The GPS System
A squadron of 24 satellites orbiting 12,000 miles above Earth comprises the heart of the GPS. Each satellite, powered mainly by solar energy, completes its orbit every 12 hours.
The Signal
Each GPS satellite broadcasts radio signals that code the exact time of the pulse and information about the satellite's course and status.
The Receiver
If the handheld GPS receives signals from at least four satellites, it can attempt to calculate its position, based on factors like the time differential between the radio pulse's emission and reception, the satellite's orbit and the speed of light. Four satellites are required to calculate position in three dimensions: latitude, longitude, and elevation.
Warning
Potential errors in the communication between satellite and receiver include satellite/handheld unit clock discrepancies, signal bounce off buildings and topography (the multipath effect) and atmospheric signal delay.
History
The first GPS satellite achieved orbit in 1978, and the entire arsenal of 24 was in place by 1994.
About The Author
Ethan Schowalter-Hay is a writer and naturalist living in Oregon, although he originally hails from the prairies and brushy woods of Wisconsin. He has written for the "Observer" newspaper (La Grande, Ore.), Demand Studios, Examiner.com and the Bureau of Land Management. He holds a Bachelor of Science in wildlife ecology and a graduate certificate in geographic information systems from the University of Wisconsin.
