Where's my bus? Radio signposts, dead reckoning, and GPS

To estimate arrival time at the next stop, current bus position, speed, and heading must be known. This information from an onboard positioning system is relayed to the fleet management center via a radio link. Updated arrival time can then be provided to transit users through a Web page, an automatic telephone answering system, or an electronic display at the bus stop. The transit system can also use the data to study effects of traffic and other factors on bus schedules, to increase efficiency. Onboard bus positioning systems typically take the form of a dead-reckoning system using an odometer coupled with an electronic compass or gyroscope. Errors of such a system grow with time and the system must be reset periodically using an accurate position fix. A radio signpost system with transmitters spaced along the bus route can perform this service. However, unless the density of signposts is sufficiently high, the dead-reckoned position error (and hence the error in the expected time or arrival at the next bus stop) will be unacceptably large. And should the bus detour from its scheduled route due to an accident or road works, the dead-reckoning system cannot be updated. This month's column describes a bus positioning system which integrates GPS with dead reckoning and radio signposts to significantly improve the system's accuracy.