Automobile Navigation Using the Global Positioning System (GPS)
By:
Jason StauchFor:
ASEN 5090 - Introduction to GPS
INTRODUCTION
The real time tracking of automobiles has been of increased interest over the past couple of decades. There are several applications of real time tracking systems. The system can provide location information in emergency situations for both the victim and response units (i.e. police/ambulance). A vehicle tracking system can also be employed to track a fleet of commercial vehicles in order to provide location information and increase dispatch efficiency. Lastly, tracking systems can be employed on private vehicles to provide location information and directions to points of interest.
The Global Positioning System (GPS) can be used to significantly improve a vehicle tracking system. The most appealing aspects of GPS s that it is globally available 24-hours a day and the solutions obtained do not degrade with time (or distance). Also, the tremendous amount of commercialization of GPS has dramatically reduced the cost of GPS receivers.
A BRIEF HISTORY OF LOCATION/NAVIGATION
Determination of ones position has been sought for many centuries. One of the earliest forms of navigation was star-pointing (which is still used today). Mechanical methods of navigation are also very useful for land navigation. One of the earliest examples of this is the
South Pointing Chariot. It uses mechanical means of determining the difference between the distance each wheel travels and keeps the arm pointing in the same direction. This concept is still used today and is called Dead-Reckoning.NON-GPS VEHICLE NAVIGATION TECHNIQUES
There are several techniques that can be used to locate/navigate a car. The most commonly used methods include Dead-Reckoning, inertial navigation systems, sign post, Terrestrial Radio Frequency (TRF), and map-matching.
Dead-Reckoning -- This method is the most widely used technique for car navigation (at least prior to the introduction of GPS). Dead-Reckoning uses the same principle as the South Pointing Chariot. It uses sensors to determine the distance each wheel travels. The wheels together determine the speed of the car and the distance it has traveled. The difference between the wheel distances determines the heading of the vehicle. Often times, a compass is used to augment the determination of the absolute heading.
There are several drawbacks of Dead-Reckoning. Errors in the calibration and irregular behavior of the wheels cause nearly unavoidable errors. Road hazards (such as pot holes and bumps) will cause an error in the distance traveled by the wheels and will result in an error in the direction of travel. The biggest problem with Dead-Reckoning is that it is a relative positioning scheme. This means that the absolute position error grows proportionally with the distance traveled. The errors associated with Dead-Reckoning are generally 2% of the distance traveled.
Inertial Navigation System -- An inertial navigation system uses 3 accelerometers and 3 gyroscopes to propagate the position at high rates. This system is used in relatively few car navigation systems. The cost of highly accurate accelerometers and gyroscopes is relatively high. As with Dead-Reckoning, it is a relative positioning scheme, thus the accuracy degrades with time.
Sign Post -- Infrared signals are transmitted from certain locations (e.g. traffic lights or light posts) to give vehicles information about their location. This system is not very robust for individual vehicle navigation, but it can be useful to provide information about traffic patterns.
Terrestrial Radio Frequency (TRF) -- This system is very similar to GPS, but it is ground based. A system of towers emit radio frequency signals, which are picked up by a receiver in the car. These signals are used to triangulate the position of the car. As with GPS, this system is an absolute scheme, thus the accuracy does not degrade with time. However, the coverage is limited to areas that are within visibility of the radio towers.
Map-Matching -- This technique is used in combination with other methods to correlate the position of the vehicle with a map. The basic principle is that if you are traveling near a road, parallel to it, then chances are that you are on that road. There are a couple of drawbacks to this technique. For an accurate map-matching scheme to work, the navigation computer must have accurate maps, which take up a lot of memory. The other problem is that map-matching does not deal well with long straight sections of road. If the distance traveled is not very well known, then when you turn off of the long, straight road, the map-matching scheme may think that you turned on the road before or after the road you actually turned on.
These methods are often used in combination with one another to provide a good estimate of where the car is located.
A BRIEF BACKGROUND OF GPS
GPS is a system of at least 24 satellites, which transmit a signal that very accurately tells what time it is when the signal leaves the satellite. This signal is picked up by a receiver, and by knowing what time it is when the signal is received, the distance from the satellite can be found by multiplying by the speed of light. There are four unknowns in the positioning solution. The position accounts for three unknowns (i.e. x, y, and z coordinates). The last unknown is the time the signal was received. This is unknown because the clock used by a receiver is not exactly in sync with the GPS clocks. Since there are four unknowns, four satellite signals are needed to provide an accurate solution.
The system of satellites was designed such that there are at least four satellites in view almost everywhere on Earth, almost all the time. This fact is very attractive for automobile navigation because the system can provide global coverage using the exact same system and software.
