On-board unit for vehicle telemetry

An on-board unit for a vehicle, configured to determine the changes in position of the vehicle and thus its route taken comprising: a map database containing constraint information that describes possible vehicle routes; an inertial measurement unit; and a data processor; the data processor being programmed to process the output of the inertial measurement unit, combined with relevant motion constraints from the map database, and from these determine the actual route that has been taken by the vehicle.