Time prediction accuracy for a space clock

For the Galileo system it is required that a space clock time prediction be performed, covering the time interval (T-p) between two uploads. The time prediction accuracy of the space clock is therefore an important issue. The predictability of the Space Passive Hydrogen Maser (S-PHM) time error is evaluated by the RMS of the predicted time errors at the prediction time T-p: DeltaT(RMS) (T-p). A linear prediction model is used, corresponding to the absence of a frequency drift. The results show that: (1) the RMS time error can be evaluated from a priori knowledge of the clock's Allan deviation; (2) conversely, it is possible to extract the Allan deviation from the measurement of Delta T-RMS versus T-p; (3) the modelling of DeltaT(RMS) (T-p;) of S-PHM based on the white frequency and flicker frequency noises appears to be particularly accurate: the difference between the model fit and the measured prediction accuracy is less than or equal to 10 ps RMS for T-m = 24 h; (4) for T-p = 4 h, the performance of the S-PHM is a factor of 4.5 better than the performance of the space rubidium frequency standard (S-RAFS). This has a dramatic effect on the probability of the Signal In Space Accuracy: assuming a Gaussian distribution the probability of a predicted time error DeltaT less than or equal to 1.5 ns for T-p = 4 h is 89% for the S-RAFS, while the same time error constitutes an absolute upper bound for the Galileo S-PHM.