The research topic of this study, improving safety in the vicinity of the level crossings based on C-ITS, informs various research problems. Among them, the formulation, development and validation of an advanced integration V2X platform served for V2V communications and networking, and V2V positioning in the vicinity of level crossings are a prerequisite for the successful deployment of the CLX warning system. Although there has been little research characterizing the reliability and integrity of C-ITS, these systems and their technologies are aggressively being improved and evaluated by automotive makers, research communities and universities, and manufacturers of communications and positioning systems.
IVC-based systems, especially the emerging CLX safety system, are not yet mature enough to be widely utilized. Communications and positioning technologies and standards used in C-ITS, such as the 5.9 GHz DSRC technology and the Radio Technical Commission for Maritime Services (RTCM) standards, have the capability of improving these systems and of addressing the safety issues on roads and at level crossings.
Figure1.1illustrates a level crossing zone equipped with the CLX warning system, which is among the most promising C-ITS, incorporated with emerging wireless technologies such as the 5.9 GHz DSRC technology, wireless ad-hoc networks and Global Navigation Satellite Systems (GNSS). The proposed system shown in Figure1.1 includes DSRC On-Board Units (OBUs) that are installed in mobile nodes (cars and trains), Road-Side Units (RSUs), and GPS receivers and digital maps connected to OBUs, as well as DGPS stations to provide precise positioning services.
Most of the safety threats and efficiency issues, such as those illustrated in“the textbox of scenarios”
on page6, as well as dangers at level crossings, can be eliminated or decreased if vehicles are advised about the presence of trains prior to reaching the dangerous zones of level crossings. This early
Figure 1.1: CLX warning system
notification is important to allow sufficient time for taking appropriate actions. Mobile Ad-hoc Networks (MANETs) are utilized by IVC systems to built VANETs in order to distribute safety and efficiency messages, as well as positioning data over required geographical areas. In the scenarios of Figure1.1, the train can disseminate safety messages (alerts) at earlier times to specific geographical locations.
These are extracted by the train’s OBU from its digital road map, along with consideration of its current position. As illustrated by the orange ovals in the figure, the train can notify the vehicles in those areas about its presence at the level crossing at a specific point in time. These messages, called geocast since they target specific geographical areas, may or may not be relevant to all vehicles presented in the targeted area. For example, cars labeled 1 and 2 are within a common targeted zone but the safety geocast message sent to that zone is not relevant to Car 2 because it travels in the opposite direction to the level crossing. This thesis focuses mainly on thepositioning and communications components of the CLX safety system, not on all the components of C-ITS.
Safety Scenarios
Diverse factors such as adverse weather or road conditions, alcohol or drug use, fatigue, unintended driver errors, excessive speed and other risk takings may lead to various dangerous situations at level crossings. There are also traffic efficiency issues at level crossings.
Train out of sight: Some of the problems at level crossings occur because drivers are not aware of the presence of trains. For instance, consider the car labeled 1 in Figure 1.1. This car travels on a road crossing the railway tracks while the approaching train is not in sight of the car’s driver.
Consequently the driver is not aware of the approaching train until reaching the level crossing, which may be too late for notification about the train. The possible solution is to warn the driver earlier to make sure they allow enough time to stop.
Poor crossing location choice / Obstacles affect sight distances and train visibility: In some cases the angles of entry to level crossings for cars, or the existence of obstacles, may lead to poor visibility of approaching trains. In Figure 1.1, the vehicle labeled 5 is in a position that does not have a clear view of the approaching train due to the existence of an obstacle. Trains in the distance are often closer than they appear.
Crossing without careful attention to the conditions in front: It is vital for drivers crossing a railway to be aware of the length of their vehicles and the space required to clear the track. Consider a situation in which the car labeled 4 stays at the level crossing when the driver is not absolutely sure about the clearance of the exit while the train approaches the level crossing. In this case, the accident may not be preventable. To avoid this kind of accident when a train approaches, cars need to be stopped in the red precincts highlighted in Figure1.1, behind the critical zones. To do so, drivers need to be notified about the existence of the train earlier, for instance when they are in a predefined zone behind the critical zones.
Second train threat: It is usually difficult for drivers to see a second approaching train, because the first train may obscure the second one.
Heavy congestion at level crossings: A train passing a level crossing may cause heavy congestion on nearby roads. In Figure 1.1, assume that cars labeled 4 and 5 wait (causing congestion) for the train to reach the level crossing while cars labeled 7 and 8 approach the congestion. In this case, if the approaching cars receive information about the existing congestion prior to their arrival, they may make a detour to save time and fuel, and at the same time avoid causing a congestion increment. This case is illustrated in the figure where car labeled 8 tries to make a detour.
