October 25, 2021
Automatic vehicle location (AVL) systems help fleets with dispatching and tracking their vehicles. From radio direction finding, to signpost, to GPS-based systems, AVL helps fleets adhere to schedules and improve response times. Learn more about how AVL works and the different types of AVL systems fleets use.
Automatic vehicle location (AVL) helps to determine and transmit the geographic location of a vehicle. A tracking system may collect this aggregate data from one or more vehicles to manage an overview of vehicle travel.
AVL is often used in dispatching and tracking transit vehicles and public transport. This is because AVL has helped improve schedule adherence and keeps passengers notified of arrival times. Similarly, emergency vehicles, law enforcement, and other first responders have used AVL to help reduce response times by improving communications between dispatchers.
There are several types of location and navigation systems fleets used in vehicle tracking.
Direction finding or radio direction finding (RDF) measures the direction from which a received signal was transmitted—also called triangulation. It’s used in amateur radio and with some cellular or wireless communications. The simplest systems calculate the bearing from two fixed sites to the mobile one, creating a triangle to pinpoint where the mobile transmitter is. In wireless phone systems, phones must transmit continually when off-hook, making continual tracking possible by collecting many location samples. This is one of the systems required by the Federal Communications Commission for 911 calls.
Signpost transmitters are used to track and locate vehicles along a fixed route. These systems are typically used on transit routes and rail lines where tracked vehicles operate on a continual, linear route. Transponders or radio-frequency identification (RFID) chips placed along the route are polled as the vehicle passes by. As the vehicle passes the transponder, the vehicle receives a signal from the signpost transmitter. The transmitter on the vehicle would then report passing the signpost to a system controller. This allows fleet managers and dispatchers to monitor vehicle route progress. These systems are an alternative when GPS signals are blocked by terrain.
With the increased reliability and lowered cost of global positioning systems (GPS), many modern AVLs use this technology. GPS-based systems don’t require fleets to install and maintain a robust infrastructure. Only a GPS receiver collects satellite signals installed in each vehicle and a radio to communicate the location data to dispatchers.
AVL systems transmit vehicle GPS data to dispatch centers and fleet managers over a private network. Depending on the central fleet management system, location data is periodically polled from each vehicle or shared real-time information. Then, location data is displayed on a map, allowing dispatchers to see the exact location of every vehicle.
We’ll focus on GPS-based systems as that’s what is most common these days. As mentioned, a vehicle’s location is determined using GPS technology. It’s then transmitted using a telemetry system. Typically, Global System for Mobile Communications (GSM) and Evolution-Data Optimized (EVDO) are the technologies used for telemetry due to the lower data rate needed for AVL.
When GPS signals are poor, AVL systems can use various technologies to determine actual location information. For example, AVL can use dead reckoning, which takes a previously determined position, and then incorporates estimations of speed, heading direction, and course over elapsed time. They can also use a signpost RFID system if the route is fixed.
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