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:
- The location of the transmitters has to be accurately known. This is particularly
important when satellites are used, which have a altering location.
- The location of the transmitters in relation to the location of the receiving vehicle has
to
be favorable.
- The transmission speed of the transmitted signal must be known. This may cause
difficulties as most signals traverse different layers of the atmosphere.
- The signals have to be received by the vehicle without reflections. Satellite signals,
which have very short wavelengths (ca. 20 cm), are hindered by large buildings which
may result in complete obstruction of certain signals or in the reception of indirect,
reflected signals. Terrestrial signals, which have very long wavelengths (ca. 3 km),
experience reflections caused by overhead wires.
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
- the German Autofahrer Leit und
Informationssystem (ALI) by Bosch/Blaupunkt,
- the AUTO-SCOUT system by Siemens,
TravelPilot from Bosch (which is based upon the ETAK system), and
- the CAR Information
and Navigation (CARIN) system by Phillips.
Early examples of such systems in the USA are
- the
ETAK Navigator (the first (1985) commercially available automobile navigation aid consisting
of an electronic map display and using map-matching to enhance the accuracy of dead
reckoning system), and
- NavMate, a prototype autonomous, in-vehicle route guidance system
developed by Zexel Corporation, including route determination, vehicle positioning, and route
guidance.
In Japan, a long history in car-navigation exists. Here, we find
- the Electro-multivision
by Toyota, including dead reckoning, car location displayed on map, colour television,
radio/cassette player and CD/CD-Rom player;
- Drive Guide by Nissan (1989), that uses a
compass and wheel sensors and displays digital road maps;
- Electro-gyrocator by Honda,
which
uses transparent map overlays to show the road network; and
- MAPIX III by Nippondenso,
using
GPS and differential oedometer and a colour display. It is one of the most
comprehensive in-car navigation systems, including features such as hotel, restaurant and
sightseeing information, an option for a receiver allowing communication with roadside
beacons, while simple route guidance can be issued on preselected routes.
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.
GPS was developed as part of the Pentagon's 12 billion dollar Strategic Defense Initiative of the 1970s.
GPS was made available to the civilian use after the 1983 incident with the Korean Air passenger aircraft.
It helped U.S. military troops to navigate through the Iraqi desert
in Operation Desert Storm. In 1995 news media broadly covered
the story of Air Force pilot Scott O'Grady using GPS to pinpoint his position and call in rescue crafts in Bosnia.