Implementation of SDR TWSTFT in UTC Computation

Two-Way Satellite Time and Frequency Transfer (TWSTFT or TW for short) is one of the primary techniques for the realization of Coordinated Universal Time (UTC), which is computed by the International Bureau of Weights and Measures (BIPM). TWSTFT is carried out continuously by about 20 timing laboratories around the world. One limiting factor of the TWSTFT performance is the daily pattern (diurnals) in the TWSTFT data. Their peak-to-peak variations were observed as up to 2 ns in some extreme cases. They must be attributed to the equipment on ground or the satellite transponder, but no clear understanding has been achieved for some years. In 2014 and 2015, it was demonstrated inlinks between Asian stations that the use of Software-Defined Radio (SDR) receivers for TWSTFT could considerably reduce the diurnals and also the TWSTFT measurement noise. In 2016, BIPM and the Consultative Committee for Time and Frequency (CCTF) working group (WG) on TWSTFT launched a pilot study on the application of SDR receivers in the Asia to Asia, Asia to Europe, Europe to Europe and Europe to USA TWSTFT networks. The very first results of the pilot study have been reported to the PTTI 2017. The results show, 1) for continental (short) links: SDR TWSTFT demonstrates a significant gain in reducing the diurnals by a factor of two to three; 2) for inter-continental (very-long) links: SDR TWSTFT displays a small gain of measurement noise at short averaging times; 3) SDR receivers show superior or at least similar performance compared with SATRE(dagger) measurements for all links. The CCTF WG on TWSTFT prepared a recommendation "On Improving the uncertainty of Two-Way Satellite Time and Frequency Transfer (TWSTFT) in UTC Generation" for the 21st CCTF meeting. One of the recommended items is to introduce the use of SDR TWSTFT in UTC generation. The recommendation was approved by CCTF during the meeting in June 2017. In the current setups, a SDR receiver and the collocated SATRE modem receive the signals transmitted by remote SATRE modems, and the two devices independently determine the arrival time of the received signal. Thus, a few setup changes are necessary to implement SDR receivers into operational TWSTFT ground stations. On the other hand, the data computation, e.g. for calibrated time transfer, needs some caution in the data processing and provision, which are not trivial. Thus, an Ad hoc Group has been established to work out a procedure for the use of the SDR TWSTFT in UTC computation. This paper reports on the progress of the work for using SDR TWSTFT in UTC computation. Section 1 introduces SDR TWSTFT and the pilot study. Section 2 presents the considerations for using SDR TWSTFT in UTC generation. Section 3 and 4 show the methods for analysing SDR TWSTFT and the analysis results. Sections 5 and 6 discusses the further improvement of SDR TWSTFT and summarizes the work towards using SDR TWSTFT in UTC generation.