Automatic Vehicle Location

Description/objective

Automatic Vehicle Location Systems (AVLS) are technologies that can be used by transport operators to provide a direct link between vehicles, operation control centres and real-time passenger information systems. These systems allow for real-time tracking of services that provides the ability to improve service efficiency, asset utilisation and customer service. Interface with AVL systems tends to be provided for the driver as part of a driver’s console and for the operator at a control centre. The primary navigational technologies utilised in AVL systems include GPS, dead-reckoning systems, station or roadside detectors, sub-surface detector loops and wireless triangulation.

Global Positioning System

Description / objective

Global Positioning System (GPS) is a technology that allows receiver devices (GPS receivers) to self-determine their location. This is based on accurate real-time location information from satellite signals for the purposes of navigation. The system is developed and operated by the United States Government, having been originally developed for military purposes by the Department of Defence (with the first satellite launch in 1978). It was made available for public use in 1991. In the context of transportation, GPS technology is primarily used to support the maximisation of service efficiency and performance, through the provision of accurate vehicle and route location information to both in-vehicle and external ITS devices and applications. The GPS receiver can be a free-standing device, or it can be an embedded card in other devices such as the on-board computer. Currently, it is by far the most widely utilised technology for the purposes of automatic vehicle location.

The technology operates using three key segments:

1. Satellites, of which there are 27 orbiting the earth twice a day.
2. Monitoring stations, which are located at various points across the world and are used to monitor the satellites, to correct errors and to update the satellites navigational message.
3. A GPS receiver, which is the device providing user access to the system.

GPS requires signals from a minimum of three satellites to determine the position of a given GPS receiver. Signals are sent out continuously by each satellite. The signals contain information relating to the location of the satellite and the time at which the signal was sent. The GPS receiver compares the time the satellite signal was received to the time that it was sent, to determine the receiver distance from that satellite. By calculating this distance for a minimum of three satellites, the receiver position can be found. The basic system is accurate to approximately 15 metres.

Generally, the location information from GPS equipped vehicles is transferred to an operations control centre or other system via dedicated wireless networks (cellular networks, satellite or terrestrial radio). Location information can be displayed on board the vehicle and to the transport user via internet, smartphones, SMS or on information displays at transport stops in various formats and in real-time.

While GPS is currently by far the most utilised satellite navigation system, other GPS-type systems currently exist or are under development. These include the Russian system GLONASS, China’s Compass system and the European Union’s Galileo system.

1. The GLONASS system is the world’s second global navigation satellite system. As of September 2011 the GLONASS system consisted of 23 operational satellites, just one short of the required amount for global coverage. GLONASS became publically available in 2007 and is currently being actively promoted throughout the Middle East, Eastern-Europe, India and South America. GLONASS can be used in combination with GPS, which increases the number of visible satellites, and hence may offer increased accuracy in dense urban areas.
2. China is also developing its own independent global navigation satellite system. Although military orientated, it will also have a civilian service available. Current expectation is that the system will have 16 of its full 35-satellite constellation in operation by the end of 2012, which will provide full coverage of the Asia-Pacific region. The system is expected to be fully operational by 2020 and to be compatible with the other global navigation satellite systems.
3. European Union’s Galileo system will be an independent system under civilian control, thus removing the dependency on the systems of other nations. Galileo is currently under development and is expected to be partially operational in 2014. Full public service will be available when the complete constellation of 30 satellites becomes operational in 2020. Galileo will also be interoperable with the GLONASS, Compass and GPS systems.

Applications

Although the primary function of GPS technology in transportation is for vehicle tracking, it can also be utilised for the provision of real-time passenger information, fleet and performance management, route selection, schedule adherence monitoring and control, emergency response and priority traffic signalling. The system can also be integrated with the vehicle to display information relating to the vehicle and driver performance.

Advantages and cautions

GPS allows for accurate monitoring of service delivery and efficiency, which allows transport providers to streamline services, efficiently allocate resources and to greatly improve customer service. The technology is widely deployed, readily available, and all relevant ITS systems on the market can integrate with it. It is now a mature, universal, low-cost technology.

There are occasional limitations to its accuracy. It is not always possible to calculate the exact location of a vehicle for various reasons, including atmospheric effects, shifts in satellite orbits, satellite clock errors and multipath effects. The multipath effect refers to the reflection of satellite signals off large obstructions such as tall buildings, which can reduce accuracy and signal strength. This can be problematic in dense urban areas affecting the quality of service that the technology can provide. Some urban bus companies supplement the GPS data with other technologies (e.g. odometer readings) to overcome such limitations.

Off-vehicle systems are dependent on communication of the location data from the on-vehicle GPS receiver. Where this data is transferred via wireless network coverage and network, availability can present a problem. For communications efficiency, vehicles transmit their location information at intervals, typically every 20 to 60 seconds. While the GPS device itself has high location accuracy, by the time the vehicle transmits the information, the vehicle may have moved some hundreds of metres. While this variance is usually not a problem for urban transport operations management and real-time passenger information systems, it usually renders GPS insufficient for traffic signal priority applications, unless additional location polls are added as the vehicle approaches the signals. It may also have implications for accident and incident investigation.

