Reference for time: UTC
For all kinds of applications, it is important that there is a global reference for the exact time. Satellite navigation is of course a well-known and important application, but the exact time also plays a major role in industrial processes, stock exchange trading, telecommunications, astronomy, and medical research. The global reference for time measurements is UTC: the coordinated universal timescale. UTC is generated on the basis of atomic clock measurements of about 90 time centres worldwide, which are interconnected.
Realising the time standard
Erik Dierikx is researcher Time & Frequency at VSL, the Dutch metrology institute. "At VSL we maintain all relevant national measurement standards, including time, mass, length, etc. This is a task we perform for the government. When it comes to time: with 4 atomic clocks we realise the national time standard of the Netherlands. And like VSL, there is a total of about 90 time centres worldwide that realise a time standard. We compare our measurements with each other, and this results in the UTC. That is a kind of "weighted average" of the time measurements of those time centres.”
Pierre Waller works as an engineer at ESTEC (European Space Research and Technology Centre), the ESA's technical centre in Noordwijk. This is where most of the ESA projects are developed. "ESTEC is also one of the time centres mentioned by Erik. We also have atomic clocks with which we can make very precise time measurements. Among other things, we need these for the monitoring of GALILEO, the civil satellite navigation system developed by ESA. But ESTEC is also the ESA's technical centre when it comes to time measurement. We need to have hands-on expertise in this area".
From radio connection to fibre optic connection
Time comparisons between the time centres for the realisation of UTC are mostly done via radio signals from communication satellites (Two-Way Satellite Time and Frequency Transfer, TWSTFT) and navigation satellites (Global Navigation Satellite System, GNSS). These time measurement systems have proven their worth, but are also sensitive to interference, because the signals from satellites are relatively weak. For this reason, alternative techniques that are less sensitive to interference have been experimented with for a number of years.
One of these alternative techniques is time synchronisation via fibre optic connections. VSL and ESTEC use this technique to carry out a proof of concept. SURF has built a fibre optic connection between VSL in Delft and ESTEC in Noordwijk, which is dedicatedly used to compare the atomic clocks of VSL and ESTEC, and to contribute to the determination of UTC.
Accurate and reliable
Dierikx: "There are already three optical fibre time and frequency transfer connections between time centres operational in Europe, with good results. We are the first to use the White Rabbit technology. This is a time and frequency transfer protocol based on standard internet protocols. This means that you can carry out the measurements via an existing network. That offers a lot of perspective. By the way, that does not mean that you can simply turn on White Rabbit. SURF has made adjustments to the optical layer of the national education and research network to make the connection between VSL and ESTEC possible. We see that the performance is good: the measurements are very accurate, and the system is very reliable. There is much less chance of external interference than with satellite connections. Perhaps by an excavator breaking a cable, but that chance is very small.” Waller adds: "What's more, the bandwidth of the connection is many times larger than with satellite-based techniques. That's another reason why the performance is so good".
What's the next step?
Waller: "Last year, VSL and ESTEC completed a project with a similar connection between the Belgian metrology service SMD in Brussels and ESTEC in Noordwijk. We achieved excellent results with that link. So, with this new connection between VSL and Noordwijk, we ultimately want to move towards a continuously operational connection. Together, VSL and ESTEC will then contribute to the determination of UTC in an even more accurate and reliable way".