Abstract:
In a detection system for detecting intruders in the vicinity of a predetermined path or line defined by a distributed antenna, for example an open transmission line, an array of discrete antennas are provided alongside the distributed antenna and within a predetermined distance therefrom. The antennas are spaced apart from each other and the distributed antenna and define a plurality of detection zones. A radio frequency transmitter is connected to one of the distributed antenna and the array of discrete antennas, and a complementary receiver is connected to the other of the distributed antenna and the array of discrete antennas. A control unit controls the transmitter, receiver and array of antennas to exchange radio frequency energy several times via the distributed antenna and selected ones of the array of antennas and to detect perturbations caused by an intruder moving adjacent the path and adjacent a particular antenna.

Description:
This application claims priority from United States Provisional patent application number 60/214,473 filed Jun. 27, 2000. 
    
    
     TECHNICAL FIELD 
     The invention relates to detection systems and methods and, in particular, to detection systems and methods which are used to detect objects or people moving in the vicinity of a predetermined path or line defined by a distributed antenna, for example an open transmission line. The invention is especially applicable to the detection of intruders or escapees. 
     BACKGROUND ART 
     Known such detection systems use at least one open transmission line, usually a leaky cable, as a distributed receiving antenna to receive a radio frequency signal transmitted from an associated antenna; or as a transmitting antenna to transmit signals for reception by a separate antenna. An intruder or escapee, or other object, moving in the vicinity of the leaky cable causes a perturbation in the coupling of continuous wave RF energy into or from the leaky cable. Detection of the perturbation indicates an intrusion or escape attempt. It will be appreciated that there is technically no distinction between an intruder traversing the path to enter a protected zone and an escapee traversing the path to leave a protected zone. For convenience, therefore, in this specification, the term “intruder” will be used to cover both. 
     It is desirable to determine, at least approximately, the location of the intruder along the length of the cable. U.S. Pat. No. 4,994,789 (Harman) issued Feb. 19, 1991 discloses a detection system in which several detection zones are provided by interposing phase-shifting modulators at intervals along the leaky cable. Each modulator can be shunted by a switch. A signal processor analyzes the signal received from the cable while the switch is operated so as to shunt the modulator or connect it in series with the cable sections, thereby allowing determination of the section in which the intrusion occurred. When such a system uses only two zones, it may be relatively economical. However, when such a system is expanded to many zones, the interdependence of the modulators, the complexities of switching them, and intricacies of signal analysis prohibitively increase cost and reduce reliability. 
     U.S. Pat. No. 4,887,069 (Maki) issued Dec. 12, 1989 discloses a detection system which uses two coaxial cables, one of them a leaky cable, extending along a perimeter of a protection zone, one coupled to a transmitter and the other to a receiver. The cables are subdivided into sections which are interconnected by oscillators and switches allowing selection of one section at a time. If a section has not been selected, the RF signal passes along its inner conductor. When a section is selected, the RF signal is switched to propagate as an external wave along the outer sheath of the cable section. Maki also discloses a system in which both of the coaxial cables are leaky cables, with zones provided by serialized switching, each zone being powered from a switched local oscillator. In either case, signal perturbations caused by an intruder are transmitted through the intervening sections to a receiver located at one end of the cable. The oscillators and switches increase complexity and reduce reliability. 
     In either of these known systems, the radio frequency energy from each cable section must pass through any preceding sections. Consequently, failure of an oscillator/switch, or modulator, as the case may be, especially near the receiver end of the cable, may compromise the system or even render it inoperative. 
     The present invention seeks to provide a detection apparatus and method with a plurality of detection zones, while eliminating or at least mitigating the aforementioned disadvantages of known detection systems. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, a detection system for detecting intruders moving in the vicinity of a defined path comprises a distributed antenna, for example an open transmission line, extending along the path and an array of discrete antennas extending alongside the distributed antenna and within a predetermined distance therefrom, the antennas being spaced apart from each other and the distributed antenna and defining a plurality of detection zones, a radio frequency transmitter connected to one of the distributed antenna and the array of discrete antennas, a complementary receiver connected to the other of the distributed antenna and the array of discrete antennas, and control means for controlling the transmitter, receiver and array of antennas to exchange radio frequency energy several times via the distributed antenna and selected ones of the array of antennas and to analyze the energy received from each selected one of the array of antennas so as to detect perturbations caused by an intruder moving adjacent said path and adjacent that particular antenna. 
     The control means may comprise switching means for selecting each one of the array antennas individually for such energy exchange. 
