GB1559734A - Intrusion sensing system - Google Patents

Description

(54) INTRUSION SENSING SYSTEM (71) We, MICROWAVE AND ELECTRONIC SYSTEMS LIMITED, of Lochend Industrial Estate, Newbridge, Midlothian, Scotland EH28 8LP, a British Company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to an intrusion sensing system.

The invention is particularly concerned with a system for monitoring the presence of an intruder at a boundary, such as the perimeter of a zone or area of high security, which is not to be crossed by unauthorised persons.

The best known form of perimeter protection is a mechanical fence of height and shape to prevent unwanted entry into a protected area. Mechanical fences alone can often be cut or circumvented and are generally insufficient to provide high security protection against unauthorised entry. In some instances local factors may make it difficult or inconvenient to erect a fence along the whole perimeter to be protected.

Another form of perimeter protection is that known as seismic sensing in which seismic sensors, e.g. geophones, are buried in the ground to detect vibrations. The sensors are laid out in a line along the boundary or perimeter in question and are sufficiently closely spaced that the detection zone of each sensor overlaps those of the sensors to each side. Although with suitable sensors, the seismic system can be made highly sensitive, this high sensitivity can itself be a disadvantage in producing false alarms if the place of installation is associated with sources of vibration that activate the sensors. Deliberately restricting the effective sensitivity of each sensor to better define the zone of protection entails the use of more sensors thereby increasing the cost of protection per unit distance. Efficient seismic sensor systems are currently expensive.

Another disadvantage of seismic sensor chains is that the electrical signals from the sensors normally have to be transmitted over some distance to a control station where the signals are processed. This requires the use of screened cables in order that the relatively weak signals from the sensors are not lost in extraneously induced noise, such as power line noise, and again adds to the costs per unit distance of a seismic system.

A seismic sensor system may be laid adjacent a fence so that added to the physical barrier of the fence, the seismic system detects an attempted intrusion near the fence or an attempt to tunnel under the fence. Both the seismic sensor chain and the physical fence involve considerable labour to install which adds to costs.

Another form of perimeter or boundary protection is that known as a microwave fence in which a microwave transmitter is located to beam radiation along the boundary or perimeter to be protected to a receiver which gives an alarm upon the beam of radiation being interrupted to a predetermined extent by an intruder entering the beam. One particular form of microwave fence described in British patent specification 1 475 111 establishes a beam of radiation up to about 2 metres high and contiguous to the ground over a substantial range, say 150 metres, and without suffering deleteriously from ground reflection effects. A microwave fence has a significantly lower cost per unit distance than a seismic system. It is commonly employed in conjunction with a mechanical fence of substantially greater height than that of the microwave fence.Neither is immune to tunnelling and even the combination could in some circumstances be scaled.

The present invention is concerned with a perimeter-type intrusion sensing system and a system of this type embodying the invention will be described hereafter which provides a high security of protection upon proper installation and achieves this with good costeffectiveness.

In broad terms the invention provides in one aspect an intrusion sensing system comprising: a microwave trnsmitter terminal located at ground level to establish a beam of microwave radiation; a microwave receiver terminal located at ground level and spaced from said transmitter terminal to receive said beamed radiation, said receiver terminal including a signal processing unit responsive to a change in level of received radiation to generate a first alarm signal; a chain of seismic sensors located below ground at intervals in a direction parallel to said beam of radiation, said seismic sensors being connected to a cable which enters said receiver terminal to transmit vibration-responsive signals from said sensors to said receiver terminal;; a signal processing unit in said receiver terminal connected to said cable to provide a second alarm signal in response to vibrationrepresenting signals received from one or more of said seismic sensors; a power supply unit located remotely from said receiver terminal and a cable connecting said power unit to said receiver and transmitter terminals to supply power thereto; and a monitoring unit and means providing communication between the monitoring unit and said receiver terminal to receive therefrom said first and second alarm signals or a signal derived from a logic function performed on said first and second signals.

In a second aspect the invention provides an intrusion sensing system comprising: a plurality of microwave sensors establishing respective zones of radiation along portions of a perimeter or section of perimeter to be monitored, each sensor comprising a microwave transmitter terminal located at ground level to direct a beam of microwave radiation along a portion of said perimeter and a microwave receiver terminal located at a ground level and spaced from said microwave transmitter terminal along said perimeter to receive radiation beamed along said portion thereof;; a plurality of chains of seismic sensors each comprising a cable having a plurality of seismic sensors connected thereto, a respective chain being laid in the ground along each of said perimeter portions so as to extend parallel to the microwave beam directed therealong, and the cable of said each seismic chain entering the receiver terminal of the microwave sensor associated with the same portion of the perimeter; each receiver terminal including a microwave receiver and first signal processing means coupled thereto and responsive to a change in the level of received radiation to generate a first alarm signal; and a second signal processing means coupled to the cable of the associated seismic chain to provide a second alarm signal in response to vibration-representing signals from one or more sensors of the chain; an a.c. supply cable extending along said perimeter or perimeter section; a power supply unit connected to said a.c.

