CRITICAL POINTS AND TECHNICAL APPROACH IN THE E-OSPF TIME SYNCHRONIZATION AND PREDICTION ACTIVITIES

摘要

Accurate satellite clock predictions are essential for navigation satellite systems. The user positioning and integrity performances are highly dependent on the level of accuracy that can be achieved when the clock estimation and prediction techniques are applied to the generation of the navigation information.This article is aimed at presenting the latest timing approaches prototyped in the E-OSPF (Experimental Orbitography and Synchronization Processing Facility), and the obtained results, related to the clock estimation technique, the clock prediction fitting strategy, and specific solutions designed for particular Galileo system clock features.Regarding the clock estimation technique, the OD&TS (Orbit Determination and Time Synchronization) E-OSPF SW module implements an algorithm that allows the existence of a backup time reference to be used in case of lack of observability from the master PTF (Precise Timing Facility). This is particularly useful in the IOV (In-Orbit Validation) configurations, which will allow the verification of the proper functioning of the overall system before entering the FOC (Full Operation Capability) phase.The clock prediction fitting strategy currently implemented in the E-OSPF SW consists of estimating the zero-order clock prediction parameter with a reduced set of clock offsets containing the final epochs in the estimation arc, whereas the first-order clock prediction parameter is computed by fitting the clock offsets in a larger time period, typically as long as the estimation period. This strategy has been demonstrated to be successful with GPS data, and is now being tested for real and simulated Galileo data as well.The fulfillment of the stringent requirements defined for the Galileo system functionality and performances imply that planned and unplanned clock events such as clock switches and resynchronizations, or clock phase jumps, among others, have to be managed by the E-OSPF in such a way the system accuracy, integrity, and availability are preserved.