Trimble provides a basic GPS Tutorial which gives a basic overview of GPS without getting too heavy into details. Global Positioning System Overview is an excellent reference (by far the best I've found on the web, and there are links to other helpful pages) explaining GPS in great detail (provided by Peter H. Dana, The Geographer's Craft Project, Department of Geography, The University of Texas at Austin).VEHICLE NAVIGATION USING GPS
There are several reasons that make GPS attractive for automobile navigation. First, the coverage is global and available 24-hours a day. This allows for the same system to be mass produced and installed on any car in the world. Another attractive aspect is that non-government interest in GPS has driven down the cost of a receiver. The absolute nature of GPS positioning allows for no degradation of position accuracy with distance. GPS can be integrated with other navigation techniques, such as map-matching, to provide the user with excellent location and navigation information.
There are a few problems associated with GPS. The first problem is that the errors associated with the GPS signal do not allow sufficient accuracy to map location. The signal error used to be SA dominated, but even with the recent removal of SA, there are still significant errors associated with GPS. Another major problem is the "Urban Canyon" phenomenon. When in a downtown city environment, the buildings can often block several, if not all, of the satellites from view. This seriously effects the ability to provide accurate location information in downtown environments.
SnapTrack gives access to a paper about their GPS technology which claims to overcome the "Urban Canyon" problem (Just follow the link on their main page to the IEEE paper).Each of the above mentioned problems has a solution. To get around the errors associated with GPS, a technique called Differential GPS (DGPS) can be employed. The basic premise behind DGPS is error correlation. Most of the errors due to GPS are highly correlated between receivers within close proximity of each other. If a ground station with a known position can use the GPS signal to calculate it's position, the error in the GPS position solution can be found. The difference between the known position and GPS position gives the error in the GPS position information. This difference can then be broadcast to the receiver in the car, the correction can be applied and an accurate position can be found. This method takes care of most of the GPS errors (as long as the tower sending the corrections is within 100's of kilometers of the vehicle). There are some errors that are not correlated, such as multi-path errors, but these are generally small.
Trimble's GPS Tutorial also has a DGPS tutorial. As before, Global Positioning System Overview offers a more technical and detailed description of DGPS.Using a combination of navigation techniques along with GPS can also help overcome the problems associated with GPS. Many existing systems use GPS to periodically correct Dead-Reckoning. Some of the newer systems use GPS as the primary navigation tool and use Dead-Reckoning (or some other form of navigation) to provide coverage in "Urban Canyons". Map-matching can be used to fit the position data to a map and provide navigation information.
GPS VEHICLE NAVIGATION PRODUCTS
Many GPS companies are making vehicle navigation products. The following table provides information about several commercially available GPS automobile navigation products, as well as, links to the product web site and manufacturer web site.
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Manufacturer |
Product Web Page |
Description of Product/Technique |
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Magellan (Their intro page is quite interesting if you have the right web-browser, definitely worth a visit) |
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SnapTrack (This page describes their general product, which can be applied to vehicle navigation) |
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SiRF GPS Technology White Paper (A description of the advantages of SiRF technology) |
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CONCLUSIONS
GPS is very well suited for use in car navigation applications. Global coverage of GPS allows for mass production and broad use of a navigation system. GPS positioning solutions do not degrade in accuracy with distance traveled. Differential GPS can be used in areas where there are fixed DGPS stations with known locations. Combining GPS with other techniques allows for location information location accurate to less than 5 meters at all times. With the cost of GPS decreasing and the interest in GPS increasing, GPS automobile navigation will be a common accessory in future cars.
REFERENCES
Czerniak, Robert J. and James P. Reilly, "application of GPS for Surveying and Other Positioning Needs in Departments of Transportation." Synthesis of Highway Practice, 258. Washingtion, DC: National Academy Press, 1998.
Krakiwsky, Edward J. "Tracking the Wroldwide Development of IVHS Navigation Systems" GPS World, Oct. 1993, pp. 40-47.
Krakiwsky, Edward J. "GPS and Vehicle Location and Navigation" GPS World, May 1991, pp. 50-53.
Gibbons, Glen, Sherrie Steward, Dan Tyler and Ling Chan, "Automatic Vehicle Location: GPS Meets IVHS", GPS World, April 1993, pp.22-26.
French, Robert L. "Land Vehicle Navigation and Tracking" American Institute of Aeronautics and Astronautics, 1995.
Torii, Hayato and Yoshiaki Takechi "Vehicle Location Using GPS in Urban Areas" Furuno Electric Co. Ltd.
Poppen, Richard and Darrell Mathis "Integration of GPS With Dead Reckoning and Map Matching for Vehicular Navigation" 1993 National Technical Meeting Proceedings.
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