In recent years, a growing interest has been witnessed in the usage of free space optics (FSO) link for satellite communication (SATCOM) scenarios, as it offers much higher data rates up to gigabits per second (Gbps) compared to existing radio frequency (RF) link. However, FSO links are sensitive to beam scintillation and pointing errors. In this paper, we consider a hybrid FSO/RF communication between ground station (GS) and satellite, where the RF link will act as a backup link to improve the reliability of FSO communication. In addition, we also consider high-altitude platform station (HAPS), which will act as a relay station, between GS and satellite to improve the end-to-end system performance. This has led to the development of space-air-ground integrated hybrid FSO/RF SATCOM networks. We analyse the performance of the proposed hybrid network considering an adaptive-combining-based switching scheme for both uplink and downlink scenarios with and without using HAPS as a relay station. In case of adaptive-combining-based switching scheme, the data is continuously transmitted over the FSO link, while maximal-ratio-combining (MRC) of RF and FSO links is performed when the quality of FSO link deteriorates. The performance analysis of adaptive-combining-based switching scheme in terms of outage and average symbol error rate (SER) is carried out and the same is compared with the single-link FSO SATCOM and single-threshold-based switching scheme proposed in the literature for hybrid FSO/RF SATCOM. In addition, the performance gain obtained by the proposed adaptive combining scheme over single-link FSO system for different channel conditions is also reported. Further, the asymptotic analysis is also carried out to obtain the diversity gain of the proposed system. ; Ministry of Education (MOE) ; Published version ; This work was supported in part by the Start-up Research Grant (SRG) of Science and Engineering Research Board (SERB), Department of Science and Technology, Government of India, and in part by the Singapore Ministry of Education Academic Research Fund Tier 1 grants.
Trabalho final de Mestrado para obtenção do grau de Mestre em Engenharia de Electrónica e Telecomunicações ; O presente trabalho estuda uma solução alternativa de navegação aeronáutica que contribua para a racionalização da infrastrutura terrestre de ajudas-rádio de navegação na Europa. O conceito designado de "Performance Based Navigation (PBN)" emerge actualmente ao nível da Organização Internacional de Aviação Civil, visando o aperfeiçoamento do sistema de gestão do tráfego aéreo ao nível da eficiência, segurançae capacidade. O conceito PBN promove a modernização da infrastrutura aeronáutica com base na utilização preferencial de sistemas de navegação por satélite, designadamente mediante o recurso a sinais disponibilizados pelas constelações "Global Navigation Satellite System (GNSS)". Face às vulnerabilidades dos sistemas GNSS a interferências RF, "jamming" deliberado ou fenómenos solares, foi decidido manter uma infrastrutura de recurso/"backup", para mitigar falhas GNSS, baseada numa redede rádio-ajudas terrestres "Distance Measuring Equipment (DME)". Visto que estes DMEs não facultam uma boa cobertura, especialmente a baixa altitude, e tratando-se de equipamentos próximos da obsolescência tecnológica e pouco eficientes em termos de espectro rádioeléctrico, a sua racionalização requer uma tecnologia alternativa. O presente trabalho explora o recurso a novas tecnologias aeronáuticas de comunicações dados ar-solo, designadamente o futuro "data link" OFDM/TDMA de banda L (LDACS), verificando a sua adequação para suportarem as funções de navegação descritas substituindo os DMEs. Pretende-se confirmar a viabilidade com base no conceito de Navegação Relativa (RELNAV) usado em contexto militar recorrendo a filtros Kalman. As características da tecnologia LDACS são descritas e são apresentados resultados de testes do seu desempenho em termos de medição de distâncias ("ranging"). Com base nas capacidades RELNAV militares são propostos melhoramentos baseadosem filtros Kalman, simulando para demonstrar que o LDACS ...
Global navigation satellite system (GNSS) spoofing poses a significant threat to maritime logistics. Many maritime electronic devices rely on GNSS time, positioning, and speed for safe vessel operation. In this study, inertial measurement unit (IMU) and Doppler velocity log (DVL) devices, which are important in the event of GNSS spoofing or outage, are considered in conventional navigation. A velocity integration method using IMU and DVL in terms of dead-reckoning is investigated in this study. GNSS has been widely used for ship navigation, but IMU, DVL, or combined IMU and DVL navigation have received little attention. Military-grade sensors are very expensive and generally cannot be utilized in smaller vessels. Therefore, this study focuses on the use of consumer-grade sensors. First, the performance of a micro electromechanical system (MEMS)-based yaw rate angle with DVL was evaluated using 60 min of raw data for a 50 m-long ship located in Tokyo Bay. Second, the performance of an IMU-MEMS using three gyroscopes and three accelerometers with DVL was evaluated using the same dataset. A gyrocompass, which is equipped on the ship, is used as a heading reference. The results proved that both methods could achieve less than 1 km horizontal error in 60 min.
