One of the wonders of modern science is a handheld electronic instrument, known as the Global Positioning System (GPS) receiver, that calculates your exact position on this planet. GPS receivers use a fleet of 24 satellites that orbit 11,000 miles above the earth with extremely accurate clocks.

When a GPS receiver is activated, it listens for radio signals from the GPS satellites. These signals contain precise information about the location of each satellite in space, along with the exact time when that information was sent out.

When the receiver has time and location information from at least four different satellites, it uses its own computer to figure out your location. It does this by taking into account how long each signal took to travel at the speed of light and exactly when it arrived.

A typical GPS receiver calculates your position to within 15 meters or less (45 feet) accuracy. Units that are WAAS-enabled can calculate your position to within 3 meters (10 feet) or less. WAAS stands for Wide Area Augmentation System, and it refers to a system of satellites and ground stations that provide GPS signal corrections, giving you even better position accuracy. At present, this system is only available in North America.

A GPS Glossary
Garmin - About GPS

A GPS primer:
The Aerospace Corporation

Datums and Projections: A Brief Guide
USGS Biological Resources

GPS World Magazine:
www.gpsworld.com/gpsworld

Makers of GPS units:
Garmin
Magellan