In laymen's terms, GPS determines the user's exact location, speed or time using radio signals transmitted from the satellites. With 24 satellites in continuous orbit, at least six of them are available at all times from anywhere in the world.

The technology was originally conceived to provide navigation capabilities for the military. Since then, it has found its way into everyday life.

Emergency services use GPS to locate the closest response vehicle to an incident. Onboard navigation and anti-theft systems available in new automobiles were made possible by GPS. Wildlife-management agencies are tracking endangered wildlife with tiny GPS receivers attached to animals.

With the development of hand-held GPS units, many recreational uses for the devices have been found. Hunters, fishermen and hikers were the first recreational users to embrace GPS.

Some uses being developed involve adding GPS technology to hand-held computers and cellular phones. How far into everyday life GPS reaches will only be limited by the human imagination.

Hand-held GPS units available today are accurate to within 5 to 30 feet and range in price from $100 to more than $1,000. The number of additional features and capabilities will determine price.

People purchasing GPS units should consider how they plan to use them. Advanced technology and competition among manufacturers has resulted in a number of specialized units.

Basic features to look for in a GPS unit begin with size and weight. Hikers in particular should look for a unit that is easily and securely carried in a pocket or pack.

Units used in situations where they are susceptible to getting wet should be waterproof.

Screen size and how easily the screen can be read should be considered, especially for those with problems reading fine detail.

Long battery life is another priority in handheld units. Although extra batteries should always be available, longer battery life will allow the user to carry fewer batteries.

External antenna capability coupled with a 12-channel parallel receiver system will allow the GPS unit to operate more efficiently under heavy tree cover or difficult terrain. Check the literature included with the unit to determine features and capabilities.

Additional features available include built-in maps, routing, storing of waypoints and the ability to upload topographic maps to the unit.

The boom in GPS units and their uses is similar to the explosive growth of personal computers over the last 20 years. With continued research and development, prices will continue to fall as available features and uses continue to rise.

An informative article detailing the inner workings of GPS can be found on the Web at www.aero.org/publications/GPSPRIMER.