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Multipath Propagation, Mitigation and Monitoring in the Light of Galileo and the Modernized GPS

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Multipath Propagation, Mitigation and Monitoring in the Light of Galileo and the Modernized GPS Dissertation Multipath Propagation, Mitigation and Monitoring in the Light of Galileo and the Modernized GPS eingereicht von Dipl.-Ing. Markus Irsigler Vollständiger Abdruck der an der Fakultät für Luft- und Raumfahrttechnik der Universität der Bundeswehr München zur Erlangung des akademischen Grades eines Doktors der Inge- nieurwissenschaften (Dr.-Ing.) eingereichten Dissertation. Vorsitzender: Prof. Dr. rer. nat. Bernd Häusler, (Chairman) Institute of Space Technology, Bundeswehr University Munich 1. Berichterstatter: Univ.-Prof. Dr.-Ing. Günter W. Hein , (1st reviewer) Institute of Geodesy and Navigation, Bundeswehr University Munich 2. Berichterstatter: Univ.-Prof. Dr.-Ing. Bernd Eissfeller , (2nd reviewer) Institute of Geodesy and Navigation, Bundeswehr University Munich 3. Berichterstatter: Prof. Dr. Per K. Enge , (3rd reviewer) Dept. of Aeronautics and Astronautics, Stanford University Die Dissertation wurde am 4. April 2008 bei der Universität der Bundeswehr München, Werner-Heisenberg-Weg 39, 85577 Neubiberg eingereicht und durch die Fakultät für Luft- und Raumfahrttechnik am 23. April 2008 angenommen. Tag der mündlichen Prüfung: 18. Juli 2008. Abstract Abstract Content of Thesis Among the numerous potential sources of GNSS signal degradation, multipath takes on a prominent position. Unlike other errors like ionospheric or tropospheric path delays which can be modeled or significantly reduced by differential techniques, multipath influences cannot be mitigated by such approaches. Although a lot of multipath mitigation tech- niques have been proposed and developed in the past – among them many receiver inter- nal approaches using special signal processing algorithms – multipath (especially multipath with small geometric path delays) still remains a major error source. This is why multi- path has been a major design driver for the definition of the Galileo signal structure car- ried out in the past years and the subsequent signal optimization activities. This thesis tries to provide a broad and comprehensive insight into various aspects of mul- tipath propagation, mitigation and monitoring (without claiming to be exhaustive). It con- tains an overview of the most important aspects of multipath propagation , including the discussion of different types of multipath signals (e.g. specular vs. diffuse multipath, sat- ellite vs. receiver multipath or hardware-induced multipath), typical characteristics such as periodic signal variations whose frequency depends on the satellite-antenna-reflector geometry and the impact on the signal tracking process within a GNSS receiver. A large part of this thesis is dedicated to aspects of multipath mitigation , first providing a summary of the most common multipath mitigation techniques with a special focus on receiver-internal approaches such as the narrow correlation technique, double-delta cor- relator implementations, the Early-Late Slope (ELS) technique or Early/Early tracking im- plementations. However, other mitigation approaches such as using arrays of closely spaced antennas or multipath-limiting antennas are discussed as well. Some of these techniques are used for subsequent multipath performance analyses considering signals of the (modernized) GPS and Galileo. These analyses base on a new methodology to estimate typical and meaningful multipath errors making use of multipath error envelopes that are scaled in a suitable way to account for different multipath environments. It will be shown that typical (mean) multipath errors can be derived from these scaled envelopes by com- putation of the envelope’s running average and that these mean multipath errors are of the same order as multipath errors obtained from complex statistical channel models. Another part of this thesis covers various aspects of multipath detection and monitoring . First, current techniques for multipath detection and monitoring are described and dis- cussed with respect to their benefits and drawbacks or their real-time capability. Among the considered approaches are techniques like “code minus carrier” monitoring, SNR monitoring, the use of differenced observations or spectral and wavelet analysis. Follow- ing this introductory overview, a completely new approach for real-time multipath moni- toring by processing multi-correlator observations will be introduced. Previously being used primarily for the detection of Evil Waveforms (signal failures that originate from a malfunction of the satellite’s signal generation and transmission hardware), the same basic observations (linear combinations of correlator outputs) can be used for the devel- opment of a multi-correlator-based real-time multipath monitoring system. The objective is to provide the user with instant information whether or not a signal is affected by mul- tipath. The proposed monitoring scheme has been implemented in the form of a Matlab- based software called RTMM (Real-Time Multipath Monitor) which has been used to verify the monitoring approach and to determine its sensitivity. 