Abstract:
An intrusion detection technique involves: emitting a wireless signal within a container; detecting the wireless signal near the container; and responding to detection of a change in a characteristic of the detected wireless signal by transmitting a wireless notification to a location remote from the container. A different technique involves: responding to a determination that a condition representative of unauthorized intrusion is present while the container is stationary by transmitting a wireless notification; and responding to a determination that the container is moving by ignoring whether the condition is present. Yet another technique involves: monitoring whether a door of a container is in a closed position; and emitting a wireless signal indicating whether the container door is in the closed position.

Description:
[0001]    This application claims the priority under 35 U.S.C. §119 of provisional application numbers 60/464,067 filed Apr. 18, 2003, 60/496,056 filed Aug. 18, 2003, and 60/504,580 filed Sep. 19, 2003.  
         FIELD OF THE INVENTION  
         [0002]    This invention relates in general to security techniques and, more particularly, to techniques for detecting unauthorized intrusion into a container.  
         BACKGROUND OF THE INVENTION  
         [0003]    Metal cargo containers are designed with provisions to secure the entry doors in order to prevent unauthorized entry into the container while it is in transit. One known approach is to secure the door handle or latch with a conventional lock. Another approach is to secure the door handle or latch with a steel bolt that is fitted with a non-removable retainer. The bolt must be cut with bolt cutters in order to release the handle or latch so that the doors can be opened.  
           [0004]    Still another approach is to provide a device which includes a steel bolt and non-removable retainer in combination with a radio frequency identification (RFID) tag. Once the retainer has been coupled to the bolt, the RFID tag monitors the retainer and bolt and, if any tampering is detected, the tag transmits a radio signal to a remote receiver known as a reader, so that an alarm condition can be brought to the attention of a human operator and/or security personnel, who will then deal with the intrusion into the container.  
           [0005]    Despite security measures of the type discussed above, it can be possible for the container doors to be opened without directly defeating security measures of the type discussed above. For example, it may be possible to cut the door handle or the latch. Similarly, where a door handle is fixedly connected to a rotatable connecting link which has at each end a dog that engages a recess provided in the container housing, it may be possible to cut through the connecting link and thus permit the ends of the link to be rotated so as to free the dogs from the recesses, thereby permitting the door to be opened. Still other approaches include drilling out the door handle joint, removing the door hinge pins, or cutting a hole in the sheet metal walls or roof, or through the wooden floor. There is a need for an effective technique for detecting any of these types of intrusion, in a manner that is reliable and avoids false alarms.  
           [0006]    A further consideration is that there are some situations in which it is helpful to take into account the effects of container movement during shipment. For example, when a container is lifted from the ground and placed on a vehicle such as a truck or a ship, the position of the cargo within the container may shift. Alternatively, after the container has been loaded on a vehicle, normal vehicle movement could cause the cargo to shift position within the container. The shifting cargo should not be misinterpreted by a security system as human movement, or the security system may produce a false indication of human intrusion.  
           [0007]    As another example, a typical cargo container is normally made of steel except for the floor, which is usually made of wood. If a monitoring system is relying on some form of electromagnetic field for the purpose of detecting unauthorized human intrusion, movement of the container may affect the electromagnetic field. For example, energy of an electromagnetic field will not readily pass through the steel walls, but will more readily pass through the wooden floor. When the container is sitting on the ground, the ground may influence characteristics such as the strength of any electromagnetic field which may be passing through the wooden floor.  
           [0008]    If the container is then lifted off the ground, for example while it is loaded on a vehicle, the wooden floor will be spaced from the ground, thereby reducing the extent to which the ground can influence characteristics such as the strength of any electromagnetic field which may be passing through the wooden floor. To the extent a security system is relying on the electromagnetic field to try to detect human intrusion into the container, this change in the electromagnetic field as the container is lifted off the ground may be misinterpreted as human activity, and may cause the security system to produce a false indication of human intrusion. With these considerations in mind, it will be recognized that there is a need for a technique which can reliably detect unauthorized human intrusion into a cargo container, with little likelihood of false indications of intrusion as a result of factors such as container motion.  
         SUMMARY OF THE INVENTION  
         [0009]    One form of the invention involves: emitting a wireless signal within a container; detecting the wireless signal in the region of the container; monitoring the detected wireless signal for a change in a characteristic thereof; and responding to detection of a change in the characteristic of the detected wireless signal by transmitting a wireless notification of the occurrence of the change to a location remote from the container.  
