Automatic vehicle location
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Automatic vehicle location or AVL is a means for determining the geographic location of a vehicle and transmitting this information to a point where it can be used. Telelocation is a synonym used in the European Union.
Most commonly, the location is determined using GPS, and the transmission mechanism is a satellite, terrestrial radio or cellular connection from the vehicle to a radio receiver, satellite or nearby cell tower. Some other possibilities for determining location, for example in environments where GPS is not usable, are dead reckoning, inertial navigation, or RFID readers; sometimes a combination of these methods can be used. The tracking data is then transmitted using any one of a variety of telemetry systems; GSM and EVDO are the most common technologies used for telemetry, because of the low data rate needed for AVL, and the low cost and near-ubiquitous nature of these public networks. The low bandwidth requirements also allow for satellite technology to receive telemetry data at a moderately higher cost, but across a global coverage area and into very remote locations not covered well by terrestrial radio or public carriers.
Automatic vehicle location is a powerful tool for managing fleets of vehicles, from service vehicles, emergency vehicles, and construction equipment, to public transport vehicles (buses and trains). It also is used to track mobile remote assets, such as construction equipment, trailers, and portable power generators.
A typical system would be land-based and would be used to simultaneously track the locations of a fleet of vehicles. The primary purpose of tracking is to provide graded service or to manage a large staff effectively. For example, suppose an ambulance fleet has an objective of arriving at the location of a call for service within six minutes of receiving the request. Using an AVL system allows dispatch personnel to evaluate the locations of all vehicles in a fleet in order to pick the vehicle that will most likely get there fastest, (meeting the service objective).[1]
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[edit] Types of systems
[edit] Direction finding
Amateur radio and some cellular or PCS wireless systems use direction finding or triangulation of transmitter signals radiated by the mobile. This is sometimes called radio direction finding or RDF. The simplest forms of these systems calculate the bearing from two fixed sites to the mobile. This creates a triangle with endpoints at the two fixed points and the mobile. Trigonometry tells you roughly where the mobile transmitter is located. In wireless telephone systems, the phones transmit continually when off-hook, making continual tracking and the collection of many location samples possible. This is one type of location system required by Federal Communications Commission Rules for wireless Enhanced 911.
[edit] LORAN-based
Motorola offered a 1970s-era system based on the United States Coast Guard LORAN maritime navigation system. The LORAN system was intended for ships but signal levels on the US east- and west-coast areas were adequate for use with receivers in automobiles. The system may have been marketed under the Motorola model name Metricom. It consisted of an LF LORAN receiver and data interface box/modem connected to a separate two-way radio. The receiver and interface calculated a latitude and longitude in degrees, decimal degrees format based on the LORAN signals. This was sent over the radio as MDC-1200 or MDC-4800 data to a system controller, which plotted the mobile's approximate location on a map. The system worked reliably but sometimes had problems with electrical noise in urban areas. Sparking electric trolleys or industrial plants which radiated electrical noise sometime overwhelmed the LORAN signals, affecting the system's ability to determine the mobile's geolocation. Because of the limited resolution, this type of system was impractical for small communities or operational areas such as a pit mine or port.
[edit] Signpost systems
To track and locate vehicles along fixed routes, a technology called Signpost transmitters is employed. This is used on transit routes and rail lines where the vehicles to be tracked continually operated on the same linear route. A transponder or RFID chip along the vehicle route would be polled as the train or bus traverses its route. As each transponder was passed, the moving vehicle would query and receive an ack, or handshake, from the signpost transmitter. A transmitter on the mobile would report passing the signpost to a system controller. This allows supervision, a call center, or a dispatch center to monitor the progress of the vehicle and assess whether or not the vehicle was on schedule. These systems are an alternative inside tunnels or other conveyances where GPS signals are blocked by terrain.[2]
[edit] GPS-based
The low price and ubiquity of Global Positioning System or GPS equipment has lent itself to more accurate and reliable telelocation systems. GPS signals are impervious to most electrical noise sources and don't require the user to install an entire system. Only a receiver to collect signals from the satellite segment is installed in each vehicle and a radio to communicate the collected location data with a dispatch point.
Large private telelocation or AVL systems send data from GPS receivers in vehicles to a dispatch center over their private, user-owned radio backbone. These systems are used for businesses like parcel delivery and ambulances. Smaller systems which don't justify building a separate radio system use cellular or PCS data services to communicate location data from vehicles to their dispatching center. Location data is periodically polled from each vehicle in a fleet by a central controller or computer. In the simplest systems, data from the GPS receiver is displayed on a map allowing humans to determine the location of each vehicle. More complex systems feed the data into a computer assisted dispatch system which automates the process. For example, the computer assisted dispatch system may check the location of a call for service and then pick a list of the four closest ambulances. This narrows the dispatcher's choice from the entire fleet to an easier choice of four vehicles.
Some wireless carriers such as Nextel have decided GPS was the best way to provide the mandated location data for wireless Enhanced 9-1-1. Newer Nextel radios have embedded GPS receivers which are polled if 9-1-1 is dialed. The 9-1-1 center is provided with latitude and longitude from the radio's GPS receiver. In centers with computer assisted dispatch, the system may assign an address to the call based on these coordinates or may project an icon depicting the caller's location onto a map of the area.
[edit] Notes
- ^ One definition of AVL exists in, "Glossary," Arizona Phase II Final Report: Statewide Radio Interoperability Needs Assessment, Macro Corporation and The State of Arizona, 2004, pp. 165.
- ^ For an example of one US signpost system, see its service manual: T1919A Metrocom II 150.8-174 MHz Vehicle Location Receiver, (Schaumburg, Illinois: Motorola Communications and Electronics, 1979).
[edit] External links
- Silicon.com report: London's bus-tracking system gets £116m update