This paper presents a novel Global Navigation Satellite System (GNSS) Avionics Based Integrity Augmentation (ABIA) system architecture suitable for civil and military air platforms, including Unmanned Aircraft Systems (UAS). Taking the move from previous research on high-accuracy Differential GNSS (DGNSS) systems design, integration and experimental flight test activities conducted at the Italian Air Force Flight Test Centre (CSV-RSV), our research focused on the development of a novel approach to the problem of GNSS ABIA for mission- and safety-critical air vehicle applications and for multi-sensor avionics architectures based on GNSS. Detailed mathematical models were developed to describe the main causes of GNSS signal outages and degradation in flight, namely: antenna obscuration, multipath, fading due to adverse geometry and Doppler shift. Adopting these models in association with suitable integrity thresholds and guidance algorithms, the ABIA system is able to generate integrity cautions (predictive flags) and warnings (reactive flags), as well as providing steering information to the pilot and electronic commands to the aircraft/UAS flight control systems. These features allow real-time avoidance of safety-critical flight conditions and fast recovery of the required navigation performance in case of GNSS data losses. In other words, this novel ABIA system addresses all three cornerstones of GNSS integrity augmentation in mission- and safety-critical applications: prediction (caution flags), reaction (warning flags) and correction (alternate flight path computation).
The European Rail Traffic Management System/European Train Control System (ERTMS/ETCS) has been developed by the European Union to enhance safety and efficiency of the railway transportation. So far, the positioning and monitoring of the trains have been based mainly on trackside equipment named balises: the introduction of Global Navigation Satellite Systems (GNSS) to implement an absolute positioning system on trains could lead to a better utilization of the line with a consequent line capacity increase, along with reliability increasing and cost reducing due to the removal of expensive infrastructures. This paper describes the insertion of a state-of-the-art channel model, named Land Mobile Satellite Channel Model (LMSCM) properly adapted to represent railways scenarios, into a hardware-in-the-loop system, where the parameters generated by LMSCM are used to configure an RF GNSS signal generator. The use of the hardware generator eases the simulation of the entire GNSS constellation, while the availability of the RF signal enables the assessment of the effects of rail-domain multipath on commercial receivers. The resulting model, named Rail-Land Mobile Satellite Channel Model (Rail-LMSCM), has been enhanced with the support of different kind of scenarios a train may encounter around the rail track, as well as with the introduction of GNSS outages due to the presence of tunnels. Such enhancements make the Rail-LMSCM a more realistic average representation of the railway environment. The implemented Rail-LMSCM has been used to test in the lab the local propagation effects on the performance of a complete GNSS receiver.
Global navigation satellite system (GNSS) spoofing poses a significant threat to maritime logistics. Many maritime electronic devices rely on GNSS time, positioning, and speed for safe vessel operation. In this study, inertial measurement unit (IMU) and Doppler velocity log (DVL) devices, which are important in the event of GNSS spoofing or outage, are considered in conventional navigation. A velocity integration method using IMU and DVL in terms of dead-reckoning is investigated in this study. GNSS has been widely used for ship navigation, but IMU, DVL, or combined IMU and DVL navigation have received little attention. Military-grade sensors are very expensive and generally cannot be utilized in smaller vessels. Therefore, this study focuses on the use of consumer-grade sensors. First, the performance of a micro electromechanical system (MEMS)-based yaw rate angle with DVL was evaluated using 60 min of raw data for a 50 m-long ship located in Tokyo Bay. Second, the performance of an IMU-MEMS using three gyroscopes and three accelerometers with DVL was evaluated using the same dataset. A gyrocompass, which is equipped on the ship, is used as a heading reference. The results proved that both methods could achieve less than 1 km horizontal error in 60 min.
