Abstract:
A system and method are provided for automatically detecting intrusion in an off-limits zone. The system includes a transmitter transmitting a signal along a path likely to encounter an intruder to the off-limits zone and a modulating reflector for receiving the transmitted signal. The modulating reflector includes a modulator receiving the received signal and generating a modulated signal having a characteristic. The modulating reflector transmits the modulated signal to be received by a receiver located to receive the modulated signal. The system further includes a processor coupled to the transmitter and to the receiver, the processor being configured to process the received modulated signal and to initiate an action as a function of the characteristic in the received modulated signal.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This is a continuation-in-part of non-provisional patent application Ser. No. 10/647,413, which in turn claims priority from U.S. provisional patent application No. 60/405,490, filed Aug. 23, 2002, each of which is incorporated by reference herein in their entirety. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The invention relates generally to a microwave detection system. More particularly, the invention relates to a system and method for automatically detecting intrusion in an off-limits zone.  
       BACKGROUND OF THE INVENTION  
       [0003]      FIG. 1  illustrates a typical prior art railroad grade crossing  100  with a single railroad track  102 . A first gate  104 A and  104 B is closed when a train approaches on track  102  thereby restricting the flow of traffic from the corresponding side of track  102 . A second gate  106 A and  106 B is closed on the opposite side of track  102  from gates  104 A and  104 B to restrict the flow of traffic from the opposite side.  
         [0004]     In  FIG. 2 , a similar prior art railroad grade crossing  200  is shown but with two tracks  202  and  204  shown as the grade crossing  200 . Similar to shown above for the single track configuration  100 , a first gate  206 A and  206 B is closed when a train approaches on track  202  or  204  thereby restricting the flow of traffic from that side of track  102 . A second gate  208 A and  208 B is closed on the opposite side of tracks  202  and  204  from gates  206 A and  206 B to restrict the flow of traffic from the opposite side. .  
         [0005]     In these prior art systems, the gates close when an approaching train is detected. In order to detect obstacles located between closed gates in the proximity of the tracks, some prior art systems rely on a transmitter/receiving system that is responsive to reflections of the transmitted signals by the obstacles themselves and do not utilize a reflector or detect the presence of a signal from the reflector. See U.S. Pat. No. 6,340,139 and U.S. Pat. No. 5,625,340.  
         [0006]     Other prior art systems rely on reflectors that reflect frequency-modulated radar which utilize the frequency and amplitude differences between the transmitted and reflected signal to determine the presence of an object in the surveillance zone. These prior art systems detect differences in signal amplitude and the signal phase. The latter results from a phase shift determined by the signal transit time as defined by a transit time component at the reflector. However, in this known implementation, the system includes a receiver, circulator, transit time element, a directional separating filter, and an amplifier, each of which incrementally adds to the complexity and cost of the system. See U.S. Pat. No. 5,775,045.  
         [0007]     Several systems have been developed which utilize microwave detection systems. However, prior art systems currently encounter problems such as false detection of obstacles, inaccurate detection of obstacles, failure to detect obstacles, detection of echoes, inadequate surveillance, and high cost associated with the initial installation and with ongoing operations. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  is an illustration of a prior art railroad grade crossing for a single track crossing.  
         [0009]      FIG. 2  is an illustration of a prior art railroad grade crossing for a two track crossing.  
         [0010]      FIG. 3  is a schematic illustrating a microwave detection system for automatically detecting intrusion in an off-limits zone in accordance with aspects of the invention.  
         [0011]      FIG. 4  is a diagram exemplary illustrating exemplary control states for a system for detecting intrusion in an off-limits zone.  
         [0012]      FIG. 5  is a diagram illustrating exemplary steps in a logic flow for a system for detecting intrusion in an off-limits zone.  
         [0013]      FIG. 6  is an illustration of a system for detecting intrusion in an off-limits zone, such as a railroad crossing having a single track crossing and indicating one exemplary embodiment of the layout of transceivers, modulating reflectors, and an exemplary surveillance zone.  
         [0014]      FIG. 7  is an illustration of a system for detecting intrusion in an off-limits zone, such as a railroad crossing having two-track crossing and indicating one exemplary embodiment of the layout of transceivers, modulating reflectors, and an exemplary surveillance zone.  
         [0015]      FIG. 8  is an illustration of a system for detecting intrusion in an off-limits zone, such as a railroad crossing having a two-track crossing and indicating one exemplary embodiment of the layout of transceivers, modulating reflectors, passive reflectors, and an exemplary surveillance zone.  
         [0016]      FIG. 9  is an illustration of a system for detecting intrusion in an off-limits zone, such as a railroad crossing having a three track crossing and indicating one exemplary embodiment of the layout of transceivers, multiple modulating reflectors, and an exemplary associated surveillance zone.  
         [0017]      FIG. 10  is an illustration of a system for detecting intrusion in an off-limits zone, such as may be defined by a perimeter and indicating one exemplary embodiment of the layout of transceivers, modulating reflectors, and an exemplary surveillance perimeter. 
