HOW YOUR GPS WORKS

The GPS receiver that you can hold in the palm of your hand is a highly complex and powerful device. You don't need to understand it fully in order to use it but some understanding can be helpful in getting the most from a GPS. First of all it is a radio wave receiver similar in some ways to a standard FM radio. It only receives signals but unlike a standard radio these signals come from satellites moving rapidly overhead. The receiver uses these satellite signals to compute a position on the earth. You need not worry that the satellites know where you are. They do not, only the receiver itself figures this out. How it does this is the subject of the next few paragraphs.

Unlike the standard FM radio which only receives one station at a time the GPS receives several signals simultaneously. As a matter of fact it needs at least three satellite signals to compute its position and to compute a full solution it needs four. Once it has computed the position it is said to have a "fix" or "lock" on the satellites and will then dynamically update the position once a second as you travel. Also, unlike the FM radio a GPS receiver needs to have a fairly clear view of the sky in order to receive these signals. Signal reception is effectively blocked by water (however, not rain because of the space between the rain drops), the human body, the metal roof on your car, a building, and many other things. The good news is that there are usually a lot more that 3 satellites overhead (sometimes as many as twelve) so if some are blocked others may be available. And just like the fact that you can see through some objects such as a window there are some objects that do not block signals for the GPS.

The GPS uses mathematical formulas based on the distance to the satellites and the position of the satellites to compute your location very much like a ship captain might take sightings on land objects or stars to compute the ship's position at sea. Because these computations are based on the current positions of the satellites the unit downloads the position data from the satellites themselves. This takes about 30 seconds to receive, but if it misses some of the data it could take longer. After the download is complete the computation can start. The computed position accuracy depends on what satellites the GPS can view, where they happen to be, and the ability of the receiver itself, but generally a consumer GPS receiver with a good sky view can compute a position that is accurate to about 30 feet. This is usually good enough to find your house or even your car but probably not good enough to settle a property line dispute.

Most of the time you will never need to worry about any of this. You will just turn the unit on and it will find the satellites and compute the position solution in about a minute or even less and everything will be ok. You can just begin using it. However, if there are problems it can be helpful to know what can be done to resolve them.