New Orbit Determination and Clock Synchronisation Modules for EGNOS

Abstract

The purpose of a Satellite Based Augmentation System (SBAS), such as EGNOS or WAAS, is to identify all range error sources and to distribute the corresponding corrections to the civil aviation user community with reliable navigation services for different flight phases. The effect of a satellite location error depends on the user location while a satellite clock error with respect to a reference time scale directly translates into a common pseudorange error to all the users. Therefore the SBAS shall broadcast a 3D vector that represents the satellite orbit error and a satellite clock correction. To achieve this objective the SBAS shall internally estimate the orbits and clocks for all the navigation satellites in view of the service area. The orbit determination function is in charge of computing the satellite ephemerides. The synchronization function computes the corresponding clock bias for each epoch and each satellite. Then the corrections are constructed from the differences between these orbits and clocks and the corresponding ones broadcasted inside the GNSS navigation messages. Starting from R&D activities, Thales Alenia Space has developed new orbit determination and synchronization modules that are part of the Thales Algorithm Navigation Chain. These modules have been designed in collaboration with the orbit determination team at CNES (the French Space Agency). The new proposed orbit determination module is based on real time processing using code carrier measurement only. This module provides a stable and metric GPS orbit performance using an SBAS set of receivers corresponding to the EGNOS service area. The new synchronization module solves clock errors directly steered to GPS reference time scale, for the stations and satellites. It uses both code carrier and phase carrier measurements as well as the orbits estimated by the orbit determination process. The clocks solution error Allan’s deviation is around 10-12 at 120s leading to 7cm of possible deviation for a prediction up to 120s. This performance is fully compatible with the needs of the SBAS mission. These modules are now fully integrated into the SPEED platform, the SBAS Operational Test-bed that fully represents EGNOS Performances in terms of accuracy, continuity, availability and integrity for Safety Of Life services. The performance evaluation shows a real improvement over the current EGNOS algorithms, particularly in terms of the distribution of the Satellite Residual Error for the Worst user location (SREW). This paper provides a high level architecture description of this new Thales solution. A set of performance figures showing the achieved improvements is also presented.