Influence of Selected External Factors on Satellite Navigation Signal Quality

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 significant 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 accuracy 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 humid-air Organization 2006). Basically, the determination humidity caused by clouds, rain, fog. of a GNSS signal availability criterion for given It should be noted that the analysis of external operations or areas, should be based on: factors, including meteorological, is of large sig- nificance also in the case of other fields of air traf- a. traffic intensity and complexity; fic. The elaborations include studies on, inter alia, b. the presence of back-up navigation aids; the impact of meteorological conditions on the

702 execution of aircraft landing operations or model- process, i.e., which control to choose for a current ling external factors in the context of air traffic. output, is available (Kacprzyk 2001). These publications evidence the great importance The construction of a model and conducting the of different factors present in air traffic. The fact aforementioned tests is subject to having a suffi- that meteorological factors should not be underes- cient number of data (Siergiejczyk & Krzykowska timated in any of the air traffic fields seems crucial 2017). The satellite signal analysis was based on (Rychlicki & Miszkiewicz 2013). EGNOS system data. A total of 181 measure- In light of the received satellite signal, one ments for a PRN120 satellite, conducted in the should adopt a relevant correction, taking into 1st half of 2014 at a station in Warsaw for an account its passage through different layers of the APV-1 vertical guidance approach operation shall atmosphere. For example, in order to determine a be used for constructing the model. The selection correction taking into account the passage of the of this period, area and satellite is justified by the signal through the ionosphere, a vertical compo- completeness of data, which is compulsory in nent of electron density TEC [el/m2] is adopted. such studies—conditioning high reliability of the TEC (Total Electron Content) is the total number results. By agreement with the Institute of Mete- of electrons concentrated between two points, orology and Water Management in Warsaw, a list along a column with a cross-section of 1 m2. of weather condition measurements (cloud cover, The studying of the TEC variable, its modelling humidity, precipitation, pressure and temperature) and forecasting became a popular issue for con- was received, for the same period of time and area. temporary science in the context of the satellite The Space Research Centre of the Polish Acad- technology development (Rius et al 1994). Due to emy of Sciences publicizes the data on solar activ- the demonstrated significant impact of the TEC ity, including the number of sunspots, number of value on the quality of the received satellite sig- observations, standard deviations. That data was nal, a lot of attention was focused on TEC pre- also used in the research. dictions, also via artificial neural networks (Paul The article presents a model regarding the satel- & Sur 2013). lite signal accuracy. Therefore, input data was iden- tified. It included: • cloud cover; 4 RESEARCH METHODOLOGY • air humidity; • precipitation; The use of fuzzy sets for controlling and model- • average air temperature; ling processes has a long history. Originally, the • atmospheric pressure; creators of fuzzy logic saw their theory in fields • solar activity expressed as the number of sun- such as economics or psychology, where human spots (daily sunspot number). perception plays a crucial role, and the phenom- ena can be described in an unclear manner, thus, At the same time, it can be concluded that a fuzzy one. However, already in the 1970s, the cloud cover, humidity and precipitation form a possibility to control processes through this tool first group of input data, which determines the was observed. Nowadays, the greater part of its humid tropospheric part, associated with water application concerns controlling, quite often, vapour content. Two other factors-temperature technical systems (Żurek & Grzesik 2015) (Rob- and pressure constitute a fragment of the dry part inson et al. 2005) (Skorupski & Uchroński 2016) of the troposphere and form a second data group. (Stańczyk & Stelmach 2014) (Losurdo et al. According to the literature (Wołoszyn 2009), the 2017). dry factor causes almost 90% of the tropospheric In principle, the literature sources define two error (delay). The last, but not least, factor-solar approaches towards fuzzy control-descriptive and activity, forms a third group of input data asso- prescriptive (Zadeh 2008). The first one is based ciated with the ionosphere and ionospheric delay on the expertise of an operator who, based on his (Siergiejczyk & Krzykowska 2017). experience, knows how to control a process. It is a Nonetheless, the determination of an output traditional approach, not based on a model. The variable remains important. It was an accuracy second approach assumes the existence of a sto- error, determined by the average value of HNSE chastic or deterministic model, and defines how, in and VNSE. an optimal manner, to control it. The traditional The next table shows a summary of linguistic approach is closer to the studies in this paper. This values for all variables and their assigned numeri- is due to the fact that a model of a process defining cal values, representing the total membership the output (satellite signal interference) as an input to a fuzzy set. This classification, for input vari- function (weather condition) is not known. This ables, was based on known literature of the sub- means that the process is a, so-called, black box. ject (Wołoszyn 2009) (Moszkowicz & Tuszyńska However, knowledge on how to correctly control a 2006). The definition of output variable ranges