supply cable to produce d.c. power for equipment in said transmitter and receiver terminals; a cable connected to the d.c. output of said power supply unit and extending along said perimeter or perimeter section, connections being made between said terminals and adjacent points of said d.c. supply cable to supply d.c.

power to said terminals from the power supply unit; a control station remote from said perimeter section; and means providing communication between each receiver terminal and said control station to transmit to the latter the first and second alarm signals generated at each receiver terminal or a signal derived from a logic function performed on said first and second alarm signals.

The invention and its practice will be further explained with reference to an embodiment illustrated in the accompanying drawings in which: Figure 1 shows a diagrammatic layout of a security installation embodying the invention to protect the perimeter of a secure area; Figures 2A and 2B show in side elevation and plan view respectively one microwave fence-seismic chain combaination installed along the perimeter of the installation of Figure 1; Figure 3 shows in an end view a microwave fence-seismic chain combaination modified for use with a double physical fence; Figure 4 is a block circuit diagram of one microwave fence-seismic chain combination showing how it is connected into the perimeter installation and to the control centre; and Figure 5 is a circuit diagram showing a preferred electrical arrangement of a seismic chain.

Figure 1 shows a diagrammatic layout of a security installation embodying the invention.

For the purposes of Figure 1 it is assumed that the perimeter of a large area 1 is to be protected and the figure is drawn on the assumption that the perimeter may extend to many kilometres in length. For simplicity of illustration the secure area 1 is given a rectangular shape.

In the secure area a central control station 10 is located. The control station includes a power supply system 12, a display and control arrangement 14 and signal logic circuits 16. The units may be used in conjunction with other security surveillance and control devices not concerned with the present invention and they will only be described in so far as material to the present invention.

Around the perimeter of the secure area a series of combined microwave fences and seismic chains are established. Current microwave fences may be used satisfactorily at a range of say 150m. so that a considerable number of them are required for perimeters of the order of kilometres. To avoid gaps adjacent microwave fences are overlapped or staggered.

This is very diagrammatically illustrated in the figure in which is shown two overlapping combinations 20a and 20b of microwave fence and seismic chain. A more detailed description of such a combination is given hereafter. Each fence combination includes a microwave transmitter terminal 22 which beams radiation as indicated by arrow R to a microwave receiver terminal 24 over the ground. Laid under the ground is a chain 26 of seismic sensors which terminates in the receiver terminal 24 as will be described later. The microwave-seismic combination may also be used in conjunction with a physical fence. This is not shown in Figure 1 in order to avoid cluttering the figure but how this is done will be described later with reference to Figure 3.

The supply of operating power to the microwave-seismic combinations will now be discussed. The long perimeter to be protected or monitored is divided into sections. By way of example a division into six sections is shown in Figure 1, each of the shorter sides of the rectangle constituting one section and each of the longer sides being divided into two sections.

The microwave fence-seismic chain combinations in each section are supplied with direct current from a power supply unit (P.S.U.) associated with that section. The power supply units are supplied by an a.c. ring main originating from the control centre. Looking now at Figure 1 in more detail the A.C. supply system 12 in the control centre 10 feeds a ring main 30, e.g. a 220 volt, single phase mains supply, which extends around the whole perimeter as shown.

Taking the section that includes fence combinations 20a and 20b as an example of how each section is treated, this section extends for over half of one of the longer sides of the rectangle. To supply d.c. power to the fence combinations along this portion of the perimeter a d.c. distribution line 32 is provided extending along the section and fed with d.c.

from a power supply unit 34. D.c. power can be distributed to the transmitter and receiver terminals 22 and 24 by connecting them onto the adjacent portion of distribution line 32 as indicated at points 35 to 38. Each of the other sections is likewise provided with d.c. power distribution for its microwave fence-seismic chain combinations. It is presently contemplated that up to ten such combinations may be connected into one d.c. distribution section.

The direct voltage from the power supply units 34 should be in excess of the maximum d.c.

supply rail voltage within the transmitter and receiver terminals to allow for stabilisation of the supplies in the terminals and for any voltage drop in distribution line 32. For example, if the maximum rail voltage in the terminals were to be 12 volts then the output voltage from units 34 might lie in the range 13 to 30 volts.

Before leaving the power supply arrangement, it should be mentioned that the central supply system 12 will normally take its power in the usual way from the public electricity supply. Voltage stabilising transformers may be incorporated in the central system 12 and the system 12 also includes stand-by generators to provide continual mains supply in the event of a failure of the public electricity supply.

