GPS is the symbol for the Global Positioning System. It is a satellite navigation system designed and built by the United States Department of Defense. It was first conceived in the 1960's and the first GPS satellite was launched in 1978. This navigation system became fully operational in 1995 and was used exclusively by the U.S. Military until the U.S. government authorized its use by the general public.
The GPS network is really just composed of three things:
Earth Monitoring Stations
The 24 satellites orbit the earth in 6 distinct trajectories. They orbit the earth twice a day at around 7,000 miles per hour at a distance of 12,000 miles. This allows their signals to be received over a large percentage of the earth's surface. The more satellite signals a GPS unit receives the more accurately it can calculate its position by triangulating the signals. The satellites orbits are placed so that nearly every part of the earth is receiving 4 signals at all times. Of course landscape and groundcover can affect this.
The GPS earth monitoring stations, of which there are 4 unmanned and 1 master stations, are responsible for relaying position data from the satellites to the master station. The master station uses this data to correct the position of the sat tiles which is then transmitted back to the satellites. This keeps the data that the satellites transmit synchronized to provide accurate coordinate information
The GPS receiver is the device that we are all familiar with. In the car, hand-held units, data collectors, and GPS surveyor equipment all make use of the Global Positioning System. They receive a low power radio signal from the satellites overhead if they are in line-of-sight.
A GPS receiver triangulates its position based on the position of at least three satellites. It calculates your position by comparing the time it takes your receiver to receive the signal from each satellite.
3 satellites will allow your GPS unit to calculate your coordinates in latitude/longitude while four or more satellites will also allow it to calculate altitude. With this information the current generation of GPS devices can calculate your trip distance, distance to your destination, how fast you are going, and your altitude.
As you can see the GPS systems in place today provide a very useful tool to anyone who wishes to tap into its network.