JPL's Wireless Communication Reference Website

Chapter: Network Concepts and Standards
Section: Road Transportation Informatics

Positioning and Navigation

Positioning Techniques

Several methods exist for determining a location. Most existing and currently utilized methods are composed of (a combination of) the standard techniques.

Dead-Reckoning

The location of the vehicle is calculated by integrating the traveled distance in various directions in relation to a known initial location, often the previous location. The traveled distance is measured by wheel sensors, the driving direction is measured by an electronic compass. The wheelsensors are also used for determining the driving direction by frequently comparing the distance traveled by the left and the right wheel. Both obtained driving directions are compared to compensate for the accumulating error of the wheelsensors as well as for fluctuations in the earth magnetic field. As the measured distance and direction remain rough approximations, dead-reckoning suffers from an accumulated error which amounts to circa 4% of the traveled distance.

Map-Matching

The location of the vehicle is recognized by matching the apparent path of the vehicle, measured by wheelsensors and an electronic compass, with the pattern of a digital road map. A disadvantage is that this location technique is susceptible to small variations in the traveled path in an fine-meshed road network (e.g. urban area), which may cause the system to fail in determining the correct location of a vehicle. Also when a traveled road is not present in the digital road map this system will fail. A combination of dead-reckoning and map-matching is used in virtually all state-of-the-art in-vehicle navigation systems (e.g. CARIN, Travelpilot).

Proximity-Beacon Technologies

The location of the vehicle is confirmed by location-coded signals from strategically located short-range-signal emitters (infrared or micr wave) that the vehicles passes. The emitters are often located on traffic lights. As the emitters have a short range, a significant large number is required for covering a large geographic area. In some systems (e.g. EURO-SCOUT), the location function of the proximity-beacons is combined with a downlink dissemination function; the beacons are also used for transmitting dynamic traffic or route information to the passing vehicles. For navigation between beacons a combination of dead-reckoning and map-matching is often used.

Trilateration Technologies

The location of the vehicle is accurately computed from simultaneous reception of signals from three or more fixed transmitters. From the mutual time delay in the received signals the distance from the receiving vehicle to each of the transmitters is computed by establishing three or more imaginary circles (or spheres). The location of the receiving vehicle is given by the point of intersection of these circles (or balls). Every additional signal improves the accuracy in the computed location. The following four conditions have to met in order to apply a trilateration technique:

Landbased Trilateration

The accuracy of the Omega system is 2-10 kilometers and is therefore only used for waterways shipping, while the accuracy of a location determined using the terrestrial systems Loran-C (operated by the U.S. Coast Guard as a public service), Chayka and the Decca Navigation System (DNS) is circa 200 meters.

Satellite Trilateration

Two major systems exist The accuracy of the trilateration satellite systems GPS and Glonass is 3 meter at best, but typically 10 meter accuracy is mentioned. For military functions GPS enables Precise Positioning Service (PPS), which provides extremely accurate latitude/longitude ground positions. For defense purposes a deliberate error (selective availability) is introduced into the civilian service, only enabling Standard Positioning Service (SPS). The accuracy of the latter is approximately 100 meters.

Differential location techniques, which correct for systematic errors by measuring the divergence occurring in a certain area in a reference station and transmitting this to the vehicles, considerably improve the performance of location techniques. In this way, the accuracy of differential GPS (DGPS) is 2 to 10 meters. In urban areas, the performance varies considerably, as also DGPS is susceptible to reflections of (large) buildings. For this reason, a combination of a trilateration location technique with the above-mentioned first two other techniques ((a) and (b)) is often used, where dead-reckoning and map-matching are used to navigate through urban areas. Consumer GPS devices cost about 150 dollars.

Car Navigation Systems

In Europe, the first in-vehicle systems were Early examples of such systems in the USA are In Japan, a long history in car-navigation exists. Here, we find

Example

For road vehicle applications such as gathering traffic flow data, a combination of differential GPS (DGPS) in combination with map-matching and dead-reckoning appears useful. The reliability of this combined location method is very high and the accuracy is about 2 to 10 meters.

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JPL's Wireless Communication Reference Website Marcel Westerman and Jean-Paul M.G. Linnartz, 1993, 1995.