The last decade has witnessed a wide spread of small drones in many civil and military applications. With the massive advancement in the manufacture of small and lightweight Inertial Navigation System (INS), navigation in challenging environments became feasible. Navigation of these small drones mainly depends on the integration of Global Navigation Satellite Systems (GNSS) and INS. However, the navigation performance of these small drones deteriorates quickly when the GNSS signals are lost, due to accumulated errors of the low-cost INS that is typically used in these drones. During GNSS signal outages, another aiding sensor is required to bound the drift exhibited by the INS. Before adding any additional sensor on-board the drones, there are some limitations that must be taken into considerations. These limitations include limited availability of power, space, weight, and size. This paper presents a novel unconventional method, to enhance the navigation of autonomous drones in GNSS denied environment, through a new utilization of hall effect sensor to act as flying odometer &ldquo ; Air-Odo&rdquo ; and vehicle dynamic model (VDM) for heading estimation. The proposed approach enhances the navigational solution by estimating the unmanned aerial vehicle (UAV) velocity, and heading and fusing these measurements in the Extended Kalman Filter (EKF) of the integrated system.
The European Union (EU) aims at making railway a more attractive transportation method by improving its efficiency and reducing its costs. These achievements could be covered with the migration from ETCS level 2 to ETCS level 3. Many projects related to new positioning systems have been funded by The European Union. Most of these positioning systems are based on GNSS, due to the key role that GNSS will play in the migration to ETCS level 3. One of the problems on using only GNSS systems is the lack of availability of them. During railway operation, there are areas with potential GNSS outages, such as urban canyons, woods or other possible signal blockers and disturbances. Moreover, it is a fact that GNSS signals are not reachable, nor reliable in tunnels or indoor environments. For GNSS to be able to have a key role in the next years in railway security, the afore mentioned lack of availability has to be solved. To cope with this issue, a multi-sensor approach with software enhancements is proposed in this dissertation. The objective of this research work deals with fusing different sensors and creating new software strategies to achieve a higher availability with the best possible accuracy. The seamless position will benefit in all the operation modes, from the train station to a harsh environment for satellites, during the train operation. The scope of the dissertation is to create a multi-sensor positioning system including GNSS, Inertial Measurement Unit (IMU), and Ultra Wide Band (UWB) with other software techniques to obtain a position estimation with a 100% availability for railway systems. This work shows the different steps from the study of the state of the art, going through the implementation, and ending with the performance analysis of the algorithm developed. This research work has been conducted under different European projects such as ERSAT-EAV, FR8RAIL or X2Rail-2, in which CEIT has participated.
En stigende efterspørgsel efter satellitbaserede kørselsafgiftssystemer er på vej i Europa. Satellitbaserede kørselsafgifter omfatter opkrævning af trafikanterne for deres vejforbrug ved at at lade køretøjerne bestemme deres position indenfor et givent afgiftsområde ved hjælp af Global Navigation Satellit Systemer (GNSS). Den forskning, der præsenteres i denne afhandling, beskæftiger sig med performanceniveauet samt de teknologiske udfordringer ved bestemmelse af køretøjets placering inden for GNSS-baserede kørselsafgiftssystemer. GNSS-baserede kørselsafgiftssystemer kan antage en række forskellige former. Afhængigt af systemets formål kan disse kørselsafgiftssystemer være udformet på forskellig vis og varieret efter både politiske og teknologiske hensyn, men de har alle til formål at taksere trafikanterne for deres brug af vejnettet. Den første del af afhandlingen præsenterer en omfattende oversigt og klassificering af forskellige former for kørselsafgiftssystemer samt mulige teknologier, suppleret med en gennemgang af forskellige eksisterende systemer. Efterfølgende defineres det grundlæggende kørselsafgiftssystem og dette sættes ind i en begrebsramme, der danner grundlag for forskningen præsenteret i denne afhandling. For at forstå GNSS-baserede kørselsafgiftssystemers struktur og virkemåde er det vigtigt at fremhæve den overordnede systemarkitektur og definere de væsentlige funktioner samt beskrive forholdet mellem dem. Arkitekturen er brugt som et middel til at strukturere diskussionen om de teknologiske udfordringer i GNSS-baserede kørselsafgiftssystemer. Afhandlingen diskuterer de overordnede krav til afgiftsprocessens ydeevne indenfor GNSS-baserede kørselsafgiftssystemer. GNSS muliggør kilometer-baseret afgiftsopkrævning, hvor