Numeric modeling of low-altitude altimeter with double frequency modulation

This study is concerned with the problem of increasing the accuracy of a low-altitude altimeter employing the frequency modulation principle. A way to suppress the “discrete error” of the altimeter by employing additional “slow” frequency modulation of the carrier wave and averaging the resulting counts is considered. The benefit of such approach is simplicity of technical implementation manifesting in minimal changes in the microwave path and the recording device, which needs to run in the averaging count mode. It is shown that, given a limited frequency band, the linear modulation form is not optimal. Results of error calculations presented are obtained via mathematical modelling of the altimeter's operation for different shapes of the additional frequency modulation. It is shown that using complex shapes of the “slow” modulation with positive third derivative and optimizing for a given altitude range allows to reduce the average measurement error 2-3 times additionally relatively to the linear modulation form without expanding the occupied frequency band.