Patent GB-1349678-A (GB)
Title: Underwater direction finding system
IPC: G01S1/72
Filed: 1971-04-19
Abstract: 1349678 Acoustic direction-finders SANDERS ASSOCIATES Inc 19 April 1971 [16 March 1970] 22889/71 Heading H4D A direction-finding system comprises a sonobuoy unit having directional hydrophones 14, 16 with their sensitive axes angled to each other, e.g. perpendicular, and an omnidirectional hydrophone 24. The ratio of the signals from 14 and 16 gives the tangent of the incoming acoustic signal bearing and their phases with respect to the omnidirectional transducer signal indicates the quadrant containing the incident signal. The signals from 14, 16, are combined in modulator 28, together with a reference signal from a compass transducer 26 coupled to the hydrophones. The resulting signal and the signal from hydrophone 24 are then fed to radio transmitter 30. The signals from receiver 32 are passed to a switch 34 for direction to either demodulator 38 or conventional signal processor 36. When the signals are passed to processor 36 all received acoustic signals are displayed at device 46 simultaneously but when passed to demodulator 38 and processor 40, a single particular acoustic frequency may be selected. The various received frequencies are indicated at display 46 with the switch 34 in the appropriate position and a cursor on display 46 moved to the frequency of interest. The switch is then moved to its second position and the signals passed to demodulator 38. Signals 100, 102 and 104 correspond to those from hydrophone 12, 14 and 16 respectively. The first stage of the processor contains heterodyning section (106) (Fig. 2A, not shown) for selecting a particular acoustic frequency the local oscillators (112) of which are controlled by generator 52. The resulting signal corresponding to signal 100 is limited at (138) and converted to a square wave which is applied together with each of the signals corresponding to 102 and 104 in synchronous detectors (140), (142) respectively. The full-wave rectified outputs of the detectors are of positive polarity if the inputs are in phase and of negative polarity if they are out of phase. The polarities of the outputs, considered together, therefore indicate the quadrant containing the incident acoustic signal. The demodulator outputs are then applied to integrators (150), (152) (Fig. 2B, not shown). The outputs of the integrators are applied as one input to a reset device (154), the second input of which has a positive or negative threshold voltage applied. When either integrator output reaches the threshold the outputs are reset to zero by (154). The outputs of (150), (152) are therefore sawtooth waveforms, the peak amplitude ratio of which still gives the bearing tangent. Further integrators (156), (158) produce D.C. signals giving the mean value of the sawtooth waveforms. The D.C. signals are combined with the signal from a reference oscillator (164) to produce signals at the reference frequency having amplitudes corresponding to the D.C. levels. The two resulting signals are phase shifted by 90 degrees, summed in network (168) and passed to a low pass filter (170). The output of the filter is a signal of constant amplitude the phase of which, relative to the reference frequency, represents the bearing angle #. The filter output is converted to a square wave in limiter (172) and applied to bi-stable (174). The bi-stable is set by the reference frequency signal leading edge and reset by the filter output. The width of the pulses from (174) is therefore proportional to the bearing angle #. The pulses are integrated at (176) to produce a D.C. signal which is applied to bearing indicator (42), e.g. a digital voltmeter. The signal may also be applied to display (46) or recorder (56) via switch (44). In a further embodiment of the reset arrangement of Fig. 2B the outputs of (150), (152) (Fig. 3, not shown) are squared in devices (178), (180). The outputs of (150), (152) may be considered as B sin 0 and A cos # respectively. The squared outputs are summed in device (182). This is then fed to the reset device.