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Advanced Topics

GPS hardware topics

How a 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 including altitude 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.

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

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.

COMPUTING A FIX

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 must

download 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.

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Mobile Crossing WayPoint Map Loader Spécifications Page 108