Abstract: An FMCW distance measuring device is disclosed which provides nearest return tracking for increased accuracy. A portion of the transmitted wave in an FMCW altimeter is mixed with a received signal indicating distance to a target to produce a beat frequency between the transmitted wave and the received wave. This beat frequency is input to a frequency discriminator. The discriminator output is controlled so that a predetermined point within the spectrum of the returned signal most nearly represents the nearest return. The frequency discriminator thus allows more accurate tracking of altitude in contrast to those conventional systems which track the average or centroid of the spectral return. The frequency discriminator may be used in two ways. In the first technique the transmitted wave is modulated to maintain a constant beat frequency by adjusting the slope of the modulation.

Abstract: An FMCW distance measuring system is disclosed which provides a search and track function for enabling searching for nearer returns during altitude tracking, and for more accurately determining altitude rate. A portion of the transmitted wave in an FMCW altimeter is mixed with the received signal indicating distance to target to produce a beat frequency between the transmitted wave and the received wave. The transmitted wave is triangularly modulated to produce separate upsweep and downsweep modulations which are maintained to produce a constant beat frequency from the return signal. The up and downsweep of the triangular modulation are independently operable so that tracking may be maintained on either or both of the sweeps or one sweep may track while the other sweep is searching for a nearer return. By comparing the difference in the modulation slopes during the up and downsweep, a reading can be provided which is indicative of the rate of change in altitude.