PHILIPPE PSAILA / SCIENCE PHOTO LIBRARY PHILIPPE PSAILA / SCIENCE PHOTO LIBRARY
Global positioning system (GPS) receiver being used by a firefighter to work out the fastest route to an emergency. The display shows the fire-engine's position as it drives towards its destination. 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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