In-orbit performance assessment of GIOVE clocks

Abstract : Two onboard atomic clock technologies have been developed for the Galileo system, one based on vapor cell rubidium technology (RAFS: Rubidium Atomic Frequency Standard) and one based on passive hydrogen maser (PHM) technology. In the years 2004-2005, both technologies have successfully passed a full qualification campaign (including shock, vibration, thermal vacuum, ...) aimed at verifying their performance in a Galileo-like environment. In the year 2005, six RAFS and two PHM flight models have been delivered to ESA in order to be installed on two experimental satellites: GIOVE-A (launched on 27 December 2005) embarks two RAFS in a redundant scheme and GIOVE-B (launched on 26 April 2008) embarks two RAFS in a redundant scheme and one PHM. The clocks onboard both GIOVE-A and GIOVE-B have been monitored on a continuous basis through the GIOVE Mission infrastructure. It includes two Ground Satellite Control Stations (one for each satellite) collecting and archiving onboard telemetries, and a network of 13 Galileo Experimental Sensor Stations (GESS) distributed worldwide that collect both GIOVE and GPS observables (pseudorange and carrier phase). One of these Ground Sensor Stations (located at INRiM, Turin, Italy) is connected to an active hydrogen maser that realizes the reference timescale for the GIOVE Mission. The Galileo Processing Centre located at ESAESTEC, Noordwijk centralizes all these data which are then processed through the Orbit Determination and Time Synchronization (OD&TS) algorithms, allowing the restitution of the phase difference between the transmitted clock signal and the ground reference. This paper presents the performance assessment of GIOVE-A and GIOVE-B clocks, based on the analysis of the clock behavior as restituted by the ODTS algorithms and GIOVE Mission infrastructure. The very first results of the PHM behavior onboard GIOVE-B are reported and show excellent performances over the analyzed period.