Abstract:
A CW radar ( 10 ) is used to detect motion of objects ( 22 ) behind a wall ( 20 ) by projecting a radar beam through the wall and by measuring the returns from the objects behind the wall, with a change in the phase difference between the transmitted and received CW signals providing an indication of motion behind the wall and thus the presence of an individual.

Description:
RELATED APPLICATIONS  
       [0001]     This application claims rights under 35 USC 119(e) from U.S. Application Ser. No. 60/537,868 filed Jan. 20, 2004, the contents of which are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention relates to through-the-wall sensors and more particularly to the use of CW radar to detect motion of objects behind a wall.  
       BACKGROUND OF THE INVENTION  
       [0003]     Oftentimes it is desirable to be able to detect individuals within, for instance, a burning building or enemy combatants or troops behind a wall. Moreover, in some instances police can utilize through-wall sensing systems to be able to detect the presence of wanted individuals from a position outside the building.  
         [0004]     Through-wall sensing can be used in military operations in urban terrain, for homeland security, for law enforcement and for fire departments. The need to sense behind walls is clear. It will be appreciated that the details of the mission and types of walls or obstructions dictate the design of the through-wall sensors.  
         [0005]     In the past, ultra-wideband devices have been used as ground penetrating and through-the-wall radars. The difficulty with ultra-wideband approaches is that one has to generate short pulses which requires fairly expensive hardware. A particular difficulty with ultra wide band is that walls have frequency dependent attenuation. This results in a distortion of the return pulses that pass through the wall. This distortion blurs the pulse making it hard to correlate. Moreover, in ultra-wideband applications one must integrate over multiple pulses in order to obtain enough processing gain to detect objects inside a building. In this regard, in ultra-wideband systems, one has to generate a stream of pulses. The problem with generating streams of pulses is the existence of clutter and with an ultra-wideband pulsed radar, one detects everything within a room. One therefore has to have a means for discriminating clutter from background, which ultra-wideband systems do not do.  
         [0006]     What is therefore required is an easily portable, low cost, low power drain, compact unit that can be positioned outside a building that can detect motion of individuals within the building and discriminate against inanimate objects.  
       SUMMARY OF INVENTION  
       [0007]     In order to obtain a through-the-wall motion detector capable of easily detecting a person within a room, is has been found that one can detect these individuals because they are typically in motion. In order to detect individuals behind a wall, the subject invention employs a simple CW radar with a directional antenna. In one embodiment, the transmitter for this CW radar employs a circulator, which is coupled to a directional antenna so that a CW beam is projected through the wall and into the room. Returns from the CW beam arrive at the same antenna and are split off by the circulator. A reduced power replica from the transmitted signal is mixed with the returns from the antenna. Changes in the phase difference between the two signals indicate motion, and thus the presence of an individual behind the wall. In one embodiment, the summing is performed at a mixer, with slight phase differences indicating motion of an object behind a wall. Thus, if there is anything behind the wall that is moving, and recognizing that people normally move, the system will detect them.  
         [0008]     In one embodiment, the CW radar transmitter includes a frequency source coupled to a power divider, with one output of the power divider coupled to the circulator and thence to the directional antenna, which may either be a YAGI or may be a planar antenna having plates spaced from a ground plane.  
         [0009]     The power divider output is also supplied to the aforementioned mixer that mixes the output signal with radiation returned from behind the wall, with the motion detection including sampling the signal representing the phase difference and applying an adaptive threshold which, when the change in phase difference exceeds a predetermined threshold, the presence of an individual is indicated. Thus if the phase difference change is greater than a predetermined threshold, the presence of an individual is declared. In one embodiment the detection threshold is adaptively determined by a microprocessor and is then used by the microprocessor to declare detections.  
         [0010]     The subject system can be made portable and battery powered and can be transported by fire, police, military troops or other individuals conducting a search of a premises. Within seconds one can ascertain whether individuals are within the premises due to the normal motion of the individuals in a room or along a hallway.  
         [0011]     It has been found that the phase difference is periodic when the object behind the wall has a constant motion, as when an individual is walking at a constant rate of speed, or is more random when the speed of the object is not constant. In either case, the change in the phase difference between the transmitted signal and the received signal being above a given threshold can be used to trigger an alarm to indicate the presence of an individual or at least some object that is moving.  
         [0012]     Thus, in the case of a fire when one does not want to enter a burning building unnecessarily, one can position one or more of these CW radars adjacent the building to see if there are victims that need rescuing. Likewise, when chasing felons, police may provide such a unit adjacent the outer wall of a building to be able to ascertain if the felon was within the premises.  
         [0013]     It will be appreciated that what has been described in one embodiment is a single-frequency CW radar in which a change in the phase difference between the transmitted and received signal is used to indicate the presence of an object in motion behind a wall. It has been found that the system operates irrespective of the type of wall material so that no adjustment need be made based on the type of wall encountered. Moreover, the system automatically discriminates against objects within the room that are stationary, usually inanimate objects. Thus, unlike ultra-wideband radars, the system does not pick up stationary objects such as furniture and the like.  
         [0014]     While some frequencies are better than others for wall penetration, it has been found that a signal in the 900 MHz band is optimal for detecting motion behind most walls. However, systems using higher frequencies are effective for longer standoff ranges through lower density walls and lower frequency units are indicated for more dense walls.  
