GIOVE-A Apparent Clock Assessment and Results

Jörg H. Hahn, Francisco Gonzalez, Pierre Waller, Daniel Navarro-Reyes, Ricardo Píriz, Álvaro Mozo, Virginia Fernández, Marta Cueto, Patrizia Tavella, and Ilaria Sesia

Abstract:	The performance of the Galileo space-borne clocks is of highest importance for navigation and integrity at the corresponding user level. Clock estimation and prediction errors driven by the onboard clock’s frequency stability and drift are directly mapped into the User Equivalent Range Error (UERE), which has to be kept below its specified limits. The clock quality also affects the integrity performance of the Galileo system: clock “feared events,” such as frequency and phase jumps, have to be characterized and adequate barriers built against them. In addition, the clocks operate in a rather harsh MEO-orbit radiation environment, and performance has to be guaranteed in all satellite mission phases, including eclipses. Two Rubidium Atomic Frequency Standard (RAFS) units developed by Spectratime (former Temextime) were launched on the first experimental Galileo satellite, the Galileo In-Orbit Validation Element A (GIOVE-A). The RAFS already demonstrated excellent performance during on-ground qualification and acceptance tests. To verify its performance onboard, an experiment was carried out as part of the GIOVE Mission Segment. Under this Mission, an infrastructure was deployed, including a network of 13 Galileo Experimental Sensor Stations (GESS) monitoring continuously the GIOVE-A signals. Using dedicated network adjustment techniques, the processing of these observables allows the restitution of the phase difference between the onboard clock and a ground reference. Such restitution, however, is affected by the noise of the measurement system (e.g., onboard and averaged on-ground group delays, orbit residuals, etc.), therefore the “apparent” clock denomination. This paper will present the GIOVE-A apparent clock results obtained since the ground infrastructure became operational for both the nominal and the redundant onboard clocks and for various signal configurations. These results will be complemented by relevant onboard telemetry data. Finally, the assessment of the onboard clock performance will be discussed, together with the contribution of the measurement system noise.
Published in:	Proceedings of the 39th Annual Precise Time and Time Interval Meeting November 27 - 29, 2007 Hyatt Regency Long Beach Long Beach, California
Pages:	95 - 114
Cite this article:	Hahn, Jörg H., Gonzalez, Francisco, Waller, Pierre, Navarro-Reyes, Daniel, Píriz, Ricardo, Mozo, Álvaro, Fernández, Virginia, Cueto, Marta, Tavella, Patrizia, Sesia, Ilaria, "GIOVE-A Apparent Clock Assessment and Results," Proceedings of the 39th Annual Precise Time and Time Interval Meeting, Long Beach, California, November 2007, pp. 95-114.
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