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Global positioning system (GPS) receiver held in a hand. The display shows the location of 12 of the 24 satellites of the GPS network. The receiver is locked on to 5 of them, with signal strengths seen at lower centre. There is a weak signal from a 6th satellite. These satellites trace precision orbits around the Earth, and transmit radio signals that are detected by this receiver. Three signals allow calculation of the user's latitude and longitude. A fourth signal allows altitude calculations. Time information transmissions allow calculation of the local time. (KEYSTONE/SCIENCE PHOTO LIBRARY/CORDELIA MOLLOY)