Attention can now be given to the manner in which operating personnel in the control centre 10 are made aware of events such as attempted intrusions on the perimeter which is a substantial distance from the control centre. To avoid the need to send analogue signals over long distances, signal processing is done locally at the periphery and alarm signals are sent to the central station 10 by d.c. signalling. The alarm signals emanate from the receiver terminals 24. Each receiver terminal provides two signals, one from the microwave fence of which it forms part and the other from the associated seismic chain. These signals are kept separate and are conveyed separately back to the control centre 10 along with the similar signals from the other receiver terminals.

Communication between the receiver terminals and the control centre is effected by a multiconductor cable having multiple conductor pairs for connection to the receiver terminals 24. This cable is also arranged along the perimeter though in the embodiment illustrated it is divided for convenience into two sections 40a and 40b.

Thus the receiver terminals 24 of the two fence combinations 20a and 20b are each connected to two pairs of conductors of cable section 40a as indicated at 41, and the cable section 40a is connected to the control centre 10 by a multiconductor highway 42a. Likewise section 40b is connected to control centre 10 by multiconductor highway 42b. It will be noted that the highways are indicated as conveying signals both to and from control centre 10. Other conductor pairs in the cables may be used to transmit tamper signals from items of equipment as is well known in the security field and to send control signals, e.g.

resetting signals, and test signals to the microwave fence-seismic chain combinations. The details of the particular procedures adopted are not pertinent to an understanding of the present invention save to point out that all the signals are at d.c. It will be appreciated that the cable runs in an installation of the size previously discussed are very lengthy and, as will be more apparent from the subsequent more detailed description, the conveying of analogue signals from sensors is kept to short distances.

The various alarm signals arriving at control centre 10 along highways 42a and b are supplied to and processed in the logic circuit 16. Again the manner of processing is not relevant to the present invention save to note that in the illustrated case all decisions are taken within the control centre 10 with the aid of the logic processing and the operator to whom the processed results are displayed on unit 14 for him to take appropriate action.

The installation of the various cables 30, 32 and 40 can conveniently be done by disposing the parallel cables within a common conduit such as indicated at 50. Still more conveniently, since trenches are dug to lay the seismic sensor chains 26 (as is further described below), the conduit 50 can be laid in the same trench. It need not be a metal conduit but can be of plastics. The power supply units 34 can be mounted at ground level or sunk into the ground.

Figures 2A and 2B show one combination 20 of a microwave fence and a seismic sensor chain as used in the installation of Figure 1.

Figure 2A is a simplified elevation along a portion of the perimeter, ground level being indicated at G while Figure 2B is a plan view.

The microwave transmitter terminal 22 and microwave receiver terminal 24 are spaced apart and establish therebetween a beam of microwave radiation 201 which constitutes a microwave "fence". The construction of the transmitter and receiver and in particular their respective antennas to shape beam 201 is described in detail in the aforementioned specification 1 475 111 to which reference can be made for further information. Typically the length of the fence 201 is 150m. and it has a height of about 2m. for a vertical antenna aperture of 1.5m. at an operating frequency in X-band (about 10 GHz). The beam 201 extends to ground level G.

In so far as material to the present system the receiver and its vertically extending antenna (e.g. a waveguide array) are housed in an elongate cylindrical housing 242 which acts as a radome and generally protects the receiver equipment. The housing structure 242 stands on and is secured to a plinth 244 which may be made hollow to house certain electrical units associated with the receiver terminal or auxiliary batteries. These are to be discussed further with reference to Figure 4. The transmitter unit and its vertical antenna are likewise housed in a protective housing 222 standing on and secured to a plinth 224. The plinths 244 and 224 can be partly sunk into the ground as shown to ensure a firm and secure base.

While the microwave beam extends above ground level G there is laid below ground level the chain 26 of seismic sensors located at intervals between the transmitter and receiver terminals. The chain comprises sensors 262, such as geophones, connected onto a cable 264. Only a few of the sensors are shown.

The connection to the cable can be done in various ways but a preferred arrangement will be described hereafter with reference to Figure 5. The cable and sensors are preassembled and laid into a trench T (Figure 2B) at a depth of say 30cms.. The trench is then filled in. The cable 264 leads into the plinth 244 of the receiver terminal and carries the analogue vibration-responsive signals from the sensors 262 back to a signal processing unit contained within the plinth 244 or receiver housing 242 and described later with reference to Figure 3.

