GPS, or Global Positioning System, is the latest thing in keeping from getting lost when trekking out to visit your Aunt Louise. Years ago you had to rely on landmarks, your handy compass or the guy at the corner gas station, assuming he knew where everything was. Today for about a hundred bucks you can get a little gadget that will tell you exactly where you are, any place on Earth. By using some basic mathematical calculations and a rather complex system, you can be in the middle of the desert, lost as can be, push a button and have your GPS tell you exactly where you are so that you can hopefully find your way back home.

So what exactly is GPS and how does it work?

GPS is actually a very complex and expensive system but the fundamental concepts of the system are very simple.

The GPS system is a group of 27 Earth orbiting satellites. Twenty-four of them are in operation and 3 are backups in case one fails. The system was originally created for the US military but was eventually opened up to everyone else.

Each one of these satellites, weighing in at about 4,000 pounds, circles the Earth at about 12,000 miles per hour and makes 2 complete rotations everyday. The orbits are arranged so that at any one point in the day 4 of these satellites are visible from any place on Earth.

The complimenting receiver that was created to receive signals from these satellites has the job of locating 4 or more of these satellites, figure out the distance to each and from that information determine where on the planet the receiver itself is located. It then transmits this location back to the person who happens to be in possession of the receiver. The operation is based on a simple mathematical principal called trilateration. To simply explain how this works, let's say someone tells you that you are 10 miles from Newark, NJ, another person tells you that you are 15 miles from Maplewood, NJ and a third person tells you that you are 20 miles from Woodbridge, NJ. By taking those three facts you can figure out exactly where you are because each radius of each distance when linked together will intersect at only one point. That point is where you are located.

In order to actually do the calculation from space the GPS receiver needs to know 2 things. The location of at least 3 satellites above you and the distance between you and each of those satellites. The reason that only 3 satellites or spheres are needed is because the Earth itself can act as a fourth sphere. The GPS receiver itself works by radio waves which are a kind of electromagnetic energy and travel at the speed of light The receiver can actually figure out how far the signal has travelled by timing how long it took for the signal to arrive from each of the satellites.

In the next article on GPS we'll go over exactly how this system works to measure distance.

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Michael Russell
Your Independent guide to Global Positioning Systems
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