Time and Frequency Transfer using Passive Techniques in Active Fiber Optic Networks

Licentiate thesis, 2010

For the modern society, high precision time and frequency transfer have become crucial since a large number of systems, such as telecommunication systems, require precise and stable synchronization. Electric power companies use synchronized systems for taking stability measurements and searching for errors in their networks. For the scientist working in radio astronomy with very long baselines interferometer (VLBI), synchronization and timing are important factors and as a result, many laboratories operates their own timescales with atomic clocks as reference. Navigation of spacecrafts within the solar system is heavily dependent on synchronized control stations on Earth. The financial market uses time and date stamps for identification of transactions in order to organize them in chronological order to increase electronic transaction speed. There are many more areas within the society that are in need of time and frequency synchronization with varying accuracy and precision. Many of these applications utilize GNSS (Global Navigation Satellite System) for these purposes of time and frequency synchronization. GNSS depends on radio transmissions that easily can be disrupted; therefore, the main objective for this thesis is to implement a system within Sweden for high precision time transfer utilizing existing infrastructure for an alternative and complementary time transfer method, with comparable accuracy and stability to existing and well established satellite based methods. An introduction to the developed technique for passive time transfer with its benefits is presented in the thesis, and is further described with its different sources of error in papers [A-D]. Precision relative to the GPS (Global Positioning System) carrier phase of < 1 ns has been obtained for all experiments independent of configuration. The technique utilizes time and frequency transfer over an asynchronous fiber optical transmission control protocol network. The system is based on passive listening to existing data traffic in a network. The most recent and still continuing performing system compares Hydrogen-MASERs separated by a distance of more than 560 km, using passive listening technique and a GPS-link as reference. This developed technique passively monitors the data bit stream generated in an optical fiber transmission network using packet over SONET/SDH technique between two core IP routers. Because the network is asynchronous, intermediate supporting clocks will be located and compared at each router for the purpose of referencing each synchronous path to the asynchronous network. These results are comparable with alternative fiber based solutions using dedicated bandwidth for transmitting timing signals. The main benefit with this system in comparison to active fiber based time transfer and time transfer in dedicated networks is that this passive time transfer not will influence the data traffic in the existing network. This thesis reports on several time transfer experiments that have been performed with different complexity and over different distances, and all these experiments utilize a developed passive technique