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When a second lasts for ever

On computers, smartphones, in the kitchen – it’s the same time everywhere. But what does «the same time» actually mean? In everyday life, a small deviation in time can go unnoticed, but with complex sensor systems it can be tantamount to an eternity. armasuisse Science and Technology is testing a new method for the most accurate time synchronisation possible: White Rabbit. This method aims to enable a more stable synchronisation than with standard methods.

Christof Schüpbach, Communications and Electromagnetic Protection, armasuisse Science and Technology

Three wrists with the same digital watch but with slightly different time data
Time synchronisation often happens unnoticed in the background. Depending on the method of measurement, the time information can be more or less accurate and thus not equally well suited for all applications.

«Synchronise your watches!» You still often see it in older spy films: The agents synchronise their watches before an important operation, so that the bomb goes off at the scheduled time.  Although less spectacular, many people still know how to adjust their watch in everyday life. While in the past we used to set the exact time according to the clock at the station or by the time signal on the radio, we hardly have to worry about it at all today. Our clocks and other gadgets are all connected and adjust themselves automatically thanks to the Internet connection. But how exactly does this make our clocks run synchronously and for what else do we need a precise time?

Time synchronisation accurate to millionths and billionths

Our intuitive understanding of time is rather limited and in everyday language «a fraction of a second» is the smallest unit for most people. The aforementioned time synchronisation of our electronics, such as smartphones and computers, is usually accurate to several thousandths of a second at the most. This has long been precise enough for our everyday life. However, in professional applications the requirements are considerably higher. In the financial sector for example, buying and selling securities needs to be precisely timed to a millionth of a second in order to optimise profits.

Military sensor systems need to be synchronised even more precisely, namely to billionths of a second, in order to locate the origin of radio signals. But how do you achieve this kind of accuracy?

Time measurement with Global Positioning System GPS

In most cases, satellite navigation systems such as the Global Positioning System GPS are used for this purpose. The time is determined together with the location in these systems. This enables a GPS receiver to have a time that is always accurate to a few billionths of a second and it can provide it to the other systems. This form of time synchronisation is basically a very attractive solution, as it can be used independently of location and also on a mobile basis. Unfortunately, it also has a few critical disadvantages. The biggest problem is the high susceptibility to radio interference. A GPS signal can be drowned out even with simple interference devices and thus rendered useless for the recipient. Another problem is that one is dependent on the respective operators when it comes to satellite systems. In addition, the latest interference technology even allows deception. Any locations and times can thus be faked and sensor systems thus deliberately fooled. It is therefore extremely important to have a stable alternative for military applications.

The biggest problem (with GPS-based time synchronisation) is the high susceptibility to radio interference. A GPS signal can be drowned out even with simple interference devices and thus rendered useless for the recipient.


White Rabbit as alternative for a more accurate time measurement

The CERN nuclear research institute in Geneva also had high requirements for precise time measurement at different locations when renewing the particle accelerators. All the technologies in existence at the start of this project require a special infrastructure for this purpose, and needed to be connected with each other via special cables. As the entire infrastructure was connected by conventional data networks anyway, the scientists set themselves the goal of achieving synchronisation via this existing data network with an accuracy of under a billionth of a second – and thus achieving an accuracy via conventional data networks which would otherwise only be achievable with a great amount of effort. This newly developed technology achieved this goal and can in fact, under favourable conditions, even achieve accuracies of a few picoseconds, in other words, trillionths of seconds. The project was dubbed White Rabbit after the rabbit in Alice in Wonderland.

Today, the White Rabbit method is already implemented in certain commercially available network devices. Despite this, its usage is not yet very widespread and there is therefore a lack of experience regarding which restrictions can be expected and under which conditions. These types of restrictions could, for example, come about through the use of certain components such as repeaters or routers in a network. With somewhat older components in particular, it is not known whether they are compatible with White Rabbit or whether they could possibly impair time synchronisation. This is exactly what employees of armasuisse Science and Technology (S+T) are investigating. They want to find out whether or with which restrictions or adjustments this new White Rabbit technology could be used in the existing networks of the Armed Forces.

First White Rabbit tests with civilian research partners

armasuisse S+T is carrying out the first tests with the College of Engineering and Architecture of Fribourg. A network topology is set up in a laboratory structure, as it would occur with several nodes in a typical, nationwide operation in Switzerland.  Components can thus be exchanged individually and their influence on accuracy tested. The findings obtained are ultimately to be used to implement White Rabbit in the Swiss command and control network, so that the Armed Forces’ sensor systems will no longer be dependent on less reliable satellite navigation systems in future.

How does the location of radio signals work?

The location of radio signals works by sensors measuring the arrival time of the signals at various locations. As a radio signal spreads at the speed of light, the point of origin of the signal can be calculated from the measured time differences. One example: Light travels 30 centimetres in a billionth of a second. If you therefore want to accurately determine the origin of the signal to within a few metres, the arrival time at the sensor must also be measured precisely to a few billionths of a second.