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MODEL RELEASED. Global positioning system (GPS). Man using a hand-held GPS receiver and a map to navigate. The GPS system utilizes a network of 24 satellites in precision orbits around the Earth. Each satellite transmits radio signals that can be detected by this receiver. Three signals allow calculation of the user's latitude and longitude. A fourth signal allows altitude calculations. Time data transmissions allow local time calculations. (KEYSTONE/SCIENCE PHOTO LIBRARY/CORDELIA MOLLOY)