Function

• All handheld GPS units start with the same basic function. They contain a receiver that is capable of receiving radio wave transmissions from GPS satellites. The receiver then uses information contained within these transmissions to calculate your location and provide you with your coordinates.
  
  

GPS System

• Information is sent from several satellites. The Global Positioning System consists of 24 individual satellites on six orbital paths around the Earth. Each satellite completes two full orbits around the Earth each day. The orbits have been carefully crafted so that anyone using a GPS on Earth will have a clear line to at least four individual satellites at any given location and time; this is very important, as we'll see later. All the satellites send out radio wave signals at frequent intervals.
  
  

Signals

• The information contained within the satellite signal includes identification for the individual satellite, its location and time of transmission. The time of transmission is quite important because there will be a difference between the time the signal was transmitted and the time it was received by your GPS. This difference in time can easily be used to calculate the distance that the satellite is away from you. It is known that the radio waves travel at the speed of light. Speed multiplied by time is equivalent to distance. So by comparing the times, your GPS unit is able to calculate the distance to the satellite.
  
  

Trilateration

• Now you might think that you're good to go. You know how far the satellite is, and now you can figure out where you are. However, it's not that simple. Knowing how far you are from the satellite really isn't that helpful in determining where you are on Earth. Imagine you and your buddy Paul become separated in the fog. Paul is stuck in a ditch and needs some help. He can see you, but you can't see him. He yells out for you to help and says that he's 50 feet away from you. Great, I'll go get him, you think. Then you spin around in a circle but can't see him through the fog; he could be in any direction (let's assume you can't tell where his voice is traveling from). Now Paul tells you that he's 50 feet from you and 30 feet from the large evergreen tree. You see the tree and know that Paul is 30 feet from it and 50 feet from you. You have a better idea of where he is, but he could still be on either side of you and the tree. Finally, in desperation, Paul lets you know that he's just 20 feet from the base of the mountain to your left. Now using the three distances Paul gave you, you know which direction to travel in and you find him quickly. This is how GPS works; it uses the distances and location of several different satellites as a reference to provide you with your location. Like in the example above, it takes three separate satellites to give you a reference on a two-dimensional plane (longitude and latitude). It takes at least four satellites to give you a three-dimensional reference, which includes elevation. The more satellites that are used, the more accurate the location is.
  
  

Timing

• One inherent issue in the GPS system concerns the very basis of its operation: time. Each GPS is equipped with an atomic clock that is accurate to the nanosecond. However, atomic clocks are far too expensive to be built into a GPS receiver, so the GPS uses a much less accurate clock. When the signal is traveling at the speed of light, it arrives in a fraction of a second, so it's essential that the clock is able to detect this difference. Because of the inaccuracy of your receiver's clock, the trilateration discussed above is not precise. However, this inaccuracy exists in the calculations to all of the satellites and becomes a variable in the equation. The GPS unit adjusts for the inherent inaccuracy and uses information from as many satellites as are available to provide a more accurate location. While the system isn't perfect and won't give you a 100 percent accurate pinpoint location, GPS is accurate within 10 to 20 meters, which gives you a very good idea of where you are.
  
  

Navigation

• Your GPS is constantly receiving transmissions, updating your location as you travel and allowing you to track your progress. Mapping GPS units include software topographic maps in which you can visually see your location on the map and navigate accordingly. Simpler GPS units provide coordinates and allow you to roughly navigate toward pre-designated locations known as waypoints.You can see your progress toward a given set of coordinates and set your bearing accordingly.