TIME TRANSFER USING AN ASYNCHRONOUS COMPUTER NETWORK: RESULTS FROM A 500 KM BASELINE EXPERIMENT
Paper in proceeding, 2007
SP Technical Research Institute of Sweden and STUPI have performed a time transfer
experiment over a 500km long baseline between Borås and Stockholm. The time transfer
technique passively utilizes the data bit stream generated in an optical fiber computer network
based on the packet over SONET/SDH technique. A small fraction of the optical signal is
monitored both at the transmitter and at the receiver. When an occurrence of a unique bit
sequence of the SDH frames is detected, an electrical pulse is generated and compared with a
resolution of 100 ps to a local clock. With data from all four positions of an optical bidirectional
link, two-way time-transfer can be achieved and any symmetrical variations in delay can
potentially be cancelled. The results presented here have been obtained over OptoSUNET, the
new Swedish University Network. In the experiment, 10 Gbit/s traffic from SP over OptoSUNET
is extended in Stockholm to STUPI, a clock laboratory which is the second node in this setup.
This reconnection enables that a communication channel is established between two nodes,
with no intermediate jump. The time-transfer experiment includes more than 500 km of fiber
transmission, of which several km is via air-lines. By comparing the results from a GPS carrierphase
link, a precision better than ± 1 ns is achieved over several months of measurements
between two Hydrogen-masers.