In accordance with the layout described with reference to Figure 1, the receiver and transmitter terminals are connected to the power distribution line 32 (this is, of course, a two core cable) and alarm signals-microwave fence and seismic-generated in the receiver terminal 24 are linked into the multiconductor cable 40 as indicated at 401, each signal being carried by a separate conductor pair. The conductor pairs lead eventually to the logic unit 16 in control centre 10. Figure 2A also shows the power supply unit 34 which powers that perimeter section which includes the microwave fence-seismic chain combination of Figure 2A. The unit 34 draws power from ring main 30 and has its output connected to distribution line 32.

As already mentioned the cables 30, 32 and 40 can be conveniently enclosed in a single conduit 50 and this is conveniently laid in the same trench as the seismic sensor chain 26.

This is illustrated in the plan view shown in Figure 2B which shows how the trench T is located parallel to but slightly to one side of the centre line of the microwave fence. The seismic sensor cable is brought directly into the plinth 244 of the receiver terminal while connections from the latter to cables 32 and 40 only require short spurs.

It will also be appreciated that both the microwave fence and the seismic chains will have zones of sensitivity extending laterally with respect to line of the fence, as will be better seen from Figure 3 where the zones are respectively designated 201 and 268, and that in the case of Figure 2B the zones are superposed, one above and one below ground level.

The combination of microwave fence and seismic chain provides a very high degree of security. The microwave fence extends from ground level to a height of about 2m. and has a greatest width half-way along it of a few meters. The seismic chain is typically established to provide a zone of sensitivity within the ground having a width (i.e. the direction normal to the plane of Figure 2A) of say 7 metres and extending to a depth of say 2m.

The combination of microwave fence and seismic chain provides advantage in regard to the total costs per unit distance of perimeter protected. Because the microwave fence provides a high security perimeter protection in its own right, the seismic chain can use a lesser number of sensors than if the chain alone were relied upon. Another advantage of the combination disclosed is that the signal processing is performed at the microwave receiver terminal.

The receiver analogue signals due to disturbance of the microwave beam are directly processed at the receiver terminal and by processing the analogue signals from the associated siesmic chain also at the receiver terminal, the length of signal-carrying-cable which is costly because it needs to be a well screened cables much reduced over conventional seismic sensor installations in which all the signals are conveyed back to a central processing station. Where, as in Figure 1, security for large areas is involved this would involve long cable runs and be difficult. In the installation, signalling from the receiver terminals 24 to the central station 10 is direct current rendering screened cable unnecessary and further eliminating cross-talk problems of the kind that occur between parallel conductors carrying analogue signals.

The d.c. alarm signals are of a simple two state kind which are efficient to transmit and detect.

The outcome of the foregoing savings is that the dual protection of microwave fence and seismic chain installed as described above can be expected to be less than the cost of a seismic chain alone installed in conventional manner.

Where a very high degree of security is demanded, the combined system thus far described is used in conjunction with a normal wire of like fence and preferably a double fence such as is illustrated in Figure 3. This shows an end view of a microwave fence-seismic chain combination such as shown in Figures 2A and B but modified to provide further vibration sensing for an attack on the fence. Figure 3, shows, by way of example, the receiver terminal 24 which is disposed between an outer fence 60 and a spaced, parallel inner fence 62.

These provide an additional physical height barrier against intruders. The microwave fence is directed along the mid-line between the fences 60 and 62. The cross-section of the microwave beam 201 is approximately indicated and substantially fills the space between the two physical fences. However, even though the microwave fence can be designed to mitigate the effects of lateral reflections as is disclosed in aforementioned specirication 1 475 111, care should be taken to avoid excess reflections from the fences 60 to 62 which may render the microwave fence liable to give false alarms.

It is common to apply vibration sensors to a wire or like fence and this form of security can be added at small extra cost by using the seismic chain 26 for this purpose as well as a sensor of vibrations in the ground. This dual function is obtained by a modification of the layout shown in Figures 2A and 2B whereby the seismic chain is displaced somewhat laterally from the microwave fence to enable its sensors to be coupled to the inner fence 62.

Thus in Figure 3, the sensor chain 26 is located closely adjacent fence 62. It is again in the same trench as the cable-carrying conduit 50 which, together with other conductors entering plinth 244, is omitted from the figure for clarity.

Preferably each sensor 262 of the seismic chain, of which one is shown in Figure 3, is coupled to the fence 62 by a respective vibration-transmitting member 266 having one end rigidly secured to the adjacent portion of the fence 62 at 268 and the other end secured to the sensor 262.

The sensors 262 are preferably of a directional kind and are disposed in the ground so as to be vertically polarized, i.e. have a vertical directivity, whereby they respond strongly to vertically propagating vibrations in the ground but have lesser response to horizontal vibrations. The coupling to the fence 62 is also designed to provide a primary response to vertical forces iri fence 62. In this way the seismic chain can be rendered relatively insensitive to horizontal forces such as generated by wind acting on the fence and the horizontal reaction forces thereby produced in the ground.

