The Global Positioning System is a worldwide radio navigation system comprised of 24 satellites orbiting earth. Each of these satellites transmits a low power signal back to earth. When your GPS receives four of these signals, it can triangulate your position with accuracy to 3 meters.

The GPS system is divided into three segments: space, control, & user segments.

GPS Satellite The space segment consists of 30 satellites (this is constantly changing as new satellites are launched and older ones are decommissioned) on 6 orbital planes (six circles around the earth) at about 11,000 miles above sea level. Each satellite transmits two distinct signals to earth. All the satellites use the same frequency but each has a unique identifier. Each satellite transmits its position, the current time, and a status message.
Satellite Dish The control segment consists of a master control station, three ground antennas, and five monitoring stations (Hawaii, Kwajalein, Ascension Island, Diego Garcia, Colorado Springs). The five monitoring stations transmit data to the master control station at Schriever AFB in Colorado.
Garmin Venture The user segment consists of devices designed to receive the satellite's signals to calculate its position. Most GPS units can provide location information with three different stellites, the more ssatellites the GPS receiver can see, the more accurate the displayed position will be. The Garmin model on the left is about mid-range in price for a handheld unit. In addition to position, the model also can provide information to the user such as altitude, ground speed, average speed, ETE (Estimated Time Enroute), ETA (Estimated Time of Arrival), time of day, and the date. It also displays the best time to go fishing at the current location.
Boat ExampleThe modern GPS receiver has 12-20 "channels" to receive signals from different satellites simultaneously. When the receiver acquires the first satellite, it can rapidly acquire more satellites because each satellite sends position information telling the receiver what satellite to "look" for. The GPS measures the amount of time, in milliseconds, it takes for the signal to reach the unit. When it receives three separate satellite signals, it triangulates its position.

Think of it as a boat in an inlet with three lighthouses, one on each side of the opening & one on in the middle. If the inlet is big enough, the distinct sound from each lighthouse's fog horn will be heard at different times, assuming they were all sounded simultaneously. Since we know that the speed of sound is just over 750 MPH, we can calculate the distance from each lighthouse by timing the length of time it takes us to hear each horn. When we intersect the three distances, we can plot that on a map to determine where we are! GPS is just the high tech version! When the GPS knows its location, it can point you to where you want to go.

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