A TEST-BED SIMULATOR FOR GPS AND GIS INTEGRATED NAVIGATION AND POSITIONING RESEARCH: - BUS POSITIONING, USING GPS OBSERVATIONS, ODOMETER READINGS AND MAP MATCHING

A test-bed application, called Map Matched GPS (MMGPS) processes raw GPS output data, from RINEX files, or GPS derived coordi nates. This developed method uses absolute GPS positioning, map matched, to locate the vehicle on a road centre-line, when GPS is known to be sufficiently accurate. MMGPS software has now been adapted to incorporate positioning based on odometer derived dist ances (OMMGPS), when GPS positions are not available. Relative GPS positions are used to calibrate the odometer observations. In OMMGPS, GPS pseudorange observations are combined with DTM height information and odometer positions to provide a vehicle position at one second epochs. Generally, odometer positioning is used much more often, to position the vehicle, than GPS. Typically the ratio is 7:3 odometer positions to GPS pos itions. In total, over 15,000 vehicle positions were computed using OMMGPS. The described experiment used GPS observations taken on a bus on a predefined route, hence the correct road is always known. Ther efore, map matching techniques are used to improve the GPS positioning accuracy, and to identify grossly inaccurate GPS positions. Calibrated odometer corrections are made using odometer count at the current epoch and relative GPS distance travelled. If a GPS position is detected to be inaccurate, it is not used for positioning the bus, or for calibrating the odometer correction factor. In general the position quality provided by GPS alone was extremely poor, due to multipath effects caused by the urban canyons of central London. In the case of one particular trip, OMMGPS provides a mean error of position of 8.8 metres compared with 53.7 metres for raw GPS alone.