Post-Mixing Phase-Modulated FM-CW Radar Altimeter

Abstract

A new FM-CW radar altimeter is devised, in which a post-mixing phase-modulation of the beat signal is employed for reducing the step error in the zero-crossing counting.

The symmetry of the beat signal waveform with respect to the returning point of the triangular frequency-modulating wave is lost by the post-mixing phase-modulation. This unsymmetry causes jumps in the phase of the beat signal and gives undesired zero-crossing. However, the error due to this unsymmetry is small.

Also, the influence of Doppler effect on the reduction of the step error is analyzed by the use of time vs. phase plots.

Effectiveness of the phase-modulation is presented through experiments with a breadboard construction of the altimeter, which operates at 4.3 GHz with the maximum frequency deviation of 60 MHz. Experimental results show that the reduction rate of step error is about 1/20, and an accuracy of ±0.2 ft is obtained.