afgifterne beregnes for hvert enkelt køretøj baseret på den kørte afstand, tidspunktet for turen samt bilens geografiske position. Kilometerbaseret afgiftsopkrævning betragtes derfor som en mere retfærdig og effektiv form for afgiftsopkrævning, da disse systemer opkræver afgiften i forhold til den tilbagelagte afstand, og afspejler dermed den forbrugsbaserede tilgang mere præcist end andre afgiftsopkrævningspolitikker. Men kørselsafgifter på grundlag af den tilbagelagte afstand er teknisk udfordrende og betragtes ofte som en af de mest komplekse ordninger. Fastsættelsen af den faktisk kørte distance er den centrale del af afgiftsopkrævningen, og de vigtigste betænkeligheder omkring pålideligheden vedrører derfor denne del af afgiftsprocessen samt resultaterne af køretøjets lokaliseringsfunktion. Denne afhandling præsenterer endvidere en grundig gennemgang af de forskellige GNSS-baserede forsøg og eksperimenter, der er udført inden for de seneste år, for at vurdere resultater og muligheder ved brug af GNSS-baserede afgiftssystemer. I 2007–2009 blev et teknisk forsøg med et GNSS-baseret kørselsafgiftssystem udført i København som en del af denne forskning i samarbejde med Siemens. Forsøget blev udført for at vurdere resultater og tekniske udfordringer ved GNSS-baserede kørselsafgiftssystemer baseret på den nyeste kørselsafgiftsteknologi. Denne afhandling præsenterer det udførte eksperiment og giver en vurdering af køretøjets lokaliseringsfunktion inden for GNSS-baserede kørselsafgiftssystemer. Tidligere forsøg og vurderinger af ydeevnen af GNSS-baserede kørselsafgiftssystemer har generelt fokuseret på mulighederne i afgiftssystemerne i stedet for begrænsningerne. Ofte har det ikke været klart beskrevet, hvilke fejl og mangler der eksisterede i de indsamlede data, men i stedet er de blot blevet udelukket som ugyldige data forud for de foretagne vurderinger, som derefter konkluderede, at mere fokus bør lægges på de fundne fejl. Derfor har det været bevidst i denne ph.d.-forskning, ikke at udelukke fejlagtige og forkerte data i vurderingen af ydeevnen. Resultaterne der er præsenteret i denne afhandling er baseret på alle de indsamlede data fra forsøget, i sin oprindelige form, som det ville være input til den automatiserede afgiftsberegningsproces i et kørselsafgiftssystem. Endvidere er der udviklet nye metoder til vurdering af køretøjets lokaliseringsfunktion i form af datapålidelighed og lokaliseringsfunktionens performance. Resultaterne af vurderingerne foretaget i denne afhandlingen viser, at selvom betydelige performanceforbedringer er sket i løbet af de sidste fem år, er der væsentlige udfordringer at overvinde i forbindelse med implementering og drift af GNSS-baserede kørselsafgiftssystemer. Det tekniske forsøg i dette ph.d.-studie viste sig at lide under forskellige tekniske udfordringer, som havde forskellig indvirkning på den samlede systemp˚alidelighed. På grund af disse udfordringer, omfatter data både unøjagtige og ufuldstændige informationer, og det konkluderes derfor at med disse høje niveauer af datainvaliditet og -mangel, kan data ikke anvendes i sin nuværende form som grundlag for en afgiftsberegningsproces. Disse resultater understreger betydningen af at anvende en dataprocesseringsfunktionalitet før afgiftsberegningen og vejforbrugsbestemmelsen i afgiftsprocessen. Vurderingen af køretøjets lokaliseringsfunktion viser en signifikant forskel i den ønskede lok´aliseringsperformance. Mens nøjagtighedskravet delvist var opfyldt i København, led kontinuiteten og dermed tilgængeligheden nødvendig for bestemmelsen af køretøjets lokalitet af alvorlige udfald i positioneringsdata. Disse udfald skyldtes både satellitutilgængelighed, forårsaget af ringe signalmodtagelse i byområder og lang signalerhvervelsestid hos modtageren, og endvidere de forskellige tekniske problemer og konfigurationsfejl, der opstod under forsøget. Da både satellitsynligheden og positioneringsnøjagtigheden gennem de seneste år er forbedret markant, viser resultaterne at de største udfordringer i forbindelse med bestemmelse af køretøjets lokalitet ikke som ofte antaget, er positioneringsunøjagtigheder, men snarere et højt niveau af positioneringsafbrydelser hovedsageligt forårsaget af GPS. På baggrund af resultatvurderingen konkluderes det endvidere, at de væsentligste bekymringer vedrørende den manglende tilgængelighed til køretøjets lokaliseringsbestemmelse bør være, hvordan den lange systemnedetid og konfigurationsudfaldene kan fjernes samt hvordan forekomsten af de mange GPS-udfald kan reduceres. Da dataudfald og -svigt kan påvirke bestemmelsen af den kørte afstand i kontinuerlige afgiftssystemer, giver denne afhandling mulighed for at vurdere og forstå forskellige forekomster, bidrag og effekter af udfald i relation til GNSS-baserede kørselsafgiftssystemer. I denne afhandling præsenteres en vurdering af distancebestemmelsens tolerance overfor disse forskellige positioneringsudfald. Vurderingen er foretaget på grundlag af en simuleringsmetode der er udviklet i denne afhandling. Metoden analyserer indflydelsen af