AVL systems are computer-based vehicle tracking systems that function by measuring the real-time position of each vehicle and relaying this information back to a central location. They are used most frequently to identify the location coordinates of vehicles for a variety of purposes. Potential benefits to a transit agency are: improved dispatch and operational efficiency; improved overall reliability of service; quicker responses to disruptions in service, such as vehicle failure or unexpected congestion; quicker responses to threats of criminal activity; and improved data for future planning purposes.
Automatic Vehicle Location (AVL) systems track the geographical location of a vehicle and transfer the information to a requester. The location of the vehicle is determined by Global Positioning Systems (GPS). Vehicle location can be viewed on electronic maps. In the early periods, signpost systems were used to track the transit vehicles. A transponder or radio frequency identification chip (RFID) along the vehicle route would be polled as the transit vehicle traverses its route. As each transponder was passed, the moving vehicle would receive an acknowledgement from the signpost transmitter. A transmitter on the mobile would report passing the signpost to a computer.
Along with data on location of the vehicle, AVL systems also provides the driver information, emergency messages, speed of the vehicle, odometer reading, route, fuel amount, engine temperature, altitude, door open close events, tire pressure, turn off ignition, headlight information, battery status, odometer, engine RPM, throttle position, etc.
Typically AVL systems are classified as active and passive. Passive devices store GPS location, speed, direction the vehicle is moving and sometimes triggers an event when a door opens/closed. Once the vehicle returns to the destination, the device is removed and the data is downloaded to a computer for analysis. Active devices also collect the same information but usually transmit the data in real-time via cellular or satellite networks to a computer or data center for analysis.
A GPS receiver calculates its position by precisely timing the signals sent by GPS satellites high above the Earth. Each satellite continually transmits messages that include the time the message was transmitted, precise orbital information. The receiver uses the messages it receives to determine the transit time of each message and computes the distance to each satellite. These distances along with the satellites locations are used to compute the position of the receiver and display the current location of the vehicle on a map.