How GPS Works

A GPS receiver helps answer the ages-old question, Where Am I?  The Global Positioning system works on a very simple concept, known as Triangulation. 

Let's that you started at a point A, and walked 5 miles.  You don't have a compass, so you don't know which direction you walked towards.  So all you know is that you are somewhere on an imaginary circle with center point A and 5 miles in radius. 

location with distance from one fixed known point

Now suppose that a little bird tells you: you are also 10 miles from another point B.  It's also well known that point B is 7 miles due East of Point A.  Now you're beginning to locate yourself better.  If you are on that other circle also, then you know that you can be at only one of two possible points, where the two circles intersect.

location with distance from two fixed known points

To know exactly where you are, you just need the distance from a third point C.

location with distance from three fixed known points; location uniquely identified

This example was for a 2D world, but the same concept can be extended to 3D, where the distance from 4 different points must be known.  The GPS system uses orbiting satellites as the "birds" supplying the distance information. There are 27 GPS satellites in orbit.  The orbits are planned so that for any Earth location, at any time, four (4) satellites can be "seen" by a receiver. 

The satellites continuously orbit the Earth, completing two trips every day.  Imagine these satellites weighing more than 3,000 pounds, 12,000 miles high, and moving at 7,000 miles an hour!  The orbit (height and speed) are precisely known.  This means that the satellite positions are precisely known, so they serve as our fixed reference points. 

But wait, you say, the satellites are constantly moving relative to a fixed receiver, so how can we get distance information?  Each satellite is continuously transmitting a signal, which is monitored continuously by the receiver.  Knowing the speed of the signal (the speed of light) and the time it takes the signal to reach Earth, allows us to calculate distance.  So this method depends on knowing the time precisely: the satellites all use expensive atomic clocks, and the time on the receiver itself is corrected for. 

Once your GPS receiver figures out your position (latitude, longitude and altitude), it can do neat things with that information.  It can show you your location on a map stored in the memory.  It can also record your location periodically (automatically or manually) as you move, from which it figures out:

Atmospheric conditions and other factors can limit accuracy of standard GPS receivers to 15 meters or so.  The accuracy of the system can be improved further (3 to 5 meters) using fixed ground stations and properly-equipped receivers.  The WAAS (Wide Area Augmentation System) and DGPS (Differential GPS) systems are based on this idea.