One of the primary functions of a road or highway network is to allow the safe passage of people and goods from their origin to their destination. Traditional sources of information (printed road maps, direction signs, route listings and journey plans) all have their place but satellite navigation systems are now used widely. Generically these are known as Global Navigation and Satellite Systems (GNSS). Specifically, they include the Global Positioning System (GPS) developed by USA, GLONASS, the Russian global satellite navigation system and GALILEO, the civilian global satellite navigation system being developed by the European Union from its precursor, the European Geostationary Navigation Overlay Service (EGNOS) (See Video).
The USA’s GPS consists of 24 satellites that are deployed and maintained by the US Department of Defence (USDoD). Originally, the system was used solely for military purposes, but since 1983 the USA has made GPS available for civilian purposes. For location determination (longitude, latitude, and elevation), a GPS receiver needs to receive signals from at least four satellites (signals from the fourth satellite are needed to correct for errors and improve accuracy).
A GPS on-board a vehicle – or a smart-phone with a GPS – can determine the location of the vehicle. The location can then be communicated via wireless communication to a central location (such as a traffic operations centre) for processing and data fusion. Besides pinpointing the location of a vehicle and communicating that location to a traffic operations centre, GPS receivers are at the core of all navigation-aid devices developed by companies such as Garmin, TomTom, and Magellan. For navigation and turn-by-turn directions, accurate digital maps are needed, in addition to the GPS receiver.
For its operation, the USA’s GPS relies on signals transmitted from the 24 satellites orbiting the earth at an altitude of 20,200 km. GPS receivers determine the location of a specific point by determining the time it takes for electromagnetic signals to travel from the satellites to the GPS receiver. A limitation of GPS is that it cannot transmit underground or underwater and signals can be significantly degraded or unavailable in urban canyons, in road tunnels and during solar storms. This is why there is continuing interest in terrestrial based radio-positioning systems using technologies such as mobile phones, Bluetooth and Wi-Fi.
GALILEO is the first complete civil positioning system to be developed under civilian control, in contrast to the USA’s GPS and the Russian Glonass systems. GALILEO has been designed with commercial and safety-critical applications in mind, such as self-guided automated cars. The first satellite was launched on 21 October 2011 and the system is scheduled to be fully operational before 2020. When fully deployed GALILEO will consist of 30 satellites (27 operational plus 3 back-up), circling the earth at an orbit altitude of 23,222 km. GALILEO will be fully interoperable with GPS and GLONASS and is expected to achieve very high levels of service reliability and real-time positioning accuracy not previously achieved.
Digital maps are a pre-requisite for satellite navigation and many other ITS applications, such as automated driving and traveller information systems. Many technologies are currently available for creating and updating digital maps. For example, digital maps can be created by collecting raw network data, digitising paper maps, from aerial photographs and other sources. An initiative called OpenStreetMap (http://www.openstreetmap.org) intends to develop digital mapping for the whole world. The maps are developed from GPS traces collected by ordinary people and uploaded to the website. Aerial imagery and low-tech field maps are often used to verify that the resulting maps are accurate and up to date.
A navigation database is a commercially developed database used in satellite navigation systems. It is often based on a Network Gazetteer (See Basic Info-structure) and will contain all the elements needed to construct a travel plan or a route from a specific origin to a specific destination. Additional criteria may be added such as the route passes through a specific point, that it avoids tolls, that it is the fastest or shortest available, or that it minimises fuel consumption or emissions.
A navigation database is multi-layered and requires more than the basic coordinates for the road network links and nodes, although that is an important starting point.
Additional features necessary for navigation include:
Navigation databases also contain information on points of interest and landmarks, such as public transport facilities, major office buildings by name, hotels, restaurants and tourist attractions, post offices, government buildings, military bases, hospitals, schools, petrol stations, convenience stores, shopping centres and malls, toll-booths – and in some countries, the location of speed cameras.
Crowd-sourcing and social networking has enabled the creation of navigation databases that are adapted to the needs of specific groups of road-users, such as truck drivers and cyclists. There are also important developments taking place in pedestrian navigation.
Infromation on pedestrian navigation: http://www.insidegnss.com/node/513 and http://www.navipedia.net/index.php/Pedestrian_Navigation
Location-based services are computer applications that use location data to control features or the information displayed to the user. They have several applications in health, entertainment, mobile commerce, and transport. In road transport, for instance, location-based services can be used to provide point of interest information (using data held in a navigation database – such as the closest fuel station or restaurant. Location-based services can also be used to display congestion or weather information according to the location of the user (See Location-Based Services).
In co-operative systems, vehicles share data with each other and with the road infrastructure using vehicle to vehicle (V2V) and vehicle to infrastructure (V2I) communications. Vehicles that are connected in this way can make use of real-time information on moving objects (such as other vehicles nearby), and on stationary objects that might be transitory (traffic cones, parked vehicles and warning signs). This highly detailed, constantly changing information is held in a data store known as a Local Dynamic Map (LDM). The LDM supports various ITS applications by maintaining information on objects that influence, or are part of, the traffic. Data can be received from a range of different sources such as vehicles, infrastructure units, traffic centres and on-board sensors. The LDM offers mechanisms to grant safe and secure access to this data by means of V2V and V2I communications.
The data structure for the LDM is made of four layers:
Location referencing and object positioning for the upper layers of the LDM is complex and requires adequate location referencing methods. Since not all ITS applications need location referenced information, the use of this data is not mandatory.
These technologies allow the location of vehicles to be ascertained in real-time as they travel across the network. AVL has many useful applications for vehicle fleet management, such as improving emergency management services by helping to locate and dispatch emergency vehicles. AVL can be used for probe vehicle detection and on buses to locate vehicles in real-time and determine their expected arrival time at bus stops.
A number of technologies are available for AVL systems including dead reckoning, ground-based radio, signpost and odometer, and Global Positioning Systems. GPS is currently the most commonly used technology.
Another system for tracking and locating vehicles uses fixed point transponders which can read and communicate with other equipment – for example, toll tags on-board vehicles. These systems can determine when a vehicle passes by a certain point, and provide useful information on travel times and speeds.
A third method for locating vehicles is through mobile phone triangulation. The location of a mobile phone user is identified by measuring the distance to several cell phone towers within whose range the user is located. Using this technology, the location of the vehicle can be identified within an accuracy of about 120 meters. In rural areas, where few cell towers are located, the tower can measure the angle of transmission, which – along with the distance – can be used to locate the phone user even though the user might only be within the range of single cell phone tower. The estimated location in this case is not very accurate (within about 1.6 km).