GPS systems work around the basic principle called trilateration. Trilateration uses the known location of a transmitting device and the distances between these devices and the person using their GPS device to calculate the exact position of the receiver. This is very much like the more widely know triangulation, except that trilateration does not use angles at all in its calculations.

To better understand how this works imagine that you are standing at a some point int he middle of nowhere. You have no idea how you got there and all you have with you are the clothes on your back, enough food and water to travel for one day, and a GPS device. You wake up wondering where you are and you look at your GPS tracking watch and read that you are at a certain latitude and longitude at 10 feet above sea level. How does you GPS know so much more information than you?

Well, traveling 12,600 miles (20,200 km) above you is a system of 24 satellites maintained by the US government that are constantly transmitting microwave signals at the speed of light. These transmissions contain information read by that watch on your wrist - information used to accurately calculate your position on the surface of the earth. The basic information contained in these messages is the time at which the signal was initiated by the satellite and the satellites position. The distance between the receiver and the satellite is then calculated by multiplying the speed of the signal by the time is took to get to the receiver.

So your GPS device has invisible, silently, and quickly calculated your position, but what is that going to do for you in the hypothetical situation outlined above? Well, if your GPS device is simply a data logger (the simplest and most widespread GPS application) then you are still just as hopelessly lost as you where when you woke up. With no map you could head out in the wrong direct and end up dying a painful, horrible death alone in the wilderness - yuck.

But if you had a data pushing GPS device you can sit back and relax because you know that your device not only knows exactly where it is in the world, but it also is transmitting that information to a central database for the good guys (family, friends, or authorities) to see. They will know where you are to within 500 feet. The is pretty close. With a data pushing GPS device you can find some shade and rest in mother nature until the rescue team arrives. In our hypothetical situation you want a this type of GPS device.

It is important to note that each type of GPS device has its own application and use. For most people in most situations a data logger will do just fine. It tracks where you have been and has applications in hiking, biking, and running. But for situations where someone else would benefit from knowing your location - in tracking down a stolen vehicle, knowing the whereabouts of a child, or finding a lost hiker - than you definitely want to invest in a data pushing GPS device.