What are Presure Altimeters and Radio Altimeters?

Altimeter is used for measuring altitudes. There are two basic types of altimeters viz. pressure altimeters and radio altimeters. Pressure altimeters are aneroid barometers calibrated to indicate altitude instead of pressure. It is based on the principle of drop of atmospheric pressure with gain of height.
The capsules inside the altimeter casings expand as we go up. This expansion is magnified to move the indicator over the dial. The mean sea level pressure is 1013.2 millibars or 29.92 inches of mercury which is taken as zero altitude and there is drop of 1 millibar for approx. every 32 feet gain of height. The instrument is calibrated accordingly to indicate loss/ gain of pressure in terms of height gained or lost. Depending on the pressure setting or datum, the instrument reads altitude (above MSL) or absolute altitude / height (above ground level – AGL).
A radio altimeter is a radar aid, which is effective only from 20 ft to 2,500 ft. It is basically used as a ground proximity warning system in mountainous terrain.