         [0015]     In summary, a CW radar is used to detect motion of objects behind a wall by projecting a radar beam through the wall and by measuring the returns from objects behind the wall, with a change in the phase difference between the transmitted and the received CW signals providing an indication of motion behind the wall and thus the presence of an individual. The system may use a microprocessor to set the threshold and declare detections.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     These and other features of the subject invention will be better understood in connection with a Detailed Description, in conjunction with the Drawings, of which:  
         [0017]      FIG. 1  is a diagrammatic illustration of a single-frequency CW radar having a frequency source, a power divider, a circulator and a mixer, with the circulator being coupled to a directional antenna that directs the CW radar beam into a building and in which a change in the phase difference between returns and the transmitted signal is measured to indicate the presence of an individual behind the wall;  
         [0018]      FIG. 2  is a diagrammatic illustration of the system of  FIG. 1  indicating a change in phase difference when the individual behind the wall is moving;  
         [0019]      FIG. 3  is a graph showing a periodic waveform of the change in phase difference versus time for constant motion;  
         [0020]      FIG. 4  is a graph of change of phase difference versus time for random motion;  
         [0021]      FIG. 5  is a graph showing change of phase difference versus time for a stationary object, indicating a straight line on the graph; and,  
         [0022]      FIG. 6  is a block diagram of a microprocessor capable of being used in the system of  FIG. 1  as a motion detector, with the microprocessor including sampling the phase difference signal and providing the output that is used to set an adaptive threshold which is then used to monitor the signal motion detection. 
     
    
     DETAILED DESCRIPTION  
       [0023]     Referring now to  FIG. 1 , a CW radar  10  includes a frequency source  12 , a power divider  14  and a circulator  16  coupled to an antenna  18 . Preferably, the antenna is a directional antenna so as to project all of the energy in a given direction, in this case through a wall  20 , so as to be able to ascertain whether an individual  22  exists behind the wall.  
         [0024]     In one embodiment, the radar is a single frequency radar set optimally in one embodiment to 900 MHz, with antenna  18  in one embodiment being a YAGI antenna, with 13 dB forward gain. While a YAGI antenna may be utilized in order to reduce back lobes and yet have a readily portable unit, a flat panel antenna with conductive elements insulated from a ground plane may be used to eliminate back lobes and is lighter and more easily transportable.  
         [0025]     As illustrated, one output of power divider  14  is coupled to circulator  16  coupled to a directional antenna  18  that forms a CW beam as illustrated at  24  which penetrates wall  20 . Energy reflected by objects behind the wall as illustrated at  26  is detected by antenna  18  is coupled to circulator  16  and thence to a mixer  30 , to which is coupled a divided-down sample of the output of frequency source  12 . The result is that power divider  14  divides the power of frequency source  12  to provide a phase reference signal to the mixer. Mixer  30  therefore mixes signals on lines  32  and  34  to derive a phase difference or Doppler transmitted on line  36  to a motion detector  40 .  
         [0026]     It is the function of motion detector  40  to ascertain when a change in phase difference on line  36  exceeds a predetermined threshold. When this occurs, a moving object behind wall  20  is indicated. Motion detector  40  may have a local alarm or display screen, whereas, as indicated by dotted line  42 , the output of motion detector  40  may be transmitted by a transmitter  44  via an antenna  46  to a remote location. In one embodiment, the same RF hardware used by the radar is used to also send the detection report to a remote location. This simplifies the design of the system. In another embodiment, the an independent RF transmitter is used to send the detection report.  
         [0027]     In this manner, the unit may be set up, for instance, on a tripod near the exterior wall of a building, with the results of the motion detection being detected at a distance from the building, either to protect troops or police from hostile action or to protect monitoring individuals from, for instance, the heat of a fire.  
         [0028]     As illustrated in  FIG. 2 , the CW radar  10  projects beam  24  from antenna  18  such that, if individual  22  is moving as illustrated by arrow  48 , there is a change in phase difference between beam  24  and returned radiation  26 .  
         [0029]     As illustrated in  FIG. 3 , for constant motion, when the phase difference change is graphed against time, there is a sinusoidal waveform  54  that results.  
         [0030]     As illustrated in  FIG. 4 , if there is random motion of the object behind the wall, then the graph of the phase difference change versus time results in a random curve  56 , whereas as illustrated in  FIG. 5 , if the object is stationary, then the phase difference change versus time is flat as illustrated by straight line  58 .  
         [0031]     Referring now to  FIG. 6 , in one embodiment, motion detector  40  may include a microprocessor  60 , which includes as part thereof a sampling circuit that samples and holds the phase-difference signal as illustrated at  62 . Changes in the sampled phase difference are calculated and applied to an adaptive threshold unit  64 , which outputs a signal on line  66  to a detector  68  that provides a signal when the phase difference change is greater than a threshold T set by adaptable threshold unit  64 . When there is a signal on line  70  one can declare that motion has occurred and that there is an individual behind the wall.  
         [0032]     What has therefore been provided is an extremely simple system for detecting the presence of an individual behind a wall, which uses a CW radar signal and a unit for detecting a change in the difference in phase between the outgoing transmitted CW signal and the reflected CW signal.  
         [0033]     It has been found that this is a very sensitive detector of motion and one for which it is not necessary to integrate pulses or, for instance, to sweep the frequency such as is the case in ultra-wideband applications.  
         [0034]     While the present invention has been described in connection with the preferred embodiments of the various figures, it is to be understood that other similar embodiments may be used or modifications or additions may be made to the described embodiment for performing the same function of the present invention without deviating therefrom. Therefore, the present invention should not be limited to any single embodiment, but rather construed in breadth and scope in accordance with the recitation of the appended claims.