Studies on Measurement Noise in the European TWSTFT Network

Two-way satellite time and frequency transfer (TWSTFT) using geo-stationary telecommunication satellites is widely used in the timing community nowadays and has also been chosen as primary means to effect synchronization of elements of the ground segment of the European satellite navigation system Galileo. We investigated the link performance in a multi-station network in dependence on operational parameters, such as the number of simultaneously transmitting stations, transmit and receive power, and chip rates of the pseudo random noise modulation of the transmitted signals with the following results: 1. TWSTFT through a "quiet" transponder channel (two stations transmitting only) leads to a measurement noise, expressed by the lpps jitter, reduced by a factor of 1.4 compared to a busy transponder carrying signals of 12 stations. 2. The frequency transfer capability expressed by the Allan deviation is reduced at short averaging times by the same amount. 3. At averaging times of > 1 day no such reduction could be observed, which points to the fact that other noise sources dominate at such averaging times. 4. Higher transmit power increases the carrier-to-noise density ratio at the receive station and thus entails lower jitter but causes interference with other stations' signals. 5. The use of lower chip rates which could be accommodated by a reduced assigned bandwidth on the satellite transponder is not recommended. The lpps jitter would go up by a factor of 2.5 when going from 2.5 MCh/s to 1 MCh/s. The two Galileo PTFs can be included in the currently operated network of 12 stations in Europe and all requirements on the TWSTFT performance can be met, provided that suitable ground equipment will be installed in the Galileo Ground Segment.