GB2167183A

GB2167183A - Direction finding arrangement

Info

Publication number: GB2167183A
Application number: GB08426942A
Authority: GB
Inventors: William Edward Brierley; Eric Dillon
Original assignee: GEC Avionics Ltd
Current assignee: Allard Way Holdings Ltd
Priority date: 1984-10-25
Filing date: 1984-10-25
Publication date: 1986-05-21
Also published as: GB2167183B; GB8509036D0; GB2168812B; GB2168812A

Abstract

An acoustic direction finding arrangement has three microphones A,B,C, connected as three sets of two. The relative time of reception of sound at each microphone is determined by adjusting a variable delay device 14, and the delay is obtained in terms of a clock frequency value Fo, from which the direction of the sonic source can be determined. A phase sensitive rectifier 16 produces a signal representative of the phase difference between the two signals A,B and controls the clock frequency VCO 17 to vary the delay imparted by means 14 so that the signals arrive at the rectifier 16 in synchronism, i.e. the phase difference is eliminated. The microphones A,B,C may be arranged as a three-dimensional array. <IMAGE>

Description

SPECIFICATION Direction finding arrangement This invention relates to a direction finding arrangement and is particularly applicable to the detection of sonic sources; this is to say sources emitting energy in the form of longitudinal compression waves, although in principle the invention is applicable to the detection of sources emitting energy in other wave-like forms.

The need arises to identify the direction of a source of wave-like energy with respect to a receiving location with a high degree of accuracy. This can be difficult to achieve if the energy is in the form of short duration bursts, and the invention seeks to provide an improved such arrangement.

According to this invention, a direction finding arrangement includes at least two receiver elements spaced apart from each other with their respective received signals (or signals derived therefrom) being fed to a time comparator via at least one variable delay device; means for varying the delay imparted by the delay device so that the two received signals arrive in synchronism at said comparator; and means dependent on the value of the varied delay for determining the direction of a remote source of waves giving rise to said received signals.

Preferably the remote source of waves is a sonic source, and each receiver is adapted to respond to sonic waves.

Preferably three similar receiver elements are provided, and which are spaced apart by equal distances in three dimensional space.

This permits the bearing of a sonic source to be precisely located in any direction in azimuth and elevation without ambiguity.

The invention is further described by way of example, with reference to the accompanying drawings in which: Figure 1 shows the arrangement in diagramatic form, and Figures 2 and 3 illustrate aspects of it in greater detail.

Referring to Figs. 1 and 2, the arrangement shown therein is suitable for accurately identifying the direction, i.e. bearing, of a sonic source. By sonic source in this example is meant a source of sound which may be fixed relative to the arrangement, or which may move around, on the ground, in the air, or under water. Three receivers A, B and C are provided, which thus conveniently take the form of small microphones. They are mounted on a framework (not shown) such that they are equi-distant from each other in three-dimensional space. This in effect, means that they are located on the surface of an imaginery sphere subtending equal angles at the centre thereof.The outputs of the microphones A, B and C are passed through respective amplifiers (not illustrated) to channel filters 3, 4 and 5 having similar passband characteristics so that the signals made available at their outputs have similar frequency distributions. The characteristics of the filters are preferably related to those of the received signals, and the filter characteristics may be fixed so as to select predetermined kinds of sonic sources, or can be variable in dependence on the nature of the received signals.

The relative phases of the signals produced by the filters 3,4 and 5 are fed to the three resolution circuits 6, 7 and 8, where their phase delays are compared and respective output signals made available, on lines 9, 10 and 11, having frequency characteristics whichare dependent on the phase differences. The bearing of the source of sonic energy is then determined by comparing these three signals at a processor 12. In practice, it is sufficient to use these three signals to address a lookup table as each combination of three frequency values will uniquely identify a particular bearing in three-dimensional space.

