Abstract:
Radar method and apparatus for detecting small, slow moving targets in the presence of high background clutter that includes a balanced bridge structure to achieve a null balance with the target and background clutter. Radar return signals from background objects as well as the targets are viewed by the radar as a composite single signature return. The invention uses a phase shifter to achieve a null balance even though the background clutter consists of many components at varying distances from the radar.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of U.S. provisional patent application Ser. No. 60/305,318, filed Jul. 13, 2001, which is herein incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to radar systems and, more particularly, to a radar apparatus and method suitable for detecting small, slow moving targets in the presence of high background clutter. 
     2. Description of the Related Art 
     In the past, radar systems have been used to detect and track relatively large, fast moving targets by the military and others. These targets have been typically artillery projectiles, rockets, people, automobiles, aircraft, etc. Detecting very small, slow moving targets at short distances, in the presence of high background clutter, has not been a high priority. 
     There have been a number of attempts to design a radar system to detect small, slow moving targets. These radar systems have not been very successful for a number of reasons. First, using a pulse radar to detect and track small, slow moving targets has been plagued with difficulties. The radar cross section of the target is very small, and any background clutter tends to blind the radar. The pulse radar has a minimum range, primarily determined by the transmitter pulse width. A typical minimum range of this type of radar is of the order of many hundreds of feet. At this range, identifying small targets is virtually impossible. Tracking a swarm of flying insects is nearly impossible due to the very small radar cross section. The target is not homogenous, and a good radar lock on a small undulating mass is very difficult. 
     Secondly, several attempts have been made to use a Continuous Wave (CW) Doppler Radar to detect small slowly moving targets. A doppler radar is sensitive to velocity, and a slow moving target generates a doppler frequency of the order of a few cycles per second. Insects normally move slowly over a few tens of millimeters during a measurement period. For reliable detection the target must generate a number of Doppler cycles by moving in a straight line towards or away from the radar antenna. An insect moving many wavelengths in a straight line in such a direction is unlikely. 
     Therefore, there is a need in the art for a radar apparatus and method that can detect small, slow moving targets in the presence of high background clutter. 
     SUMMARY OF THE INVENTION 
     The disadvantages associated with the prior art are overcome by a radar apparatus and method for detecting small, slow moving targets wherein the radar return signals from background objects as well as the target are viewed by the radar as a composite single signature return. The apparatus comprises a balanced bridge structure that can achieve a null balance even though the background clutter consists of many components at varying distances from the radar. The target sensitivity of the radar depends upon how deep a null of the clutter signal the radar can achieve. A very small change in the target position changes the composite clutter signature and causes the null to shift. The change in the null is then more easily detected. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     So that the manner in which the above recited features, advantages and objects of the present invention are attained and can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings. 
     It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
     FIG. 1 depicts a block diagram of one embodiment of a balanced bridge radar system; 
     FIG. 2 shows a graph of the output of the balanced bridge radar system versus the line stretcher adjustment for a fixed target position; 
     FIG. 3 shows a graph of the radar cross section versus wavelength for a spherical reference target; 
     FIG. 4 shows the far field antenna pattern of the balanced bridge radar system; 
     FIG. 5 shows the near field antenna pattern of the balanced bridge radar system; 
     FIG. 6 depicts a block diagram of another embodiment of the balanced bridge radar system; 
     FIG. 7 illustrates a waterfall display of the output of the radar system in FIG. 6 when detecting targets; 
     FIG. 8 illustrates a waterfall display of the output of the radar system in FIG. 6 in the absence of targets 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention is an implementation of a continuous wave (CW) microwave radar system that can detect targets that are very small. These targets move slowly and randomly over short distances in the presence of high background clutter. There are many uses for such a radar system including, but not limited to, the detection of very small insect targets such as termites, carpenter ants, weevils, and the like in structures, in materials such as grain, and in other environments. 
     FIG. 1 depicts a block diagram of one embodiment of a balanced bridge radar system  100  in accordance with the present invention. The radar system  100  comprises a microwave energy generator  102 , a transformer  104 , a line stretcher  106 , a transmitter antenna  108 , a receiving antenna  110 , and a mixer  112 . Transmitter antenna  108  and receiving antenna  110  collectively comprise an antenna system  109 . 
     In operation, the energy from the microwave generator  102  is split into two paths  114  and  116  by the transformer  104 . The transformer  104  can also be a directional coupler, or other suitable means as are known in the art. A small amount of energy flows in path  114  to supply power into the local oscillator port of the mixer  112 . The mixer  112  comprises a direct current (dc) coupled microwave mixer. The input port of the mixer  112  is connected to the receiving antenna  110 . The output port of the mixer  112  is the dc coupled output signal representing the difference between the signals at the local oscillator and input ports. 
