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

Abstract

7LPH )UHTXHQF\ DQG *UDYLPHWU\ 6HFWLRQ %XUHDX ,QWHUQDWLRQDO GHV 3RLGV HW 0HVXUHV %,30 3DYLOORQ GH %UHWHXLO ) 6(95(6 &('(; )UDQFH ]MLDQJ#ELSPRUJ $EVWUDFW 87& generation includes the computation of 87&-87&(N) and its uncertainty estimation. A significant part of the uncertainty of the 87& approximations 87&(N) 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 87& 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 [87&(N) -87& (O)] between any participating laboratory and that chosen as a pivot (at present the PTB). In the 1980s, GPS C/A technique dominated the 87& 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 u B 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 [87&(N) -87& (O)] bigger the respective u B 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 u A has been reduced by a factor of 10 since a dozen of years. The state of the art of u A is 0.5 ns for TW and 0.3 ns for GPS PPP. The u B calibration uncertainty is dominant in the total uncertainty of [87&-87&(N)], and several authors have investigated how to improve the calibration of time transfer equipment to decrease its value.