     
    
       [0018]     Corresponding reference characters and designations generally indicate corresponding parts throughout the drawings.  
       DETAILED DESCRIPTION  
       [0019]     Aspects of the present invention are directed to a microwave detection system, such as may be used for automatically detecting intrusion to an off-limits zone using a modulated microwave signal. The description below will first describe one embodiment such as may be used for automatically detecting the presence of obstacles within the zone of a railroad track grade crossing. The description will then describe another embodiment such as may be used for automatically detecting intrusion through one or more perimeters that define an off-limits zone, such as may used at an airport.  
         [0020]      FIG. 3  is a simplified block diagram of one embodiment of a system  300  for automatically detecting intrusion in an off-limits zone, such as detecting the presence of an obstacle within the zone of a railroad track grade crossing using a microwave transmitter/receiver  302  and a modulating reflector  308 . Transmitter/receiver  302  is equipped with an antenna  304 . As shown, transmitter/receiver  302  may be a combined transceiver  302 , or may be a separate transmitter  302 A and a separate receiver  302 B. In such a latter case, transmitter  302 A and receiver  302 B may each be equipped with an antenna  304 . Transceiver  302  provides received signal  338  to a preamplifier  312  that provides a processed signal to a demodulator  314 . Demodulator  314  provides a demodulated received signal  338  to a processor  316  for signal analysis.  
         [0021]     Processor  316  may be a single processor, or may in another embodiment be configured as a multiple processor  316 . In one embodiment, processor  316  is a dual-processor  316  configuration. Processor  316  may be comprised of a memory (not shown), hardware, software and/or firmware. The functions described with regard to processor  316  may be configured and performed by one or more of software, firmware, or hardware.  
         [0022]     Transmitted signal  332  is transmitted by transmitter  302 A and received by one or more modulating reflectors (MDR)  308 . Modulating reflector  308  receives transmitted signal  332  and introduces a characteristic to create modulated signal  330 . Modulated signal  330  is transmitted or reflected by modulating reflector  308  and is received by receiver  302 B. System  300  provides enhanced definition of surveillance zone  334  as defined by transceiver  302  and a modulating reflector  308  and associated transmitted signal  332  and modulated signal  330 . Transmitted signal  332  and modulated signal  330  define surveillance zone  334  such that the detection of an obstruction in surveillance zone  334  is a function of the disruption of either the transmitted signal  332  or modulated signal  330  as will be further discussed below.  
         [0023]     In one embodiment, transceiver  302  operates in band X at a frequency of 9.2 GHz to 10.6 GHz, e.g., 10.0 GHz with a 22.0 MHz FM sweep/bandwidth. In one embodiment, this is a continuous-wave microwave signal. The power of transmitter  302 A may be in the range of  10  mW, plus or minus 1 mW. Other power levels of transmitter  302 A may be in the range of 20 mW, plus or minus 2 mW. Receiver  302 B may be, in one embodiment, the originating site which is transceiver  302 . In another embodiment, receiver  302 B may be separate from transmitter  302 A. In yet another embodiment, dual receivers  302 B may be used wherein their received signals  338  are combined and the combined signal is analyzed. This later embodiment may be applicable where the frequency of transmitted signal  332  may result in a null signal such as results from phase shifts or other signal patterns that result in the transmitted signal  332  negatively affecting the modulated signal  330 , thereby negatively affecting the ability to detect modulating signal  330  and any characteristic introduced by the modulating reflector  308 .  
         [0024]     In another embodiment, transceiver  302  transmits a frequency modulated transmitted signal  332  rather than a continuous or single frequency signal. In such an embodiment, frequency modulation with a bandwidth between 5.0 and 25.0 MHz may be introduced in transmitter  302 A. By introducing frequency modulation into transmitted signal  332 , the frequency of unwanted amplitude modulation is increased to a level that enables improved detection of a peak of received signal  338  and/or the sidebands in received signal  338 .  
         [0025]     In one embodiment, antenna  304  may be a directional antenna that provides for the formation of transmitted signal  332  such as to define surveillance zone  334 . The selection of the type of transceiver antenna  304  is dependent on the shape of the desired surveillance zone  334 , the intended distance required for surveillance of surveillance zone  334 , and the frequency of transmitted signal  332 . For instance, a parabolic antenna may provide a beam angle of 5 degrees whereas a horn antenna may provide a beam angle of 30 degrees. In addition, in one embodiment, transceiver antenna  304  may have a TX/RX ø=35 cm.  
         [0026]     Modulating reflector  308  is responsive to transmitted signal  332 . Modulating reflector  308  may comprise or include a modulating reflector antenna  336 . In one embodiment, modulating reflector  308  is a modulating horn reflector with a horn reflector size of 12.5×9.5×15 cm. In another embodiment, modulating reflector  308  is a pyramidal horn reflector resulting in a maximum distance between modulating reflector  308  and transceiver antenna  304  of 100 meters. In yet another embodiment, modulating reflector  308  is a parabolic reflector that provides for a maximum distance between modulating reflector  308  and transceiver antenna  304  of 200 meters.  
