Radar altimeters
The simplest air radar altimeter is only a single antenna, which in turn assume the functions of receiving and broadcasting. Electromagnetic waves generated in the transmitter and then sent by the antenna are passed on at a speed of 300,000 km / s. When you reach the surface of the earth already, some of them is reflected back and reaches out to the aircraft. The antenna receives the signal, which is then amplified and sent to electronic circuits, and these on the basis of time elapsed between the giving and receiving the signal, determine the distance between a plane surface. The speed of these electromagnetic waves are sent is so huge that sent the signal comes back almost immediately from the receiver.