CORDELIA MOLLOY / SCIENCE PHOTO LIBRARY CORDELIA MOLLOY / SCIENCE PHOTO LIBRARY
Global positioning system (GPS) receiver held in a hand. The display shows a compass pointer pointing towards a known location. GPS satellites trace precision orbits around the Earth. Each satellite transmits radio signals that can be detected by this receiver. Three signals allow calculation of latitude and longitude. A fourth signal allows altitude calculations. Transmission of time data allows the calculation of local time.
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