As discussed previously, CLXs may employ geocasting to improve safety at level crossings. With the intention of supporting geocast in addition to other safety applications of C-ITS such as lane departure assist, precise positioning of cars (and trains) is crucial. The GPS as a fully operational GNSS was designed to provide two levels of performance to military and general (civilian) users. Military users who had access to the coded signals on two L-band frequencies, enabling ionospheric correction, were provided the highest accuracy by the Precise Positioning Service (PPS). Civilian users were provided a lower level of accuracy by the Standard Positioning Service (SPS). The accuracy achieved by the SPS is in the few dekameter range (10 m), which is not accurate enough for navigation and positioning services offered by C-ITS. Nevertheless, the basic SPS is enhanced with local or regional augmentation systems. Additionally, new civilian signals and frequencies are being added to the GPS satellites through the ongoing GPS modernization program, which enables ionospheric correction for civilian users as well, and which boosts civilian GPS accuracy. To ensure the performance of IVC systems, it is vital to study important implications of GNSS-based positioning and its required communications protocols in VANETs. To this end, GNSS data streaming for precise cooperative positioning in the context of geocasting, for instance, must be investigated.
In addition to the positioning requirements of C-ITS, numerous factors must be considered in the design of any reliable network protocol for CLXs. These include the network nature (e.g. ad-hoc vs infrastructure), its associate layer(s) (e.g. physical, MAC, network, etc.) and possible cooperation between the layers (cross-layer design), the transmission medium, and the routing strategies. For instance, a packet centric topological approach was originally used to design the Internet Protocol (IP) suite. The need to interconnect networks with heterogeneous technologies was considered in the packet centric topological approach, which delivers information irrespective of its content. Generic Internet access purposes were the basic idea behind the Internet protocol suite design, whereas geocast protocol design requires specific requirements since this design has to meet its own particular constraints. There are several approaches to design geocast protocols. For this purpose, IP can be extended to support geocast; however, this approach is rather complex and inefficient in VANETs. Therefore, geocast needs to be regarded as an independent protocol of vehicular networks, so that it may offer more design freedom to fulfill the requirements of VANETs. VANETs must still support IP-based communications, since these communications protocols are fundamental for IVC and particularly for Roadside to Vehicle Communications (RVC). Geocast protocol development can be studied from three aspects: routing, transmission and positioning. These areas of research are not comprehensively studied in respect to the requirements of each other.
1.2.1 Research Hypothesis
It is hypothesized that cooperative vehicular communications systems integrated with efficient networking schemes and global positioning satellite systems will provide more competent, effective, flexible and cost-effective technological solutions to improve safety on roads, rail networks and in the vicinity of level crossings for land transport systems than autonomous vehicular safety systems.
The design considerations described in this section regarding the communications, networking and positioning components of vehicular safety systems impose significant technical challenges on the adoption of WAVE Short Message (WSM) for IVC concerning geocasting and Inter-Vehicle Positioning (IVP) for safety mission applications. The following challenges are therefore recognized regarding the integration of wireless communications with networking and routing strategies, and positioning techniques:
• Design of an integration mechanism for communications, networking and positioning to support reliable, scalable and cooperative networking and positioning using geographical information of VANETs.
• Design of topological addressing and packet forwarding mechanisms using cooperative networking and positioning with geographical information of VANETs.
• Design of efficient positioning data sharing and streaming mechanisms using cooperative networking and positioning with geographical information of VANETs.
1.2.2 Research Questions
The gaps identified above have individually received some attention to date in the literature; however, the majority of studies have not collectively considered the communications, networking, routing, addressing and positioning requirements of such mechanisms. Therefore this research study focuses mainly on the requirements of the CLX system, and C-ITS in general, in terms of communications and networking (e.g. geocast transmission and route management) and precise positioning. Each node of VANETs supporting geocast must be able to target geographical areas, therefore packets need to carry the geographical position information of the intended receivers. In addition to this, additional GNSS data may be required by some C-ITS applications to be exchanged among cooperative vehicles and the infrastructure. GNSS data is extensively used by C-ITS safety applications, so supporting swift transmissions between cooperative vehicles and the infrastructure is vital to the integrity of the system.
In this regard, the radio communications aspects of precise positioning using low cost GPS receivers and their performance monitoring are not yet adequately studied in the literature. The following research questions have been identified to address the objectives of this research study.
Research Question (1) How can communications, networking and positioning be used collectively to provide a more efficient routing experience to C-ITS, e.g. CLXs, by extending topological addressing and forwarding of packets with precise positioning solutions?
Research Question (2) How can IVC and Differential GNSS (DGNSS) positioning be cooperatively utilized by C-ITS to meet the stringent communications and positioning requirements of C-ITS safety applications?
Research Question (3) How does the performance monitoring of cooperative communications, networking and positioning systems served for C-ITS and of autonomous positioning systems differ?