         [0027]     In another embodiment as shown in  FIG. 3 , a passive reflector  310  is positioned to receive transmitted signal  332 A from transmitter  302 A, and passively reflect transmitted signal  332 B to modulating reflector  308 . Additionally, passive reflector  310  may be positioned to receive modulated signal  330 A from modulating reflector  308  and to passively redirect modulated signal  330 B to receiver  302 B. By positioning passive reflector  310 , surveillance zone  334  may be shaped, expanded, or designed to particular railroad crossing applications and designs to more effectively monitor the desired surveillance zone  334  for obstructions. Passive reflector  310  may also be used to form two segments of transmitted signal  332  that define two separate surveillance zones  334 . For example, in one embodiment, passive reflector  310  defines a second surveillance zone  334  that is at an angle of up to 60 degrees from the first surveillance zone  334 . In other embodiments, the angle between the two surveillance zones  334  created by passive reflector  310  may be greater than 60 degrees. In such embodiments, the reflected energy is reduced and thereby the zone defined by the transmitted signal  332  and the modulated signal  330  is reduced. However, by using passive reflector  310  with an angle less than or equal to 60 degrees, the total surveillance zone  334  covered by transmitted signal  332  and modulated signal  330  may be expanded to survey more complex zones and to provide more complete surveillance coverage.  
         [0028]     The selection of the transceiver antenna  304  and modulating reflector antenna  336  defines the size of surveillance zone  334  including a distance (or length) between transceiver  302  and modulating reflector  308 . In one embodiment where transceiver antenna  304  is a horn antenna and modulating reflector antenna  336  is a horn, the distance between antennas  304  and  336  to define surveillance zone  334  is between 10 and 28 meters. In another embodiment where transceiver antenna  304  is a horn antenna and modulating reflector antenna  336  is a parabola, the distance is between 18 and 28 meters. In yet another embodiment where transceiver antenna  304  is a parabola antenna and modulating reflector antenna  336  is a parabola, the distance is between 28 and 60 meters. Similarly, when passive reflector  310  is included in the system. In one embodiment where transceiver antenna  304  is a horn antenna and modulating reflector antenna  336  is a parabola, the distance is between 10 and 25 meters. In another embodiment where transceiver antenna  304  is a parabola antenna and modulating reflector antenna  336  is a parabola, the distance is between 25 and 50 meters.  
         [0029]     In one embodiment, modulating reflector  308  receives transmitted signal  332 . Modulating reflector  308  modulates the received transmitted signal  332  and re-transmits modulated signal  330  with a modulation characteristic  340  by reflection to receiver  302 B. In one exemplary embodiment, the modulation characteristic may be a phase modulation. It will be appreciated, however, that any modulating technique may be used for imparting a modulation characteristic to the signal  330 . Illustrative examples of analog and digital modulation techniques that may be utilized include the following: amplitude modulation (am), frequency modulation (fm), pulse modulation (pm), pulse-code modulation (pcm), differential pulse coded modulation (dpcm), delta modulation (dm), continuously variable slope delta modulation (cvsd), minimum shift keying (msk), etc. Modulating reflector  308  may be a passive device or may be an active device. In one exemplary embodiment, modulating reflector  308  produces modulated signal  330  by introducing characteristic  340 , such as a phase modulation, to received transmitted signal  332  with a phase modulation of between 0° and 180° at a frequency of around 10.0 KHz. The modulation frequency may be at 4.0 KHz, 4.7 KHz, 5.7 KHz, 6.7 KHz, 9.0 KHz, or 12.0 KHz. Other frequencies for the phase modulation in the range of 4.0 KHz to 13.0 KHz may also be used. In yet another embodiment, modulating reflector  308  is a multiphase or continuous phase shift-modulating reflector with eight (8) or more different phases. Such an embodiment may be beneficial in eliminating unwanted amplitude modulation of modulated signal  330 .  
         [0030]     The modulation by modulating reflector  308  results in one or more uniquely identifiable characteristics  340  in modulated signal  330  which provide for the detection of obstacles. For example, frequency or phase modulation may create sidebands in the modulation signal  330  that are not present in the transmitted signal  332 , e.g., the transmitted carrier signal. The amplitude, energy, frequency, or number sidebands may define various embodiments the characteristic.  
         [0031]     Receiver  302 B is responsive to signals in the frequency range of transmitted signal  332  and modulated signal  330 . Received signal  338  as received by receiver  302 B may or may not contain characteristic  340  as introduced by modulating reflector  308 . Received signal  338  is converted into base band using a portion of the carrier signal from transmitter  302 A in transceiver  302 . Preamplifier and filter  312  amplifies and filters received signal  338  and passes the conditioned received signal  338  to demodulator  314 . Received signal  338  is demodulated by demodulator  314  to process received signal  338  for signal analysis by processor  316  for analysis of the amount of characteristic  340  as introduced by modulating reflector  308 . This amount can be indicative of an obstacle in surveillance zone  334 .  
