Long-term instability in UTC time links

Calibration and link stability are the key issues for UTC time transfer. This study is made in the framework of a joint task group of the CCTF Working Groups on TWSTFT and the Working Group on GNSS Time Transfer to investigate the long-term stability of the UTC time transfer links with our suggestions to monitor and limit the instabilities. For a UTC time link, stability is as important as the calibration. We investigated the site-based and link-based instabilities of UTC time transfer. Using the zero-baseline common clock difference analysis, we compared co-located GPS receivers that use common reference signals from the same clock to directly investigate the long-term instability of the individual C/A, P3 and PPP techniques. Using the double clock difference between separated UTC laboratories, we compared GNSS and TWSTFT time links to indirectly investigate the long-term instability of GPS C/A, P3, PPP, GLONASS L1C and TWSTFT over the period 2011-2016. Our study is interested in instabilities at the nanosecond level, since the combined (statistical and systematic) calibration uncertainty of a TW link (using mobile TW station or GPS calibrator) is 1-1.5 ns and that of the GPS equipment calibration is 1.7 ns, cf. Circular T 345 [1].