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The Technical Note presents the possibility of applying the classical onboard aviation radar altimeter in the function of a simple anticollision system. Currently, classical radar altimeter offers two important pieces of information for pilots: actual flight altitude and aircraft descent at the level of decision making. The goal here is to draw attention to the innovative technique of using the radar altimeter to generate further crucial information for the pilot of a helicopter or small aircraft for predicting the threat of collision with the terrain. This new technique is based on the evaluation of the Doppler frequency in the differential signal of the radar altimeter. The theoretical assumptions have been verified by simulation and experiment.Nomenclature c speed of light, m/s F D differential signal frequency of radar altimeter corresponding to Doppler effect, Hz F M frequency modulation, Hz F r differential signal frequency of radar altimeter corresponding to helicopter's altitude, Hz F rD resulting differential signal frequency shaped by radar altimeter, Hz f 0 center frequency sweep of radar altimeter, Hz H helicopter altitude, m s distance flown, m t c time remaining to the collision with terrain, s t cr critical time, s U D voltage proportional to difference of the measured frequencies at each half-period of the modulation frequency, V U H voltage proportional to altitude, V v speed of aircraft flight, m/s v v vertical component of speed, m/s α terrain inclination angle, deg β hill inclination, deg f frequency deviation, Hz

The paper focuses on the new trend of increasing the accuracy of low altitudes measurement by frequency-modulated continuous-wave (FMCW) radio altimeters. The method of increasing the altitude measurement accuracy has been realized in a form of a frequency deviation increase with the help of the carrier frequency increase. In this way, the height measurement precision has been established at the value of ±0.75 m. Modern digital processing of a differential frequency cannot increase the accuracy limitation considerably. It can be seen that further increase of the height measurement precision is possible through the method of innovatory processing of so-called height pulses. This paper thoroughly analyzes the laws of height pulse shaping from the differential frequency presented by the number that represents the information about the measured altitude for this purpose. This paper presents the results of the laboratory experimental altitude measurement with the use of a so-called double-channel method. The application of obtained results could contribute to the increase of air traffic safety, mainly in the phase of the aircraft approaching for landing and landing itself.

This manuscript focuses on the analysis of a critical height of radio altimeters that can help for the development of new types of aeronautical radio altimeters with increased accuracy in measuring low altitudes. Altitude measurement accuracy is connected with a form of processing the difference signal of a radio altimeter, which carries information on the measured altitude. The definition of the altitude measurement accuracy is closely linked to the value of a critical height. Modern radio altimeters with digital processing of a difference signal could shift the limit of accuracy towards better values when the basics of the determination of critical height are thoroughly known. The theory results from the analysis and simulation of dynamic formation and the dissolution of the so-called stable and unstable height pulses, which define the range of the critical height and are presented in the paper. The theory is supported by a new method of derivation of the basic equation of a radio altimeter based on a critical height. The article supports the new theory of radio altimeters with the ultra-wide frequency deviation that lead to the increase the accuracy of a low altitude measurement. Complex mathematical analysis of the dynamic formation of critical height and a computer simulation of its course supported by the new form of the derivation of the basic equation of radio altimeter guarantee the correctness of the new findings of the systematic creation of unstable height pulses and the influence of their number on the altitude measurement accuracy. Application of the presented findings to the aviation practice will contribute to increasing the accuracy of the low altitude measurement from an aircraft during its landing and to increasing air traffic safety.

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