Patent GB-633490-A (GB)
Title: Improvements in or relating to echo sounders and like devices for distance measuringin water
IPC: G01S13/70
Filed: 1947-01-15
Abstract: 633,490. Echo sounding and distance measuring apparatus. MARCONI'S WIRELESS TELEGRAPH CO., Ltd., and TWATT, J. Jan. 15, 1947, No. 1407. [Class 118 (ii)] A pulsed echo sounder or like subaqueous distance-measuring device comprises means for limiting reception to a succession of subperiods such that two successive sub-periods do not together exceed the pulse transmission period, means which ensure that during the absence of received echoes the received energy content of alternate sub-periods averaged over a time which is long relative to the transmission period is equal to the energy content of the intermediate sub-periods, means for comparing the two energy contents and means actuated in dependence upon an inequality therein for altering the time of commencement of each sub-period until equality is restored, and means for indicating or recording depth or distance in terms of the time difference between these times of commencement and transmission pulses. Curve (a) of Fig. 1 shows the rectified receiver output including the effect of a transmitted pulse B and an echo D, both superimposed upon the general noise level. In the present invention the time between transmitted pulses is twice the greatest echo time to be dealt with, the receiver is suppressed during transmission as at AC on curve (b), and the polarity of the rectified output is reversed during half the transmission period BJ, as at EH. In addition the receiver is suppressed at FG and in the absence of an echo positive and negative current are delivered for equal periods and, with random noise, the currents tend to cancel out. The reception of an echo D upsets this balance, but if the phase of the current reversals is advanced with respect to the pulse transmission so that the change in polarity E is moved to the centre of the echo pulse D, curve (c), the currents again cancel out, and the necessary change in phase is a measure of the position of the echo pulse and hence of the depth being sounded. Instead of inserting a " spurious " suppression FG, the receiver sensitivity may be reduced over the period EH or this period may be slightly shortened. In a first embodiment (Fig. 2) the rectified output of a subaqueous receiver is periodically reversed by cam-operated contacts (not shown) and applied to an integrating circuit 1, 6 whose time constant is long compared with the time between transmitted pulses. The output is amplified and applied to a cathode-follower valve 7 forming one arm of a balanced bridge 7, 11, 12, 13. When an echo is received, unbalance occurs and current flows in one direction or the other in the field winding 14 of a D.C. motor 15. The motor rotates the reversing contacts in relation to their cam and so carries the point in a transmission cycle at which reversal takes place until balance again results and no current flows in the field winding 14. The short receiver suppression periods AC (Fig. 1) are provided by further cam-operated contacts (not shown). The extent of rotation of the motor is utilized to indicate or record the distance of the reflecting object in,any known convenient manner. In a purely electrical embodiment (Fig. 3) a transmitting multivibrator 18 operates a trigger circuit 19 which pulses the transmitter 20 at two-second intervals. Echoes received at 21 are amplified, the amplifier 22 being suppressed during transmitted pulses by the trigger circuit 19, and fed to two amplifiers 23, 24. These are rendered operative in turn by opposite halfcycles of a second multivibrator 25 of the same frequency as the first 18. The rectified outputs are fed in opposition to a condenser-resistance integrating circuit 28 whose output is used to vary slightly the frequency of the multivibrator 25 and thereby produce a change in the relative phase of the two multi-vibrators. The arrangement is such that the change effected is in the direction to make the rectifier outputs of equal average value, i.e. to bring a received echo equally within the successive periods of operation of the two gated amplifiers 23, 24. Distance is indicated or recorded by means of a circuit 29 which is responsive to the phase difference of the two multivibrators 18, 25.