           [0010]    A different form of the invention involves: monitoring a container for a condition representative of unauthorized intrusion into the container; determining whether the container is stationary; responding to a determination that the condition is present while the container is stationary by transmitting a wireless notification of the presence of the condition; and responding to a determination that the container is moving by ignoring whether the condition is present.  
           [0011]    Yet another form of the invention involves: monitoring whether a door of a container is in a closed position; and emitting a wireless signal indicating whether the container door is in the closed position. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    A better understanding of the present invention will be realized from the detailed description which follows, taken in conjunction with the accompanying drawings, in which:  
         [0013]    [0013]FIG. 1 is a diagrammatic view showing a cargo container of a known type, with a transmitter disposed inside the container, a receiver disposed on the outside of the container, and a stationary reader disposed in the region of the container;  
         [0014]    [0014]FIG. 2 is a diagrammatic view similar to FIG. 1, but showing an alternative embodiment which includes a cargo container of a known type, a transmitter disposed inside the container, two receivers disposed on the outside of the container, an oscillator which synchronizes both receivers, a transmitter coupled to both receivers, and a stationary reader disposed in the region of the container;  
         [0015]    [0015]FIG. 3 is a diagrammatic view similar to FIG. 1, but showing yet another alternative embodiment, which includes the addition of a motion detector coupled to the transmitter;  
         [0016]    [0016]FIG. 4 is a diagrammatic view similar to FIG. 2, but showing still another alternative embodiment, which includes the addition of a motion detector coupled to the transmitter;  
         [0017]    [0017]FIG. 5 is a diagrammatic perspective view of another alternative embodiment, showing a cargo container of a known type, with a transmitter mounted on the exterior of the container, and a stationary reader disposed in the region of the container;  
         [0018]    [0018]FIG. 6 is a diagrammatic fragmentary perspective view of the transmitter of FIG. 5, in an enlarged scale; and  
         [0019]    [0019]FIG. 7 is a diagrammatic fragmentary perspective view of a sensor arrangement which is provided within the container of FIG. 5. 
     
    
     DETAILED DESCRIPTION  
       [0020]    [0020]FIG. 1 is a diagrammatic view showing a cargo container  10  of a known type. In FIG. 1, the container  10  has a wooden floor, and the remainder thereof is made of steel. However, the container could alternatively be made of other materials. The container  10  has two doors  11  and  12 , and a handle and lock assembly which is shown diagrammatically at  14 . A low power (for example 1 mW) radio frequency (RF) transmitter  21  is installed inside the container, near the junction of a wall and the roof. The transmitter  21  transmits a beacon pulse encoded with an identification number unique to that particular transmitter. A radio frequency receiver/transmitter  23  is fixed to the outside of the container  10 , and is configured to detect only RF signals transmitted by the transmitter  21  inside the container. When the container  10  is in the region of a stationary RF reader of a known type, one of which is shown diagrammatically at  26 , the receiver/transmitter  23  can engage in RF communication with the reader  26 .  
         [0021]    When the container doors  11  and  12  are closed and the metal skin is intact, emissions from the internal transmitter  21  are shielded by the metal skin, except for skin effect currents which are induced on the inside surfaces and conducted around the edges of openings such as cracks at the door joints, or where the bottoms of the side walls meet the wooden floor. The receiver/transmitter  23  on the outside of the container  10  receives, measures, and records the power level of this attenuated RF signal from the transmitter  21 .  
         [0022]    If a door  11  or  12  is opened, or if a hole is cut through the metal skin of the container (for example as shown diagrammatically at  31 ) radio frequency emissions will increase by radiation through the area of the opening, and by skin effect conduction around the edges of the opening, and the resulting increase in the signal level will be detected by the outside receiver/transmitter  23 . If the increase in signal power exceeds a selected threshold, which is adjustable and chosen to suppress RF noise-induced false alarms, the outside transmitter  23  will sense and log the change in state, and may perform one or more additional tasks.  
         [0023]    In more detail, the receiver/transmitter  23  will log the change in state and the time it occurred, and will retain that data until the container  10  is in proximity to an RF reader such as that shown at  26 , and the reader interrogates that receiver/transmitter  23  and retrieves the data. This may occur almost immediately if the container  10  is already near a reader  26 , or may occur later, as soon as the container  10  first comes near a reader  26 . Alternatively, the receiver/transmitter  23  may log the change in state and the time it occurred and immediately begin to transmit periodic RF pulses encoded with that data. If the container  10  is already near a reader  26 , then these pulses will be detected almost immediately by the reader  26 . Otherwise, the pulses will be detected after a period of time, as soon as the container  10  again comes near a reader  26 .  
