GB2111679A - Sonar intruder detectors - Google Patents

Abstract

A sonar intruder detector for protecting a harbour mouth, river entrance or coastline which has a string of fixed beam sonar transducers (1) arranged along the line to be protected. These transducers each project a pulse-modulated sonic beam up towards the surface, and are sequentially triggered by a pulse on a cable interconnecting them. Any object (2) which crosses the sonar "fence" thus provided reflects the sonic pulse and this for reflection is detected and processed as in radar to give an indication that the "fence" has been crossed and where. To indicate whether the object is entering or leaving two such fences parallel to each other are used. The transducers may operate at different frequencies. <IMAGE>

Description

SPECIFICATION Sonar intruder detectors The present invention relates to the protection of harbours, shore lines, rivers, etc., against intruders, human or not.

Water provides a medium in which such intruders can conceal themselves from visual and possibly also radar detectors, and even radar can miss non-metallic surface vessels.

Any water borne intruder at least penetrates or disturbs the water or its surface, so that a sub-surface acoustic detection system is usable. This invention has as its object the provision of a simple and relatively inexpensive detection system of the above type.

According to the invention there is provided an underwater sonar detection system, which includes a number of fixed sonar transmitters arranged in a line or other configuration so as to define a boundary the crossing of which is to be detected, and receiving means associated with the sonar transmitters, wherein each said transmitter is a fixed-beam transmitter so located that its sonic output is beamed upwards towards the surface of the water under which the transmitters are placed, the receiving means being associated with the transmitters so as to receive signals therefrom after reflection from a body which comes under the influence of the sonic signals from the transmitters.

Such a system is suitable for use where it is considered sufficient to know that an intruder has entered the protected area and to determine the point of entry. In such a case adequate coverage is given by confining the sonar emissions to a narrow ribbon which defines the protected area so as to form a sonar intruder "fence", which is defined by sonars on the sea or river bed, and which respond to an object passing over or through it. Such a system only needs to be effective at short range, and the processing equipment only needs to deal with objectes penetrating the fence.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which Figure 1 shows schematically the arrangement of the sonars in a simple embodiment of the invention.

Figure 2 shows the use of two "sonar fences" each as shown in Fig. 1.

Figure 3 shows schematically the geometry of a single one of the sonars.

Figure 4a, 4b and 4c shows the type of displays expected with a system such as that of Fig. 1.

Figure 5 shows schematically the elements of a complete system of the type shown in Fig. 1.

The basic principle of the "sonar fence" is shown in Fig. 1, where we have a chain of short-range active sonars 1 extending across the mouth of a harbour or other inlet. As can be seen, these sonars are coupled to an analysis centre (not shown), and some objects to be detected are indicated, e.g. at 2. Such a system can be used on its own, or to supplement a volume coverage surveillance system by easing the latter's detection tasks. The system is not infallible, and is intended for use where the higher grade of protection given by a volume surveillance system is unnecessary or too expensive.

Each sonar transmits a fixed beam of sonic energy, amplitude modulated by short-duration pulses, which pulses are reflected from the surface of an intruding object. Fig. 3 is an idealised polar diagram of one of the sonars of Fig. 1, located at 4 on the water bottom, so that the insonified volume 5 is a right-circular cone of semi-angle 8 and at a depth d. As can be seen at 6, the insonified surface is circular.

Such individual sonars can be no more complex than a simple "yacht-type" echo sounder.

In the simplest form of the system, no attempt is made to determine the bearing of a target, the object being to detect its presence as it passes more or less over the sonar. The receivers for the sonars are each adjacent to or incorporated with one of the sonars, and the detection criterion is that the echo has to be distinguishable from surface echoes, which are usually the only permanent echoes.

The system can measure target distance from the sonar transducers in terms of the time between emissions of a sonic pulse and its reception after reflection from the target.

With send and receive transducers collocated, a target is indicated as being in a hemisphere centred on the transducers. Thus the system indicates approximately the targets' range in the horizontal plane if the sea is shallow compared with the sonar's maximum range, and the targets' depth if the sea is relatively deep. Some complication of the geometry occurs if the send and receive transducers are not collocated.

Fig. 4 shows typical waveforms seen on an A-scope display for various target situations, Fig. 4a showing the display with no target, Fig. 4bwith two targets and Fig. 4cwith a shoal of fish. The complex returns indicated at 10, which are due to multiple bounce effects between water bottom and surface, are normally gated out. In the diagram the transmitted pulse is shown at 11, the surface pulse at 12, and the next transmissed pulse at 13.

Fig. 4b shows echoes 1 4 and 1 5 from two targets, one deep and one shallow, while Fig.

4c shows at 16 the return due to a shoal of fish. This shows clearly the difference between the return from a single target and a shoal of fish.

As indicated above, in its simplest form the "fence" consists of a single chain of sensors, see Fig. 1, with the insonified volumes overlapping. Such a "fence" can be placed across a harbour entrance, or across a river, or parallel to a coastline, so that entry is impossible without passing over a sensor, and that the intruder is detected. The polar diagrams are not as perfectly conical as shown in Fig.

