Patent GB-1514157-A (GB)
Title: Method of and apparatus for determining the direction of a moving target from a moving vessel
IPC: G01S15/60
Filed: 1966-02-03
Abstract: 1514157 Sonar direction finding apparatus ATLAS WERKE MASCHINENBAU GmbH 3 Feb 1966 [6 Feb 1965] 4877/66 Heading G1G A method of determining the direction of a moving target from a moving vessel comprises the steps of transmitting a scanning wave of predetermined frequency from the vessel, receiving a reflected wave the frequency of which differs from the transmitted frequency by a self-Doppler frequency due to motion of the vessel and also by a target Doppler frequency due to motion of the target, and superimposing on the reflected wave a compensating wave the frequency of which differs from the transmitted frequency by the negative of self-Doppler frequency present in the reflected wave to obtain an information wave in which the self-Doppler frequency is absent. The method is intended for horizontal echo-sounding in water. In the embodiment described with reference to Figs. 1a, 1, 2, a ship is fitted with a transmitter 1 which can be rotated about a vertical axis to perform a scanning operation: The transmitter is shown in the drawing as operating at an angle α in relation to the direction of the velocity # of the ship. The transmitter functions both as a transmitter and as a receiver of sound waves in water. A second transmitter/receiver 2 disposed in backto-back relation with the transmitter 1 operates in a direction α + 180 degrees. Both transmitters have a directional polar response of, for example, Œ5 degrees. Receivers 1 and 2 receive signals fs + f DE  + (f DF ) and f S  - f DE  (+ fDF), where the target Doppler frequency f DF  is placed in parentheses to indicate that it only occurs in the signal received by receiver 1 and 2 when a target is present in the path of the beam transmitted from the respective transmitter. The received signals are fed from the receivers through conventional preamplifiers 5 and 6 to a mixer stage 7 which forms a cumulative frequency signal and a difference frequency signal. On the assumption that it is extremely unlikely that targets will be simultaneously present in the path of the beams from both transmitters 1 and 2, the target Doppler frequency will occur in only one of the received signals and the cumulative frequency will be 2f S  (+ f DF ) and the difference frequency 2f DE  (Œ f DF ). It is also possible, instead of the transmitted frequency f S , to form an intermediate frequency in the amplifiers. Following the mixer stage 7, the cumulative and difference frequency signals 2f S  (+ f DF ) and 2f DE  (Œ fDF) are filtered respectively in a band-pass filter 8 and a low-pass filter 9 respectively to filter out any spurious signals. The cumulative frequency signal emerging from the band-pass filter 8 is a self-Doppler flee signal containing only target Doppler information. This cumulative frequency signal is used in normal manner to indicate the presence of a target in a given direction. Thus a mixer stage 10 is employed to form a difference signal from the cumulative frequency signal 2f S  (+ f DF ) and an auxiliary frequency signal 2f S  - 1000 C/S, so as to give an information signal having the form 1000 (+ fDF) C/S at the output 11. Mixer stage 10 is shown in Fig.2, where its output information signal is fed directly to a transformer T 1  and through a frequency-dependent phase-shifting device 12 to a transformer T 2 . The device 12 produces no phase-shift between input and output for an input signal of 1000 C/S and an approximately linearly rising or falling phase shift for higher or lower frequencies respectively. When the switch S1 between the secondary windings of transformers T 1  and T 2  is closed, the voltage difference between the outputs of the transformers is formed and fed to an indicator. As long as no target Doppler frequency f DF  is present, a signal of 1000 C/S is generated at the output of mixer stage 10. For 1000 C/S, the input and output voltages of the phase-shifting device 12 are in phase and the inter-connected outputs of T 1  and T 2  give a zero resultant output, i.e. no reading is obtained. Only when a target Doppler frequency f DF  is present is a frequency different from 1000 C/S generated by mixer stage 10, so that the input and output of device 12 are no longer in phase. The inter-connected outputs of T 1  and T 2  no longer cancel each other and thus the signal of a moving target, containing the target Doppler frequency f DF , produces a reading. Switch S1 may be opened when it is desired to observe all targets, both stationary and moving, with one transformer T 1  or T 2 . When a target Doppler frequency f DF  appears, it cannot be immediately determined whether it comes from receiver 1 or receiver 2, for, whether the target Doppler frequency is picked up by receiver 1 or receiver 2, a positive f DF  value, that is a high target Doppler, will result when a target is approaching, and a negative f DF  value, that