New Concept for the On-Board Master Clock Generation Unit for Future Galileo Satellites

F. Soualle, T. Beck, H. Trautenberg, D. Felbach, L. Stopfucken, J. Wendel, Francisco Amarillo Fernandez, A. Fernandez, M. Sanchez Nogales

Abstract: Measurements based on the Galileo experimental sensor stations have confirmed the expectations regarding the stability of the atomic clocks on-board the GIOVE-A and GIOVE-B satellites. These results are very encouraging for the performance of the Master Time Reference (MTR) generated by the Clock Monitoring and Control Unit (CMCU) on-board the future Galileo satellites. This CMCU will host two Passive Hydrogen Masers (PHM) and two Rubidium Atomic Frequency Standards (RAFS), see [10]. Such a redundancy will allow switching of atomic clock when an abnormal behavior of the MTR is observed. This operation will however require the isolation of the satellite from the constellation, which will degrade the system availability performance. To overcome this situation, it is proposed in this paper to implement a time scale based on an ensemble of clocks on-board the satellite. Time scales indeed allow smooth transitions during clock switching. Furthermore, they combine clocks with different stability characteristics in order to synthesize an average clock which shows better stability performance than each individual clock for both short and long terms. The stability performance of two time scale algorithms, based on a Kalman filter implementation (see [1] and [3]), will be analyzed as well as their behavior during maintenance operations such as clock removal or re-introduction. The second main evolution proposed for the Galileo CMCU is the implementation of barriers which should guarantee that any clock used for time scale generation does not suffer of anomalies. For the detections of anomalies affecting the clock stability (alteration of the Allan Variance), similar performances for the Dynamic Allan Deviation (DADEV) and the Generalized Likelihood Test Ratio (GLRT) algorithms (see [14]) have been observed. A similar work, now devoted to the detection of anomalies affecting the clock trend like frequency jumps, has shown that the averaged instantaneous frequency (AIF) algorithm was more sensitive than the GLRT algorithm. Note that these results are only preliminary because they are based on a reduced set of test scenarios. The final part will focus on the implementation of this new CMCU concept on-board the satellite. Two candidate architectures will be described, as well as their main characteristics. The technology readiness for their implementations will also be addressed. The overall objective of this paper is to show that the implementation of a time scale on-board the satellite together with efficient barriers for the detection of clock anomalies will improve the accuracy (better stability), the integrity (prompt detection) and availability (no satellite isolation required) of the critical timing sources of the Galileo system.
Published in: Proceedings of the 2010 International Technical Meeting of The Institute of Navigation
January 25 - 27, 2010
Catamaran Resort Hotel
San Diego, CA
Pages: 880 - 894
Cite this article: Soualle, F., Beck, T., Trautenberg, H., Felbach, D., Stopfucken, L., Wendel, J., Fernandez, Francisco Amarillo, Fernandez, A., Nogales, M. Sanchez, "New Concept for the On-Board Master Clock Generation Unit for Future Galileo Satellites," Proceedings of the 2010 International Technical Meeting of The Institute of Navigation, San Diego, CA, January 2010, pp. 880-894.
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