G. Petit

GPS All in View time transfer for TAI computation

Since the 1980s, GPS time links have been essential to the TAI computation and, until 2006, the Common View (CV) technique has been used for this purpose. Recent advances in obtaining precise satellite orbits and clock parameters now permit us to obtain better results using another technique, which we name All in View (AV). By comparing the GPS CV and AV with the independent and more accurate TW and PPP time transfer techniques, we quantify the gain that can be obtained on a given time link. The AV technique also allows us to choose a more efficient network of GPS links between the tens of laboratories participating in TAI, which further improves the uncertainty in the access to UTC. The BIPM TAI software has been updated and the AV technique has been effectively used since the computation for the month of September 2006.

Precise Point Positioning for TAI Computation

We discuss the use of some new time transfer techniques for computing TAI time links. Precise point positioning (PPP) uses GPS dual frequency carrier phase and code measurements to compute the link between a local clock and a reference time scale with the precision of the carrier phase and the accuracy of the code. The time link between any two stations can then be computed by a simple difference. We show that this technique is well adapted and has better short-term stability than other techniques used in TAI. We present a method of combining PPP and two-way time transfer that takes advantage of the qualities of each technique, and shows that it would bring significant improvement to TAI links.

The TAIPPP pilot experiment

Time transfer techniques using GNSS carrier phase and code signals are increasingly used in time metrology. In order to study the introduction of these techniques for TAI computation, the BIPM initiated a pilot experiment, named TAIPPP, where time laboratories contribute GPS phase and code data and where the BIPM uses the Precise Point Positioning technique to generate monthly solutions, in slightly deferred time after the regular TAI computation. This paper reports on results obtained from this experiment after one year.

The long term stability of EAL and TAI (revisited)

International atomic time TAI gets its stability from some 300 atomic clocks worldwide that generate the free atomic scale EAL and its accuracy from a small number of primary frequency standards (PFS) which frequency measurements are used to steer the EAL frequency. While the long-term (above one year) stability of TAI is mostly driven by the PFS and therefore directly related to its accuracy, its stability for averaging times from one to a few months is mostly driven by EAL. Based on the analysis of 8 years (1999-2006) of TAI data, the paper presents several results. We estimate the long-term stability of EAL and TAI with respect to the latest realization of TT(BIPM), and the one-month stability of EAL from the statistics of the weighted clocks. We study the two principal classes of clocks which form EAL and evidence systematic rate variations in clock ensembles. Their effect on the long-term stability of EAL is discussed.

Relativity in the IERS Conventions

In the last years, a fully general relativistic definition of reference systems and of their application to astronomy and geodesy has been passed into Resolutions of the scientific unions, following work of several working groups and of the community at large. In this community, the role of the International Earth Rotation and Reference systems Service (IERS) is to generate the terrestrial and celestial reference systems and the transformation between them, and the IERS Conventions provide the set of models and procedures used in the generation of IERS products. It is therefore essential that the IAU framework for relativity is introduced in the IERS Conventions, and that this is done consistently and completely throughout the document. The paper reviews relativistic aspects in the IERS Conventions and presents recent and on-going work aiming at providing a complete and consistent presentation for a new reference edition of the IERS Conventions, expected to appear in the next year.

Precise Point Positioning for TAI computation

We discuss the introduction of a new time transfer technique for computing time links for TAI. Precise point positioning uses GPS dual frequency carrier phase and code measurements to compute the link between a local clock and a reference time scale with the precision of the carrier phase and the accuracy of the code. The time link between any two stations can then be computed by simple difference. The paper presents results of link comparisons as well as a general procedure to be implemented in the regular TAI computation and shows that this technique is well adapted to this usage.