These systems are known as automatic vehicle locating (AVL) systems. AVL systems do not rely exclusively on GPS, but they may use other technologies such as beacons and radio transmitters, or dead reckoning systems. However GPS based
systems are the most common and widespread ones nowadays. Most common problems of GPS based AVL systems are the canyoning effect in urban areas from high-rise buildings. Several research projects within the E.U. in the last 5–6 years have dealt with these issues reaching to useful conclusions.