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Time and frequency comparisons

The computation of a Universal Time Coordinated (UTC) requires time and frequency comparisons of clocks that are distributed over the whole globe. Time and frequency transfer methods are thus an important part of time and frequency metrology. For comparisons of remote clocks various methods have been developed. They can be classified in one-way and in two-way methods.

One-way methods

To put in place a one-way method, all participating stations receive a common signal and record the state of the clock to be compared opon arrival of this signal. Subsequently, the recorded data are exchanged among the partner stations and the clock differences at the moment of measurement can be computed. The common signal is only used as trigger. To participate in a one-way method, only signal receiving capabilities are required. Therefore these methods do often not required a heavy infrastructure. The drawback is that the differences of the time of flight of the common signal to the participating stations can not be measured. This will limit the achievable accuracy. Several well established one-way methods are using GPS-signals as common trigger.

Two-way methods

In a two-way method, the times of flight of the signals between two partner stations are measured and can therefore be taken into account. However, two-way methods require a heavier infrastructure as all participating station must be equipped with reception and transmission capabilities. Time and frequency laboratories use routinely a two-way method over a commercial telecom satellite (Two Way Satellite Time and Frequency Transfer, TWSTFT).

Comparison methods at METAS For the comparison of its clocks with those of other time laboratories and to tie its clocks into the Universal Time Coordinated (UTC), METAS is equipped with several time and frequency methods.

GPS Common View The best known one-way method is GPS Common View. This method is realised at METAS with a multi-channel GPS receiver. The collected data are transmitted regularly to BIPM from where they can be retrieved FTP-Server.   Are neutrinos faster than light? A calibration performed by Metas contributes to the synchronization between Rome and Geneva.   GPS Carrier Phase METAS has collaborated with the Astronomical Institute of the University of Bern to develop a time and frequency transfer method which exhibits a better short term stability than GPS Common View. This methods relies on a network of stations each of which record GPS-Code and GPS Carrier Phase with respect to the local clock to be compared. The individual clock differences from the participating stations can then be obtained in a post-processing step that is carried out in a analysis center. The satellite orbit data needed in the processing of the data are obtained from theInternational GNSS Service (IGS). METAS hosts on its roof its own IGS-Site (WAB2).   TWSTFT METAS also operates a two way satellite time and frequency transfer station for the comparison of its clocks with other institutes over telecom satellites. Since January Januar 2005 METAS has participated on a regular basis in the comparisons defined by the CCTF WG on TWSTFT (CCTF: Comité consultatif du temps et des fréquences) teil.     Detailed description of the TWSTFT-setup at METAS (in german 170 kB)