The Method of Evaluation of Radio Altimeter Methodological Error in Laboratory Environment

Abstract

The presented article is focused on the evaluation of aviation radio altimeter (ALT) methodological error in order to increase air traffic safety. It briefly explains the background of methodological error at the theoretical level and offers practical conclusions to understand the issue. A radio altimeter provides information on an aircraft or helicopter’s instantaneous (radar) altitude or UAV to the pilot and another assistance system, such as an autopilot or an anticollision system. The height measurement of the most common used ALTs is realized with an accuracy of from ±0.30 m to ±0.75 m. This error rate corresponds to and is caused by the radio altimeter’s methodological error (ΔH). The ALT operating parameters are defined by carrier frequency, modulation frequency, and frequency lift. The methodological error of ALT can be obtained in three ways—calculated on a theoretical level, simulated in a suitable simulation environment, or evaluated in laboratory conditions. The ambiguity of ALT methodological error measurement causes bias in its presentation. This often leads to an incorrect determination of measurement inaccuracy (too optimistic statement of error value). The article’s primary goal is to present a new method for determining the value of the methodological error and its effect on the resulting error of measurement of the radio altitude (radar altitude). It presents a new experimental laboratory method for measuring ΔH and the resulting accuracy of height measurement with a radio altimeter. Thanks to this method, it can be verified that the information obtained by measuring the height above the ground corresponds to the standard specified by the manufacturer.

Evaluation of Doppler Effect Error Affecting the Radio Altimeter Altitude Measurements

“…The methodological error has the largest share in this inaccuracy (70%); frequency modulation parameter instability error is next (20%); an aircraft/helicopter flight dynamics error of 6% and remaining measurement errors such as parasitic modulation or Doppler shift error represent the remaining 4%. Data for such a statistical evaluation were obtained through many years of research by the authors in the field of radio altimeters [ 11 , 12 , 13 , 14 ].…”

“…The initial measurement (using the depolarization panel through electromagnetic waves) of discrete height values in indoor laboratory conditions with the RV-5 radio altimeter was successful and well documented in [ 11 , 12 ]. Subsequently, they also proved to be successful measurements of dynamic height change in indoor laboratory conditions in the same assembly of the measuring chain [ 11 , 12 ].…”

“…The initial measurement (using the depolarization panel through electromagnetic waves) of discrete height values in indoor laboratory conditions with the RV-5 radio altimeter was successful and well documented in [ 11 , 12 ]. Subsequently, they also proved to be successful measurements of dynamic height change in indoor laboratory conditions in the same assembly of the measuring chain [ 11 , 12 ]. The success of the measurement relates to a new method, using a patented depolarization panel, through which the reflections of electromagnetic waves from surrounding obstacles in the interior laboratory environment were significantly suppressed.…”

“…Therefore, the laboratory measurements were moved from the interior to the exterior, where it was planned to use a car’s movement for the simulation of a dynamic change in height. In this type of measurement, it was assumed that if the parasitic reflections of the radio altimeter transmitted high frequency (HF) carrier signal were eliminated in the interior using the depolarization panel, the parasitic reflections in the exterior would also be eliminated (which was also confirmed in the exterior measurements) [ 11 , 24 , 25 ].…”

“…Altitude measurements by radio altimeters are affected by several measurement inaccuracies, which were discussed in [ 11 , 12 ]. Some of these inaccuracies cannot be influenced and are fixed by the circuit design of the radio altimeter.…”