         [0032]     In the transceiver  302 , transmitted signal  332  or the carrier components thereof is mixed with received signal  338  wherein in one exemplary embodiment the carrier signal is canceled thereby leaving the sidebands for analysis by processor  316 . The sidebands may be analyzed for determination of the desired characteristic  340  and thereby the presence or absence of an object in surveillance zone  334 .  
         [0033]     In one exemplary embodiment, the signal analysis process by processor  316  includes detecting and comparing the amount of energy in the sidebands of received signal  338 , such as represented by the amplitude of the peak of the sideband. Received signal  338  is filtered by preamplifier filter  312  to remove echoes that may be due to Doppler effects from moving objects. After such filtering, received signal  338  only includes, in the absence of an object in surveillance zone  334 , characteristic  340  as introduced by modulating reflector  308 . In one exemplary embodiment, the modulation frequency is selected at a frequency that is higher than Doppler-effect frequencies that result from an object moving in surveillance zone  334 . As noted above, frequencies of 4 KHz, 4.7 KHz, 5.7 KHz, or 6.7 KHz may be used when a carrier frequency of transmitted signal  332  of 10 GHz is used.  
         [0034]     As noted in the example given above, the desired characteristic  340  may be a specific amplitude, frequency, and/or phase of the sidebands contained in received signal  338 . The received signal and its sidebands may be analyzed and compared against predefined values, thresholds, or models. For example, if the received signal has a sideband with amplitude peak or energy level that exceeds a predefined value, processor  316  may determine that an obstacle is not present in surveillance zone  334 . However, if the amplitude peak of the sideband of the received signal is below the predefined value or threshold, then processor  316  would determine that an obstacle is within surveillance zone  334 . In one embodiment, it may be determined that a decrease of more than 3 dB in the peak amplitude of the first sideband indicates that an object is in surveillance zone  334 .  
         [0035]     The amount of energy in the sidebands of the sidebands in received signal  338  may also be utilized to determine the presence or absence of an object. If the determined energy level is found to be below a predetermined level, processor  316  may determine that an object is present in surveillance zone  334 . In one embodiment, the system may detect and determine the amount of total energy in the first, second, and third sidebands of received signal  338 . The total energy level of such sidebands is compared to a predetermined energy level. In one embodiment, when the total energy level is 80 percent of the normal level, e.g., a reduction of 20 percent, processor  316  determines that an obstacle is present in surveillance zone  334 . In other embodiments, the one or more sidebands may be analyzed and/or the deviation may range from 5 percent to 50 percent for the energy or peak amplitude of the sidebands.  
         [0036]     In one exemplary embodiment, the predetermined comparison levels for peak amplitude or energy level detection are established during product development, product design, and/or product deployment based on testing and operation, and are dependent on the transmitted frequency. In some embodiments, system  300  includes a variable input function (not shown) that enables an operator to adjust the sensitivity or threshold levels of processor  316  used to determine whether received signal  338  contains the desired characteristic  340  and thereby determine whether or not an object is detected within surveillance zone  334 .  
         [0037]     If received signal  338  contains the desired amount of characteristic  340  as introduced by modulating reflector  308  as described above, system  300  provides an indication that surveillance zone  334  is free of obstacles. The presence of desired amount of characteristic  340  as generated by modulating reflector  308  indicates that received signal  338  is that which was originally transmitted as transmitted signal  332 , modulated by modulating reflector  308 , and re-transmitted as modulated signal  330  with characteristic  340 . The receipt of the desired amount of characteristic  340  in modulated signal  330  also ensures that improper or false signals that are received do not provide a false indication that surveillance zone  334  is clear.  
         [0038]     In an alternative embodiment, system  300  may be comprised of two or more transceivers  302  each operating at a separate frequency. In this embodiment, it may be viewed as having two separate received signals  338  being received by receiver  302 B, or that one received signal  338  is received, but the received signal  338  having more than one signal component. In one view two transmitted signals  332  are transmitted two transceivers  302 , and two modulated signals  330  with two characteristics  340  are generated by modulating reflector  308 . In either case, the signal conditioning, demodulation, and analysis process described above is applied with regard to each received signal  338 . The determination by processor  316  with regard to the presence of an object in surveillance zone  334  is determined by a combination of the signal analysis for each of received signals  338 .  
         [0039]     In another exemplary embodiment, transceiver  302  separately detects a plurality of modulated signals  330  and characteristics  340  from a plurality of modulating reflectors  308 . In such an embodiment, each modulating reflector  308  may be tuned to frequency or phase modulate transmitted signal  332  at a unique and separate modulated frequency. Each receiver  302 B is tuned to demodulate the signal to determine the characteristics  340 , thereby determining the presence of obstacles in each of the defined surveillance zones  334 . In such an arrangement, each set of transmitters  302 A, modulating reflectors  308 , and receivers  302 B, define separate surveillance zones  334  that may include multiple paths as defined by the zones between each set of communicating transmitters  302 A, modulating reflectors  308 , and receivers  302 B. For example, see  FIG. 9 .  
