MARTIN RIEDL / SCIENCE PHOTO LIBRARY MARTIN RIEDL / SCIENCE PHOTO LIBRARY
Global positioning system (GPS) receiver held in a hand. 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. This is a Garmin eTrex GPS receiver that is receiving signals from eight GPS satellites and giving a location in the far north and west of Canada, just within the Arctic Circle.
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