Archive for the ‘Global Navigation Satellite System (GNSS)’ Category

What is Automatic Vehicle Location?

Friday, March 26th, 2010

Automatic vehicle location (AVL) is a computer-based system used for tracking vehicles, primarily transit buses but also fleets of trucks (see freight) and automobiles. For transit, the actual real-time position of each vehicle is determined and relayed to a control center. Actual position determination and relay techniquesvary, depending on the needs of the transit system and the technologies employed. Typically, vehicle position information is stored on the vehicle for a specific time, which can be as short as a fewseconds or as long as several minutes. Position information can be relayed to the control center in raw form or processed on-board the vehicle before its transmission. Transit agencies often incorporate AVL with other operational functions such as computer-aided dispatch, mobile data terminals and emergency alarms. Increasingly, transit agencies are also using AVL for services that directly benefit riders such as:-

  • Real-time passenger information
  • Automatic passenger counters
  • Automated fare payment systems

Other components that may be integrated with AVL systems include:-

  • Automatic stop announcements
  • Automated destination signs
  • Vehicle component monitoring
  • Traffic signal priority

Benefits of AVLAVL systems can help transit agencies increase fleet utilization and reduce fuel, labor and capital costs. Key benefits of AVL include improved schedule adherence and timed transfers, more accessible passenger information, increased availability of data for transit management and planning, and the efficiency/productivity improvements in transit services. A 2003 survey of transit agencies using AVL, conducted by the Transit Cooperative Research Program (TCRP), found that, while many transit agencies deployed AVL systems for the purpose of increasing operational efficiency, an additional benefit was improved customer service due to the ability of AVL systems to provide customers with real-time predictions of bus arrivals. Thus, transit agencies are increasingly integrating real-time information systems into their existing AVL systems. Respondents of the survey indicated that the area of greatest improvement with AVL was improved customer service, while increased customer satisfaction was the second greatest benefit.

Furthermore, due to supplemental technology such as automatic passenger counters, transit agencies are better able to analyze transit service performance in real time and historically, to gather information needed for system planning, and to locate vehicles for emergency repairs. The 2003 survey also reported that transit agencies implementing AVL can expect ridership and revenue to increase and that a modal shift toward public transportation may result.

Benefits of AVL include:

  • Operations
    • Transit firm productivity gains: increased passenger trips, capital savings (potential reductions in fleet size due to better utilization of vehicles), lower annual maintenance costs and generally a lower vehicle cost per mile.
    • Improved schedule adherence, accuracy in schedule adherence monitoring and transfer coordination.
    • Increased transit ridership.
    • Labor savings: reduced need for additional road supervisors and manual data entry.
    • Improved ability of dispatchers to control bus operations as well as better monitoring of driver performance.
    • Effective tracking of off-route buses as well as paratransit vehicles and drivers.
  • Communications
    • Improved communications between supervisors, dispatchers, and operators
    • Reduced voice radio traffic and loss of radio calls
  • Passenger Information
    • Provides capability to inform passengers of predicted bus arrival times enhancing the quality of transit service and allowing travelers to make better travel decisions.
    • Reduces customer complaints and the need to add customer information operators.
    • Improves image of agency.
  • Scheduling and Planning
    • Provides more complete and accurate data for scheduling and planning.
    • Allows for potential reduction in schedule preparation time and staff.
    • Aids in effective bus stop placement (when combined with a G.I.S. database and automatic passenger counters).
  • Safety and Security:
    • Enhances driver and traveler security (particularly when coupled with silent alarm technology) by allowing quick location of vehicles and faster security response.
    • Enhances driver and traveler safety: accurate and quick location information allows for faster response to accidents.
    • Provides better operational response during detours caused by accidents, roadway closings or bad weather.

