GPS Solutions

, Volume 11, Issue 4, pp 227–237

Single epoch estimation of the Galileo integrity chain sensor station clock offsets

  • Marek Ziebart
  • Paul Cross
  • Antony Sibthorpe
  • Peter Arrowsmith
  • Washington Ochieng
  • Shaojun Feng
  • Umar Bhatti
  • Peter Niemann
Original Article


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, but by the distortions induced by noise and bias in the integrity chain. These distortions increase the minimum bias the integrity chain can guarantee to detect, which is reflected in the user protection limit. A contributor to this distortion is the inaccuracy associated with the estimation of the offset between the Galileo sensor station (GSS) receiver clocks and the Galileo system time (GST). This offset is termed the receiver clock synchronization error (CSE). This paper describes the 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). In the study we simulate open access to a time datum using IGS data. Two methods are compared for determining CSE and the corresponding uncertainty (noise) across a global network of tracking stations. The single-epoch single-station method is an ‘averaging’ technique that uses a single epoch of data, and is carried out at individual sensor stations, without recourse to the data from other stations. The global network solution method is also single epoch based, but uses the inversion of a linearised model of the global system to solve for the CSE simultaneously at all GSS along with a number of other parameters that would otherwise be absorbed into the CSE estimate in the averaging technique. To test the effectiveness of various configurations in the two methods the estimated synchronisation errors across the GSS network (comprising 25 stations) are compared to the same values as estimated by the International GPS Service (IGS) using a global tracking network of around 150 stations, as well as precise orbit and satellite clock models determined by a combination of global analysis centres. The results show 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 12 ns (3.7 m), this value being largely driven by the satellite CSE errors. The global network approach is capable of delivering CSE errors at the level of 1.5 ns (46 cm) depending on the number of parameters in the linearised model. The International GNSS Service (IGS) receiver clock estimates were used as a truth model for comparative assessment.


GNSS Galileo Integrity Clock modelling 

List of Abbreviations


Antenna reference point


Broadcast ephemeris


Concatenated BE parameter files


Clock synchronisation error


Day of year


Earth centred inertial


European space agency


GPS time


Galileo sensor station


Galileo system test bed


International earth rotation service


International terrestrial reference frame


International GPS service


BE parameters in RINEX format


US national geodetic survey


Orbitography and synchronisation processing facility


Pseudorange observable on the L1 carrier


Pseudorange observable on the L2 carrier


Receiver independent exchange format


Solution independent exchange format


US national geodetic survey standard GPS orbit format


Coordinated universal time

Copyright information

© Springer-Verlag 2007

Authors and Affiliations

  • Marek Ziebart
    • 1
  • Paul Cross
    • 1
  • Antony Sibthorpe
    • 1
  • Peter Arrowsmith
    • 1
  • Washington Ochieng
    • 2
  • Shaojun Feng
    • 2
  • Umar Bhatti
    • 2
  • Peter Niemann
    • 3
  1. 1.Department of Geomatic EngineeringUniversity College LondonLondonUK
  2. 2.Department of Civil and Environmental EngineeringImperial College LondonLondonUK
  3. 3.LogicaCMGLondonUK

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