Question

The radio altimeter on an aircraft is a
A) MTI radar
B) Continuous wave radar
C) FM-continuous wave radar
D) Doppler radar

Answer 1

First let us see what a radio altimeter is:

The radio altimeter is an aerial electronic instrument capable of determining the aircraft’s height above the ground directly below the aircraft.

Complete step by step solution:
Early radio altimeters measured the altitude by measuring the time between sending the radio signal from the aircraft and receiving the reflected signal. Modern devices use other means, such as calculating the difference in phase between the emitted and the reflected signal. The FM radio altimeter on the aircraft is a continuous wave radar.
In nearly all situations, the show of radio height stops as the aircraft rises through $2500'$ above ground level and starts again as it descends through $2500'$ agl. This is physically supported by the appearance/disappearance of the ‘OFF’ flag and the presence of a cursor from behind a mask or by triggering of a graphical monitor.

Radio altimeter calls can be announced by one of the flying pilot or monitoring pilot either in conjunction with the SOP or on an ad hoc basis, or automatically produced by a synthetic voice.

Many operators have SOPs that enable a “Rad Alt Live” pilot call to be made during the descent as soon as possible after height signals reappear at $2500\,feet$ agl in order to increase the crew’s knowledge of proximity to the terrain.

The use of the radio altimeter is an integral part of both the role of ground proximity warning systems and the operation of aircraft during Cat $\dfrac{2}{3}$ approach where it is used to determine the location off the aircraft in relation to the applicable decision height.

Hence, option C is correct.

Note: Here we may get confused between options B and C but the FM continuous wave radar is used to measure the height of the aircraft from the ground by passing a beam. And also FM is much more effective than any other communication system.