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Find Your Way Easily and Inexpensively with GPS

The Global Positioning System (GPS) is a fully functional satellite navigation system that is becoming widely accepted by drivers everywhere.

How GPS Works
The GPS consists of 24 well-spaced satellites orbiting the earth and enables people to determine their geographic location with the use of ground receivers. Accuracy is within one to 100 meters (328 feet), depending on the type of equipment. The U.S. Department of Defense owns and operates the GPS, which is available worldwide for general use.

Each GPS satellite contains an atomic clock, a radio and a computer. Each satellite, with an understanding of its own orbit and the clock, continually broadcasts its position, which is constantly changing, and time. The satellites are situated so that no matter where you are, four satellites are above the horizon.

GPS receivers on the ground, such as those in cars, work because they contain a computer that gets time signals from three of those four satellites and thus calculates its own position, via a process called trilateration. The result is a geographic position of latitude and longitude.

Geographic positions are then shown on a map in receivers that are equipped with a display screen. If a fourth satellite signal can be received, then the computer or receiver can also determine its altitude.

In order to calculate an accurate geographic position of an object, a GPS receiver must know the location of at least three satellites above you, as well as the distance between you and each of those satellites. It does this by analyzing the low-power, high-frequency radio signals from satellites. Higher-end GPS units have multiple receivers that can pick up signals simultaneously from several satellites.

GPS in Cars
Many people enjoy the convenience of having a Global Positioning System receiver in their car to provide them with turn-by-turn directions to their destination.

For a GPS receiver located in a moving vehicle, the receiver may be able to calculate direction of travel and speed, which will help it provide an estimated time of arrival to a specific destination.

Civilian GPS receivers have limits on the altitudes and velocities at which they will report coordinates, which prevents them from being used to create improvised missiles.

Other Uses for GPS
The Global Positioning System is also used in the following fields:

• agriculture
• geology
• geophysics
• marine
• military
• navigation
• surveying.

GPS is especially beneficial to the military because it enables accurate targeting of precision-guided munitions, cruise missiles and other weapons.

Hand-held GPS receivers are also being used in a recreational game called Geocaching, which has become popular with players of all ages. It involves the use of a hand-held receiver to search for objects hidden by other players at a specific latitude and longitude. 

Cost of GPS
As GPS technology becomes more common its cost decreases. GPS receivers are available at all electronic stores and at most discount stores that sell electronics.

Today, the cost of a pocket-sized GPS that will tell you your location at any given time, in any given place on earth is less than $100.

Handheld GPS receivers can also be used in cars. The disadvantage is that they are not mounted in place, like ones specifically made for use in cars. The advantage is that a handheld receiver can be carried along when engaging in outdoor activities.

GPS receivers for use in vehicles provide self-contained voice guidance and turn-by-turn directions. All of them contain a color screen for map viewing. Some also come with an MP3 player and digital photo viewer.

Vehicle-mounted GPS receivers with basic navigation systems cost between $250 and $900. The higher-end receivers that come with additional functions, like an MP3 player or a larger screen, can cost more than $1,000.

Disadvantages of GPS
GPS can be inaccurate. Radio signals change speeds as they pass through the earth's ionosphere and troposphere. Radio signals may also bounce off tall buildings and other large objects on earth, which gives the receiver the impression that a satellite is farther away than it really is. Additionally, satellites sometimes broadcast poor or inaccurate almanac data and thus misreport their own position.

Advantages of GPS
The main advantage of a GPS receiver is its practical application in finding one's location and providing directions to a specific destination. Another advantage is its use in the game of Geocaching, which appeals to all ages and is a source of good, clean outdoor family fun.

GPS receivers are fairly affordable and can be used in a wide variety of other applications, including hiking, biking, mountain climbing, camping and other outdoor activities.

Of course, GPS' best-known advantages are those which have been advertised widely: being lost in a strange or dangerous place; being late for an important meeting; or for determining location and time needed to reach a destination.