3, and also there are side lobes; hence a deep-swimming intruder would be most unlikely to avoid detection by passing between the two sensors.

A single fence as described above merely detects that an intruder passes overhead and gives the approximate position of the intrusion. To indicate direction of movement, two fairly closely-spaced fences can be used, see Fig. 2. As long as there are not too many intruders in close formation, this not only discriminates between entering and leaving, but can also give a measure of speed and a crude indication of direction of motion. The speed element is significant, and gives one form of discrimination between hostile and innocent intruders. The latter, for instance seals and large fish, are likely to swim more quickly than the former.

Another difference between animals and humans (or man-made objects) is the noises they make. Hence the association of a passive or "listening" sonar with the active "fence" may be desirable. When the active system detects an intruder, it enables a passive system which listens to the intruder's noise and decides whether or not it is hostile. With practice this can be done manually, but automatic systems exist which compare the spectrum of a received sound with a "library" of "signatures" of typical objects. The alarm system can then be instructed to ignore returns from apparently innocent or friendly intruders. The passive system can be separate from the active one, or it could be organised in a linear fencelike pattern, perhaps using the same hydrophones as the active system.

Another way to decide if an intruder is friendly, is for friendly targets to have a simple IFF transponder, which picks up an incident sonic pulse, and emits it after a slight delay, so modified that the receiver can distinguish it from a simple skin reflectqr. Hence the central processing circuitry can separate such friendly signals from returns from nonequipped targets. The modification may also identify the target. Although it may not be economic to fit all friendly targets likely to use the protected waters, if the constant visitors were fitted, the number of false alarms would be reduced. The modification can be as simple as doubling or tripling the received pulse, or could be a sonic carrier at a different frequency, as in the system of Fig. 5.In such a case the transponder can also "tell" a ship's crew that they are crossing the "fence", which gives an approximate position indication, useful in poor visibility conditions.

A simple primary radar could be added to the system to detect very shallow draught vessels which could in some sea states escape detection by the sonar "fence". Thus the sonar "fence" of Fig. 1 could be supplemented by a simple short range fixed-beam primary radar on one side of the harbour entrance.

The line of sensors also constitutes a linear array whose signals can be processed using known beam-forming techniques to extract positional information from the received signals. Other techniques similar to those used in radio navigational aids can also be used, although there may be some problems due to multipath conditions. Generally two or more neighbouring sensors co-operate in a "rhorho" mode, establishing a reflecting target as being on the intersection of two spheres, or on a hyperboloid, etc.

Although each sonar's coverage is limited, it is inevitable that responses due to one transmitter will sometimes be picked up by the receiver of others. This can provide useful information if the receivers "know" the origin of a received echo. This can be done by causing the transmitter to radiate on differentfrequencies, or by synchronising all emissions, such that a response at any receiver in a specific time position could only come from a specific transmitter. This causes no difficulty if each receiver is only concerned with its own transmitter's signals, as any other signals are then gated out. Thus the sensors should be activated in sequence so that only one operates at any time. Thus the string of sensors is caused to 'blink" sequentially along its length, which gives simple cabling.

In the simplest form of system, each sensor is connected to its own cable to the shore, where all processing is done. However, sequenced operation, as described above, effectively involves time division multiplexing, so that all sensors can share a single cable, with economy in cabling. In this system, the sequencing is done by a sync pulse applied to one end of the cable, which triggers the first sensor. After a delay, the first sensor sends on the sync pulse to the second sensor, which after a delay triggers the third sensor, and so on. When all sensors have been triggered, a second sync pulse is applied to the cable end.

The receiver pulses can return on a separate cable, but it is also possible to use the same cable, reversing polarity relative to the sync pulse, with by-passing of the delay circuits.

The cable used can be coaxial, or twisted pair for short systems. Optical fibres could also be used.

We now consider the schematic representation, Fig. 5, of both the "off shore" equipment and the "shore" equipment. It is desirable to minimise the amount of electronics installed beneath the surface, and in some short systems it may be possible to eliminate electronics and merely have the hydrophone (for both send and receive) coupled by lowloss cable to the shore equipment. However, in most cases the receiver front end needs to be close to the hydrophone, in which case the transmitter's output stage can also be there present. With the sync pulse method described above, the carrier frequency source for the receiver, and the "hand-on" delay would all be in the sensor, Fig. 5.

In Fig. 5, the sensor includes a pulse modulator 20 which receives sync pulses from the cable 21, and applies its output to a transducer 22 for both send and receive. Responses from the water surface and an intruder are indicated at 23 and 24. Also the response from a vessel 25 with an IFF transponder is indicated at 26. As shown this reponse includes both skin response and the coded response from the IFF transponder.

This latter includes a coder 27 and an alarm 28 to advise the crew that an interrogation is in progress.