Despite these issues, GPS provides a dependable means of continuously tracking vehicles, and when integrated with another navigation system as back-up, such as inertial navigation or roadside detectors, signal strength and accuracy problems can be overcome. The continued development of the different global navigation satellite systems and their mutual compatibility will allow for much improved accuracy with devices that can recognise multiple systems. GPS receiver units themselves have undergone substantial development also so that GPS cards can be inbuilt to low cost devices, so that inexpensive ITS platforms can be provided.

Relevant case studies

Dublin, Florence, Mysore, Zurich

References

http://www.esa.int/esaNA/index.html

http://insidegnss.com

http://www.kowoma.de/en/gps/index.htm

http://www.hel2.fi/liikenteenohjaus/eng/index.asp

Gyroscope and odometer

Description / objective

The method of using gyroscopes and odometers for vehicle location is termed ‘dead-reckoning’ and is form of inertial navigation. This navigational method is most commonly used in conjunction with GPS as part of an integrated in-vehicle system, but it could also be used with other vehicle location systems such as roadside detectors. In most cases a dead-reckoning system is used to provide back-up vehicle location information to a GPS system, when the GPS signal is unavailable.

The dead-reckoning system can be used once an initial reliable location is known (e.g. last reliable GPS co-ordinate; signal from roadside beacon; location input from driver at known point such as a terminus). From this point, the dead-reckoning system can provide heading and distance information until the GPS signal becomes available again. Within the dead-reckoning system, the odometer sensor provides information on the travel distance of the vehicle, while the gyroscope (typically a fibre optic gyroscope) provides directional information. By counting the number of revolutions of the vehicles wheels, which are of a known radius, the travel distance is computed.

Applications

Dead reckoning with gyroscope and odometer is mainly used to provide backup vehicle location information. This can be used to display real-time passenger information to service users and as schedule adherence and fare collection support to the vehicle driver. Typically the technology is integrated with another vehicle navigation system such as GPS or roadside detectors due to navigational inaccuracies that accrue over time. Typically the data from the dead-reckoning system sensors is interpreted by the on-board computer, where it can be sent to the control centre or used to update on-board passenger information and fare collection systems.

Advantages and cautions

The system is most useful in areas where GPS signal is intermittently unavailable as backup in an integrated GPS/dead reckoning system. The dead reckoning system can supplement location information from the location at which the GPS signal was lost to the location at which it becomes available again. At this point the GPS can take over again and confirm the vehicle location at the next known point such as a bus stop.

While a dead-reckoning navigational system using a gyroscope and odometer may represent a low cost solution for tracking vehicles once an initial GPS co-ordinate is known, reduced accuracy overtime can cause false location information. Although this technology can be implemented as a stand-alone system there is a necessity for external verification of the vehicles location at certain points and for frequent maintenance.

While this method of using an odometer sensor to calculate distance is initially accurate, this accuracy reduces over time with tyre wear, variations in vehicle speed, and tyre pressure and tyre skidding. In Singapore, where an integrated GPS/dead reckoning system has been in operation on public buses, it has been shown that location inaccuracies due to tyre wear can be as much as 5%. Accuracy of the gyroscope can also decrease over time and this effect combined with odometer errors can result in significant errors in the vehicles estimated navigational position. Due to this there is a requirement for error compensation and frequent location data correction.

Relevant case studies

Dublin

Station or roadside detector

Description / objective

Station or roadside detectors can be used to perform a number of different functions including access control, priority routing, vehicle tracking, performance monitoring and real-time passenger information. There are a variety of different systems available with different capabilities and levels of complexity.

The most common station or roadside detector systems use radio beacon transmitting sign posts. In this system, receivers are located on-board vehicles. Beacons are located at intervals along the roadside, for example at bus stops or stations. Beacons continuously transmit a coded signal, which is then picked up by the vehicles on-board receiver. The coded signal identifies the exact position of the roadside beacon and the receiver records the time at which the signal was received. This provides the real-time location of the vehicle, which is interpreted by the on-board computer and passed to the operations control centre via GSM or dedicated wireless network where the vehicle location is displayed on a map or as a co-ordinate listing. Other systems can operate in the opposite way with beacons located on-board vehicles and receivers located at the station or roadside.

Applications

The information obtained each time an equipped vehicle passes a station or roadside detector unit is primarily used to plot the vehicles location. It can also be used for a number of other applications including calculating journey times, schedule adherence, priority signalling and provision of real-time passenger information. With high vehicle frequency and system coverage, the technology can be used to warn vehicles of congested areas and be used to find alternative routes.

In relation to schedule adherence and priority signalling, it is possible to track the vehicle’s position relative to its schedule and request priority at junctions that have been integrated with detectors, usually when vehicles are full or running late. This system is suited to dedicated routes where other traffic cannot present an obstacle between the point at which priority is requested and the junction itself.

