Accurate timekeeping is a matter of strategic importance. Not only today, but already since the 17th century, when determining longitude at sea required precise knowledge of a reference time. Today, although highly accurate clocks are available, they are not always located where they are needed. Therefore, time transfer methods play a crucial role. These are often implemented together with related applications, collectively referred to as PNT (Positioning, Navigation, and Timing), with typical examples being global navigation satellite systems such as GPS, Galileo, GLONASS, and BeiDou. The problem is that such systems can be easily disrupted or their receivers can be fed with incorrect data.

This vulnerability has long been known, but it has largely been ignored, and relatively little attention has been devoted to developing alternative solutions. The situation has changed dramatically since the beginning of the invasion of Ukraine, when reception in surrounding regions—from Romania to Scandinavia—has become heavily degraded and at times impossible. This fundamentally changes the rules of the game: users of systems such as Galileo or GPS are now becoming aware of their own vulnerability and the need for a backup plan. One possible solution is the transfer of precise time and frequency via optical fibers. This presentation provides an overview of the available methods and their key parameters. While these approaches have the obvious disadvantage of requiring a physical connection to a node via an optical fiber, they offer significant advantages in terms of achievable accuracy—on the order of nanoseconds—and strong resilience to interference.

Vladimír Smotlacha graduated from the Faculty of Mathematics and Physics at Charles University and the Faculty of Electrical Engineering at the Czech Technical University in Prague, where he also obtained his PhD. He works as a Senior Researcher at CESNET and also lectures part-time at the Faculty of Information Technology of the Czech Technical University in Prague. At CESNET, which operates the Czech national research and education network, he has been working for over 20 years on network protocols for time distribution. In recent years, his work has focused on optical time transfer with sub-nanosecond accuracy, making it suitable not only for the distribution of highly precise time information but also for the comparison of atomic clocks. In the past, he designed and deployed time scale transfer systems between national time laboratories in Prague and Vienna.