bild

Global positioning system (GPS) receiver held in a hand. The display shows a compass pointer pointing towards a known location. 24 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. (KEYSTONE/SCIENCE PHOTO LIBRARY/CORDELIA MOLLOY)