EGNOS Network Time and Its Relationships to UTC and GPS Time

摘要

EGNOS, the European Satellite Based Augmentation System, generates its own system time scale ENT (EGNOS Network Time). ENT is steered by the EGNOS Ground Control Segment to GPS time (GPST). The offset between ENT and UTC is broadcast in the EGNOS navigation message. To compute that offset, an EGNOS ground station was set up in the Observatoire de Paris 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. The aim of this paper is to assess the closeness of ENT to UTC and GPST. First, the paper recalls the timing aspects of EGNOS and describes the connection of ENT to UTC (OP). Then the broadcast time difference ENT - UTC (OP) is analyzed and compared to an independent method of computing that offset. This independent method makes use of the calibrated OPMT IGS station that is indirectly connected to UTC (OP). The agreement between the broadcast information and the independent method is discussed with respect to EGNOS requirements. Using the BIPM Circular T that provides UTC - UTC (OP), we can then easily derive the offset between ENT and UTC. This independent method can also be carried out using the IENG IGS station connected to UTC (IT), providing therefore ENT - UTC (IT). Moreover, an assessment of the time offset ENT - GPST is also carried out by two methods. The first method combines the broadcast offset ENT - UTC (OP) to the time differences UTC - UTC (OP) and UTC - GPST both extracted from BIPM Circular T. The second method combines the broadcast offset ENT - UTC (OP) to the REF - GPS obtained in the CGGTTS files coming from measurements made by the OPMT IGS receiver at the Observatoire de Paris referred to UTC (OP). Both results are compared and discussed.