         [0040]     In another exemplary embodiment, a GPS system  322  receives data signals from a Global Positioning Satellite (GPS) system (not shown). In this embodiment, system  300  receives and stores in a memory (not shown) the time and/or synchronization signals from the received GPS data. Processor  316  may utilize received GPS data to enhance the reporting, administration, and/or diagnostics capabilities of system  300 .  
         [0041]     In operation, the surveillance operation of system  300  is initiated when a gates closing signal is received from the crossing gate system  324  indicating that the gates have closed. Upon receipt of the gate closing signal, system  300  begins to transmit transmitted signal  332  and to receive received signal  338  to monitor surveillance zone  334  for obstacles in the crossing after the closing of the gates. In one embodiment, system  300  discontinues checking the crossing or surveillance zone  334  after the activation of the track open signal. In another embodiment, system  300  continues to survey the surveillance zone  334  if the surveillance zone  334  is not interrupted by an expected obstruction such as a passing railway vehicle.  
         [0042]     When no obstruction is detected, system  300  generates a consent action  326  that in one embodiment is an initiation of a relay that is energized by processor  316 . When an obstacle is detected in the crossing zone or surveillance zone  334 , an open zone indication is not generated and further action is taken. In one such embodiment, an alarm action  328  is initiated by processor  316  such as the energizing of an alarm relay. In another exemplary embodiment, the event or action data is stored in a memory (not shown) so that the data events can be analyzed at a later time or by a remote administration system (not shown).  
         [0043]     In another exemplary embodiment, processor  316  is configured to provide one or more operational functions. These include receiving information relative to the lowering or rising of the gates for the gates open system  324 . Processor  316  may initiate the transmission of transmitted signal  332  by transmitter  302 A when receiving information or a gates closing signal from gates open system  324  indicating that the gates have been lowered. When demodulator  314  has received the processed received signal  338 , processor  316  analyzes the received signal for characteristic  340 . When processor  316  determines from received signal  338  the desired amount of characteristic  340  as described above, processor  316  may generate consent signal  326 . When processor  316  determines that received signal  338  does not contain the desired amount of characteristic  340  and therefore determines that an obstacle is present in surveillance zone  334 , processor  316  generates the occupied zone alarm  328 .  
         [0044]     In other exemplary embodiments, processor  316  optionally acquires and verifies the integrity of the internal components of system  300 . Processor  316  may also initiate and provide self-diagnosis and check on efficiencies of operations of all system components (see  320 ) including providing automatic self-test of transmitters  302 A and receivers  302 B. Processor  316  may also provide for administration and management of various inputs and outputs to system  300  such as communication ports/links (not shown) including the acquisition of the time reference signal from GPS system  322 . Processor  316  also may manage an anti-intrusion sensor associated with system  300  equipment cabinets containing transmitter  302 A, receiver  302 B, modulating reflector  308 , passive reflector  310 , and other system equipment. Processor  316  may also provide a system failure alarm either as a local alarm or to a remote administrative entity or system (not shown). Processor  316 , in conjunction with a memory (not shown), may record or store the actions or events as determined by processor  316  and generate the communication of such events, actions, and status to remote sites, systems, or entities.  
         [0045]     In  FIG. 4 , operating states of one embodiment of the invention are illustrated. The first state is a system off state  402 . When power is initially provided to system  300 , processor  316  shifts to an initialization state  404 . In this state, processor  316  verifies its configuration and operating status. If the configuration is not present, processor  316  shifts to a configuration state  406  to obtain configuration information or data from an external source. In one embodiment, this information could be obtained from a remote administration system via a communication link (not shown). If correct configuration data is present, processor  316  controls the presence of repetitive errors that occurred before the last reset of processor  316 . If an error exists, then processor  316  shifts to unavailability state  408  and waits for an external command via a communication link to restart surveillance by system  300 . If there is an error in the system, processor  316  may also shift to unavailability state  408 , and an alarm or notification is made to an external system or administration system indicating the need for repair. In another embodiment, unavailability state  408  may automatically initiate a system restart (not shown).  
         [0046]     If processor  316  passes the tests and configuration diagnostics of initialization state  404 , processor  316  shifts to a stand-by state  410 . In this state, the system is operational and is awaiting an external indication to enter an analysis state  412 . During stand-by state  410 , the system is operating correctly without any errors and is awaiting the “gates closed” signal. Processor  316  monitors the safety and self-diagnostics of the system for changes to the systems operability. Processor  316  updates the time and synchronization data received from GPS system  322 . The external indication to enter analysis state  412 , in one embodiment, is the receipt from an external source that the gates of the railroad grade crossing have been lowered. Additionally, during stand-by state  410 , processor  316  receives information from Global Positioning Satellite (GPS) receiver system  322 . This information may include any of the available GPS satellite provided information. In one embodiment, this information includes time and/or synchronization information. Once the system receives an activation signal such as the gates closing signal, processor  316  shifts from stand-by state  410  to analysis state  412 .  
