A 21st Century Satellite Orbiting Earth Most satellites have internal sensors to measure voltages, currents, temperatures and other information on the health of equipment. The measurement data is encoded into a telemetry signal and relayed by transmitter to ground controllers on Earth. Communications satellites have additional radio transmitters and receivers to send many signals to and from Earth. Observation satellites often have optical and infrared cameras and radar for altitude measurements.
Navigation satellites have highly-stable radio receivers and transmitters.
Communications satellites relayed military TV from Somalia in 1992 and later years, while U.S. spysats were moved into position over the Balkans to stare down at the former Yugoslavia. Satellites usually are powered by the Sun, but nuclear-powered electricity generators sometimes are built into spacecraft. Today Online is edited by Anthony R. Curtis who welcomes comments via e-mail .