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Global Positioning System (GPS) Technology and Cars
(Released April 2002)

  by Chao-Hsu Yao  


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Over the years, the technology involved in manufacturing an automobile has become more advanced, as automakers shift their focus from basic transportation to the design of features that make a vehicle safer, more comfortable, and more easily operated. One such feature is the global positioning system (GPS).

A GPS unit consists of a space segment, a control segment, and a user segment. The space segment is a constellation of two dozen satellites orbiting the earth twice every 24 hours, at approximately 10,900 nautical miles above the earth's surface (1). These satellites are funded and controlled by the U.S. Department of Defense. The control segment is a series of monitoring stations located at different sites on earth. These stations update and correct errors in the navigational message of the satellites. The user segment is a receiver that receives radio waves from the satellites in orbit. It can determine how far away it is from each satellite by keeping track of the time it takes for a radio wave to travel from the satellite to the receiver (2).

Four satellites are used simultaneously to pinpoint the precise position of the receiver on the earth. Information from the first three satellites narrows down the range of possible locations to two points; one of these is usually illogical and indicates a point not on the earth. A fourth satellite is used to confirm the target location (3).

The accuracy of a typical GPS receiver is about 10-15 meters. This may not be practical for locating a small object such as an automobile, which is about three meters long. Differential GPS (DGPS) is a system that improves the accuracy of the GPS receiver to about one to two meters (4). Several reference GPS receivers are placed at stationary locations, whose coordinates are known. These receivers compare their known locations to the location information they receive from satellites, and broadcast the range errors they detect from each other and from every satellite. A DGPS receiver can pick up this range error information and correlate it with the satellite signals it is receiving, to find out its true position (5). The accuracy is dependent on how fast the reference receivers broadcast their signals.

When installed in a car, a GPS unit can provide useful information about the car's position and the best travel routes to a given destination by linking itself to a built-in digital map (6). A monitor in the car shows the relevant portion of the map. The driver can enter the target location, and the computer will calculate the optimal route and display it instantly. It can respond to user preferences and map a route that avoids highways or avoids local roads. If the map is detailed enough, it will also provide the locations of the nearest gas station, supermarket, restaurant, hotel, and ATM machine. Some GPS units can issue auditory directions (i.e., "Turn left,") to guide drivers as they travel (7).

GPS also tracks the distance traveled on a particular trip, vehicle mileage, and speed. It can keep a record of driving activity, including the address of each destination, names of streets traveled, and how long the vehicle remained at each location, to allow owners to monitor the use of their cars by other drivers (8). Some systems issue warnings when the car is speeding and when the car is approaching a speed trap (9).

GPS can aid in the recovery of a stolen car. Integrated with the automobile security system, GPS can notify the car owner by phone or e-mail when the car alarm is triggered, and indicate the location of the car (10).

An owner can contact the GPS unit from outside the car using a cell phone or via the Internet, and can start the car engine remotely and unlock the door, solving the problem of frozen locks.

GPS also monitors the car condition, and issues warnings when the battery is low or when it is time for an oil change (11).

The Ford automobile company has developed a new telematic system through GPS on the Ford Focus, that will alert emergency services when an airbag deploys. The emergency services can locate the vehicle quickly and provide assistance to an injured driver or passengers (12).

In addition to its applications in private vehicles, GPS is being used by commercial shipping companies to speed the delivery of cargo. It allows companies to track their fleets, record the movement of their trucks, and control route planning (13). Car rental companies can equip their cars with GPS technology to find stolen rental cars and charge customers for dangerous conduct as speeding (14).

Commercial fishermen can use GPS receivers to keep track of the best spots where they have caught fish. GPS technology is also a valuable aid to targeting locations for military personnel, firefighters, and construction workers (3).

The development of GPS technology opens the door to the design of an unmanned vehicle. Equipped with laser and infrared scanners to "see" its environment, such a vehicle could be used for tasks that are considered too time consuming or hazardous for human drivers. Students in the electrical and electronic engineering department of the Nanyang Technological University in Singapore are working on this technology (15).

Though GPS technology is gaining popularity in cars, some problems still exist. The biggest problem is the blockage of signal transmission by obstacles such as mountains, high buildings, tunnels, urban canyons, or thick-branched trees. Multipath signals generated by reflections from nearby surfaces or fences can also interfere with the GPS data. Some methods have been introduced to improve these disadvantages. Dead Reckoning (DR) is a way to keep tracking vehicles during periods of blocked transmission. DR combines directional and distance information from a heading sensor in the car and from the car's odometer, and calculates the current position of the vehicle by computing the course steered and speed over the ground from a last known position. DR is used to improve accuracy when GPS transmission is available, in addition to being a backup when transmission is blocked. However, its accuracy tends to drop if the car travels for an extended period without receiving GPS signals (16).

GPS technology is advancing very quickly. New models are being developed every year to correct problems in the older versions, focusing on improved accuracy, better reception, and even more user-friendly features.

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