Here's A simplified GPS tutorial on
How GPS Works
How GPS works? This short GPS tutorial should give you the basic idea of just how GPS works. The premise on which the GPS device works is rather simple. The actual implementation, however, is not. Fortunately for us, understanding the technical aspect of a portable GPS device is not a requirement to owning and operating one. Portable GPS systems are now a way of life. You find them almost everywhere. They’re in golf bags, fastened to your dog’s collar, on ships, trucks, airplanes, automobiles, motorcycles, wristwatches, cell phones and just about anywhere you can think of. There are so many makes and models of GPS receivers available, that being able to make a decision on which one to buy can be a real challenge. This web site has been created to give you a wide range of information including GPS reviews and GPS ratings to help you decide what make and model of GPS unit is best for you. History of the GPS In the 1960’s the U.S. Navy and the Army were developing navigation systems for use in the field. Unfortunately these units were not compatible with one another. In 1973 the Department of Defense directed the Air Force to head up a program to be used by all branches of the service. This new system would be based on an earlier U.S. Navy program in which atomic clocks would be carried on satellites. This new program was named Navstar Global Positioning System. And like all things in the military, it was reduced to the acronym GPS. The first portable GPS device was not as portable as we have today. This is a MANPACK receiver used by the military between 1988 and 1993. Although the program was designed for both military and civilian use, the civilian version was not allowed to have the precision as that of the military version. The signal sent to civilian GPS devices was purposely modified to reduce its accuracy. With the downing of Korean Air Lines flight 007 in 1983, an incident that may not have occurred if the pilots had better navigation information, President Reagan issued a directive that GPS signals would be made available to the world. This helped open up the civilian market we witness today. The first wide spread use of the GPS device came with the Persian Gulf War, during which, the portable GPS was used extensively. How GPS works?
First let me say that the portable GPS device is a passive device. That is, it receives signals only. It does not transmit anything. That takes an additional device such as a cell phone. The simplified version of how GPS works is fairly easy to comprehend. There are 24 active satellites that encompass the globe at one time. Thus at any given time, theoretically, 12 satellites should be “in view” of a GPS receiver anywhere on earth. The GPS “sees” these satellites and calculates its’ position on earth based on its’ relative position to the satellites. It is sort of like triangulation that a surveyor would use. But instead of angles the portable GPS systems use time to calculate its' earthly location. Each satellite transmits data that gives its’ location and time. All satellites are synchronized to transmit at exactly the same instant. Because the satellites' distance to a given point on earth is different for each satellite, the time it takes the signal to reach the GPS device also varies depending on its' location. Based on the time and satellite location information, it is then possible for the GPS device to calculate spheres of location on earth from each satellite. After information from four satellites has been "acquired" the location of the GPS device can be calculated. This position is based on where the four spheres intersect.  The portable GPS then converts that information (latitude and longitude) into an easy to understand display on your GPS device. Today most portable GPS receivers advertise an accuracy of +/- 49 ft, but many perform much better than that. If you would like a much more detailed explanation of how GPS works, this web site has an excellent tutorial: Trimble.com
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