703 Table 1. Linguistic values of input and output variables.

I/O data Linguistic variable Linguistic value

I1 Cloud cover - small average big -

I2 Humidity v. low low average high v. high

I3 Precipitation - small average big -

I4 Temperature - low average high -

I5 Pressure - low average high -

I6 Solar activity v. low low moderate high v. high O Accuracy error - small average big -

Table 2. The function of linguistic variable membership.

I/O Measurement Numerical data Measure value range value

I1 cloud cover 0–8 0 (in octane 4 scale) 8

I2 % 42–100 42 57 72 Figure 1. Graphic representation of the impact of tem- 86 perature and solar activity on the accuracy error. 100

I3 mm water 0–25 0 head 12.5 25

I4 average air (-15) – 24 -15 temperature 4.5 in °C 24

I5 hPa 985–1024 985 1005 1024

I6 sunspot 40–222 40 number 87 Figure 2. Graphic representation of the impact of 110 cloud cover and precipitation on the accuracy error. 150 222 O average HNSE 5–8 5 significant impact on the accuracy of a satellite and VNSE 6.5 signal. value in 8 In the presented model, solar activity (I ) was metres 9.5 6 more important than temperature (I4), nonetheless, the impact of low temperature on the increase of the accuracy error is noticeable. The configu- was solved otherwise. It applied the expertise and ration of precipitation (I3) with cloud cover (I1) applicable requirements regarding the use of a and their impact on the satellite signal is interest- satellite signal in civil aviation imposed by ICAO ing. Figure 2 shows an increase of the accuracy (International Civil Aviation Organization 2006). error in conditions of big and small cloud cover The input and output variables were assigned and rather low precipitation. This leads to a very the Gaussian membership function according to important conclusion—I are able to indicate the the ranges given in Table 2. 126 rules were set out. day, during which it was not the weather factor, which impacted the increased signal accuracy

error, because pressure changes (I5) remained 5 RESEARCH RESULTS insignificant, whereas humidity (I2) contributed to the error to a small extent. Such was the situ- The following visualizations present selected con- ation, for example, on 14 March 2014, when the figurations of atmospheric factors with the most applied principle (no. 78) had the following form:

704 If the cloud cover is average and humidity is Equatorial Ionization Anomaly in the Indian longitude very low and precipitation is low and temperature sector during the low and moderate solar activity levels is average and pressure is average and solar activity of the 24th(...), Advances in Space Research, tomvol. 52, is moderate then the accuracy error is high. no. 5. Rius, A. & Zarraoa, N. &Sardón, E.1994. Estimation of In most other cases, when the accuracy error the transmitter and receiver differential biases and the reached a linguistic value of “high”, solar activity ionospheric total electron content from Global Position- would assume a linguistic value of “high” or “very ing System observations. Radio Science, vol. 29, no. 3. high”. The presented example, in some ways, con- Robinson, V.B., Cobb, M.A. & Petry, I.E. 2005. Fuzzy firms the effectiveness of the model and the tool. Modeling with Spacial Information for Geographic Problems. Springer. Rychlicki, M. & Miszkiewicz, A. 2013. Ocena dokładności 6 SUMMARY danych lokalizacyjnych odbiorników GPS. [The assess- ment of GPS receiver localization data accuracy] Prace The described cases are undoubtedly a big advan- Naukowe Politechniki Warszawskiej. Transport, no. 92. tage of the model. In many cases, the knowledge Siergiejczyk, M., Krzykowska, K & Rosiński, A. Evalua- tion of the influence of atmospheric conditions on the that the error source should not be sought among quality of satellite signal, Proceedings of the Conference weather conditions is more important. Especially, on Marine Nagivation and Safety of Sea Transporta- if the factors are not accompanied by explicit- tion (TransNav 2017) Gdynia, Poland, 21–23 June 2017, ness regarding the impact on signal interference. Editor: Adam Weintrit, CRC Press Taylor&Francis However, it can be concluded that the tool and the Group, London, UK 2017. 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