Performance Assessment of the Time Difference Between EGNOS-Network-Time and UTC

J. Delporte, N. Suard, P. Uhrich

Abstract:	The European SBAS (Satellite Based Augmentation System) EGNOS (European Geostationary Navigation Overlay Service) is now operated and provides in its navigation message the time difference between EGNOS Network Time (ENT) and UTC to users. For this purpose one EGNOS Earth station has been installed in the Observatoire de Paris (OP) providing a physical link to UTC(OP). Applying EGNOS corrections on GPS measurements provides a precise time and navigation solution referenced to ENT. Assessment of the time difference between ENT and UTC is therefore a key issue for time users. A new EGNOS system release has been tested since the beginning of 2008 including some improvements for the EGNOS time to UTC offset. This paper deals with the evaluation of the very good performances obtained with EGNOS and its new release in this time domain. First the paper details some time aspects of EGNOS and describes the connection of ENT to UTC(OP). EGNOS system time requirements are also reminded. Then two methods of performance assessment are described. The first method is based on the time difference between ENT and UTC obtained by combination of broadcasted information in Message Type 12 and BIPM Circular T: ?? ENT-UTC(OP) is broadcasted in EGNOS Signal In Space (SBAS Message #12) using internal delays of EGNOS Earth station located in the Observatoire de Paris. By analysing the content of the different MT#12, we can derive the ENT-UTC(OP) evolution. On an other side, UTC-UTC(OP) is computed by the BIPM and available in their Circular T. By comparing these two pieces of information, an estimation of ENT-UTC can be established. The second method consists in an independent estimation of ENT at user level, in locations where EGNOS corrections are valid: ?? For this user, an estimate of the difference between its receiver time and ENT can be produced using GPS measurements and EGNOS corrections. If the receiver clock is connected to a UTC(k), an estimation of ENT-UTC(k) is obtained. UTC(OP) and UTC(IT) have been chosen in this assessment by using the data from an IGS station connected to these UTC(k). This independent estimation of ENTUTC( OP) is obtained and then compared to the one broadcasted in EGNOS SIS. The results of the two methods are given and checked in regards to EGNOS system requirements. In BIPM Circular T, there are also information on the offset between GPST and UTC enabling a differed estimation of ENT-GPST from which results will be presented. To conclude, as EGNOS Network Time is by construction referenced to UTC(OP) and not directly to UTC, the expected gain in this chain due to the on-going improvements in the modernisation of UTC-UTC(OP) in order to have UTC(OP) closer to UTC is briefly discussed.
Published in:	Proceedings of the 22nd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2009) September 22 - 25, 2009 Savannah International Convention Center Savannah, GA
Pages:	3329 - 3337
Cite this article:	Delporte, J., Suard, N., Uhrich, P., "Performance Assessment of the Time Difference Between EGNOS-Network-Time and UTC," Proceedings of the 22nd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2009), Savannah, GA, September 2009, pp. 3329-3337.
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