Figure 3 also serves to indicate by chain line 268 the approximate cross-section of the zone of sensitivity of seismic chain 26 within the ground. The above and below ground zones of sensitivity 201 and 268 of the microwave fence and seismic chain are not fully superposed here due to the relative lateral displacement but nonetheless a substantial area of overlap remains.

The interconnection of the units contained within the receiver and transmitter terminals of a microwave fence, the associated seismic chain and the control centre 10 will now be further explained with reference to Figure 4 which is block circuit diagram.

The housing structure of the transmitter terminal 22 contains a vertical antenna array 226 energised by a microwave transmitter 228.

The d.c. power to the transmitter 228 is obtained from distribution line 32 through a stablizer unit 230. To allow the latter to function properly the voltage from line 32 is in excess of that supplied to transmitter 228.

As already discussed, the line 32 is supplied with d.c. power from a power supply unit 34 energised from ring main 30 connected to the supply unit 12 in the central control station 10.

The housing structure 242,244 (Figure 2) of the receiver terminal contains a vertical antenna array 246 which feeds microwave energy received from transmitter 22 to a receiver 248. The output of the receiver 248 in turn feeds a signal processing circuit 250 that provides an alarm signal on line 252 upon a change in the received signal that satisfies prescribed conditions.

The associated seismic chain 26 feeds a signal processing circuit 270 which provides an alarm signal on line 272 upon prescribed conditions being satisfied. Various forms of signal processing are known in both microwave and seismic sensing systems and the details thereof are not pertinent to an understanding of the present invention.

The power supply for the microwave receiver units 248 and 250 and the seismic unit 270 is drawn from d.c. distribution line 32 via a stabiliser unit 254 which derives the required voltage rails for the units. As the seismic system is likely to require different voltage supplies from the microwave system, unit 254 is shown in two parts 254a and b separately supplying the two systems.

Also connected to the power supply is an auxiliary battery arrangement 256 which is constantly float charged from the supply line 32 and which in the event of an interruption in the normal supply via line 32 not only energises the receiver terminal but feeds power to the transmitter terminal 22 via line 32. A similar action occurs in all the microwave fence systems connected to the same section of distribution line 32 (Figure 1). It will be appreciated that auxiliary batteries can alternatively or additionally be located in the transmitter terminals.

The alarm signal lines 252 and 272 are connected to respective pair lines of the multiconductor cable 40 already described and as indicated these lines lead into the logic unit 16 in central station 10 via the highway 42. The alarm signals from processing units 250 and 270 are obtained by switching direct voltages derived from the d.c. power supplies by means of transistor output circuits though obviously devices such as relays could be used. It will again be noted that the length of screened cable 264 required to carry the analog vibrationrepresenting signals from sensors 262 is small and that at the major distances over which signals are carried, i.e. along cables 40 and 42, d.c. signalling is used, the signals being simply two state. Thus cheaper unscreened cable can be used for this purpose and since only two d.c.

levels have to be detected at the logic processor 16, the system can be made highly immune to interference due to extraneous noise pick up or to induced voltages due to various ground currents that may flow in the large secure area 1 (Figure 1).

The installation described has been particularly concerned with a large secure area having a long perimeter. It will, however, be recognised that the principles taught about could be applied on a smaller scale. For example, a single d.c. power unit 34 might suffice and this could be contained within the remote control station.

The advantage of having a seismic chain part of a combined microwave fence-seismic chain powered from the receiver terminal is maintained as is the advantage of the signalling from the terminal being at d.c. Although all the logic functions are performed in the remote control station 10 in the described embodiment, this does not preclude logic functions being performed within the receiver terminal if desired and the logic result being d.c. signalled back to the control centre.

A preferred seismic chain system will now be described with reference to Figure 5 which is a schematic circuit of one such chain 26.

In Figure 5, the seismic sensors 262 are connected in repetitive sections of like construction. The sections are denoted 281 to 285 respectively and each comprises a group of five sensors connected in parallel, interconnections between adjacent sections and between the first section and the cable leading to the microwave receiver terminal 24 being made through respective junction boxes. The junction boxes are referenced 286 to 290 respectively. The sensors are typically spaced at intervals of say 3 metres or more and in a given section the connection between adjacent sensors is via portions of the screened cable generally indicated at 264 in Figures 2A and B.Taking section 281 by way of example, the screen of cable 264 is indicated at 291 and contains three conductors 292,293 and 294 one of which, 292, is a common conductor to the whole installation while the sensors 262 are connected between conductors 293 and 294. The portion of cable 264 connecting the first junction box of the chain back to the relevant microwave receiver terminal only requires two conductors of the screened cable, one being a continuation of conductor 292 the other 295 being connected to one of the conductors 293, 294 and, as in the embodiment shown, being connected to conductor 293 of section 821.

It will be seen that the conductor 292 extends along the whole seismic chain passing through each junction box via a respective tamper switch 297. This is a switch, such as a microswitch, which is only closed to complete the circuit when a lid (not shown) by which access is gained to the junction box is properly closed. The conductor 292 is not directly connected to any of the sensors in sections 281 to 284 but is connected via the tamper switch in junction box 290 to one of the conductors, viz. 293, between which the five sensors in the end section 285 are connected. The conductors 293,294 of the sections 281 to 285 are connected by links 298 in the junction boxes, one conductor being connected to the link in the junction box at one end of the section and the other conductor being connected to the link in the junction box at the other end of the section. This sequence is repeated from the far end section 285 until in the first junction box 272 the connection through the link is made to conductor 295. It will be seen that the interconnection scheme, thus connects the conductor 293 in one section to the conductor 294 in the preceding section and that while the group of five sensors in each section are parallel connected, the groups are serially connected between conductors 292 and 295.

The connection scheme shown enables a proper impedance match to be presented between the seismic sensors and the signal processing circuit 270 (Figure 4). Assuming the sensors have a substantially resistive impedance, in general if each section contains n sensors each having a resistance R and there are m sections, the resistance of the whole seismic chain is m.R./n. In this case where m n(= 5 in the illustrated embodiment), the seismic chain resistance is R. The use of a combined series parallel connection arrangement ensures that reasonable values of resistance are maintained between the extremes of R/ma. if all the sensors were simply connected in parallel to m.n.R. if they were all in series.Apart from the conductors 292 and 295 feeding the seismic signals to the signal processing circuit 270, a test d.c. signal can be sent through the seismic chain to sense variations from the established resistance level nR/m due to tampering with some portion of the seismic chain or a failure of some part of it. Any attempt to open the lid of a junction box will of course break the connection by its microswitch opening.

In order to compensate for variation of the sensors resistance with temperature the sensor chain of preferably each section can be shunted by a resistor combination connected in the associated junction box. Such a combination is shown, by way of example, in junction box 288 (shown enlarged) where a series combination of resistor Rc and thermistor Rt is shown connected across conductors 293 and 294. This additional shunt resistance will of course modify the value of resistance exhibited by section 283, the thermistor having a value and temperature coefficient chosen to best compensate for the temperature dependence of the parallel sensors.

As can be seen from Figure 5 the sections 281 to 285 are of like construction and the necessary interconnections are effected in the junction boxes. The sections are laid in the trench (T in Figure 2B) with the sensors vertically polarized as previously described, and the junction boxes are sunk and staked in the ground but with their lids accessible for access to the boxes. Of course, an attempt at unauthorised access will set off the seismic chain alarm apart form the tamper-proofing at the box itself.

WHAT WE CLAIM IS: 1. An intrusion sensing system comprising: a microwave transmitter terminal located at ground level to establish a beam of microwave radiation; a microwave receiver terminal located at ground level and spaced from said transmitter terminal to receive said beamed radiation, said receiver terminal including a signal processing unit responsive to a change in level of received radiation to generate a first alarm signal; a chain of seismic sensors located below ground at intervals in a direction parallel to said beam of radiation, said seismic sensors being connected to a cable which enters said receiver terminal to transmit vibration-responsive signals from said sensors to said receiver terminal;; a signal processing unit in said receiver terminal connected to said cable to provide a second alarm signal in response to vibrationrepresenting signals received from one or more of said seismic sensors; a power supply unit located remotely from said receiver terminal and a cable connecting said power unit to said receiver and transmitter terminals to supply power thereto; and a monitoring unit and means providing communication between the monitoring unit and said receiver terminal to receive therefrom said first and second alarm signals or a signal derived from a logic function performed on said first and second signals.

2. A system as set forth in Claim 1 in which said communication means comprises a cable connecting said monitoring unit to said receiver terminal.

3. A system as set forth in Claim 2 in which signalling over said communication cable is effected at d.c.

4. A system as set forth in Claim 1,2 or 3 in which signalling by said communication means is by two state signals.

5. A system as set forth in any one of claims 1 to 4 further comprising a control station in which said monitoring unit is located, and said control station including means for providing an a.c. power supply, said power supply unit being located remote from both said control station and said receiver and transmitter terminals and being connected to said a.c. power supply means in said control station by a cable and being adapted to supply d.c. power to said transmitter and receiver terminals by the cable by which the power supply unit is connected thereto.

6. A system as set forth in any one of claims 1 to 5 in which said seismic sensors are connected onto the cable of said seismic chain in a combination of series and parallel connections of the sensors.