         [0047]     In analysis state  412 , processor  316  sets a timer and initiates a transmission of transmitted signal  332  from transmitter  302 . In one embodiment, the timer is set for 5 seconds. The system receives signals from receiver  302  that are analyzed to determine the characteristic  340  as introduced by modulating reflector  308  as described above. If the modulated signal  330  containing the desired amount of characteristic  340  is received by receiver  302  and continues to be received by receiver  302  as described above until the timer terminates, processor  316  determines that surveillance zone  334  is clear of obstacles. When this occurs, processor  316  shifts to a zone clear state  414 . Zone clear state  414  initiates the consent action  326  and, after receiving a signal indicating the gates have been opened (not shown), processor  316  is returned to stand-by state  410 . In one exemplary embodiment, consent action  326  is the setting of an “all clear” relay but may be other actions including the sending of a message to a remote site or system via a communication link (not shown).  
         [0048]     Processor  316  analyzes the received signal  338  from receiver  302  and determines the presence of an obstruction in surveillance zone  334 . In one exemplary embodiment, once an obstruction is determined (as described above) during the period of the timer, the system shifts to a zone occupied state  416 . In zone occupied state  416 , received signal  338  continues to be monitored to determine whether the obstacle continues to be located in surveillance zone  334  or whether the obstacle has moved out of surveillance zone  334  and the zone is no longer obstructed. If this is determined and the timer has expired, the system shifts to zone clear state  414 . If the obstacle is determined by processor  316  to be moving within surveillance zone  334  (as will be discussed below), the system continues to monitor for the presence of the obstacle. To determine this, filter algorithms are used in conjunction with repeated scanning of surveillance zone  334 . If after a defined period of time, which in one embodiment may be the period of the timer, then zone occupied state  416  initiates alarm action  328 . In one embodiment, alarm action  328  may be the activation of an alarm relay (not shown). In another embodiment, alarm action  328  may be other actions including the sending of an alarm message to a remote site or system via the communication link (not shown).  
         [0049]     If during analysis state  410 , zone occupied state  416 , or zone clear state  414 , processor  316  receives a signal that the gates are no longer closed, processor  316  de-energizes any consent or alarm actions and returns the system to stand-by state  410 .  
         [0050]     If during stand-by state  410 , analysis state  412 , zone clear state  414 , or zone occupied state  416 , an error is detected or occurs in the system or in the operation of the system, the system shifts to a vital error state  418 . Whenever the self-diagnostics of the system identifies a failure of transmitter  302 A or receiver  302 B, system components, or control logic or software operated by processor  316 , the system also shifts to the vital error state  418 . In vital error state  418 , the diagnostic error is logged into a memory (not shown) and a system restart (not shown) may be initiated. In another embodiment, the system shifts to initialization state  404  for further analysis or system restart (not shown).  
         [0051]     One exemplary embodiment of a method  500  for automatically detecting intrusion in an unauthorized zone, such as detecting the presence of an obstacle located within surveillance zone  334  associated with a railroad grade crossing, is described in  FIGS. 5A and 5B , collectively referred to as  FIG. 5 . The system being in an idle state  502 , receives information from GPS system  322  on a scheduled, periodic, or continuous basis. The system awaits an actuating event or a command. In one exemplary embodiment, the system is activated automatically when the gates are closed such as upon receipt of a gates closed signal as at block  506 . When gates closed signal  506  is received or an indication is received from a gates closed system  508 , processor  316  initiates or sets a timer  510 . Additionally, processor  316  initiates the transmission at block  512  of transmitted signal  332  by transmitter  302 . In one exemplary embodiment, transmitted signal  332  is received directly by modulating reflector  308  at block  514 . In another embodiment, transmitted signal  332  is received by passive reflector  310  and reflected from passive reflector  310  to modulating reflector  308 . In either case, modulating reflector  308  receives transmitted signal  332  at block  514 . Modulating reflector  308  modulates, using any suitable modulation technique, received signal  338  at block  518  and reflects or transmits the modulated signal  330  at block  520 .  
         [0052]     Modulated signal  330  is reflected back towards receiver  302 B or is transmitted as modulated signal  330 A to passive reflector  310  which then reflects modulated signal  330 B containing characteristic  340  to receiver  302 B. In either case, receiver  302 B may receive signal  338  at block  522  which may or may not contain the desired amount of characteristic  340  as introduced by modulating reflector  308 . Received signal  338  is processed at block  528  to determine the presence of the desired amount of characteristic  340  within received signal  338  as described above. In one optional embodiment, received signal  338  is first processed by preamplifier and filter  312  at block  526  to obtain a processed signal such as a base band signal.  
