Nearest return tracking in an fmcw system

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. The modulation slope is controlled in response to a signal derived from the frequency discriminator to provide nearest return tracking. The system maintains the beat frequency constant based on that nearest return through control of the modulation slope. Nearest return altitude is inversely proportional to modulation slope. In the second technique, modulation slope is held constant or at least is not directly controlled by the frequency discriminator. The frequency discriminator is used in a phase/frequency locked loop to provide an output signal from the VCO that is locked onto the predetermined point within the spectrum of the returned signal. Nearest return altitude is proportional to the frequency of the VCO output signal.