     The majority of the power from the microwave generator  102  flows in path  116  to the input port of the line stretcher  106 . The line stretcher  106  comprises a microwave phase shifter that can adjust the delay of a microwave signal from zero to a minimum of ¼ wavelength at the radar&#39;s operating frequency. The line stretcher  106  produces a phase adjustment in the signal at the input port. The output port of the line stretcher  106  is connected to the transmitter antenna  108 . The microwave energy exits the transmitter antenna  108  and proceeds along path  118  to a target  120 . Some of the energy is reflected from the target  120  along path  122  and is picked up by the receiving antenna  110 . As described above, the energy is coupled to the input port of the mixer  112 . 
     FIG. 2 shows a graph  200  of the output of the balanced bridge radar system  100  versus the phase adjustment for a fixed target position. Since the present embodiment of the system  100  is dc coupled, the output voltage from the mixer  112  output port (out) is a dc signal  202  that varies as a sinusoid from zero through a plus or minus value and back to zero as the line stretcher  106  is adjusted through each quarter wavelength. For an arbitrary target position, the line stretcher  106  can be adjusted so the output of the mixer  112  is nulled to zero at the ¼, ½, and ¾ wavelength points, designated  204 ,  206 ,  208 , respectively. With the signal  202  nulled to zero, the sensitivity of a measuring system amplifier (not shown), which could be connected to the output port of the mixer  112 , can be increased to a high level without saturating. The increase in gain of such an amplifier compensates for the very small return from the target. Using this principle, a very small target displacement in path  118  to path  122  is easily measured. 
     FIG. 3 shows the radar cross section  302  versus wavelength  304  for a spherical reference target. Most radar systems operate in the Optical Region  306 , where the radar sensitivity is independent of wavelength. When the target  120  is very small compared to the wavelength, the radar operates in the Rayleigh region  308 . In this region the return signal decreases rapidly with decreasing target size. Scaling the drawing shows that the radar return signal decreases by nearly three orders of magnitude (over 1000) for each six to one reduction in target size. Using the balanced bridge radar system  100  and nulling out the large background clutter signal allows the system gain to be increased to the point that a small target can be observed. 
     Clutter is the radar cross-section return of the background not associated with the target of interest. One of the major advantages of a CW radar is that it discriminates against the stationary clutter signal while showing the moving target. The effect of the clutter is to reduce the radar&#39;s sensitivity by the ratio of the radar cross-section of the target divided by the radar cross section the background clutter. 
     A small target, such as an insect moving along the floor results in a large background clutter (floor, ground, etc.) associated with the target signal. Using the line stretcher  106  to null out the target and clutter combination allows the mixer  112  output signal to be greatly amplified. This makes it possible to view the target&#39;s position movement in the presence of background clutter without saturating the radar&#39;s detection system. 
     The antenna system  109  used with this type radar can operate either in the near or far field. The distance in front of the antenna  109  where it changes from near to far field is approximately as follows: R=2×D 2 /λ, where R is the distance from the antenna  109 , D is the antenna aperture, and λ is the wavelength of the transmitted signal. 
     The far field antenna pattern is shown in FIG. 4, and its extent corresponds to the equation R&gt;2×D 2 /λ. The concentric circles  402  show the ¼, ½, and ¾ null semicircles of revolution (designated  404 ,  406 ,  408 , respectively) about line  410 . A null point is passed for each semicircle that the target  120  moves through. The target  120  can move either axially or laterally with respect to the antenna  109  to pass into or out of a semicircle null and be detected. 
     The near field radar antenna system is shown in FIG. 5, and its extent corresponds to the equation R&lt;2×D 2 /λ. Here the transmitter and receiver antennas  108  and  110  are shown at one or more wavelengths apart. In the present embodiment, the two antennas  108  and  110  separated by two wavelengths. Those skilled in the art will understand that other spacing is also possible that will have different lateral sensitivities. 
     Operating in the near field has the following advantages: (A) The transmitter cross leakage to the receiver antenna is reduced by up to 60 dB reducing saturation problems and increasing the system&#39;s sensitivity; and (B) An interference pattern is set up such as shown in FIG.  5 . The intersection of two sets of concentric circles  502  and  504  shows where the radar system can achieve a system null. The axial lines  506 A through  506 G are drawn through the null points. In the near field, the radar is very sensitive to axial and lateral target movement and the lateral distance sensitivity is greater. Again the distance between semicircles is ¼ wavelength. The near field can vary from two to thirty inches, depending upon the design of the antenna system  109 . The position of the null semicircles can be moved towards or away from the radar antennas  108  and  110  by changing the phase adjustment of the line stretcher  106 . 
     The basic radar implementation described above shows the radar operating with a continuous wave unmodulated carrier. With this type of system the output information from the mixer  112  is a direct current signal that can be observed on a slow moving dc type voltmeter. This is the simplest implementation of the balanced bridge radar system  100 . The radar system  100 , however, is sensitive to any conducting medium that changes with time. Fluorescent lamps, for instant, have a mercury vapor the length of the lamp with a conduction cycles that goes on and off 120 times a second. The fluorescent lamp presents a large interfering target to the radar system  100 . There are many other sources of interfering signals to a radar system with frequencies in the vicinity of the sixty hertz power frequency. 
