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Influence of Selected External Factors on Satellite Navigation Signal Quality

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Influence of Selected External Factors on Satellite Navigation Signal Quality Safety and Reliability – Safe Societies in a Changing World – Haugen et al. (Eds) © 2018 Taylor & Francis Group, London, ISBN 978-0-8153-8682-7 Influence of selected external factors on satellite navigation signal quality K. Krzykowska, M. Siergiejczyk & A. Rosiński Warsaw University of Technology, Warsaw, Poland ABSTRACT: Signal monitoring is one of the basic tasks, which are included in the satellite system maintenance. Currently, the civil aviation, in terms of navigation, above all, develops solutions based on satellites, indicating them as future-orientated. This activity is coordinated by the ICAO (International Civil Aviation Organization), which oversees the operations of the Global Navigation Satellite System (GNSS). The analysis of satellite system errors is a major aspect limiting the operational functioning of such systems in air transport. From the point of view of this study, the tropospheric and ionospheric errors deserve special attention. It turns out that the time of year and even time of day can have a signifi- cant impact on the quality of the satellite signal and, therefore, on the operational safety of aircraft. Rela- tionships occurring between selected external factors (temperature, pressure, cloudiness, precipitation, air humidity) and their very effect on the signal interferences—will be tested using fuzzy reasoning. 1 INTRODUCTION ability for a determined position should be at least 95% – the measurement error is then within The high safety level in aviation is placed on top the specified accuracy; of the pyramid of industrial challenges for mod- • integrity – is characterized as a measure of con- ern operators and service providers’. The rationale fidence in the validity of information provided for the selection of the research problem is the fact by a system; it covers the capability of a system that the satellite systems are considered to be the to deliver appropriate warnings (alarms) to a future of navigation and surveillance in aviation. user within a predetermined time, which include Failure to meet the requirements set out for satellite information on when not to use the system; signals prevents their operational use (Siergiejczyk • continuity – is the ability of a system to utilize & Krzykowska 2014). The satellite systems play a the assumed function without unplanned inter- significant role in programmes relating to the devel- ruptions during an executed flight operation; opment of the aviation technology, including the • availability – can be defined as a percentage of SESAR programme (Single European Sky ATM time, during which a satellite system can be used Research), which is a technological component for navigation, and during which reliable infor- of the SES (Single European Sky) project imple- mation is passed on to the crew, a control system mented in the EU (Kierzkowski & Kisiel 2016). The or other aircraft flight management systems. conditions set out for the use of satellite systems The requirements in relation to accuracy indi- in, for example, air traffic operations are, therefore, cate that in a large set of independent samples, at associated with four defined, main signal parame- least 95% should meet specified conditions (stated ters: accuracy, availability, continuity and integrity. in metres, per each satellite system type). Such accu- racy must be satisfied in relation to the worst geom- etry of a satellite constellation, for which the system 2 SATELLITE SIGNAL PARAMETERS is to be available. It should be noted that position errors, in the case of, e.g. a GPS system, consist of The satellite signal used in aviation is subject to par- satellite clock and ephemeris errors. They do not ticularly stringent functional requirements (Siergie- include ionospheric and tropospheric delays, multi- jczyk et al. 2015). Therefore, it is crucial to satisfy path errors or self-receiver noise. The latter are in them. The aforementioned requirements are deter- each case included in the standards regarding receiv- mined with navigational parameters (International ers (International Civil Aviation Organization 2006). Civil Aviation Organization 2006). They include: In the context of integrity, in order to deter- • accuracy – defined by an error in determined mine whether a location error is acceptable—an position; in GNSS it is the difference between alarm limit is specified, which allows to reflect the the determined and actual position; the prob- maximum, permissible position error that will not 701 undermined the executed flight operation. It should c. covering an area with primary and secondary be noted that satellite system navigation, thus, a surveillance; satellite signal, is simultaneously transmitted to d. guidance procedures to another airport; many objects (aircraft) over a large area—often one e. navigation system used at a back-up airport; or more continents. Therefore, the impact of los- f. duration of interruptions in signal availability; ing integrity of a satellite system on an air traffic g. geographical range of interruptions. management system will be much more significant In addition, according to the International Civil than in the case of conventional navigation meth- Aviation Organization, GNSS availability should ods. Hence, the stringent requirements regarding be determined through engineering, analysing and the parameters. An information about the loss of modelling processes, and not only by measuring signal integrity (or exceeding the permissible values them. The signal availability model should take of other parameters) delivered sufficiently early, into account, among others, ionospheric and trop- should result in an abandonment of using satellite ospheric errors, as well as receiver faults, which it navigation or discontinuation of the operation (in utilizes to determine integrity via calculated HPL case of a take-off or landing). Furthermore, an indi- (Horizontal Protection Level) and VPL (Vertical vidual, as well as a unique GNSS navigation feature Protection Level) indicators (Januszewski 2012) is adapting the navigation capabilities over time, (Januszewski 2013). depending on the changing satellite constellation. The impact of changes in the space segment may be increased with an additional fault in the ground seg- 3 LITERATURE STATE OF THE ART ment, e.g., damage to one of the components (Inter- national Civil Aviation Organization 2006). The research problem in the presented paper is not In the case of en-route, approach and landing only the analysis of satellite signal parameters, but operations—the continuity of the service is associ- also a search for external factors causing interfer- ated with the capability of a navigation system to ence of that signal. Weather conditions will be cer- deliver output data with a specified integrity and tainly among those observed. The matter and its accuracy over the course of the operation, assum- essence are already known in the domestic subject ing that the data were available at the beginning of literature. R. Zieliński sets forth the issue of thermal the operation. Due to the fact that the length of noise and their presences in Earth—satellite and sat- individual operations is variable, the requirement ellite—Earth links (Zieliński 2009). Ground receiv- regarding the continuity is defined as a range of ing antenna receives noises through sky luminance signal discontinuity probability values per hour. temperature (sky radiation), whereas for a satellite The bottom range value is the minimum continuity antenna—the noises are the Earth’s surface with a value, at which a system may be used in areas with defined thermodynamic temperature. Attention was low traffic and a complex airspace structure (these also paid to additional losses arising as a result of are areas with a low number of navigation system precipitation. Signal attenuation they cause, depends failure per the number of aircraft). The top value on the extent of the precipitation itself, most often enables the application in area with heavy traffic expressed in mm/h. The European Broadcasting and a complex airspace structure (these are areas Union elaborated on measurement results, which with a higher number of navigation system failures present the phenomenon of signal attenuation, per the number of aircraft). It is worth noting that depending on the magnitude of precipitation, for flight planning may not be approved if it is based the frequency of 11.5 GHz. This statistic provides solely on GNSS navigation, in which a signal is attenuation distribution function values expressed burdened with a high risk of a continuity loss at in dB for 99% and 99.9% of the time for the worst the time of planning the executed operation (Inter- month in Europe. The dependency of meteorologi- national Civil Aviation Organization 2006). cal conditions and GNSS in the context of the posi- Defining the requirements concerning GNSS tion determination accuracy can be also found in availability should be considered in terms of the elaboration of renowned scientific journals (Wilgan expected level of the provided service. Certain et al. 2015). However, in most cases, the impact of requirements will be set out for a system, which is weather on satellite signal propagation has been to replace the existing navigation infrastructure, described in the literature in light of tropospheric and different ones for a system supporting the cur- errors. The following factors are determined: dry-air rent infrastructure (International Civil Aviation density, pressure and temperature, and
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