From the moment they leave the launch pad, unmanned spacecrafts are controlled by coded radio signals. The two-way communication is maintained with the help of large dish antennas located at earth stations. Two kinds of signals are used. Instruments on board the spacecraft continuously monitor the health of the various systems such as cameras, antenna, and batteries. These also check its orientation and flight path and send back telemetry signals to ground stations. These signals tell the scientists if the spacecraft is behaving the way it is designed to. If any change in the path or orientation is necessary to keep the spacecraft on course, command signals are sent which operate tiny gas jets on board to turn the spacecraft in any desired direction. Pictures and other data collected by on-board instruments are also transmitted back to earth via radio signals.
