Radio Altimeter Mechanism

If you guys have ever watched the landing videos through the perspective of cockpit then I am sure you've heard 50..40..30..20... Retard.. Retard in the case of Airbus. So, this post is to clear the curiousness regarding the meaning of what you've heard. 

The thing is at the downward and back section of the airplane, there lies the Transmitter Antenna and Reciever Antenna.

The transmitting antenna transmits the radio waves towards the surface of the earth and abruptly it is reflected back and receiving antenna grabs the radio waves signals on the way back. Then, Radio Altimeter Computer calculates the time and then distance is calculated by multiplying the speed of light ( i.e. 3 * 10^8 m/s) with the time taken and then divided by 2. So, it starts from 2500m
and recommences as it descends. So, the announcement 2500..1000..500..100..50..40..30..20..10. RETARD. The numbers define the altitude from the ground level.

You may have confusion regarding Retard. But retard means the alert to the pilot to reverse the throttle and apply reverse thrust when you are just 10 ft short of the runway.  There are still lame jokes regarding Airbus telling pilot retard. But they are for the absurd ones.