GB1139494A - Method of locating sound-reflecting objects in water - Google Patents

Abstract

1,139,494. Acoustic wave transmission systems. ATLAS-WERKE A.G. 1 Feb., 1966 [6 Feb., 1965], No. 4436/66. Heading H4D. In a method of locating objects underwater using an acoustic echo arrangement in which comparatively narrow band acoustic pulses are transmitted whilst the received reflected acoustic pulses have a comparative broad band due e.g. to relative movement between the transmitter and the objects or when a broad reception band must be employed in arrangements in which the frequency of the transmitted pulses is not foreseen in advance the receiver is arranged to produce an output signal with a fixed centre frequency. Fig. 2 shows an embodiment in which signals received by a transmit-receive transducer 1 are supplied to a band-passfilter 17 which selects the frequency range in which useful signals are expected to occur and supplies them to a mixer 5 together with a constant frequency from a source 7 and to a further mixer 4 having the output of mixer 5 as a second input. The output of mixer 4 thus comprises the spectrum of the received signals having a constant centre frequency equal to that of source 7. The useful signal in this spectrum, assumed to be very narrow, is then selected by band-pass-filter 11 and fed to a known indicator or control device 12. The effective reception band-width is thus equal to that of filter 17 whilst the signal-to-noise ratio is determined by the narrow passband of filter 11. The receiver may form part of an echo sounding arrangement in which switch 16 alternatively connects the transducer to the receiver and to a transmitter 13 controlled by a pulse generator 14 or the receiver may operate with signals received from an independent source. In addition to the rejection of broad-band interference by filter 11 the specification proposes a method of reducing interference due to reflections of the transmitted energy by the sea-bed and water surface (termed area reverberation) and by, distributed particles (termed volume reverberation) utilizing as fact that the intentionally sounded object is usually of smaller dimensions than the surfaces or volumes giving rise to such interference and that the smaller the object the more similar will be the echo to the transmitted sounding pulse. Thus by employing constant-amplitude frequency modulated pulses for the sounding pulses the greater degree of amplitude modulation imparted by reflection at the interference producing surfaces etc. than by the smaller intentionally sounded object results in the band-width of the reflected signal spectrum due to the latter being much smaller than that due to the reflections by the former so that filter 11 rejects most of the latter. An arrangement employing a second mixer and constant frequency source in the path 3 between the band-pass-filter 17 and mixer 4 is described with reference to Fig. 1 (not shown).