         [0024]    In either case, when a reader receives from the receiver/transmitter  23  an RF signal containing data indicating that an intrusion has occurred, the reader  26  will convey the information through a not-illustrated data network of a known type to monitoring equipment of a known type, in order to notify operators of the occurrence of the intrusion.  
         [0025]    As a practical matter, radio frequency noise and changes in the electromagnetic properties of the environment may produce false signals. Consequently, the receiver/transmitter  23  has a processor executing an adaptive algorithm of a known type in order to recognize and eliminate such false signals. In association with this, the receiver/transmitter  23  is self-calibrating, in that it measures the initial state of the received RF power level, which will depend upon the environment of each installation, and thereafter looks only for changes from that baseline condition.  
         [0026]    A different technique for sensing intrusion into a container involves detection of changes in radio frequency electrical path lengths inside the container of interest. In more detail, FIG. 2 is a diagrammatic view of an alternative embodiment which includes a cargo container  10  that is identical to the container  10  of FIG. 1, except that it has a different arrangement of RF transmitters and receivers, as explained below. In particular, a low power radio frequency transmitter  21  placed at one end of the container  10  emits waves in all directions. At these radio frequencies, the RF wavelength is short in comparison to the dimensions of the container  10 , and radio waves traveling along different paths inside the container will have different total phase delays. Two RF receivers  24  and  25  are placed at the opposite end of the container, on opposite comers, and receive the signal after it propagates through different paths in space and by conduction through the container walls, such that the signal has a unique and different phase delay at the location of each of the receivers  24  and  25 . The receivers  24  and  25  are synchronized by a common local oscillator (shown diagrammatically at  27 ), and the difference in phase between the two signals can therefore be reliably detected. When the RF electrical path length from the transmitter  21  to either receiver  24  or  25  is changed by intrusion of a mass sufficient to change the dielectric and magnetic properties of that path, a change will occur in the phase difference between the two signals received by the receivers  24  and  25 , and can be detected. When a hole is cut in the container skin, for example as shown diagrammatically at  31 , it will change the electrical paths in a manner which also causes a change in the phase difference of the signals detected by the receivers  24  and  25 .  
         [0027]    The receivers are each coupled to a transmitter, for example as shown diagrammatically at  28 . In a manner similar to that described above for the transmitter  23  in FIG. 1, the transmitter  28  in FIG. 2 will log the change in phase difference in the two signals detected by the two receivers  24  and  25 , and the time it occurred. The transmitter  28  will retain that data until the container  10  is in proximity to an RF reader such as that shown at  26 , and the reader interrogates the transmitter  28  and retrieves the data. This may occur almost immediately if the container  10  is already near a reader  26 , or may occur later, as soon as the container  10  first comes near a reader  26 . Alternatively, the transmitter  28  may log the change in phase difference and the time it occurred, and immediately begin to transmit periodic RF pulses encoded with that data. If the container  10  is already near a reader  26 , then these pulses will be detected almost immediately by the reader  26 . Otherwise, the pulses will be detected after a period of time, as soon as the container  10  again comes near a reader  26 .  
         [0028]    In either case, when a reader  26  receives from the transmitter  28  an RF signal containing data indicating that an intrusion has occurred, the reader  26  will convey the information through a not-illustrated data network of a known type to monitoring equipment of a known type, in order to notify operators of the occurrence of the intrusion.  
         [0029]    As a practical matter, the configuration shown in FIG. 2 is also subject to radio frequency noise and changes in the electromagnetic properties of the environment that can produce false signals. Consequently, the receivers  24  and  25  each have a processor executing an adaptive algorithm of a known type, in order to recognize and eliminate such false signals. In association with this, the transmitter  28  is self-calibrating, in that it measures the initial phase difference between the RF signals received by the receivers  24  and  25 , which will depend upon the environment of each installation, and thereafter looks only for changes from that baseline condition.  