         [0053]     If desired amount of characteristic  340  is detected at block  530  (as discussed above), processor  316  checks to see if the timer has expired at block  532 . If the timer has not expired, processor  316  continues to analyze received signal  338  at block  528 . If desired amount of characteristic  340  continues to be detected at block  530  and the timer has expired at block  532 , processor  316  initiates a clear zone consent action at block  534 . Once the consent action is initiated, the system returns to the idle state at block  544 .  
         [0054]     If during the analysis at block  528 , processor  316  determines that desired amount of characteristic  340  is not present at  530 , processor  316  checks the timer to ensure that it has not expired. If the timer has expired at block  536 , processor  316  initiates alarm action  328  at block  542 . Once alarm action  328  is initiated at block  542 , the system returns to the idle state at block  544 .  
         [0055]     However, if during the analysis at block  528  processor  316  determines that received signal  338  does not include desired amount of characteristic  340  at block  530  and the timer has not expired, processor  316  determines whether the detected object or obstruction is moving within surveillance zone  334  or whether it is stationary at block  538 . Processor  316  determines whether the detected object is moving or is stationary within surveillance zone  334  by comparing one received signal  338 B with another received signal  338 A and determining and analyzing the changes or differences between the two signals. A first received signal  338 A may be compared to a second received signal  338 B. Changes between first received signal  338 A and second received signal  338 B may be compared to a threshold, model, or signature to determine whether the object is the same object as detected in the second received signal  338 B as the first received signal  338 A, and if so, changes may be indicative of movement of the object with surveillance zone  334 . For example, where changes in amplitude of the first sideband is lower than the threshold amplitude for a period of time shorter than 2 seconds, processor  316  may determine that the object is moving in surveillance zone  334 .  
         [0056]     In the alternative, a change in the amplitude peak of the first sideband of received signal  338  by 20 percent may be indicative of a moving object. Processor  316  can make this determination by evaluating received signal  338  over time to identify variations in the amplitude, frequency, or energy of the sidebands in received signal  338 . Additionally, two or more received signals  338  may be analyzed in the embodiment where two or more transceivers  302  are utilized to define a single surveillance zone  334  as described above. In such an embodiment, movement may be indicated by analyzing changes in two or more characteristics  340  from the two or more modulated signals  330 .  
         [0057]     If processor  316  determines that the obstruction or object is moving or in motion within surveillance zone  334 , processor  316  checks the timer at block  540 . If the timer has expired at block  540 , processor  316  initiates an alarm action at block  542 . However if the timer has not yet expired at block  540 , the system continues to analyze received signal  338  at block  528 . If it is determined at block  538  that the object is not moving in surveillance zone  334 , the system continues to analyze received signal  338  to determine the modulation characteristic at block  528 . This process continues until the timer expires.  
         [0058]      FIG. 6  illustrates an exemplary railroad grade crossing detector system for a single track crossing indicating one embodiment of the layout of the transceivers  302 , modulating reflectors  308 , and resulting surveillance zones  334 . A single track  602  is enclosed by crossing gates  604 A and  604 B and gates  606 A and  606 B. A first transceiver  608  transmits a first transmitted signal  332 A (not shown) to first modulating reflector  610  and modulating reflector  610  reflects a first modulated signal  330 A (not shown) to first transceiver  608  thereby defining a first surveillance zone  612 . A second transceiver  614  transmits a second transmitted signal  332 B (not shown) to a second modulating reflector  616 , wherein second modulating reflector  616  reflects a second modulating signal  330 B to second transceiver  614  thereby defining a second surveillance zone  618 . In this single track railroad grade crossing, the system-defined surveillance zones  334  are surveillance zones  612  and  618 .  
         [0059]      FIG. 7  illustrates an exemplary railroad grade crossing detector system for a two-track crossing indicating one embodiment of the layout of the transceivers  302 , modulating reflectors  308 , and associated surveillance zones  334 . Tracks  702  and  704  are protected by gates  706 A and  706 B and gates  708 A and  708 B. A first transceiver  710  transmits a first microwave beam  714  to a modulating reflector  712 . A first surveillance zone  334  is defined by beam  714 . A second transceiver  716  transmits a second microwave beam  720  to a modulating reflector  718 . A second surveillance zone  334  is defined by beam  720 . In this two-track railroad grade crossing, the system-defined surveillance zone  334  is the zone defined by  714  and  720 .  
         [0060]      FIG. 8  illustrates an exemplary railroad grade crossing detector system for a two-track crossing indicating one embodiment of the layout of the transceivers  302 , modulating reflectors  308 , passive reflectors  310 , and surveillance zone  334 . Tracks  802  and  804  are protected by gates  806 A and  806 B and gates  808 A and  808 B. A first transceiver  810  transmits a first microwave beam  816  that is received by a passive reflector  812 . Passive reflector  812  reflects the received beam  816  to modulating reflector  814  thereby creating a second beam  818 . The resulting surveillance zone  334  of the first transceiver is the zone defined by beams  816  and  818 . A second transceiver  820  transmits a third microwave beam  828  to a passive reflector  822 . A passive reflector  822  reflects the received beam  828  to a modulating reflector  824  thereby creating a fourth beam  826 . The resulting surveillance zone  334  of the second transceiver is the zone defined by beam  828  and  826 .  
