Every Nano-Second Counts: Estimating the Galileo Integrity Chain Clock Offsets Globally in a Single Epoch

M.Ziebart, P.Cross, A.Sibthorpe, P.Arrowsmith, W.Ochieng, S.Feng, U.Bhatti and P. Niemann

Abstract:	The information content of the Galileo integrity chain depends on a number of key factors, one of which is contamination of the Signal-In-Space Errors with residual errors other than imperfect modelling of satellite orbits and clocks. A potential consequence of this is that the user protection limit is driven not by the errors associated with the imperfect orbit and clock modelling (i.e. the accuracy of the Signal In Space Accuracy), but by the level of noise in the integrity chain. This noise increases the minimum bias the integrity chain can guarantee to detect, which is reflected in the user protection limit. One contributor to the noise in the integrity chain is the inaccuracy associated with the estimation of the offset between the Galileo Sensor Station (GSS) receiver clocks and the Galileo System Time (GST), termed the receiver clock synchronization error (CSE). This paper describes research carried out to determine both the CSE and its associated error using GPS data as captured with the Galileo System Test Bed Version 1 (GSTB-V1). The CSE is the difference between a receiver clock and system time (in this case GPS time) – whilst it is common to refer to it as an error, in fact, as long as it is perfectly known, it does not lead to any degradation of the system performance. What is important is the error in the CSE, that is, it is vital to determine the CSE as accurately as possible as any error in the CSE leads directly to equivalent range error. The aim of this research was to compare two methods for determining GSS CSE and the corresponding uncertainty (noise) across a global network of tracking stations. The two techniques are: 1: Single station-satellite pair method - an ‘averaging’ technique using a single epoch of data and carried out at individual sensor stations, without recourse to the data from other stations. 2: Global network solution method - also single epoch based, but using the inversion of one simultaneous linearised model of the system to solve for the CSE and a number of other parameters that would otherwise be absorbed into the CSE estimate in the averaging technique. To evaluate the two methods the estimated synchronisation errors across the GSS network (25 stations) are compared to the values estimated by the International GPS Service using a ~150 station tracking network, as well as precise orbit and satellite clock models. The study shows that the averaging technique is vulnerable to unmodelled errors in the satellite clock offsets from system time, leading to receiver CSE errors in the region of 3.0 metres, this value being largely driven by the satellite CSE errors. The global network approach was trialled with various parameter configurations. Using the station network and the available satellite constellation derived from the Orbit and Synchronisation Processing Facility (OSPF) orbit and clock products we show the global network approach is sensitive to the model redundancy. Very low redundancy (typically, for the network tested, 114 parameters estimated from 120 observations) leads to strong correlations between the parameters and weak solutions, which were no better than the ‘averaging’ technique, and at some times substantially worse. Moreover, the noise characteristics of the time averaged CSE errors were very weak. However, reducing the number of parameters by reformulating the model led to greatly improved results. Solving only for the receiver and satellite CSE simultaneously across the whole network gave excellent results: the receiver CSE error (that is, the difference between the network solution and the IGS truth model) reduced to circa 0.2m with an associated uncertainty of the same magnitude. The global network method yields a significant improvement, both in terms of accuracy and robustness, over the techniques trialled in the GSTB commissioning report. Looking forward to Galileo, and the potential quality of the clock and orbit products generated by the OSPF, it is likely that the global network solution to the CSE problem may be better still using a full constellation of satellites and a more complete station network.
Published in:	Proceedings of the 18th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2005) September 13 - 16, 2005 Long Beach Convention Center Long Beach, CA
Pages:	1381 - 1390
Cite this article:	M.Ziebart,, P.Cross,, A.Sibthorpe,, P.Arrowsmith,, W.Ochieng,, S.Feng,, U.Bhatti,, Niemann, P., "Every Nano-Second Counts: Estimating the Galileo Integrity Chain Clock Offsets Globally in a Single Epoch," Proceedings of the 18th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2005), Long Beach, CA, September 2005, pp. 1381-1390.
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