         [0030]    In the embodiment of FIG. 2, the receivers  24  and  25  are shown mounted at spaced locations on the exterior of the container  10 . However, it would alternatively be possible to mount them in some other configuration in which they are spaced. As one example, the receivers  24  and  25  could be mounted at spaced locations within the container  10 . A further consideration is that, for clarity, FIG. 2 shows the receiver  24 , the receiver  25 , the local oscillator  27  and the transmitter  28  as physically separate units. Alternatively, however, one or more of these components could be integrated into a single physical unit.  
         [0031]    Although operation of the embodiments of FIGS. 1 and 2 is satisfactory for many applications, there are other applications in which their operation may be influenced by container movement. For example, when the container  10  is lifted from the ground and placed on a vehicle such as a truck or a ship, the position of the cargo within the container may shift. Alternatively, after the container  10  has been loaded onto a vehicle, normal movement of the vehicle could cause the cargo within the container  10  to shift position. With reference to FIG. 1, such a cargo shift may affect the power level of the RF signal emitted by the transmitter  21 , as measured at the receiver/transmitter  23 . Consequently, a false intrusion detection alarm may be issued by the receiver/transmitter  23 . With reference to FIG. 2, a cargo shift may change the paths of travel of one or both of the signals detected by the receivers  24  and  25 , thereby resulting in a change in their phase difference which causes the transmitter  28  to issue a false intrusion detection alarm.  
         [0032]    Another example of a situation in which container movement may be a problem is where the container  10  is made almost entirely of steel, but has a wooden floor. The intrusion detection arrangements shown in each of FIGS. 1 and 2 emit RF energy within the container  10 , which will not readily pass through the steel walls, but may pass readily through the wooden floor. When the container is sitting on the ground, some of the RF energy may pass through the wooden floor, be reflected from the ground, pass back through the wooden floor, and then influence the overall characteristics of the RF energy within the container. If the container is lifted off the ground, for example as it is loaded onto a vehicle, the RF energy which passes downwardly through the wooden floor will no longer be reflected by the ground, and thus will not travel back into the container through the wooden floor. Consequently, the overall characteristic of the RF energy within the container can change as the container is lifted off the ground.  
         [0033]    With reference to FIG. 1, the reduction in reflected RF energy may affect the power level of the RF energy emitted by the transmitter  21 , as measured at the receiver/transmitter  23 . Consequently, a false intrusion detection alarm may be issued the receiver/transmitter  23 . With reference to FIG. 2, the reduction in reflected RF energy may influence one or both of the signals detected by the receivers  24  and  25  in a manner changing their phase difference, thereby causing the transmitter  28  to issue a false intrusion detection alarm.  
         [0034]    [0034]FIG. 3 is a diagrammatic view similar to FIG. 1, but showing an alternative embodiment which is identical to the embodiment of FIG. 1, except for the addition of a motion detector  41  which is fixedly mounted on a wall of the container  10 , and which helps to reduce the likelihood of false intrusion detection alarms as a result of container movement. In more detail, the motion detector  41  is a commercially available device of a known type, which can sense physical movement, and which can produce an electrical signal indicating whether or not movement is being detected. The output of the motion detector  41  is coupled to an input of the receiver/transmitter  23 .  
         [0035]    When the motion detector  41  is outputting a signal that indicates it is detecting container motion, the receiver/transmitter  23  responds to this signal by disabling its evaluation of whether there has been intrusion into the container  10 . Stated differently, the receiver/transmitter  23  evaluates whether there has been unauthorized intrusion in the same manner discussed above in association with FIG. 1, but only when the motion detector  41  is indicating that the container  10  is stationary. Unauthorized human intrusion into the container  10  is most likely to occur when the container is stationary, and thus the accuracy and reliability of intrusion detection is high in the embodiment of FIG. 3, because false intrusion detection alarms due to container movement are avoided. For simplicity and clarity, the motion detector  41  is shown in FIG. 3 as a separate component, but it will be recognized that it could alternatively be integrated with one or more of the other components, such as the receiver/transmitter  23 .  
         [0036]    [0036]FIG. 4 is a diagrammatic view similar to FIG. 2, but showing an alternative embodiment which is identical to the embodiment of FIG. 2, except for the addition of a motion detector  41  which is equivalent to the motion detector discussed above in association with FIG. 3. The motion detector  41  in FIG. 4 is fixedly mounted on a wall of the container  10 , and helps to reduce the likelihood of false intrusion detection alarms as a result of container movement. The output of the motion detector  41  is coupled to an input of the transmitter  28 .  
         [0037]    When the motion detector  41  is outputting a signal that indicates it is detecting container motion, the transmitter  28  responds to this signal by disabling its evaluation of whether there has been intrusion into the container  10 . Stated differently, the transmitter  28  evaluates whether there has been unauthorized intrusion in the same manner discussed above in association with FIG. 2, but only when the motion detector  41  is indicating that the container  10  is stationary. Unauthorized human intrusion into the container  10  is most likely to occur when the container is stationary, and thus the accuracy and reliability of intrusion detection is high in the embodiment of FIG. 4, because false intrusion detection alarms due to container movement are avoided. For simplicity and clarity, the motion detector  41  is shown in FIG. 4 as a separate component, but it will be recognized that it could alternatively be integrated with one or more of the other components, such as the transmitter  28 .  
         [0038]    [0038]FIG. 5 is a diagrammatic perspective view showing a cargo container  110  of a known type, which is similar to the container  10  of FIGS. 1-4. The major parts of the container  110  are all made of steel, except that the floor of the container  110  is made of wood. The container  110  could alternatively be made of other suitable materials. The container  110  has a pivotally supported door  111 , and a latch assembly  114 . The illustrated configurations of the container  110  and its latch assembly  114  are exemplary, and each could alternatively have any one of a number of other configurations. As one example, the latch assembly  114  could alternatively include a vertical cylindrical rod which is rotatably supported on the door  111 , which is fixedly coupled to the handle of the latch assembly  114 , and which has at each end a dog that can engage a respective recess provided in the exterior surface of the container  110 .  
         [0039]    The latch assembly  114  is maintained in a locked configuration by a security device  116  of a known type. The security device  116  includes a steel bolt which cooperates with the latch assembly  114 , and also includes a housing that cooperates with the bolt and that contains a battery-operated radio frequency identification (RFID) tag of a known type. The RFID tag can send and/or receive radio signals  119 , in order to communicate with a nearby stationary reader  121  of a known type.  
         [0040]    If the security device  116  detects that it is being subjected to some form of tampering, after its steel bolt has been engaged with the latch assembly  114 , the security device  116  transmits a radio signal at  119  to the reader  121 . The reader  121  can then present an alarm condition to a human operator and/or security personnel, who can then deal with the intrusion into the container  118 . It will often be the case that security personnel can reach the container  110  while the intrusion is still in progress, and apprehend the guilty person.  
         [0041]    In the event that the security device  116  does not happen to be within radio range of a reader  121  at the point in time when it first detects tampering, the security device  116  will continue to transmit the radio signal  119  which indicates that there has been tampering. Therefore, if the security device  116  later comes within radio range of a reader  121 , the reader  121  will receive the signal  119  and raise an alarm condition at that point in time.  
         [0042]    A transmitter  141  is fixedly secured to the upper portion of the exterior surface of the door  111 . The transmitter  141  includes a battery-operated RFID tag of a known type, which is generally similar to the tag provided within the security device  116 . The transmitter  141  can transmit radio frequency (RF) signals  144  to the reader  121 .  
         [0043]    [0043]FIG. 6 is a diagrammatic fragmentary perspective view of the transmitter  141  of FIG. 5, in an enlarged scale. Two bolts  146  and  147  fixedly mount the transmitter  141  on the door  111 . In this regard, the door  111  has three spaced openings which extend completely through it. Two of these openings receive the shanks of the bolts  146  and  147 . The third opening through the door  111  is shown diagrammatically by broken lines at  151 . There are not-illustrated wires which extend from the transmitter  141  through the opening  151  to a sensor assembly which is disposed within the container  110 , and which is discussed below.  
         [0044]    More specifically, FIG. 7 is a diagrammatic fragmentary perspective view of a portion of the interior of the container  110 , and shows at  161  the sensor assembly which was mentioned above. The sensor assembly  161  includes a base plate  163 , which is secured to the door  111  just below an inwardly extending metal flange  164  provided at the top edge of the door  111 . The base plate  163  has two openings which each receive the threaded shank of a respective one of the bolts  146  and  147 . Two nuts  167  and  168  engage the ends of the threaded shanks of the bolts  146  and  147 .  
         [0045]    The sensor assembly  161  includes a sensor support plate  171 , which extends perpendicular to the base plate  163 , and which has one edge fixedly secured to the base plate  163  in any convenient manner, for example by welding or by bolts. The support plate  171  has a horizontal cylindrical opening  172  therein, and a magnetic sensor  173  of a known type is fixedly mounted within the opening  172 . The magnetic sensor  173  is electrically coupled to the transmitter  141  (FIG. 6) by the not-illustrated wires which extend through the opening  151  in the door  111 .  