     The control means may select the antennas in turn in such a sequence that, if the energy from a particular antenna when previously selected within a prescribed time period showed a perturbation, that antenna would be selected more frequently than those antennas which had not shown such a perturbation within said time period. 
     Preferably, the array of antennas are each connected to a respective one of a plurality of taps distributed along a transmission line extending alongside the distributed antenna The control means then may comprise a plurality of switching devices for connecting respective ones of the antennas to the transmission line and switch control means for controlling operation of the switching devices to select the antennas individually. 
     The switch control means may comprise a means for transmitting antenna addresses selectively onto the transmission line and each switching device then may comprise an address decoder for detecting the address of the associated antenna and an RF switch operable by the decoder to connect the antenna to the transmission line. 
     According to a second aspect of the invention, there is provided a method of operating an intrusion detection system having a distributed antenna extending along a path to be monitored and an array of discrete antennas extending alongside the distributed antenna and within a predetermined distance therefrom, the method comprising the steps of: 
     exchanging radio frequency energy several times between the distributed antenna and each of the array of discrete antennas, in turn, and analyzing energy received by each selected antenna to detect perturbations in the received radio frequency energy indicative of an intruder moving near the path and adjacent that antenna. 
     Preferably, the array antennas are selected in turn in such a sequence that, if the energy from a particular array antenna when previously selected within a prescribed time period showed a perturbation, that array antenna will be selected more frequently than those array antennas which had not shown such a perturbation within said time period. 
     Thus, the selection sequence may comprise alternate selection of perturbation-indicating antennas with non-perturbation indicating antennas. 
     Alternatively, the selection sequence may select all of the perturbation-indicating antennas after each selection of a non-perturbation-indicating antenna. 
     In addition to selecting a perturbation-indicating antenna for which a perturbation was previously recorded, the selection sequence may also be arranged to select one or each of the antennas adjacent thereto. 
     In embodiments of either aspect of the invention, the array antennas may be fastened to a fence, trees, a wall or a roof and the distributed antenna may be a leaky cable or the like buried nearby. 
     Preferably, each array antenna is a “small” antenna, i.e. no dimension of the antenna exceeds one tenth of the wavelength of the operating signal and the antenna has substantially no directivity. 
     When such a small antenna is mounted upon a fence or other generally unstable support, it may be susceptible to movement of the fence or other support. A third aspect of the invention addresses this problem by means of a series resistance of between 150 ohms and 300 ohms between the tap and the antenna element, preferably about 220 ohms. 
     According to a third aspect of the invention, a small radio frequency antenna suitable for mounting upon an unstable support, such as a wire fence, comprises an insulating board having first and second opposite surfaces, a conductive layer on the first surface, means for attaching the antenna to the support with the second surface opposed to the support, and a resistance means for connecting the conductive layer to a feedline, the resistance having a value between about 150 ohms and about 300 ohms. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further features and advantages of the invention will be apparent from the following description of preferred embodiments which are described by way of example only and with reference to the accompanying drawings, in which: 
     FIG. 1 is a pictorial drawing of a detection system; 
     FIG. 2 is a simplified block schematic drawing of the detection system illustrated in FIG. 1; 
     FIG. 3 is a schematic diagram of one of an array of antennas illustrated in FIGS. 1 and 2; 
     FIG. 4 is a block schematic diagram of a switching device coupling the antenna to a transmission line; 
     FIG. 5 is a perspective view of a patch antenna useful in the detection system illustrated in FIGS. 1 to  4 ; and 
     FIG. 6 is a flow chart diagram illustrating a method of operation of the system. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     For convenience of illustration, FIG. 1 illustrates only a portion, including several detection zones, of the detection system. The system comprises a leaky coaxial cable  102  or other suitable open transmission line means, either laid upon the surface of the ground or buried a short distance beneath the surface, which defines a detection path or line to be monitored. A transmission line  103 , conveniently a regular coaxial cable, is shown mounted along a security fence  104  (but alternatively may be buried along the base of the fence  104 ). The cable  102  is depicted, for purposes of illustration only, as having a detection field  102   a  extending radially around it. It will be appreciated that, if the cable  102  is connected to a receiver, the detection field  102   a  will be induced rather than generated directly. The transmission line  103  has a plurality of taps  103 / 1  . . .  103 /n spaced apart along its length. The taps are connected by switching devices  107 / 1  . . .  107 /n, respectively, to a corresponding plurality of small antennas  108 / 1  . . .  108 /n, respectively. Each tap is a T-junction allowing the switching devices to select the antennas individually for connection to the transmission line  103  without the continuity of the transmission line  103  being interrupted. The patch antennas  108 / 1  . . .  108 /n are spaced from the cable  102  to provide a required degree of coupling therebetween while giving some room for a body to intrude into the detection zones. In operation, electromagnetic fields between the leaky cable  102  and the plurality of taps  103 / 1  . . .  103 /n define a corresponding plurality of overlapping detection zones depicted, for purposes of illustration only, by lines  109 / 1 ;  110 / 1 ; . . .  109 /n;  110 /n, respectively. 