positioneringsudfald i forhold til bestemmelsen af kørte kilometer i både distancebaserede og distancerelaterede kørselsafgiftssystemer. Udfaldstolerancen i afstandsbestemmelsen i begge typer afgiftssystemer er vigtig for kørselsafgiftssystemets evne til at opfylde kravene til ydeevne samt i forhold til at taksere brugerne korrekt for deres vejforbrug. Simuleringsanalyserne af indflydelsen af udfald på bestemmelsen af den kørte afstand viser at afstandsbestemmelsesfunktionen er forholdsvis robust over for småudfald på mindre end 10 sekunder i positioneringen. Men med flere mellemstore og store udfald i turene, har begge afstandsbestemmelsesmetoder svært ved med at reproducere den kørte afstand med afstandsafvigelser på mere end 1 % fra den sande værdi. Det vigtige ved disse resultater er, at den kørte afstand kan fastlægges med mindre end 1 % afvigelse for hovedparten af turene eftersom forekomsten af småudfald er hyppigst i de kørte ture. GNSS-baserede kørselsafgiftssystemer anses for at være ansvarskritiske syste-mer, hvor nægtet service samt uopdagede fejl og mangler har betydelige retslige eller økonomiske negative konsekvenser. Fejl eller mangler, der fører til ukorrekt afgiftsopkrævning, kan forårsage økonomiske tab eller føre til forkerte juridiske afgørelser, idet det økonomiske ansvar er knyttet til de juridiske aspekter som følge af potentielle klager. Derfor introducerer denne ph.d.- afhandling anvendelsen af begrebet systempålidelighed i forbindelse med GNSS-baserede kørselsafgiftssystemer. Begrebet systempålide lighed, som er tilpasset fra informationsteknologien, er en effektiv metode til håndtering af de forskellige betænkeligheder i forbindelse med kørselsafgiftssystemer inden for en fælles begrebsramme. Pålidelighed er en vigtig forudsætning for et GNSS-baseret kørselsafgiftssystem, som skal levere en retfærdig afgiftsopkrævning og sikre brugernes tillid ved at sikre systemets p˚alidelighed og ansvar. Denne afhandling diskuterer konsekvenserne af vurderingsresultaterne i forhold til systemets driftssikkerhed og præsenterer en kvalitativ risikomatrix for driftssikkerheden af køretøjets lokaliseringsfunktion. For at sikre afgiftsprocessen en høj driftssikkerhed, skal fejltolerant design derfor overvejes i forhold til de mange forskellige komponenter og funktioner der indgår i processen. Baseret på fejltolerante metoder, giver denne afhandling retningslinjer for hvordan man kan opretholde korrekt service ved tilstedeværelsen af forskellige fejl, der skyldes tekniske problemer i forbindelse med bestemmelse af køretøjets lokalitet. Hovedformålet med fejl-tolerant design inden for afgiftsprocessen er at sikre retfærdig taksering af brugerne. Det betyder, at redundante systemer, procedurer og komponenter bør indføres for at sikre, at når fejl og svigt forekommer inden for afgiftsprocessen vil kørselsafgiftsfundamentet stadig være pålideligt og give retfærdige resultater for både brugerne og kørselsafgiftssystemet. Denne afhandling konkluderer derfor, at selv om performanceniveauet for bestemmelsen af køretøjets lokalitet er fair, bør opmærksomheden omkring systemets performance rettes imod hvordan fremtidige GNSS-baserede kørselsafgiftssystemer kan designes til at virke driftsikkert med forekomsten af både data invaliditet og datamangler. Det er derfor vigtigt at udvide fokus fra tekniske udfordringer og komponentunøjagtigheder alene til et fokus på pålideligheden af systemet som helhed. Der er dog stadig nogle teknologiske udfordringer at overvinde, som i højere grad elimineres ved bedre samarbejde på tværs af de mange forskellige involverede fagområder. Som med mange andre ITS-systemer, kan vellykket design, implementering og drift af et system kun opnås, når de mange forskellige interessenter forstår hinandens krav til systemet. Systemarkitekturen som konceptuelt design sammen med den systemteoretiske metode kan bidrage til at involvere alle de forskellige parter i systemudviklingen og dermed minimere de misforst˚aelser, der i sidste ende kan blive meget dyre for systemet. Baseret på de mange resultater af dette ph.d.-studie, er nogle generelle retningslinjer endeligt formuleret for fremtidige GNSSbaserede kørselsafgifts-systemer. De foreslåede retningslinjer, der er beskrevet i afhandlingen omfatter både GNSS-baserede kørselsafgiftsforsøg i almindelighed og et fremtidigt GNSS-baserede kørselsafgiftsssystem i Danmark. ; An increasing demand for satellite-based road charging systems is developing in Europe. Satellite-based road charging involves charging road users for their road usage by allowing the vehicles to locate themselves within a certain charge area using Global Navigation Satellite Systems (GNSS). The research presented in this thesis deals with the performance and technological challenges of vehicle location determination within GNSS-based road charging systems. GNSS-based road charging systems may take on a number of different forms. Depending on the charging objective, these road charging systems can be designed in various forms and varied by both policy and technology but