Galileo IOV system initialization and LCVTT technique exploitation

Abstract

Satellite-based navigation systems uses one-way ranging measurements for system orbit estimation and timekeeping, due to its operational advantage when compared with the two-way ranging technique, in terms of complexity of ground monitoring stations (completely passive and requiring a simple omni-directional antenna to track all the satellites in view). However, a sufficient number of simultaneous independent measurements is required to solve the system unknowns: in particular simultaneous visibility of multiple stations by an individual satellite (allowing separation of the ground station's clock contributions, since the SVs clocks disappear), as well as simultaneity of observations from the same monitor stations of a large number of satellites (allowing recovery of the SV's clock parameters, since the GSS clocks drop out) is the key to an effective separation in the solution of the clock contributions from the pseudo-ranges. In the Galileo IOV phase (consisting of 4 satellites on two orbital planes and a ground network of 20 Stations), the first condition is clearly fulfilled; however, the second condition is not met for a considerable part of the time. If two GSSs do not see simultaneously a single Galileo satellite, they will not be able to estimate their clocks' time and frequency drifts, i.e. they will not be synchronized. The free-running clocks will essentially enter a holdover mode, where the relative time between the two stations will be slowly drifting as a function of the initial conditions and the stability of the clocks. The ground station's synchronization will gradually degrade with time and, when a satellite rises on the horizon, they will be essentially not synchronized to the extent required to carry on a one-way-based Orbit Determination & Time Synchronization (OD&TS). During the IOV phase, the limited number of satellites available and the peculiar characteristics of the Galileo orbits will make it difficult for the Orbit Determination and Time Synchronization to start producing meaningful data; therefore, some form of intermediate operational configuration must be sought to help in the OD&TS process initialization. The paper addresses the proposed solution to overcome the problem of Galileo system initialization, starting from the intermediate configuration with the first two satellites (first IOV Launch) up to the final IOV Configuration after the second IOV launch. The proposed solution will be based on a limited use of GPS to insure the synchronization of the Galileo Sensor Stations, relying on the exploitation of the Linked Common View Time Transfer (LCVTT) Technique, while the Galileo Orbit Determination and SV's clock characterization will be carried on autonomously and independently by GPS, in a two-step process, up to the achievement of the IOV Configuration with four satellites, when the nominal Orbit Determination and Time Synchronization process will be operated. Moreover, the paper addresses the development of the LCVTT Algorithm, carried on as part of the development of the infrastructure aiming to support the Galileo Verification Phase currently under definition as part of the Galileo Phase C/D/E1 contract. The algorithm design and implementation is presented, together with the validation carried out (both for LCVTT and MLCVTT) to verify that the synchronization accuracy is adequate to support the Galileo System Initialization.