Galileo IOV System Initialization and LCVTT Technique Exploitation
Signals and Communication Technology
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
... r 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 Report Documentation Page Form Approved OMB No. 0704-0188 Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number.