Vehicle Location Technologies

AVL systems use one of four types of navigation technology, or may combine two of these technologies to compensate for inevitable shortcomings of any one technology. The four principal technologies employed for AVL systems are:

* Global Positioning System (GPS Satellite Location)
* Signpost and odometer
* Radio navigation/location
* Dead-reckoning

Global Positioning System
GPS is the newest of these and is by far the most popular choice for transit agencies implementing new AVL systems today. GPS employs the signals transmitted from a network of satellites orbiting the earth. These signals are picked up by a receiver onboard the bus. The satellite system covers almost all of North America, eliminating the need to place transmitters/receivers along any route. The existence of the satellite system means that the main cost for the agencies result from purchase of the GPS receivers and equipment to transmit to dispatch. The accuracy and reasonable cost of GPS make it the most appealing option, though it too has some problems. Foliage, tall buildings, and tunnels can block the satellite signal, and at times satellite signals do not reach specific locations. Some agencies use dead reckoning in conjunction with GPS to fill in such gaps.

Link to story on how GPS works: http://www.gpsworld.com/gpsworld/article/articleDetail.jsp?id=102387

Satellite Technology in Europe
Until recently, Europe has depended on information derived from GPS satellites and from the Russian GLONAS satellite systems, combing satellite technology other, older systems. Without a true alternative to GPS, however, certain areas of Europe, particularly in northern Europe, were not well covered by satellite technology. However, major changes are underway in Europe. A system called EGNOS began a test phase in early 2005. This system comprises a network of more than 40 European ground stations that record, correct and improve data coming from the US global positioning system (GPS). The modified signals are communicated to users via geostationary satellites. More significantly, Europe is developing its own satellite technology, known as Global Navigation Satellite System (GNSS), or Galileo. Galileo is based on 30 satellite constellations supported by ground stations. Galileo is currently being built by the European Union (EU) and European Space Agency (ESA) With the addition of these 30 satellites, positions will be determined far more accurately for most places on Earth, even in cities where buildings obscure signals from satellites low on the horizon.

Galileo is intended to provide more precise measurements than available through GPS or GLONASS (Galileo will be accurate down to the metre range) including the height (altitude) above sea level, and better positioning services at high latitudes. The political aim is to provide an independent positioning system upon which European nations can rely even in times of war or political disagreement, since Russia or the USA could disable use of their national systems by others (through encryption).

Like the US GPS, use of basic (low-accuracy) Galileo services will be free and open to everyone. However, the high-accuracy capabilities will be restricted to military use and paying commercial users.

Dead-Reckoning
Some transit agencies use dead reckoning systems in combination with GPS. Dead reckoning systems, among the oldest navigation technologies, determine vehicle position by measuring distance traveled from a known location and direction of travel. Dead reckoning sensors can measure distance and direction from a fixed point (under the most basic setup, an odometer and compass could be used to calculate position from a specific stop on a route). Typically, these systems act as a backup to another AVL system. This relatively inexpensive system is self-contained on the bus. Dead reckoning, however, has a number of drawbacks. Uneven surfaces and hills can compromise the positioning information. Should the vehicle leave a fixed route, its location will no longer be known since there will be no waypoints off the fixed route. Also, accuracy degrades with distance traveled, and regular recalibration is required (tire circumference changes with wear).

Signpost/Odometer Systems
The signpost/odometer system was the most common navigation technology until the advent of GPS. In this system, a receiver is mounted on the bus, while transmitters are placed along the busÂ’ route. Utility poles and signposts are most commonly used as mounting locations for these transmitters. The bus picks up a low-powered signal from these transmitters as it passes by and the mileage is noted. When the bus reports its location, the distance from the last pole is used to locate the vehicle’s position on a route. The system can be run in reverse, with the transmitter on the bus and multiple receivers mounted along the route. However, should the bus need to leave the route, there will be no information about the bus, so most agencies prefer to have a receiver on the bus. This older technology has some drawbacks. Creation of new routes requires the placement of new transmitters, and the system is maintenance intensive due to the relatively high number of transmitters and receivers involved.

