Performance assessment of the time difference between EGNOS-network-time and UTC

The European SBAS (Satellite Based Augmentation System) EGNOS (European Geostationary Navigation Overlay Service) provides in its navigation message the time difference between EGNOS Network Time (ENT) and UTC. For that purpose, an EGNOS ground station was installed in the Observatoire de Paris (OP) and is connected to UTC (OP). Applying EGNOS corrections on GPS measurements provides a precise time and navigation solution referenced to ENT. Therefore the assessment of the time difference between ENT and UTC is a key issue for time users. A new EGNOS system release has been tested since the beginning of 2008, it includes some improvements in the timing functions. This paper deals with the evaluation of the performances obtained by these functions. First the paper recalls the timing aspects of EGNOS and describes the connection of ENT to UTC (OP). Then two methods of performance assessment are described. The first method is based on the time difference between ENT and UTC obtained by the combination of broadcast information (EGNOS Message Type 12) and BIPM Circular T. The EGNOS Message Type 12 contains an estimation of ENT-UTC (OP) computed by the EGNOS ground segment, using the Observatoire de Paris ground station measurements. The BIPM Circular T provides UTC-UTC (OP). The second method consists in an independent estimation of ENT at user level (in locations where EGNOS corrections are valid). In that case, we use the GPS measurements of a receiver connected to a UTC (k). We apply EGNOS corrections and station delays on these measurements so that we get ENT-UTC (k). With the BIPM Circular T, we have therefore access to an estimation of ENT-UTC. The results of the two methods are checked with respect to EGNOS system requirements. Information is also given on the current status of EGNOS for the operational Open Service and on a daily model of ENT-UTC (OP) that is provided in the RINEX SBAS navigation files. To conclude, as EGNOS Network Time is by construction linked - to UTC (OP) and not directly to UTC, the on-going improvements of UTC (OP) will have a positive impact on the prediction performance of UTC-UTC (OP) and are briefly discussed.