Accuracy of tracking radio altimeters working over a 2D rough surface

Abstract

Low altitude tracking radio altimeters use periodic frequency modulated continuous signals and the altitude measurement is based on estimation of the mean frequency of the beat signal. To realize the estimation, a closed automatic control loop is organized in radio altimeter. The loop maintains the frequency of the beat signal at a fixed value by adjusting some parameters of the transmitted signal. There is one more way to build up a tracking contour for the altitude estimate, the operation of which is based on the use of a measuring phase locked loop. The circuit adjusts the reference signal ‑ a copy of the beat signal ‑ to maximize the phase structure of the beat signal and the reference signal. When measuring the height over a rough surface, in addition to errors caused by the internal noise in the receiver, the range noise begins to affect the accuracy of the estimate. The range noise is due to the formation of a signal scattered by a rough surface as sum of reflections from stationary points of the surface where the local normal is directed towards the phase center of the radio altimeter antenna. The article aim is a comparative analysis of the accuracy of three types of tracking radio altimeters working over a rough two-dimensional surface: radio altimeter with a phase-locked loop and tracking altimeters with adjustment of the frequency deviation or the modulation period of the transmitted signal. The signal reflected from a two-dimensional rough surface was simulated by the method of stationary points, which is a particular case of the Kirchhoff method. As a result of the simulation, the biases and the root-mean-square errors of height estimates were obtained for the three types of tracking altimeters and different values of the surface roughness parameter. For all altimeters, the estimate bias is small and does not depend on the roughness parameter. The root-mean-square error of height estimation for all considered radio altimeters increases with increase of the roughness parameter. The best results were shown by the radio altimeter with the measuring phase locked loop. Its error is approximately 3 times less than that of the radio altimeters with adjustment of the transmitted signal parameters.