     In one experimental setup, the leaky cable  102  and the transmission line  103  were spaced about 20 feet apart and up to 2 miles in length with the antennas at intervals of 50 feet or so. Thus, typically, each antenna forms a perimeter sub-zone about 50 ft long, each sub-zone overlapped with its neighbouring sub-zone to obtain full coverage. 
     As shown in FIG. 2, the transmitter  105  and the receiver  106  are connected to, and controlled by, a microprocessor  111 . The transmitter  105 , receiver  106  and processor  111  may be constructed and operate generally in a manner known to persons skilled in this art and so will not be described in detail here. For examples, the reader is directed to International patent applications numbers PCT/CA91/00050, PCTICA98/00551 and PCT/CA96/00840, which are incorporated herein by reference, 
     The transmitter unit  105  includes a radio frequency transmitter (continuous wave source)  112  and a switch address encoder and transmitter  113  connected in common to the transmission line  103 . The switch address encoder transmitter  113  relays encoded address signals, each comprising an address unique to one of the antennas produced by the processor  111 , for selectively operating the switching devices  107 / 1  . . .  107 /n and hence connecting the patch antennas to the transmission line  103  individually and sequentially. The processor  111  “scans” the patch antennas in turn, i.e., it causes the RF transmitter  112  to transmit a continuous wave radio frequency signal onto the transmission line  103  and the address transmitter  113  to transmit the address of the selected one of the antennas  108 / 1  . . .  108 /n. The switching device associated with the selected antenna detects the address and connects the selected antenna to the transmission line  103  to receive the radio frequency signal and radiate it towards the leaky cable  102 . The receiver  106  detects the corresponding radio frequency signal received by the leaky cable  102  and determines whether or not there is a perturbation indicating that an intruder has affected the electromagnetic coupling between the selected antenna and the leaky cable  102 . This constitutes “scanning” or “sampling” of that antenna. How the perturbation is detected will depend upon the transmitter/receiver chosen. A simple amplitude measurement technique is preferred for its simplicity, but a “synergistic radar” technique might be used instead. If such a perturbation is detected, the microcontroller  111  records in memory (not shown) that the selected antenna is in a perturbation-indicating or “pre-alarm” condition. The microcontroller  111  will not signal an actual “intruder” alarm until a prescribed number of such “pre-alarm” conditions have been detected, as will be described later. 
     Referring to FIGS. 3 and 4, one of the patch antennas  108 /n is shown mounted on a near side  114  of the fence  104  by a base  115  of an electrically insulating material. The transmission line  103  is mounted on a far side  116  of the fence  11 . The switching device  107 /n comprises an address decoder  117 /n and a radio frequency switch  118 /n. The switch  118 /n is coupled via a capacitor  119 /n to the transmission line  103  and via a decoupling resistance element  120 /n to the patch antenna  108 /n. Branches of the T-coupling are labelled IN and OUT as a matter of convenience but energy may traverse the coaxial transmission line  103  in either direction. The address decoder  117 /n and switch  118 /n are housed within an RF shielding enclosure/shroud  121 /n. The switch  118 /n may be either any electromechanical relay or a solid state device suitable for coupling RF energy to the antenna  108 /n. The antenna address decoder  117 /n is connected to the transmission line  103  via an RF blocking device  122 /n to receive DC power and the address signals transmitted by the address transmitter  113  and controls operation of switch element  118 /n, via a lead  123 /n, in response to the receipt of the associated address control signal coupled from the transmission line  103 . When the switch  118 /n is closed, energy is exchanged between the transmission line  103  and the antenna  108 /n via the blocking capacitor  119 /n and the decoupling resistor  120 /n. 
     Alternatively, the transmission line  103  and the switching devices  107 / 1  . . .  107 /n may be concealed beneath the earth with short coaxial coupling lines extending between each of the switches  118 / 1  . . .  118 /n and the associated one of the resistors  120 / 1  . . .  120 /n, the latter each being located with the corresponding one of the antennas  108 / 1  . . .  108 /n. 
     Referring to FIG. 5, a suitable patch antenna  108 /n comprises a conductive rectangular patch  130 /n, typically having a length of 12″ along one major side  131 /n and a width of 4″ along a minor side  132 /n; in other words an oblong with a length to width ratio of about 3:1. The patch  130 /n is mounted upon insulating substrate  115 /n which is half an inch thick and is larger than the patch  130 /n so as to provide a margin around the patch  130 /n. Attachment holes  134 /n in the margin facilitate easy fastening, perhaps by loops of cord, to the fence  104 . As shown in FIG. 3, the resistor  120 /n extends through an opening in the insulating layer  115 /n and is connected at one end to the opposed surface of patch  130 /n and at the other end to the tap. Alternatively, the insulating substrate  115 /n can be any of several structures which will support the patch  130 /n spaced and insulated from any conductive surface, such as when the fence  104  is made of metallic chain link or the antenna is connected to some other conductive structure. 
     Problems of false alarms and missed alarms resulting from movement of the fence  104  or other supporting structure were substantially mitigated during experimentation by selecting the value of the decoupling resistors  120 / 1  . . .  120 /n to be between about 150 ohms and about 300 ohms. Eventually it was found that the illustrated embodiment, used with an operating frequency of about 80 MHz., was generally optimized with decoupling resistors  120 / 1  . . .  120 /n of about 200 to 220 ohms. This gave an antenna gain of −25 dBd with noise immunity to tolerate lateral displacement of the fence fabric up to +/−¼ inch. This is not to suggest that these values are optimal for other operating wavelengths or for other forms of patch antennas, for example circular or triangular. Furthermore, although illustrated separately, the resistor  120 /n may be included in the switching device  119 /n or the latter may be so constructed as to dynamically provide a suitable value of coupling resistance. 
     Operation of the system will now be described with reference to the flowchart shown in FIG.  6 . In function step  6 . 1 , the processor  111  initialises the system, e.g. counters, memory, and so on, and then begins the scanning process by scanning the first antenna  108 / 1 . Such scanning entails the selection, energisation and received signal detection steps as previously described. In decision box  6 . 2 , processor  111  accesses its memory (not shown) to determine whether or not there are any antennas in a “pre-alarm” state as a result of a previous scanning cycle detecting a perturbation in its signal. There will be no “pre-alarm” antennas because there were no previous scans, so, in steps  6 . 4  and  6 . 5  the processor  111  will select and scan the next antenna  108 / 2 . Step  6 . 6  determines whether or not that is the last antenna in the array. Because it is not, loop  6 . 7  returns the processor to decision step  6 . 2 , whereupon it checks again whether or not any antennas now are in the pre-alarm state. So long as none of the antennas are in the pre-alarm state, this loop will repeat steps  6 . 2  to  6 . 6  until all of the antennas have been scanned, whereupon decision step  6 . 6  and loop  6 . 8  cause the processor  111  to repeat the complete scanning cycle beginning with antenna  108 / 1 . The scanning cycle will repeat until the scanning of at least one of the antennas detects a perturbation indicating that an intruder is present, i.e. a “pre-alarm” condition. When that happens, the processor  111  will record that antenna&#39;s identity in memory. During the next scanning cycle, decision step  6 . 2  will show that one or more antennas are in the pre-alarm state and, in function step  6 . 3 , the processor  111  will cause all of those pre-alarm antennas to be scanned in turn. In practice, the processor may be programmed to scan not only each antenna that is in the pre-alarm state, but also its immediate neighbours Once these “pre-alarm” and neighbouring antennas have been scanned, the processor  111  will return to the main path of the flowchart shown in FIG.  6  and perform steps  6 . 4 ,  6 . 5  and  6 . 6  to scan the next antenna that is not in the pre-alarm condition. It will be appreciated that, each time the processor  111  completes the scanning of one of the antennas that are not in the “pre-alarm” state, it will scan all of the antennas that are in the pre-alarm condition again before it scans the next one of the antennas not in the pre-alarm state. 
     Preferably, the processor  111  processes the results of the scanning process statistically to determine a probability of an actual intrusion and raise an alarm contingent thereupon. If the pre-alarm indication for a particular antenna turns out to be only a random event, as determined, for example, because neither of its immediate neighbours also registers a pre-alarm, the processor  111  will not raise an alarm but rather will delete the record from memory and continue the scanning process with that antenna treated as a “normal” antenna. This procedure is useful to increase the sensitivity of the system while avoiding frequent false alarms which might otherwise reduce the response of attendant personnel. 
     Various modifications to the above-described embodiment are envisaged. For example, the scanning routine described with reference to FIG. 6 could be arranged to scan the “pre-alarm” antennas alternately with the antennas not indicating a “pre-alarm”. 
     It is also envisaged that the number of “pre-alarm” antennas selected for scanning could be limited to a predetermined maximum in order to avoid reducing their sampling rate to an unacceptable level. 
     Although, in the above-described detection system, the antennas transmit and the leaky cable receives, it will be appreciated that antenna reciprocity applies, meaning that the receiver and the transmitter are interchangeable. 
     It should be noted that, because the antennas are isolated electrically from the leaky cable, mechanical continuity of the coaxial cable  103  can be maintained, which affords a high level of reliability. 
     Advantageously, embodiments of the invention allow time- multiplexed scanning of as many antennas (sub-zones) as possible while maintaining a sampling rate per antenna high enough to obtain a reasonable resolution of the intrusion profile signature. 
     For example, to illustrate the advantages of time multiplexing, the following practical assumptions may be made: 
     a) minimum electronic sampling rate: 
     2 millisec/sample; and 
     b) typical sample per intrusion profile: 
     5 samples/intrusion; 
      for a minimum sampling rate per intrusion of: 
     2×5=10 millisec/intrusion, or 0.01 sec/int. 
     c) maximum intruder running speed: 
     8 meters/second (18 miles/hour); and 
     d) typical width of the detection envelope: 
     2 meters (6 to 7 feet); 
      for a minimum intrusion time of: 
     2/8=0.25 second 
     Therefore, the over-sampling ratio for a running intrusion (worst case scenario) is: 0.25/0.01=25 
     This means that a maximum of 25 sub-zones can be time multiplexed without sacrificing the probability of detection, even for a fast running intruder. 
     As a practical example, a typical correctional facility perimeter might comprise a dual fence spaced apart by 20 ft and 12 ft high, with 25 antennas spaced apart by 50 ft, for a total perimeter of 1,250 ft (350 m) maximum per electronic module with sub-zone resolution of 50 ft. 
     The concept of a variable sampling rate further improves detection capability. In the absence of an intrusion (target), the sampling rate per antenna may be reduced, which increases the over-sampling ratio, which typically increases the number of antennas to “time-multiplex” per electronic module by a factor of ten (10). Detection of a pre-alarm condition may be determined according to a pre-alarm threshold, that is normally set midway between the alarm threshold and background noise. Under quiet operation (i.e. in the absence of an intrusion), each antenna is switched at a lower rate on an even basis. But when one antenna response exceeds the pre-alarm threshold, more time is spent sampling that antenna. This technique presents a variable sampling rate condition for no intrusion, first intrusion, second intrusion, etc., which means that the probability of detecting a single intrusion is higher than detecting a double intrusion and so on. This compromise will usually be acceptable to increase the number of sub-zones per single electronic module, though the acceptance of such a compromise depends on the threat level and the applications. 
     Finally, an even finer resolution for the intrusion location may be obtained by comparing intrusion alarm conditions between adjacent antennas/sub-zones. This technique allows a resolution accuracy equal to one third of the antenna spacing to be achieved, for a final location resolution of 50 ft/3=17 ft (5 m). 
     Advantageously, each antenna/sub-zone may have an individual alarm threshold, which means that the noise of a difficult sub-zone is not integrated with the entire perimeter noise. Also any sub-zone can be temporarily disabled to allow authorized personnel to cross the perimeter without shutting down the entire perimeter. 
     The increased intrusion resolution allows a surveillance video camera, where applicable, to be pre-set to a finer resolution for more efficient video assessment. 
     A wide range of miniature antennas can be used so long as the loaded impedance is high in reference to the coaxial cable impedance. A high impedance, like a test probe, extracts a very small portion of the RF signals that propagate within the coaxial cable in order to minimize the cable loss along the array of antennas. Mini-whip antennas, say 10 inches long, or other small, non-directional antennas, may be used instead of patch antennas. 
     It is also envisaged that, where the antenna cable is deployed in an elevated location, such as along a fence or on the roof of a building, one or more surveillance cameras could be installed at intervals along its length. The or each camera could be embedded into a respective one of the antennas, preferably so that it is hidden. Video signals from the cameras to a monitoring station could be transmitted via the coaxial antenna cable. The power supply to the cameras could be via the coaxial cable. 
     An advantage of detection systems embodying the present invention is that they maintain a uniform RF signal strength all along the secured perimeter and allow the intrusion/crossing location to be obtained by simply activating only one antenna at a time, i.e., by time multiplexing the antennas. Therefore, each individual antenna forms its own detection zone.