Barometric altimeter short-term accuracy analysis

Although it is mandatory to have at least one barometric altimeter installed in each and every aircraft flying within ally air traffic controlled regions to assure vertical separation, the absolute accuracy of such an instrument is rather poor and limited. Basically, a barometric altimeter is a pressure sensor. It measures the atmospheric static pressure and translates it to the vertical height using a predetermined formula. However, as long as all aircraft are equipped with the same kinds of instruments, the vertical separation in any controlled air space is assured. Until recent years, all flight levels (FL) above 290 must be separated by 2000 feet. It was accomplished this way primarily because of the unknown performance of barometric altimeters. By reason of increasing congestion in the air traffic routes near the North Atlantic, the Federal Aviation Administration (FAA) of the US = conducted a series of flight tests near Cardiff Wales, UK, to determine whether the separation above FL 290 can be safely reduced to 1000 feet, which would result in doubling the available routes. During these flight tests, a Boeing 727 (tail number N40) was equipped with many recording devices as well as Global Positioning System (GPS). The GPS receiver had capabilities of measuring carrier phase of the received signals so that 3D positions with respect to WGS-84 during the entire flights can be determined down to centimeter accuracy. Among the four (4) takeoff-landing tests, the aircraft conducted these tests using two flying patterns alternatively: Figure-8 with flight level gradually increased from FL 290 to 410 and Racetrack with constant flight level. We observed that the barometric altimeter measurements resemble the kinematic GPS altitude closely except an apparent low frequency difference. In other words, the barometric altimeter had short term accuracy, which would be as good as the kinematic GPS. The objective of this paper is to document these findings. Based on the actual data, we conclude that barometric altimeters are extremely accurate sensors for altitude in the sense of short-term accuracy.