         [0061]      FIG. 9  illustrates an exemplary railroad grade crossing detector system for a three track crossing indicating one embodiment of the layout of the transceivers  302 , multiple modulating reflectors  308 , and surveillance zone  334 . Tracks  902 ,  904  and  906  are protected by gates  908 A and  908 B and gates  910 A and  910 B. A first transceiver  912  transmits three microwave beams. A first beam  916  of transceiver  912  is transmitted to a first modulating reflector  914 . A second beam  920  of the first transceiver  912  is transmitted to a second modulating reflector  918 . A third beam  924  of the first transceiver  912  is transmitted to a third modulating reflector  922 . As such, surveillance zone  334  of the first transceiver  912  is the zone defined by beams  916 ,  920  and  924 . In a similar manner, a second transceiver  926  transmits three microwave beams. A first beam  930  of transceiver  926  is transmitted to a first modulating reflector  928 . A second beam  934  of the second transceiver  926  is transmitted to a second modulating reflector  932 . A third beam  938  of the second transceiver  926  is transmitted to a third modulating reflector  936 . As such, the surveillance zone  334  of the second transceiver  926  is the zone defined by beams  930 ,  934  and  938 .  
         [0062]     In the embodiment as shown in  FIG. 9 , transceivers  912  and  926  each transmit more than one transmitted signal  332 , each such transmitted signal  332  being directed to a separate modulating reflector  308 . Each modulating reflector  308  is configured to uniquely modulate transmitted signal  332  by introducing unique characteristics  340  to generate the associated unique modulated signal  330  based on the received transmitted signal  332  as received by each modulating reflector  308 . Receiver  302 B receives signals from one or more modulating reflectors  308 . Receiver  302 B, preamplifier  312 , demodulator  314 , and processor  316  are configured to identify each of the unique modulated signals  330  and characteristics  340  as described above to determine the unique characteristics  340  in each received modulated signal  330  and therefore the presence or absence of an object. Each of these are determined separately in order to separately determine whether or not the desired amount of each and every characteristic  340  has been received, thereby determining the presence or absence of an obstacle for each and every surveillance zone  916 ,  920 ,  924 ,  930 ,  934  and  938 . In this exemplary embodiment, the system and method operate to detect the amount of each and every characteristic  340  in each modulated signal  330  for the particular configuration and embodiment. In such an embodiment, the method and processes defined in  FIG. 5  are performed for each and every separate modulated signal.  
         [0063]      FIG. 10  illustrates is an illustration of a system for detecting intrusion in an off-limits zone  1001 , such as may be defined by a perimeter.  FIG. 10  indicates one exemplary embodiment of the layout of the transceivers, modulating reflectors, and a resulting surveillance perimeter. A first transceiver  1002  transmits a first transmitted signal (not shown) to a first modulating reflector  1004  and modulating reflector  1004  reflects a first modulated signal (not shown) to first transceiver  1002  thereby defining a first surveillance perimeter section  1006 . A second transceiver  1012  transmits a second transmitted signal (not shown) to a second modulating reflector  1014 , wherein second modulating reflector  1014  reflects a second modulating signal to second transceiver  1012  thereby defining a second surveillance perimeter section  1016 . A third transceiver  1022  transmits a third transmitted signal (not shown) to a third modulating reflector  1024  and modulating reflector  1024  reflects a third modulated signal (not shown) to third transceiver  1022  thereby defining a third surveillance perimeter section  1026 . A fourth transceiver  1032  transmits a fourth transmitted signal (not shown) to a fourth modulating reflector  1034  and modulating reflector  1034  reflects a fourth modulated signal (not shown) to fourth transceiver  1032  thereby defining a fourth surveillance perimeter section  1036 . It will be appreciated that this layout may be used for many surveillance applications where an off-limits area may be defined by a perimeter, such as may be the case in airports, seaports, bridges, tunnels, industrial sites, military sites, housing complexes, etc. It will be appreciated that the off-limits area need not be fully circumscribed by a closed perimeter. Moreover, the configuration shown in  FIG. 10  is merely illustrative since the shape of the off-limits area may take any geometrical configuration. Also the number the number of transceivers, modulating reflectors, and passive reflectors, if any, will vary depending of the requirements of any given application.  
         [0064]     Those skilled in the art will note that the order of execution or performance of the methods illustrated and described herein is not essential, unless otherwise specified. That is, it is contemplated that aspects or steps of the methods may be performed in any order, unless otherwise specified, and that the methods may include more or less or alternative aspects or steps than those disclosed herein.  
         [0065]     As various changes could be made in the above exemplary constructions and methods without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.  
         [0066]     When introducing elements of the present invention or preferred embodiments thereof, the articles “a”, “an”, “the”, and “said” are intended to mean that there are one or more of the elements. The terms “comprising”, “including”, and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements.