Toward new procedures in TWSTFT and GNSS delay characterization for UTC time transfer?

Abstract

UTC generation includes the computation of UTC-UTC(k) and its uncertainty estimation. A significant part of the uncertainty of the UTC approximations UTC(k) in national contributing laboratories is based on accurate metrological measurement of time transfer equipment delays (so called “equipment calibration”). Organizing and maintaining the calibration of the time transfer facilities contributing to UTC is among the responsibilities of the BIPM. At present, the time transfer techniques used for UTC generation are based on the two-way satellite time and frequency transfer (TWSTFT or TW) and the global navigation satellite systems (GNSS), i.e. GPS and GLN (Glonass). They are used for calculating the differences [UTC(k) - UTC (1)] between any participating laboratory and that chosen as a pivot (at present the PTB). In the 1980s, GPS C/A technique dominated the UTC time transfer. Since 2000, TW and GPS MC, P3 and PPP techniques as well as GLN have been successively introduced in the UTC generation. In consequence, the calibrations of the different time transfer equipment were introduced and are performed separately. Today, there are four parallel types of independent calibrations based on different strategies that can be defined either as site-based or link-based. The BIPM has assigned values of uB of about 1 ns for the link-based TW calibrations, based on the values reported by those performing the calibration. However, for the sited-based GNSS calibrations, a conventional value of 5 ns has generally been assigned. This choice has been motivated by studies which found that values of [UTC(k) - UTC (1)] bigger the respective uB may exist when the link as calculated by different techniques, and because long-term instability of the standard receivers may cause inconstancy in individual calibrations carried out in different periods. On the other side, due to the development in technology, the statistical uncertainty uA has been reduced by a factor of 10 since a dozen of years. The state of the art of uA is 0.5 ns for TW and 0.3 ns for GPS PPP. The uB calibration uncertainty is dominant in the total uncertainty of [UTC-UTC(k)], and several authors have investigated how to improve the calibration of time transfer equipment to decrease its value. Also the BIPM has undertaken studies for improving the current calibration policy. The goal of this studies are: 1) to reduce the inconsistency between different techniques by making a combined use of the respective calibrations; 2) to reduce influence of the long-term instability of the BIPM standards by a special designed schedule; 3) to obtain more realistic uB values than the conventional 5-ns and, in consequence, to reduce the total uncertainty of UTC-UTC(k); 4) to easy the calibration organization and reduce its cost; 5) to simplify the calibration monitoring and the combination of different time transfer techniques. Uncertainty estimation is an important part of the UTC computation and hence carefully discussed. We present hereafter a study, which in no way means the adoption of a new calibration policy at the BIPM.