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 (Tp) 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 Tp: ?TRMS(Tp). 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 ?TRMS versus Tp; (3) the modelling of ?TRMS(Tp) 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 ?10?ps RMS for Tm = 24?h; (4) for Tp = 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 ?T?1.5?ns for Tp = 4?h is 89% for the S-RAFS, while the same time error constitutes an absolute upper bound for the Galileo S-PHM.