The way in which the three output signals on leads 9, 10 and 11 are generated is illustrated in Fig. 2 for output signal 9 which has a frequency fl, and only the relevant parts of the arrangement are shown. The outputs of the filters 3 and 4 are fed via respective delay devices 14 and 15 to inputs of a phase sensitive rectifier 16. Whilst delay device 15 exhibits a fixed overall delay, that of delay device 14 is variable under the control of a variable frequency oscillator 17.

Both delay devices 14 and 15 are of the kind often referred to as bucket brigade delay devices; that is to say, a cascaded sequence of delay cells with the rate at which a bit of information is transferred from one cell to its neighbour being dependent on the frequency of a clock signal. In this case, the frequency f1 of the oscillator 17 represents a clock frequency for delay device 14, whereas a fixed oscillator 18 is used to generate the clock signal at frequency fO for the fixed delay device 15. Thus, the output of the phase sensitive rectifier 16, which is dependent on the degree of mis-match between the two input signals is used to generate an output signal having a magnitude dependent on the ratio of the phase difference of the two signals A and B.This signal is used to control the frequency of the oscillator 17 which forms part of a negative feedback loop, so as to bring the outputs of the delays 14 and 15 into synchronism.

The system tracks so as to produce phase coherence at the input to the phase sensitive rectifier. The bearing is dd resolved by SIN O = -, where dd is D the differential delay between signals and D is the delay between the arrival of signals at A and B from a direction in line A-B.

Thus the output frequency fl is directly indicative of the degree of delay present between the two signals received at the receivers A and B. From a knowledge of the spacing apart of the two elements and the centre frequency of the bandpass filters 3 and 4, the bearing of the sonic source can be determined.

As the delay of the line is a function of the clock frequency, measurement of the clock indicates delay and thus bearing. Any ambiguity as a result of the sign of SIN 6 is resolved by the use of at least two pairs, in that only one common bearing will appear.

Fig. 3 shows an expanded version of the invention in which the technique described in Fig. 2 is extended to three receiving elements so as to be able to identify the bearing in three-dimensional space without ambiguity.

The use of three pairs of elements improves discrimination at angles greater than + 60 degrees for any pair, and also enables hemispherical determination.

Using the array of fixed delays 20, 21 and variable delay devices 22, 23, 24 illustrated, and three phase sensitive rectifiers 25, 26, 27 and the three voltage controlled oscillators 28, 29, 30, a unique matrix of three frequencies fl, f2 and f3 is produced. These three frequencies are applied to the processor circuit 12 in order to provide an output directly indicative of the angular bearing in terms of azimuth and elevation.

It will be appreciated that in principle, the technique is applicable to any sound source having a frequency component whose wavelength is usefully related to the practical distance by which three sonic receivers can be spaced.

Claims

1. A direction finding arrangement including at least two receiver elements spaced apart from each other with their respective received signals (or signals derived therefrom) being fed to a time comparator via at least one variable delay device; means for varying the delay imparted by the delay device so that the two received signals arrive in synchronism at said comparator; and means dependent on the valve of the varied delay for determining the direction of a remote source of waves giving rise to said received signals.

2. An arrangement as claimed in claim 1 and wherein each receiver is adapted to respond to sonic waves.

3. An arrangement as claimed in claim 1 or 2 and wherein three similar receiver elements are provided, and which are spaced apart by equal distances in three dimensional space.

4. An arrangement as claimed in any of the preceding claims, and wherein said variable delay device exhibits a delay which is dependent on the frequency of a variable frequency clock signal.

5. An arrangement as claimed in claim 4 and wherein the variable frequency clock signal is determined by the output of the time comparator, with the value of the clock signal frequency being adjusted to bring said received signals into synchronism at said time comparator.

6. An arrangement as claimed in claim 5, and wherein three time comparators and three variable frequency oscillators are provided, so as to generate three variable clock signals, each of which is related to a different combination of pairs of the three receiver elements.

7. An arrangement as claimed in any of the preceding claims and wherein signals dependent on the value of the varied delay are used to address a store at which the bearing of said source is held.

8. A direction finding arrangement substantially as illustrated in and described with reference to Figs. 1, 2 and 3 of the accompanying drawings.