10.12 GPS for transit systems Transit system authorities in many countries are faced with a challenging trend of fiscal constraints, which limits their capabilities to expand existing services and to increase ridership. Until recently, transit systems used old technologies such as odometer/compass sensors and signposts for position determination [22]. Odometers are sensors that measure the number of rotation counts generated by the vehicles wheels, which are then used to estimate the distance traveled by the vehicle. With the help of a compass, the vehicles direction of travel can be determined at any time. Combining the measurements from the odometer and the compass, the vehicles posi- tion can be determined with respect to an initial (known) position. Unfor- tunately, both the odometer and the compass drift over time, which causes significant error in the estimated position. Signposts, in contrast, are radio beacon transmitters that are placed at known locations along the bus routes [23]. Each beacon transmits a low-power microwave signal, which is detected by a receiver on the bus, to account for the odometers drift error. Unfortunately, this system has a number of limitations, including its inca- pability of knowing the exact location of a vehicle in between two sign- posts. In addition, it is not possible to track a vehicle that goes off-route as a result of, for example, a road closure [22].