ALTIMETER. In aerial navigation, an instrument employed to determine the height of aircraft above the ground or, more accurately, above sea level. It is a form of aneroid barometer, for it is based upon the changes of air pressure which are registered as an aeroplane ascends or descends. All altimeters are set at zero when the aircraft leaves the ground and they are calibrated in accordance with a standard atmosphere which has been adopted internationally. The standard atmosphere assumes that, at sea level, the temperature is 15° C and that the barometric height reduced to o° C is 76omm. of mercury. The decrease of tempera ture with increase of height is assumed to be 6.5° C per kilometer. Altimeters so calibrated are usually accurate to within 2% to 3%. For some purposes however a greater accuracy is necessary; this can be achieved by applying a correction obtained from a knowledge of the temperature on the ground and at the operating height. Carefully designed altimeters, so corrected, are capable of an accuracy of the order of 1 % so long as the conditions of atmospheric temperature and pressure do not vary. An altimeter can not distinguish a change of height from a general fall of atmos pheric pressure caused by an approaching depression.
In principle the solution is clear, the height above the ground or sea must be measured directly and not inferred from a knowl edge of the atmospheric conditions at the operating height. No in strument for this purpose however is yet in general use. Perhaps the nearest approach to success has been achieved by the Behm height-finding apparatus developed in Germany, and tested during the flight to America of the airship ZR3. In this instrument a very sensitive timing mechanism is started automatically at the instant at which a sound is emitted from the airship. The sound wave travels to the ground and is reflected back to the airship, and the timing mechanism is automatically stopped when the reflected sound wave reaches the airship. The weight of the apparatus—an all important factor when considering the design of aircraft instru ments—compares unfavourably with the almost negligible weight of an altimeter.