is a low target Doppler, when it is receding. Therefore, in order to determine which receiver has picked up the target Doppler frequency, a discriminator device is used. It should be noted that, if the target Doppler is high and comes from the receiver 1, then the difference frequency 2f DE  (Œ f DF ) from mixer stage 7 will increase as soon as the echo from the target arrives; if it comes from receiver 2, the difference frequency will decrease. If, however, the target Doppler is low and comes from receiver 1, then the difference frequency from mixer stage 7 will decrease on arrival of the echo from the target; if it comes from receiver 2, the difference frequency will increase. These characteristics suffice to enable the discriminator to make a distinction. Thus the cumulative frequency signal, after mixing to give the information signal 1000 C/S (Œ f DF ) at the output of mixer stage 10, is fed into a 1 Kc/s ratio detector which produces the necessary 90 degrees phase-shift between phase transformer 13 and quadrature transformer 14 by means of a transit delay of 0À25 ms. The output voltage from the ratio detector is fed to a bi-stable multi-vibrator 16, to whose output is connected a relay K2 having two windings. As long as no target Doppler frequency f DF  is present, i.e. only the frequency 1000 C/S appears in the ratio detector, both relay windings are equally energized and a switch blade is held by relay K2 in a neutral position. If a high target Doppler frequency f DF  appears, the signal is connected to position I, and, if a low target Doppler frequency f DF  appears, to position II. The difference signal 2f DE  (Œ f DF ) is similarly utilized to detect the jump in frequency which occurs when a target echo appears. To this end, the difference signal is first separated after low-pass filter 9 in order that only frequency information shall be processed. This operation is effected by two limiter diodes at the input of an amplifier valve 17, which produces a frequency-proportional A.C. voltage in coil 18. This A.C. voltage is rectified by rectifier 19 to give a frequency-proportional D.C. voltage across resistor 20. A capacitor 21 is connected in parallel with resistor 20 and the time constant of the circuit 20/21 corresponds approximately to the pulse length which is known from the transmitted signal. The D.C. signal is amplified and differentiated in a differentiating circuit 22/23 which feeds a bi-stable multi-vibrator 24 having its output connected to a relay K1 provided with two windings. If the jump in frequency, due to the arrival of an echo containing a target Doppler frequency, is positive, relay Kl is switched into position III; if the jump in frequency is negative, Kl assumes position IV. The connection in series of the two relays Kl K2 causes a visual indicator D S1  to operate if the echo containing a target Doppler frequency is received by receiver 1, while a visual indicator D S2  is operated when the target echo is received by receiver 2. In a second embodiment of the invention which is shown in Fig. 3 and which uses only a single transmitter and receiver, a signal f S  + f DE  (+ f DF ) received by a receiver 25 is fed after amplification in amplifier 26, which is provided with timedependent automatic amplification control, to a mixer stage 29 where it is mixed with an auxiliary signal of higher frequency such as 2f S . The value 2fs. The value 2f S  is selected in this case to give similar working to the first embodiment, using the same frequencies. The signal 2f S  can be obtained, for instance, from an oscillator 27 in the electric transmitting generator, the latter's transmitting frequency of f S  being doubled in a frequencydoubling stage 28. The mixer stage produces a difference frequency f S  - f DE  (-f DF ). The difference frequency supplied by the mixer stage 29 is filtered in band-pass filter 30 to remove spurious signals. The filtered signal is then retarded in a delay line 31, and the delay period is preferably approximately equal to the signal pulse length. The signal S of frequency f S  + f DE  (+ f DF ) coming from amplifier 26 and the delayed difference signal Ss of frequency f S  - f DE (-f DF ) are fed into a mixer stage 33, the signal S also being fed through the delay line 31 to provide a delayed signal S v  of frequency f S  + f DE  (+ f DF ) at the input of mixer stage 33. The mixer stage produces the cumulative frequency of the three input signals, such cumulative frequency having the value 2f S  (+ f DF ) in view of the time sequence of the signals as illustrated in Fig.4, where the abscissa represents the duration of a sounding period, a portion of this period being suppressed immediately following the transmitted signal, and the ordinate represents the frequency. Starting at the beginning of the sounding period, the reflected signal S has a frequency greater than f S  by the self-Doppler frequency f DE  and it retains this frequency until the appearance of an echo containing a target Doppler frequ