Advantages and cautions

As the station or roadside detector system is organised to detect a given vehicle at points along its route it therefore does not provide a means of continuously monitoring vehicle location, unless integrated with other technology such as dead reckoning. It is an accurate vehicle location solution but there is a requirement for infrastructure at stops and stations in addition to in-vehicle components and control facility equipment meaning that it may represent a relatively high cost.

There is also a need for regular maintenance as there is no backup system if equipment fails. In addition to this, the infrastructural requirement means that it may not be viable to implement the system on every route. Due to this it may only be possible to achieve limited coverage and as such it may be more appropriate to utilise a different technology.

References

http://www.fhwa.dot.gov/ohim/tvtw/natmec/00020.pdf

http://www.mountain-plains.org/pubs/html/mpc-03-154/index.php

Sub-surface detector loop

Description / objective

Sub-surface detector loops, also known as inductive loop detectors, are used to detect the passage of vehicles in a given location. This technology can detect both stationary and moving vehicles and can be installed across single or multiple lanes of traffic. A range of outputs are possible from the system depending on its complexity and integration with other systems, including the number of vehicles passing, vehicle speed and junction control.

The inductive loop detector consists of a wire loop embedded in the surface of the roadway, which is connected to an electronic unit housed in a controller cabinet. The presence of a conductive metal object is sensed as it passes over the detector as a reduction in loop inductance, which is ultimately interpreted by the controller as a vehicle.

Applications

Sub-surface detector loops are not capable of distinguishing between different vehicles of the same type and as such they are not suited to the monitoring of specific vehicle locations. While this technology can be integrated with other systems to achieve priority signalling at junctions, the inability of the detectors to recognise specific vehicles means that dedicated lanes are necessary. This type of system is more suited to the monitoring of general traffic conditions and for the most part this is the reason for which they have been implemented.

Advantages and cautions

1. Sub-surface detector loops are simple technology but are relatively expensive to install.
2. They require a nearby source of electrical power.
3. They can be expensive to maintain as they are subject to various forms deterioration including freeze/thaw cycles and mechanical stress.
4. In the case of public transport detection and priority signalling there is a requirement for reserved/dedicated lanes.
5. Installation of the system requires cutting into the road surface, which decreases pavement life and requires road closure.
6. With the use of conventional inductive loop detectors inaccuracies in vehicle counts/detection can occur in instances of congestion. In this case the detector may not recognise more than one vehicle if two vehicles are over the device at the same time. However, advanced detectors can overcome this problem.
7. Advances in other technologies such as wireless sub-surface detectors and GPS may mean that these systems would be better suited to most situations and also provide more valuable information and greater potential for integration with other systems.

Wireless triangulation

Description / objective

Wireless triangulation is a Wi-Fi based technology that aims to pinpoint vehicle/user location more accurately than GPS technology. This technology is particularly suited to use in dense urban areas where GPS may not perform well in terms of consistent service provision. This system is dependent on the availability of multiple wireless networks, which is increasingly a feature of urban areas.

The technology operates using a device (typically a smartphone) with a wireless triangulation application that detects surrounding wireless networks in a particular area. The device measures the signal strength of the detected wireless networks and compares these results with results from a database of known networks in the area. This data is used to calculate the user’s distance from the given network access point. The device does not actually connect to these networks, but using the calculated distances from multiple access points, position can be triangulated for display to the user.

The technology in its simplest form is not operated by the Wi-Fi provider; rather, it uses an application that takes advantage of how the Wi-Fi network operates. However, more accurate and advanced versions available from independent suppliers require continuous updates and may be integrated with GPS, and can be available on a subscription basis. These service providers continuously update databases of known networks confirming the location of existing access points. This technology could be used by a vehicle/driver using any device that can detect wireless networks and run the triangulation application. Alternatively, the technology could be used by a person in the street to detect his/her own location. Regardless of how the system is applied, it represents a comparatively low-cost navigation solution as it does not require additional infrastructure.

Wireless triangulation can also be achieved using cellular towers. However, the level of accuracy is wholly dependent on the density of cell towers in a particular area. The position of a mobile phone user is calculated by measuring the time it takes for the mobile phone signal to reach the nearest three cellular towers. These three cell towers create a triangle surrounding the user, which provides an approximate location. The smaller this triangle, the more accurate the location information.

Applications

1. Real-time passenger information
2. Drivers aid

Advantages and cautions

Wireless triangulation technology is not subject to the same signal issues as GPS and with a high density of wireless network access points this can be an effective means of providing location information. However, this technology depends on wireless network access points being consistently available and in the same location. With a large number of access points this for the most part does not present a significant issue. Occasionally, access points can be moved and this may result in the presentation of incorrect location information as the database information will be incorrect. While techniques have been developed to minimise these effects, the potential for inaccuracies means that there is a requirement for service providers to periodically reassess areas to update their database of networks. Some service providers are using wireless triangulation data in conjunction with GPS data to mitigate or reduce these potential errors. This method of tracking is said to have an accuracy of between 10 and 20 metres.