Currently there are more than a dozen Global Positioning System manufacturers turning out units. Not only does the list grow annually, but the product itself becomes more technologically sophisticated as well as more user friendly. Nowadays you can purchase a GPS unit for as little as $99. By the same token you can shell out as much as $1600 for a top of the line, fully loaded model. Fortunately, there are plenty of choices that fall between to accommodate both your budget and needs.

Evenly-spaced satellites deliver
good geometric quality...

As good as GPS is, there are certain limitations you need to be aware of. For one thing, GPS does not work inside most buildings. For another, it does not operate in caves or other subterranean areas. That's because satellite signals cannot be received inside buildings or below ground.

...while poorly spaced satellites
result in poor geometric quality

Another facet of GPS operation you need to know about is called Geometric Quality. This refers to the position of the satellites. Simply speaking, Geometric Quality is excellent when the four satellites needed for triangulation are located at 90-degree intervals. If the satellites are clumped together or something is blocking one or more, Geometric Quality becomes poor, interfering with the GPS unit's ability to provide an accurate reading.

On May 1, 2000, the US Government stopped selectively degrading the GPS signal received by civilian GPS users. Previously, only the military had access to the most precise GPS signals.

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