2 Abstract Major Scientific Contributions and Achievements The major scientific contributions and achievements provided through this thesis can be summarized as follows: General achievements . The majority of the multipath-related knowledge is spread in conference papers, theses and journal articles. There is almost no textbook that covers this topic in a comprehensive and exhaustive way. This thesis combines a multitude of important aspects of multipath propagation, mitigation and monitoring in one coherent piece of work. Scientific Contributions . Besides the general achievement of carrying together all impor- tant multipath-related work, the thesis contains the following significant scientific con- tributions: • Detailed analysis of occurring frequencies of multipath oscillations for different geo- metric conditions. Expected frequencies are computed and visualized for ground mul- tipath scenarios and for varying reflector locations (see section 3.1.4, “The Frequency of Multipath Variations”, p. 37 - 44). • Analysis of the suitability of the carrier smoothing approach for the purpose of multi- path mitigation and elaboration of limitations and shortcomings of this approach. It is shown that carrier smoothing does not ensure efficient multipath mitigation in any situation (see section 4.1.7, “Carrier Smoothing”, p. 66 - 72). • Development of a new and very efficient methodology to obtain typical and meaning- ful multipath errors from common multipath error envelopes. The proposed method- ology makes use of the concept of scaling the envelopes in a suitable way to account for different multipath environments. The typical (mean) multipath errors that can be derived from these scaled envelopes are of the same order as multipath errors ob- tained from complex statistical channel models (see section 5.1, ”Criteria for Multi- path Performance Assessment”, p. 81 - 97). This methodology was used to compute expected multipath errors for the future Galileo signals and the signals of the mod- ernized GPS (see sections 5.2 and 5.3, “Expected Code Multipath Errors”,” Expected Carrier Multipath Errors”, p. 97 - 104). • Development of a real time multipath monitor based on multi-correlator observations. The proposed monitoring approach allows instant detection of multipath signals and can either be used to detect multipath-affected observations and exclude them from the subsequent positioning process or to de-weigh the affected observations. By de- termining the optimum metric (i.e. a suitable combination of correlator peak observa- tions), the monitor can be made very sensitive, so that even weak multipath signals can be detected (see section 7, “Development of a Real-Time Multipath Monitor (RTMM)”, p. 122 - 195). The development of this method is the principal topic of this thesis. 3 Zusammenfassung Zusammenfassung Inhalt der Arbeit Die Qualität eines Satellitensignals wird durch eine Vielzahl potenzieller Fehlerquellen negativ beeinflusst. Neben atmosphärischen Einflüssen wie ionosphärischen oder tro- posphärischen Laufzeitfehlern tragen Mehrwegeeinflüsse einen wesentlichen Anteil zum Gesamtfehlerbudget der Satellitennavigation bei. Während eine ganze Reihe von Fehler- einflüssen (z.B. die bereits erwähnten atmosphärischen Fehler) durch geeignete Modellie- rung oder differenzielle Verfahren deutlich reduziert werden können, ist dies durch die räumliche Dekorrelation der Mehrwegeeffekte nicht möglich. Obwohl in der Vergangen- heit eine Vielzahl von Verfahren zur Mehrwegereduzierung vorgeschlagen und entwickelt wurden – darunter eine Reihe spezieller empfängerinterner Signalprozessierungsalgo- rithmen – stellen Mehrwegesignale noch immer eine wesentliche, stets zu berücksichti- gende Fehlerquelle dar. Aus diesem Grund spielten die zu erwartenden Mehrwegefehler auch eine sehr wichtige Rolle im Zuge der Definition sowie der Optimierung der Galileo- Signalstruktur und können somit als wesentliches Design-Kriterium angesehen werden. Die vorliegende Arbeit gibt einen umfassenden Einblick in verschiedene Aspekte der Mehrwegeausbreitung, -reduzierung sowie der Detektion und der Überwachung (Monito- ring) auftretender Mehrwegeeffekte, ohne dabei allerdings Anspruch auf Vollständigkeit zu erheben. Die Arbeit beschreibt zunächst die wichtigsten Aspekte der Mehrwegeaus- breitung , wobei beispielsweise unterschiedliche Arten von Reflexionen (gerichtete vs. diffuse Reflexion) oder unterschiedliche Entstehungsarten (Reflexionen am Satelliten vs. Reflexionen in der Empfängerumgebung sowie
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