phase ionosphere-free measurements of GPS and BDS satellites are used, and the time transfer results from network processing are compared with PPP results provided by Bureau International des Poids et Mesures (BIPM) and used for international atomic time (TAI) computation (TAIPPP) and universal time coordination (UTC). It is shown that the time offsets of three different time links are almost continuous and the day-boundary discontinuities are sharply eliminated by network processing, although a little extent of day-boundary discontinuities still exist in the results of UTC(USNO)-UTC(PTB). The accuracy of time transfer has been significantly improved, and the frequency stability of UTC(NTSC)-UTC(PTB) can be up to 6.8 × 10−15 on average time of more than one day. Thus, it is suitable for continuous multi-GNSS time transfer, especially for long-term clock stability comparison.
Highlights
global navigation satellite system (GNSS) are widely used in many national timing laboratories for time and frequency transfer or clock comparison [1,2,3,4,5,6,7,8,9] because of the high accuracy level and more economical from individual stations
SPT0 is an international GNSS Service (IGS) station and and a Bureau International des Poids et Mesures (BIPM) station used for TAI calculation, it is located in the Swedish National Institute of Technology and is driven by universal time coordination (UTC)(SP)
PTBB is an IGS station located in PTB (Braunschweig, Germany), it is directly driven by UTC(PTB).It is about 620 km between
Summary
GNSS are widely used in many national timing laboratories for time and frequency transfer or clock comparison [1,2,3,4,5,6,7,8,9] because of the high accuracy level and more economical from individual stations. The code-pseudorange noise is sometimes and for some stations not white noise, due to near-field multipath effects or variation of instrumental delays The averaging of this colored pseudorange noise induces clock datum changes between daily batches at the level of a few hundred picoseconds to a few nanoseconds. In order to reduce the effect of the day-boundary discontinuities to PPP for continuous time transfer, the easiest way is to concatenate the daily solutions by using overlapping computation batches [18,19]. There is a drawback of this approach because it induces an undetected error in a single batch solution on the entire continuous time series To overcome this problem, increasing the length of one batch from 24 h to longer batches, for example over three days, was introduced and tested. It is not practicable due to the large number of clock parameters in a stacked normal equation
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