On-Board Signal Monitoring Unit for Future Galileo Satellites

F. Soualle, T. Beck, H. Trautenberg, D. Felbach, L. Stopfkuchen, J-J. Floch, F. Amarillo Fernandez, A. Fernandez, M. Sanchez Nogales

Abstract:	Distortions affecting the transmitted navigation signals have to be regarded as one of the major threats which may lead to an erroneous receiver position. Both Galileo and EGNOS integrity concepts propose to monitor the received signals with a network of sensor stations to detect any signal quality degradation and warn promptly the user. One of the advantages of this solution is that the number of sensor stations can be scaled to fulfil the integrity requirements. However, these measurements are strongly attenuated by the free-space losses and affected by perturbations either during the signal propagation (e.g. scintillation), or at the sensor station itself (e.g. local multipath or interferences), which might degrade the detection performances. In this paper, it is therefore proposed to monitor the signal quality at the transmission source itself, i.e. on-board the satellite. Basically, a satellite Signal Monitoring Unit (SMU) captures the transmitted signal and compares it with an ideal replica to reveal any potential deviation. The algorithms applied in the SMU are derived from the signal quality monitoring (SQM) concept already used for the integrity verification of the WAAS systems: a multicorrelator GNSS receiver continuously tracks the transmitted signal with one pair of correlators, while all other correlators (including the prompt channel) sample the correlation function at different early-late spacings. Then, metrics are built by combining the different correlator samples to monitor the quality of the correlation function (flatness, asymmetry). Detection of signal distortions is achieved by comparing these metrics with thresholds adapted to the signal characteristics and to the integrity and continuity system requirements. This paper will first describe the general architecture of the SMU. Several options either for capturing the monitored signal or for the generation of the replica will be proposed. It will also be shown that the available (C/No) lies around 100 dB-Hz. This characteristic, which will significantly ease the detection of any signal distortion, will be one of the main justifications for the SQM migration from on-ground to on-board. An example based on the monitoring of both Galileo Safety of Life signals (SoL) will serve to describe the main steps of the SQM concept. This concerns the definition of the ensemble of modelled signal distortions, called “threat space”, the definition of the ensemble SoL receiver settings, called “user space” and the determination of the detection thresholds. It will be shown that 100 % of the signal distortions thus defined can be detected and that the high sensitivity of the SMU will offer more flexibility in the setting of the working point. An additional section will highlight other potential benefits of the SMU to improve the integrity performance. This especially concerns the possibility to detect any relevant degraded code carrier coherency, any relevant reduction of the Equivalent Isotropically Radiated Power (EIRP), or any corruption of the disseminated data.
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:	1002 - 1013
Cite this article:	Soualle, F., Beck, T., Trautenberg, H., Felbach, D., Stopfkuchen, L., Floch, J-J., Fernandez, F. Amarillo, Fernandez, A., Nogales, M. Sanchez, "On-Board Signal Monitoring Unit for Future Galileo Satellites," Proceedings of the 2010 International Technical Meeting of The Institute of Navigation, San Diego, CA, January 2010, pp. 1002-1013.
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