Evaluation of the OD&TS system noise in the Galileo Giove mission

In satellite navigation systems, the onboard clocks are a key element from which timing and navigation signals are generated. The performances of the navigation system rely, among others, on the performance of the clocks, as well as on the capability of the system to estimate and predict the clocks. In the Galileo system, the phase offset of each system clock with respect to a ground reference time is estimated by an Orbit Determination and Time Synchronization (OD&TS) algorithm which, based on the L-Band pseudorange measurements, estimates the satellite orbital parameters and the space and ground clocks, together with additional quantities. The clock estimates obtained from this system are to be considered "apparent" clock estimates, because they contain the true clock behavior but also the contribution of many additional delays and instabilities due to the on board signal generation chain, the antennas, ionosphere and troposphere, ground sensor stations, etc. For example, the clock phases are estimated by OD&TS with respect to a reference point which is the antenna phase center, while typically in time metrology the measurement reference point for a clock or time scale is physically located inside the laboratory at a certain point in the time scale generation chain. The difference between a clock "as seen" at the center of phase of an antenna and a reference point inside the lab is given by the delays and instabilities of the receivers, antenna, cables, among others, and the calibration of these delays ask for a careful procedure. In the frame of the current Galileo experimental phase named GIOVE mission, several activities have been carried out for evaluating the capability of the OD&TS algorithm to estimate the clocks and for understanding which is the noise added by this complex network system when clocks are estimated. The aim of this paper is to report on these experimental activities, explaining the different experimental situations, as well as comparing the noise of the Galileo OD&TS measurement system with respect to other "state of the art" clock comparison systems.