7. A system as set forth in claim 6 in which said seismic sensors are connected in groups, the sensors of a group being connected in parallel and the groups being connected in series.

8. A system as set forth in claim 7 in which each group of sensors and the conductors by which they are connected in parallel form a section of the seismic chain, and the seismic chain including interconnecting units disposed between adjacent sections and by which the connections are made between the adjacent sections to form the series connection of the groups.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (1)

1. **WARNING** start of CLMS field may overlap end of DESC **.

   group of five sensors in each section are parallel connected, the groups are serially connected between conductors 292 and 295.

   The connection scheme shown enables a proper impedance match to be presented between the seismic sensors and the signal processing circuit 270 (Figure 4). Assuming the sensors have a substantially resistive impedance, in general if each section contains n sensors each having a resistance R and there are m sections, the resistance of the whole seismic chain is m.R./n. In this case where m n(= 5 in the illustrated embodiment), the seismic chain resistance is R. The use of a combined series parallel connection arrangement ensures that reasonable values of resistance are maintained between the extremes of R/ma. if all the sensors were simply connected in parallel to m.n.R. if they were all in series.Apart from the conductors 292 and 295 feeding the seismic signals to the signal processing circuit 270, a test d.c. signal can be sent through the seismic chain to sense variations from the established resistance level nR/m due to tampering with some portion of the seismic chain or a failure of some part of it. Any attempt to open the lid of a junction box will of course break the connection by its microswitch opening.

   In order to compensate for variation of the sensors resistance with temperature the sensor chain of preferably each section can be shunted by a resistor combination connected in the associated junction box. Such a combination is shown, by way of example, in junction box 288 (shown enlarged) where a series combination of resistor Rc and thermistor Rt is shown connected across conductors 293 and 294. This additional shunt resistance will of course modify the value of resistance exhibited by section 283, the thermistor having a value and temperature coefficient chosen to best compensate for the temperature dependence of the parallel sensors.

   As can be seen from Figure 5 the sections 281 to 285 are of like construction and the necessary interconnections are effected in the junction boxes. The sections are laid in the trench (T in Figure 2B) with the sensors vertically polarized as previously described, and the junction boxes are sunk and staked in the ground but with their lids accessible for access to the boxes. Of course, an attempt at unauthorised access will set off the seismic chain alarm apart form the tamper-proofing at the box itself.

   WHAT WE CLAIM IS:

   1. An intrusion sensing system comprising: a microwave transmitter terminal located at ground level to establish a beam of microwave radiation; a microwave receiver terminal located at ground level and spaced from said transmitter terminal to receive said beamed radiation, said receiver terminal including a signal processing unit responsive to a change in level of received radiation to generate a first alarm signal; a chain of seismic sensors located below ground at intervals in a direction parallel to said beam of radiation, said seismic sensors being connected to a cable which enters said receiver terminal to transmit vibration-responsive signals from said sensors to said receiver terminal;; a signal processing unit in said receiver terminal connected to said cable to provide a second alarm signal in response to vibrationrepresenting signals received from one or more of said seismic sensors; a power supply unit located remotely from said receiver terminal and a cable connecting said power unit to said receiver and transmitter terminals to supply power thereto; and a monitoring unit and means providing communication between the monitoring unit and said receiver terminal to receive therefrom said first and second alarm signals or a signal derived from a logic function performed on said first and second signals.

   2. A system as set forth in Claim 1 in which said communication means comprises a cable connecting said monitoring unit to said receiver terminal.

   3. A system as set forth in Claim 2 in which signalling over said communication cable is effected at d.c.

   4. A system as set forth in Claim 1,2 or 3 in which signalling by said communication means is by two state signals.

   5. A system as set forth in any one of claims 1 to 4 further comprising a control station in which said monitoring unit is located, and said control station including means for providing an a.c. power supply, said power supply unit being located remote from both said control station and said receiver and transmitter terminals and being connected to said a.c. power supply means in said control station by a cable and being adapted to supply d.c. power to said transmitter and receiver terminals by the cable by which the power supply unit is connected thereto.

   6. A system as set forth in any one of claims 1 to 5 in which said seismic sensors are connected onto the cable of said seismic chain in a combination of series and parallel connections of the sensors.

   7. A system as set forth in claim 6 in which said seismic sensors are connected in groups, the sensors of a group being connected in parallel and the groups being connected in series.

   8. A system as set forth in claim 7 in which each group of sensors and the conductors by which they are connected in parallel form a section of the seismic chain, and the seismic chain including interconnecting units disposed between adjacent sections and by which the connections are made between the adjacent sections to form the series connection of the groups.

   9. A system as set forth in any one of claims

   1 to 8 in which a physical fence is located parallel to said microwave beam and in which at least some of said seismic sensors are coupled to said fence by vibration-transmissive means.