they all share the overall function of charging vehicle users for their road usage. The first part of the thesis presents a comprehensive overview and classification of the various forms of road charging systems and enabling technologies; supplemented with a review of different worldwide examples. Next the system fundamentals are defined and presented in a conceptual framework which forms the basis for the research presented in this thesis. In order to understand the structure and behaviour of GNSS-based road charging systems, it is important to highlight the overall system architecture and define the essential system functions and describe the relationship among them. The framework is used as a means to structure the discussion about the technological challenges of GNSS-based road charging systems. The thesis discusses the overall performance requirements for the road charging process within GNSS-based road charging systems. GNSS allows for time-distance-place charging, where charges are calculated for each individual vehicle based on the distance driven, the time of the trip and the vehicle's geographic position. Timedistance- place charging is therefore considered a more fair and efficient way of charging as these systems levy charges proportionally to the distance travelled, and thereby reflects a usage-based approach more accurately than other charging policies. However, road charging on the basis of the distance travelled is technically challenging and is seen as one of the most complex schemes. Determining the distance driven is the key part of the charging process and the main dependability concerns therefore revolve around the road charging process and the performance of the vehicle location determination function. The thesis provides a thorough review of the different GNSS-based trials and experiments conducted within recent years to assess the performance and possibilities of GNSSbased charging systems. In 2007–2009, a GNSS-based road charging experiment was conducted in Copenhagen as part of this research in cooperation with Siemens to assess the performance and technical challenges of GNSS-based road charging systems based on state of the art road charging technology. This thesis presents the experiment conducted and provides an assessment of the vehicle location determination function within GNSS-based road charging systems. Previous trials and performance assessments of GNSS-based road charging systems have generally focused on the possibilities of the charging systems rather than on the impossibilities. Often it has not been clearly described which errors and shortages existed in the collected data, but instead they have just been excluded as invalid data prior to the assessments which then concluded that more focus should be placed on the errors occurred. Hence, it has been deliberate in this PhD research not to exclude faulty and incorrect data in the assessment. The results presented in this thesis are based on all the collected data from the experiment, in its original for, as it would be used as input for the automated charge calculation process in a road charging system. Furthermore, new methodologies are developed for assessing the performance of the vehicle location determination function in terms of data reliability and navigation function performance. The results from the assessments conducted in this thesis demonstrate that although significant performance improvements have happened during the last five years, there are significant challenges to overcome in relation to implementation and operation of GNSS-based road charging systems. The technical experiment conducted in this PhD study proved to suffer from different technical challenges which had different impacts on the overall system dependability. Due to these challenges, data includes both inaccurate and incomplete data information, and it is hence concluded that with these high levels of data invalidity and deficiency, data could not be used in its current form as basis for a road charging process. These results underline the importance of a data processing functionality prior to the road charge calculation and usage determination in the road charging process. The assessment of the vehicle location determination function show significant difference in the required navigation performance. While the accuracy requirement in Copenhagen was partly met, the continuity and hence availability required for vehicle location determination suffered from severe gaps in the positioning data. These gaps were due to both satellite unavailability, caused by poor urban signal reception and long receiver acquisition times, and furthermore due to the various technical problems and configuration faults which occurred during the experiment. As both the satellite visibility and the positioning accuracy had improved significantly, the results indicate that the main challenges related to vehicle location determination are not as often stated due to positioning inaccuracies but rather due to a high level of positioning interruptions mainly caused