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Abstract

1. In sonic ranging the method which comprises transmitting a first sonic pulse having a predominant frequency in the upper sonic range, transmitting a second sonic pulse after a time delay substantially less than the length of said first sonic pulse where said second sonic pulse has a frequency in the upper sonic range but substantially different than the frequency of said first sonic pulse, separately sensing the return echoes of the sonic pulses in two frequency bands to produce a pair of received signals, a first of which includes components representative of echoes of said first sonic pulse and the second of which includes components representative of echoes of said second sonic pulse with the frequencies of the echo components modified relative to the frequencies of said sonic pulses in dependence upon the relative velocities between the point of transmission thereof and the object producing echoes, separately recording said pair of received signals, reproducing the recorded signals to generate a pair of high frequency signals respectively representative of the first and second received signals with the first high frequency signal shifted in time relative to the second high frequency signal by an amount representative of said time delay to bring like components thereof into time coincidence, generating a timing function which varies monotonically from a predetermined level occurring in known time relative to the instant of transmission of the second of said sonic pulses, generating a pair of reference signals which vary in frequency in dependence upon said timing function, separately mixing said reference signals and the first and second of said high frequency signals to produce side band components, filtering said side band components to generate a pair of output signals of narrow band width centered at frequencies respectively representative of the frequencies of said sonic pulses, separately detecting said output signals to produce a pair of demodulated signals, and from the product of said demodulated signals generating a signal appearing along a time scale dependent upon said timing function.

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