Basic ElementsAll artificial
satellites have certain features in common. They include radar for altitude measurements, sensors
such as optical devices in observation
satellites, radio receivers and transmitters in communications satellites, and stable radio-signal sources in navigation satellites. Solar cells generate power from the Sun, and storage batteries are used when the
Satellite is blocked from the Sun by the Earth. These batteries in turn are recharged by the solar cells. In special cases nuclear power sources are utilized. Attitude-control equipment is needed to keep the satellite in its desired orbit and, in some cases, to point the antennas or sensors properly. Telemetry encoders measure various quantities that describe the condition of the onboard equipment and relay this information to Earth.