by GPS. From the performance assessment it is furthermore concluded that the main concerns regarding the unavailability of the vehicle location determination should be how to eliminate the large downtime and configuration gaps and reduce the occurrence of the many GPS gaps. As data outages and failures may affect the determination of the distance driven in continuous charging schemes, the thesis provides means to assess and understand the positioning gap occurrence, contribution and effects in relation to GNSS-based road charging systems. Hence, an assessment of the driven distance determination tolerance towards these different positioning related outages is provided. The assessment is conducted on the basis of a simulation methodology developed in this thesis. It analyzes the influence of positioning gaps on the determination of the driven distance in both distance-based and distance-related GNSS-based road charging schemes. The gap tolerance of the distance determination in both types of charging schemes is important for the road charging system's ability to meet the performance requirements and charge the road users correctly for their road usage. The simulation analyses of the gap influence on the driven distance determination show that the distance determination function is relatively robust against small gaps of less than 10 seconds in the positioning. However, with several medium and large gaps in the trips, both distance determination methods have trouble in reproducing the driven distances with distance deviations more than 1 % from the truth. The importance of these results is that for the majority of trips the distance driven can be determined with less than 1 % distance deviation as the occurrence of small gaps is most frequent in trips. GNSS-based road charging systems are considered liability-critical systems, where denial of service and undetected fault and failures generate significant legal or economic negative consequences. Any fault or failures that lead to incorrect charging may cause economic loss or provoke wrong legal decisions as the economic liability is associated to the legal aspects due to the repercussion of potential claims. Hence, the thesis introduces the use of system dependability of GNSS-based road charging systems. The concepts of system dependability, adapted from computer engineering, provide an effective means of managing various concerns for road charging systems within a single conceptual framework. Dependability is an important requirement for a GNSS-based road charging system as the system must provide fair charging and gain user trust by ensuring system reliability and liability. This thesis discusses the impact of the assessment results in relation to system dependability and provides a qualitative dependability risk matrix for the vehicle location determination function. To ensure high dependability of the road charging process, fault tolerant design should hence be considered in relation to many different components and functionalities within the process. Based on fault tolerant methodologies, this thesis provides guidelines of how to maintain correct service in the presence of different faults caused by technical problems related to vehicle location determination. The main objective of fault tolerant design within the road charging process is to ensure fair charging of the road users. This means that redundant systems, procedures and components should be implemented to ensure that when fault and failures occur within the road charging process, the road charge foundation will still be dependable and provide fair results towards both the road users and the road charging system. This thesis therefore concludes that though the vehicle location determination performance is fair, the focus of the system performance concerns should be placed on how future GNSS-based road charging system can be designed to work reliably with the occurrence of both data invalidity and data deficiency. It is therefore important to widen the focus from technical challenges and component inaccuracies alone to a focus on the system dependability as a whole. There is however still some technological challenges to overcome, which to a greater extent are remediated by better collaboration across the many different subject areas. As with many other ITS systems, a successful design, implementation and operation of a system is only achieved when the many different stakeholders understand each other's requirements to the system. The system architecture as a conceptual design together with the system engineering methodology can help to involve all the different parties in the system development and hence minimize the misunderstandings which at the end can become very costly for the system. Based on the several findings of this PhD research, some general guidelines are finally formulated for future GNSS-based road charging systems. The proposed guidelines described in the thesis address both GNSS-based road charging trials in general and a future GNSS-based road charging system in Denmark.
