Time Transfer (TT) – or synchronization – techniques are essential in time metrology, mainly for comparing remote UTC(k) time scales, disseminating time to users and granting for the interoperability among navigation satellite systems. Presently, TT techniques are mainly based on satellite systems, namely the GNSS and the geostationary ones (i.e. TWSTFT, Two Way Satellite Time and Frequency Transfer), that allow the synchronization of time scales with an uncertainty of one nanosecond, in a continuous international effort to improve their accuracy, stability and reduce latency.
“Time and Frequency Transfer” techniques based on geodetic code and phase multichannel GNSS receivers, in particular, are among the most useful tools for the remote comparison of atomic clocks and time scales, and are currently used worldwide in combination with the TWSTFT system, for the realization of UTC and TAI time scales.
The basic idea of this approach is to use the code and phase measurements produced by dedicated geodetic GNSS receivers physically connected to the atomic clocks (through the clock 1PPS and Frequency electrical signals) and to relate their phase to an intermediate common time scale (basically the GPS Time or a time scale having reference to that).
In order to achieve this result, the measurements generated by the GNSS receivers have to be processed with dedicated algorithms, implementing proper geophysical models and solutions allowing to get the required timing measurement with acceptable characteristics in terms of instability and accuracy. Among these algorithms, the P3, compensating for the ionosphere effect by means of the combination of the GPS signals in both the L1 and L2 carriers, and the PPP (Precise Point Positioning) can be used. Beyond what is done at P3 level, the PPP method processes receivers code and phase measurements along with IGS precise satellite orbits/clocks and modeling for site displacement (e.g. Solid Earth Tides, Ocean Loading) and satellite attitude effects (e.g. Satellite Antenna offsets, Phase Wind-Up Corrections).