apparatus for use with a system in which successive pulses , each composed of a train of high frequency oscillations , are emitted by a transmitting transducer , propagated through a transmission medium , and received by a receiving transducer having a given transient behavior , the apparatus serving to measure the time interval between reception of successive pulses by the receiver transducer and to prevent erroneous measurements resulting from propagation of the pulses over indirect paths through the transmission medium , prevention of erroneous measurements being achieved by effecting time interval measurements only with respect to those pulses for which the output signal from the receiving transducer in response to reception of each such pulse exceeds a selected threshold value during a predetermined time interval subsequent to passage of that output signal above a given comparison value which is less than the threshold value .