Radio Navigation/Location
Radio navigation systems also tend to be combined with other systems. Radio location systems use a low-frequency signal to cover the system, and the buses are located as they receive the signal. Loran-C (Long Range Aid to Navigation) is the most common type of land-based radio location. Despite the simplicity of the system, it is subject to some major drawbacks. Overhead power lines or power substations can cause signal interference, and signal reception is typically very poor in canyons.
Integrating AVL with Other Systems

Buses equipped with AVL offer many possibilities for transit interface with highway and traffic organizations or transportation management centers. Opportunities include: providing transit buses with traffic signal priority; obtaining traffic congestion data at the dispatch center to allow rerouting of buses or informing customers of delay; incorporating transit information in traveler information systems; developing multi-application electronic payment systems; using buses to automatically communicate traffic speed; and reporting of roadway incidents by transit vehicle operators.

Traffic signal priority on arterials and at freeway on-ramps can substantially improve the schedule adherence of transit vehicles and reduce run times. This effort requires cooperation between transit and highway departments because traffic signals are normally the responsibility of highway departments, and giving transit vehicles priority affects other vehicle movements.

Transit information should be an important element of any regional traveler information system. Adding real-time transit information to available highway information can be helpful to travelers in making mode choice decisions and would be expected to increase transit ridership.

Electronic fare payment may be one of the more appealing adjuncts to an AVL system for potential riders because of the convenience it offers the user. The greatest benefits of electronic payment systems would result from the inclusion of multiple transit agencies and integration with other activities, such as toll collection, and payment for parking and retail purchases.

AVL-equipped buses can be used as probes for determining travel speeds on freeways and arterial roadways—a valuable information resource for a transportation management center, especially one with limited traffic detection or observation capabilities, particularly on arterials. Bus operators can also be useful in reporting incidents they see during their trips. Using the known location of the bus at the time of an incident report, the response of arterial, freeway, and incident management systems and emergency services can be more quickly provided. Paratransit dispatchers would be able to more efficiently route their vehicles if they have real-time information on freeway and arterial speeds and incidents.

Data Transmission to Dispatch

The two most common methods of transmitting bus location data to dispatch are through polling and exception reporting via wireless communications.Many agencies use a combination.

Under polling, the computer at dispatch operations polls each bus, in turn, asking for its location. This method requires the bus to be able to read or calculate its position. The bus location is then transmitted by radio to the dispatch center. Once all the buses have been polled, the computer starts again with the first bus and repeats the cycle. The amount of time it takes to complete a cycle will increase as the number of buses to be polled increases. However, because the computer can poll different buses simultaneously over different radio channels, the time to complete a polling cycle depends on the number of radio channels that are utilized.

In exception reporting, each bus reports its location to dispatch at only a few specified locations or where the bus is running off-schedule beyond selected tolerances. Exception reporting makes more efficient use of available radio channels.

Data Use at Dispatch

Training employees is a key to maximizing the use of an AVL system. When coupled with mapping software, AVL information can be analyzed to anticipate and address bus failures, monitor schedules and direct emergency response They can also trigger location-specific announcements, either visual or auditory, to comply with the Americans with Disabilities Act (ADA).
Implementation and Operational Challenges

Early adopters of AVL systems experienced many technical and institutional problems. The biggest challenge for agencies implementing AVL today is the potentially lengthy procurement and installation period (particulalry software development and integration of technical components). For this reason, agencies procuring an AVL system may want to use an existing design, with customization capabilities. Such an approach would substantially limit potential risks and problems. Other implementation and operational challenges to consider are:

Implementation:
o Institutional relationships may be difficult.
o Development of new software or extensive customization of existing software can result in deployment difficulties.
o Considerable effort may be required to establish an accurate geographic information system database.
o Systems should be consistent with the National ITS Architecture.
Operations:
o New technical expertise is usually required at the transit agency.
o Some existing staff may be reluctant to learn the new technology.
o The schedule adherence function design requires careful thought.
o A global positioning system signal reception problem may occur in certain areas.
o The huge volume of data that an AVL system can record may overwhelm existing agency analysis capability.