     The ultimate sensitivity of the radar depends upon the phase noise of the microwave source. For a typical solid state microwave oscillator, the phase noise is of the order of minus eighty decibels below the carrier level (−80 dBc). The phase noise decreases as the radar receiver operating point is moved away from the carrier frequency. Therefore, the further the receiver is operated from the carrier frequency (zero frequency), the lower the phase noise and the more sensitive the radar. 
     FIG. 6 shows a balanced bridge radar  600  with the addition of an electronically controlled line stretcher  602  that is placed in the transmitter path  116 . The line stretcher  602  is either a zero or ¼ wavelength phase shift that is controlled by the frequency of a modulator  604 . From FIG. 2, it can be seen that adding a ¼ wavelength to the system path length reverses the polarity of the signal on the output port of the mixer  112 . If the output signal is stable at point  210 , adding ¼ wavelength to the system path will move it to point  212  on the diagram. With a square wave modulation driving the line stretcher  602 , the output of the mixer  112  will be a square wave of an amplitude twice the value of point  210  at the modulation frequency. On the other hand, if the output signal is stable at point  204 , then adding ¼ wavelength to the system path will now move it to point  206  on the diagram. Since point  204  has a zero value, moving it to point  206  will also have a zero value. 
     The effect of the modulation is to move the signal information from dc up to the modulation frequency. Moving the information away from zero frequency reduces the interference from external sources such as fluorescent lamps, and reduces the phase noise of the microwave source. Moving the signal away from dc allows using ac instead of dc coupled components. The following are the trade off parameters for choosing the modulating frequency: (A) Most of the low frequency interfering signals are located below a couple hundred hertz (200 Hz); (B) The phase noise from most microwave sources decreases with increasing frequency, leveling off about 100 kilohertz (100 kHz) from the carrier; and (C) Above several hundred kilohertz the composite clutter signal starts to have a range related signature components making it difficult to get a good system null. Thus, the optimum radar line stretcher  602  modulation frequency lies somewhere in the range of several hundred hertz to about 100 kilohertz. 
     To eliminate the low frequency interference signals, it is desirable to move the balanced bridge radar target signal information away from zero frequency. A band pass filter at the modulation frequency with a large rejection of the low frequency interference signals is necessary. This filter can either be a hardware or software filter. The software approach seems to have the most merit. 
     A Fast Fourier Transform with a Waterfall Display of the radar signal is shown in FIG.  7 . The figure shows the amplitude modulation of the radar signal by one or two small targets walking through the null points of the radar&#39;s antenna system. The time duration of FIG. 7 is approximately forty seconds (40 seconds). The balanced bridge radar was pointed at a mason jar approximately three feet (3 ft) in front of the radar. The mason jar contained approximately ten small targets (rice weevils), each three millimeters (3 mm) long. 
     FIG. 8 shows the output from the balanced bridge radar with the same mason jar setup and no small targets present. The waterfall display shows no amplitude modulation, indicating the absence of any rice weevils. 
     The above invention has been described primarily within the context of detecting insects. Broadly speaking, the invention is applicable to any moisture-bearing entity. For example, the balanced bridge radar can detect and quantize aerosol type targets. To verify this application, an experiment was performed in which aerosol clouds generated by a small air brush type spray gun were introduced into the balanced bridge radar&#39;s field of view. The average size of the aerosol cloud generated was approximately three feet (3′) long by one foot (1′) in diameter with droplets in the order of fifty (50) microns in diameter. It is noted that with greater radar sensitivity very small droplet sizes are detected. Using water as a testing liquid, a signature similar to that shown above with respect to FIG. 7 was obtained. Each time the spray was generated and introduced into the radar&#39;s field of view, a drop in amplitude of the waterfall display was observed. After a short time, the display returned to its original amplitude. Tests using a second liquid, Methyl Alcohol yielded similar results to the water test. A third liquid was tested, Trichlorofluoromethane (Refrigerant R-11), which also gave results similar to that obtained for water. These tests confirm that a wide variety of liquids in aerosol form provide a usable signal that is detected by the balanced bridge radar of the invention. 
     In another test, the radar was pointed vertically at a roof of a dwelling thirty feet (30′) above the test platform and through a ceiling ten feet (10′) above the floor. A light rain was causing a small amount of water to run down the roof. This movement of water was detected and measured using the balance bridge radar of the invention. Additionally, a small leak from a water pipe behind a concrete wall or below a concrete slab is also detectable and measurable using the invention. 
     In one testing scenario, a signal under a concrete floor slab was detected. The detected signal turned out to be a ground hog that had burrowed under an eight inch (8″) thick concrete floor. The ground hog was about thirty feet (30′) away from the testing area and provided a strong radar return signal. In general, in addition to detecting insects and aerosols as previously described, the invention is well adapted to the detection of small animals in walls, under floors or behind the foundations of buildings. 
     While the foregoing is directed to the preferred embodiment of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.