Study of an Optimal Algorithm for Combing TWSTFT and GPS CV Comparisons

Two-way satellite time and frequency transfer (TWSTFT) and GPS C/A code based common view (GPS CV) are the two primary methods for international time and frequency transfer. TWSTFT technique has been made significant improvement in the last several years, and demonstrated to show a higher precision of T&F transfer than that of GPS CV. So it has been utilized for in the computation of TAI (international atomic time) and the GPS CV link is regarded as a back up if the links have both TWSTFT and GPS CV time transfer links. Actually, the TWSTFT is unavailable frequently for some unknown reasons or failure of instrument. More important, in some links, the comparison result of GPS CV has valuable information in the calculation of TAI and UTC (coordinated universal time). Considering these two cases, we have been used GPS CV C/A code and TWSTFT observations in attempt to form a more stable and reliable composite time transfer links. In this study we have focused on the unavailability when the system has malfunction or abnormality, in this case, the GPS CV data will be used in TAI computation which will cause a systematic error that should be deduct in some links. Finally, to avoid the above mentioned shortcomings, a new optimal algorithm for combining the two types time transfer data is given, the details of the optimal algorithm has been presented in this paper. A more stable and reliable time comparison result has been achieved in time comparison between some national time laboratories. The UTC(USNO)-UTC(PTB) and the UTC(ROA)-UTC(PTB) are given as a example