The machine may range from a television set to a guided missile, and the distance may be a few metres or hundreds of kilometres. Every remote control system has a device called a command unit, by which the operator sends signals that control the machine.
Most command units send signals by radio waves.
Others use ultrasonic waves, laser or electromagnetic waves of different frequencies. As the machine runs, the operator checks its performance and, if necessary, sends signals to keep it working correctly. Generally, remote control systems improve the ease and efficiency of operating various mechanisms. They also help perform certain tasks that otherwise would be too difficult or dangerous for human beings.