The sensor includes a receiver whose input amplifier is controlled via gating represented by the connection 29, and the output from this amplifier passes via a detector 30 to the cable. The sync pulse is passed on after a delay by a delay device 31, provided with a by-pass 32 for response pulses.

The shore equipment includes a sync pulse generator 35, which sends sync pulses to the cable, and also to a signal processor 36, e.g.

demultiplexer, range measurement, target present, etc. This receives the return pulses via an amplifier 37. The outlets of this goes to a display 38, on which transmitted pulses 39, intruder response 40, primary sonar response 41 from a friendly vessel and surface responses 42 are indicated. Similar displays may occur in the display traces for other sensor cells. The second output does to an alarm panel 43, with an "intruder in" alarm 44 and a lamp per cell to indicate location.

The third output goes to an optional IFF unit 45 which includes an IFF decoder 46 and identity display 47.

It should be noted that all those circuits are simple, e.g. the carrier generator, modulator and output power generator can all be done with one transistor, gating and delay by a second, and the receiver front end by a third.

The whole consumes very little power and the housing would be the main expense, which can be minimised by the use of one box to house the electronics for several adjacent sources.

The received pulses are processed ashore where greater complexity is acceptable, using techniques well known in sonar and radar.

Claims (11)

1. An underwater sonar detection system, which includes a number of fixed sonar transmitters arranged in a line or other configuration so as to define a boundary the crossing of which is to be detected, and receiving means associated with the sonar transmitters, wherein each said transmitter is a fixed-beam transmitter so located that its sonic output is beamed upwards towards the surface of the water under which the transmitters are placed, the receiving means being associated with the transmitters so as to receive signals therefrom after reflection from a body which comes under the influence of the sonic signals from the transmitters.

2. A system as claimed in claim 1, and wherein the transmitters all operate at different frequencies.

3. An underwater sonar detection system, which includes a number of fixed sonar transmitters arranged in a line or other configuration so as to define a line the crossing of which is to be detected, and receiving devices each associated with one of said transmitters, wherein each said transmitter is a fixed-beam transmitter so located that its sonic output is beamed upwards towards the surface of the water under which the transmitters are placed, wherein each of said receiving devices is so located as to receive signals from its transmitter after reflections from a body which comes under the influence of the sonic signals from its said transmitter, and wherein the transmitters are caused to operate one at a time and in sequence.

4. A system as claimed in claim 3, wherein the transmitters and the receivers each uses a single transducer to emit the sonic waves into the water and to receive the reflected sonic waves.

5. A system as claimed in claim 3 or 4, wherein the transmitters are interconnected by a single cable from a shore installation, wherein to initiate the operation of the transmitters a sync pulse is sent along the cable, and wherein when such a pulse reaches a transmitter it triggers that transmitter to emit a sonic pulse, whereafter the sync pulse is passed to the next transmitter (if there is one) via a delay circuit which introduces a preset delay.

6. A system as claimed in claim 5, wherein the responses received by the receivers are sent back to the shore installation via the same cable as that used for the sync pulse, and wherein at each said transmitter a by-pass circuit is provided so that such received references do not pass through the delay circuit.

7. An underwater sonar detection system, substantially as described with reference to the accompanying drawings.

CLAIMS (13 Jul 1982)

8. An underwater sonar detection system, which includes a number of fixed sonar transmitters arranged in a line or other configuration so as to define a line the crossing of which is to be detected, and receiving devices each associated with one of said transmitters, wherein each said transmitter is a fixed-beam transmitter so located that its sonic output is transmitted upwards towards the surface of the water under which the transmitters are placed, the beam of a said transmitter being a narrow-angle conical beam, wherein at the surface of the water the beams overlap so as to define the line to be protected, wherein each said receiving device is so located with respect to its said transmitter as to receive signals from its transmitter after reflection from a body which comes under the influence of the sonic signals from its said transmitter, and wherein the transmitters are caused to operate one at a time in sequence, the arrangement being such that when an intruder is detected the approximate position of that intruder is determined from the identity of the receiving device which responded.

9. A system as claimed in claim 8, and which includes a passive responder which receives the sounds made by an intruder as it progreses through the water and compares that sound with a stored library of sounds so as to determine the nature or the identity of the intruder.

10. A system as claimed in claim 1, 2, 3, 4, 5, 6, 8 or 9, and which includes a second line of fixed sonar transducers, located parallel to and adjacent to the first said line receiving means associated with the tranmitters of the second line and means responsive to the detection of the order in which the two lines respond to the same intruder to determine the direction and also the speed of movement thereof.

11. A system as claimed in claim 1, and in which the transmitters operate at different frequencies so that the indentity of the transmitter from which a response has been produced can be indicated.

1 2. A system as claimed in any one of claims 1 to 11, and which includes means responsive to the receiption of a modified sonar response from an object such as a ship whose crossing of the line is permitted, which response is modified by an IFF responder associated with such a permitted object.