A New Algorithm to Eliminate GPS Carrier-Phase Time Transfer Boundary Discontinuity

We report on a study of the Global Positioning System (GPS) carrier-phase (CP) time transfer boundary discontinuity by the use of the precise point positioning (PPP) technique. We first demonstrate that the pseudorange measurement noise leads to the boundary discontinuity by simulation. We also find that the boundary discontinuity is reduced by 10%-30% when executing PPP with the input of the IGS 30-sec clock product, instead of the IGS 5-min clock product, by analyzing the real Receiver Independent Exchange Format (RINEX) data. The average of several GPS receivers at the same station can also reduce the impact of pseudorange measurement noise on the boundary discontinuity. The improvement is typically 15%-20%. Besides, a bad data point in the RINEX file can affect not only the time at that specific epoch, but also the whole time at all epochs and thus the boundary discontinuity, especially when the bad point happens at the boundary of the data-arc. The RINEX-Shift algorithm is designed to eliminate the boundary discontinuity. A new program, called "NEW PPP", is then developed based on the above analysis and conclusions. The time comparison between NIST and PTB by NEW PPP matches the Two Way Satellite Time and Frequency Transfer (TWSTFT) result better than the conventional PPP result, in terms of the long-term (e.g., 10 days) tendency. This shows that the result of NEW PPP is closer to the true value, compared to the conventional PPP result. The comparison between two distant fountains shows that we are able to observe the UTC(k) clock behavior for an averaging time of greater than 1 day. This fountain comparison also sets up the upper limit of NEW PPP time transfer noise. It is 3x10(-16) for an averaging time of 10 days.