         [0046]    The sensor assembly  161  also includes a further base plate  177 , which is fixedly secured to the roof or ceiling of the container  110  by two screws  178  and  179 . A metal support part  182  is fixedly secured to the base plate  177  in any convenient manner, for example by welding or by bolts. The support part  182  has a horizontal cylindrical opening  183  therein, and a permanent magnet  184  is fixedly mounted within the opening  183 . In the disclosed embodiment, the parts  163 ,  171 ,  177  and  182  are all made of aluminum, but they could each alternatively be made of some other suitable material.  
         [0047]    In FIG. 7, the door  111  is shown in a closed position, where the magnetic sensor  173  is relatively close to the magnet  184 . The sensor  173  will detect the magnetic field of the magnet  184 , and will communicate this to the transmitter  141  of FIG. 6 through the wires which extend through the opening  151 . In contrast, if the container door  111  is opened, the sensor  173  will be moved to a position in which it is spaced from the magnet  184 . Consequently, the sensor  183  will no longer be detecting the magnetic field of the magnet  184 , and will communicate this to the transmitter  141  through the not-illustrated wires that extend through the opening  151 .  
         [0048]    With reference to FIG. 5, when the security device  116  is in place and its steel bolt has been engaged with the latch assembly  114 , the security device  116  will periodically transmit at  119  a radio signal which notifies the reader  121  that the security device  116  is in place and has not detected any tampering. Since this relates to an operational state in which the container door  111  is closed, the magnet  184  will be in fairly close proximity to the magnetic sensor  173 . The magnetic sensor  173  will thus detect the nearby magnet  184 , and will provide notice of this to the transmitter  41  through the not-illustrated wires within the opening  151 . The transmitter  141  will transmit radio signals at  144  to relay this knowledge to the reader  121 . Consequently, the reader  121  will know that the security device  116  is engaged and not been tampered with, and will also know that the container door  111  is in a closed position.  
         [0049]    In the event that a thief manages to open the door  111  without tampering with the security device  116 , the magnetic sensor  173  will detect that it has been moved away from the magnet  184 , and will supply this information to the transmitter  141  through the not-illustrated wires which extend through the opening  151 . The transmitter  141  will then relay this information to the reader  121 , through use of radio signals  144 . Since the door  111  will have been opened without any tampering with the security device  116 , the security device  116  will still be sending radio signals at  119  which indicate to the reader  121  that the security device  116  has not been tampered with. Therefore, since the reader  121  will be receiving an indication from the transmitter  141  that the door  111  has been opened, at a point in time when the security device  116  is indicating it has not been tampered with and that the door  111  is thus still closed, the reader  121  will know from the inconsistency that there has been an unauthorized intrusion. The reader can then present an alarm condition to a human operator and/or security personnel, who will deal with the intrusion into the container  110 .  
         [0050]    It is optionally possible for the reader  121  to periodically transmit a radio signal at  144  to the transmitter  141 , in order to request that the transmitter  141  send back a radio signal confirming that the magnetic sensor  173  is still detecting the magnetic field of the magnet  184 , representing a condition in which the door  111  is still closed. If the reader  121  receives no reply to this inquiry from the transmitter  141 , the reader  121  can make the assumption that someone has damaged or disabled the transmitter  141 . The reader  121  can then assume that the door  111  has probably been opened, and present an appropriate alarm condition so that a person will be sent to investigate whether there has been an intrusion into the container  110 .  
         [0051]    The magnetic sensor assembly  161  represents a robust approach to intrusion detection, because electromagnetic fields originating externally of the container  110  will be substantially unable to penetrate the steel walls of the container  110 . As mentioned above, containers of the type shown at  110  often have a wooden floor rather than a steel floor, but the sensor assembly  161  is intentionally positioned near the roof of the container, or in other words far above the container floor, so that any magnetic field emitted from below the wood floor of the container  110  will not have sufficient strength at the magnetic sensor  173  to override the effect of the magnetic field of the magnet  184 .  
         [0052]    Although selected embodiments have been illustrated and described in detail, purely by way of example, it should be understood that a variety of substitutions and alterations can be made therein without departing from the spirit and scope of the present invention, as defined by the following claims.