   10. A system as set forth in any one of claims 1 to 8 further comprising two spaced parallel physical fences, said microwave transmitter and receiver terminals being disposed in the space between said fences to establish said beam of radiation therebetween and parallel thereto.

   11. A system as set forth in claim 10 in which at least some of said seismic sensors are coupled to one of said fences by vibrationtransmissive means.

   12. An intrusion sensing system comprising: a plurality of microwave sensors establishing respective zones of radiation along portions of a perimeter or section of perimeter to be monitored, each sensor comprising a microwave transmitter terminal located at ground level to direct a beam of microwave radiation along a portion of said perimeter and a microwave receiver terminal located at ground level and spaced from said microwave transmitter terminal along said perimeter to receive radiation beamed along said portion thereof;; a plurality of chains of seismic sensors each comprising a cable having a plurality of seismic sensors connected thereto, a respective chain being laid in the ground along each of said perimeter portions so as to extend parallel to the microwave beam directed therealong, and the cable of said each seismic chain entering the receiver terminal of the microwave sensor associated with the same portion of the perimeter; each receiver terminal including a microwave receiver and first signal processing means coupled thereto and responsive to a change in the level of received radiation to generate a first alarm signal; and a second signal processing means coupled to the cable of the associated seismic chain to provide a second alarm signal in response to vibration-representing signals from one or more sensors of the chain; an a.c. supply cable extending along said perimeter or perimeter section; a power supply unit connected to said a.c.

   supply cable to produce d.c. power for equipment in said transmitter and receiver terminals; a cable connected to the d.c. output of said power supply unit and extending along said perimeter or perimeter section, connections being made between said terminals and adjacent points of said d.c. supply cable to supply d.c.

   power to said terminals from the power supply unit; a control station remote from said perimeter section; and means providing communication between each receiver terminal and said control station to transmit to the latter the first and second alarm signals generated at each receiver terminal or a signal derived from a logic function performed on said first and second alarm signals.

   13. A system as set forth in claim 12 in which the communication means comprises a multiconductor cable extending along said perimeter or perimeter section, conductors leading from said receiver terminals to locally adjacent points of said multiconductor cable and said multiconductor cable having a portion thereof extending to said control station for transmission of the first and second alarm signals or signals derived therefrom to said control signals, the transmission thereof being by d.c. signalling.

   14. A system as set forth in claim 12 or 13 in which said control station including means connected to said a.c. power supply cable to supply power thereto.

   15. A system as set forth in claim 14 in which the perimeter has a number of sections each provided with a plurality of microwave sensor and seismic chains arranged as specified in claim 12, each section also having an associated power supply unit and d.c. power supply cable connected to the microwave receiver and transmitter terminals of that section in the manner specified in claim 12, and there being means providing communication between each receiver terminal in each section and the control station in the manner specified in claim 12, the aforesaid a.c. supply cable being arranged as a ring main extending along all the sections and to which the respective power supply units are connected.

   16. A system as set forth in claim 15 when appended to claim 13 in which said multiconductor cable extends along at least some of said sections and the receiver terminals of those sections have conductors leading thereto in the manner specified in paragraph 13 for d.c.

   signalling of the first and second alarm signals or the signals derived therefrom to the control station.

   17. A system as set forth in claim 16 in which said multiconductor cable extends along some of said sections and the other sections or each of other groups of sections are provided with a multiconductor cable providing communication between the receiver terminals associated with the other sections of group of sections, as the case may be, and the control centre in the manner specified in claim 13.

   18. A system as set forth in any one of claim 12 to 17 in which along the or each section the a.c. supply cable and d.c. power cable extending therealong are disposed in a common conduit and in the case where a multiconductor cable extends along a section it is disposed in the same conduit.

   18. A system as set forth in any one of claims 12 to 18 in which the seismic chain is arranged as specified in any one of claims 6 to 8.

   20. A system as set forth in any one of claims 12 to 19 in which a physical fence extends along said perimeter or section thereof and in each seismic chain at least some of said seismic sensors are coupled to the fence by vibration-transmissive means.

   21. A system as set forth ill any one of claims 12 to 18 further comprising two spaced, parallel physical fences extending along said perimeter or section thereof, said microwave sensors being disposed in the space between said fences to establish their beams of radiation therebetween and parallel thereto.

   22. A system as set forth in claim 21 in which in each seismic chain at least some of the seismic sensors are coupled to one of said fences by vibration-transmissive means.

   23. An intrusion sensing system substantially as hereinbefore described with reference to Figures 1,2A, 2B and 4; or Figures 1,2A, 2B,4 and 5; or Figures 1, 2A, 2B,3 and 4; or Figures 1, 2A, 2B, 3,4 and 5 of the accompanying drawings.