CA2265457C - Concealed weapons detection system - Google Patents

Abstract

A weapons detector (12) and method utilizing radar. The system comprises a transmitter for producing an output (14) of frequencies of a set of self-resonant frequencies of weaponry; an antenna directing the transmitter output toward locations potentially having weaponry and collecting backscattered signals (15); a receiver receiving the backscattered signals (15) and operating over a range of the self-resonant frequencies; and a signal processor for detecting the presence of a plurality of the self-resonant frequencies in the backscattered signals (15). Accuracies of greater than 98 % can be obtained at distances, preferably between 4-15 yards. The weapons detector (12) is capable of detecting metal and non-metal weapons (16) on a human body (13) in purses, briefcases and under clothing; and discerning weapons (16) from objects such as belt buckles, coins, keys, calculators, cellular phones.

Description

?101520CA 02265457 2005-04-22-1-CONCEALED WEAPONS DETECTION SYSTEMBACKGROUND OF THE INVENTIONField of the Invention (Technical Field):The present invention relates to apparatuses and methods for remotely detectingconcealed weapons.Background Art:There are no known weapons detection systems on the market today that can detect aconcealed weapon from a distance of more than three feet. Virtually every device in operationis an electromagnetic device that requires the use of a portal. An eddy current flows through theportal and when metallic objects create a change in the magnetic ?ux, it activates a warningsignal. This type of system cannot discriminate between concealed weapons and other metallicobjects such as belt buckles, jewelry, coins, watches, or calculators.Millitech Corporation of South Deerfield, Massachusetts, may be developing a passivemillimeter system. The system apparently would require the use of a portal, a ?oor plate, avideo camera and a cathode ray tube to view the area being searched. Millitech has claimed itwas developing a 300 mm aperture camera for ?xed entrance-way surveillance to demonstratetheir technology. The company claims its passive millimeter wave imagers will not require thesubject to be exposed to any man-made electromagnetic fields or other radiation from animaging system.?1015202530CA 02265457 2005-04-22.2.Infrared technologies, have great dif?culty penetrating layers of clothing. in addition, inwarmer climates, a gun carried on the body will take on the same temperatures as the body,making infrared virtually useless.The Raytheon Company of Portsmouth, Rhode island, bases its weapons detectionsystem on low frequency electromagnetic radiation. Their concept is based on illuminating thesubject with a low intensity electromagnetic pulse known as a Heaviside pulse and measuringthe time decay of the reradiated energy from metal objects carried by the person. The intensityand the time decay of the secondary radiation can be characterized and the signatures identifiedas a gun or non-threatening metal objects.Idaho National Engineering Laboratory uses technology based on passive sampling ofthe earth's magnetic field. Local aberrations in the magnetic field are produced byferromagnetic objects such as guns and knives. In the gun detection system being developedby Idaho National Engineering Laboratory, the magnetic aberrations or anomalies were to besensed and measured by magnetic gradiometers. They were planning to construct a scannerusing a multiple magnetometer design that could be a standalone unit, much like an airportscanner system. The scanner would be triggered electronically by a threshold detector. Datawould be collected simultaneously from all sensors in the system providing a top to bottommagnetic profile of the targeted person. Reasonable suspicion about the presence of aconcealed weapon would be dictated by the location and magnitude of magnetic anomalies.Other attempts to provide a useful weapons detection system (or solve somemarginally related detection problem) include U.S. Patent No. 5,552,705, entitled“Non-Obstrusive Weapon Detection System and Method for Discriminating Between aConcealed Weapon and Other metal Objects,” to Keller; U.S. Patent No. 5,519,400, entitled“Phase Coded, Micro-Power Impulse Radar Motion Sensor,” to McEwan; U.S. Patent No.5,512,834, entitled “Homodyne lMpu|se Radar Hidden Object Loc:ator,” to McEwan; U.S.Patent No. 5,457,394, entitled “lmpulse Radar Stud?nder," to McEwan; U.S. Patent No.5,381,153, entitled "Portable FM-CW Radar Device with Frequency Conversion by Firstand Second Frequencies,” to Saito et al.; U.S. Patent No. 5,365,237, entitled “MicrowaveCamera,” to Johnson et al.; U.S. Patent No. 5,345,240, entitled “Handheld Obstacle?1015202530CA 02265457 l999-03- 10WO 98/12573 PCTIUS97/16944-3-Penetrating Motion Detecting Radar," to Frazier; U.S. Patent No. 5,337,053, entitled “Method andApparatus for Classifying Targets," to Dwyer; U.S. Patent No. 5,334,981, entitled "Airborne MetalDetecting Radar," to Smith et al.; U.S. Patent No. 4,905,008, entitled “Radar Type UndergroundSearching Apparatus," to Kawano et al.; U.S. Patent No. 3,707, 672, entitled “Weapon DetectorUtilizing the Pulsed Field Technique to Detect Weapons on the Basis of Weapons Thickness," toMiller et al.; and Demma et al., entitled "Remote Concealed Weapon Detection by ElectromagneticImaging Techniques."Miller and Keller employ magnetic field sensors and so will not detect guns made of non-magnetic materials such as aluminum, brass, and copper. McEwan ‘400 employs monodyneimpulse radar and cannot discriminate between object types. McEwan ‘834 and ‘394 employimpulse radars to locate large objects behind dielectric media and do not attempt to identify objectsdetected. Saito et al. employs Doppler radar to determine existence and motion of an object, butnot the nature of the object. Johnson et al. is essentially a microwave ultrasound imager and doesnot measure or examine backscatter. Frazier is a conventional moving target indicator (MTI) radar.Dwyer does analyze radar backscattering but does so with respect to unobstructed objects, notconcealed ones. Smith et al. uses radar cross polarization scattering for camouflaged metaldetection, but does not rely on spectral content. Kawano et al. is a simple radar system for seekingobjects or pockets below ground.The present invention solves the deficiencies of the prior art. It illuminates a subject with alow intensity short pulse radar. Objects made of metal or high dielectric constant non~conductivematerial are nearly all backscattered. If a handgun is present, a unique spectral signature isreceived. Signatures can be prestored or learned by a computer employing artificial intelligencetechniques.The invention has an operating distance of at least between four yards to 20 yards. Theinvention, in its portable, hand—held form, is useful by law enforcement agencies, correctionalfacilities, the military and private security companies in the United States and throughout the world.The door-mounted embodiment is useful by federal, state and local governments, as well asfinancial institutions, convenience stores and other retail businesses, airports, schools and owners of?1015202530CA 02265457 l999-03- 10WO 98/12573 PCTIUS97/16944-4-private office and apartment buildings. Each of these entities has a critical need for a low-cost,highly dependable weapons detection system. The Bureau of Alcohol, Tobacco and Firearmsestimates there are between 60 million to 200 million firearms in the United States today. More than65,000 people were killed by firearms in the United States between 1988 and 1992. In 1993,homicides were the second leading cause of job-related fatalities in the United States, following onlyhighway accidents. In 1993, the Federal Bureau of Investigation reported there were 11,876 bankrobberies in the United States. resulting in a loss of $39.3 million. There were almost 35,000 armedrobberies to convenience stores in the United States, accounting for a $15.7 million loss. Inaddition, the National Education Association reports an estimated 100,000 students carry a gun toschool. Gunshots now cause one in every four deaths among American teenagers. Other countriesare facing similar problems. The present invention seeks to lower these appalling statistics.SUMMARY OF THE INVENTION (DISCLOSURE OF THE lNVENTlOl\DThe present invention is directed to a weapons detection system. The preferred weaponsdetector comprises: a transmitter for producing an output of frequencies of a set of self-resonantfrequencies of weaponry; an antenna directing the transmitter output toward locations potentiallyhaving weaponry and collecting backscattered signals; a receiver receiving the backscatteredsignals and operating over a range of the self-resonant frequencies; and a signal processor fordetecting presence of a plurality of the self-resonant frequencies in the backscattered signals.A range finder is preferably used for normalizing the backscattered signals. The transmitterpreferably produces an output of frequencies between approximately 1 GHz and approximately 10Ghz. The time resolution of the receiver is preferably less than approximately 10 nanoseconds.The minimum signal detection capability of the receiver is preferably less than approximately 1millivolt .The weapons detector makes a prediction about presence of weaponry on a human body.The accuracy is better than or equal to approximately 75%, preferably better than or equal toapproximately 95%, and most preferably better than or equal to approximately 98%. Accuracies of99.75% have been obtained.?1015202530CA 02265457 l999-03- 10W0 98/12573 PCT/US97Il6944-5.The weapons detector may be portable, hand-held, mounted on a wall or in a doon/vay.Weaponry is detected preferably between 3-20 yards and most preferably between 4-15 yards.Upon detection, the doorway could be activated (e.g., to lock) or a camera could take a picture ofthe suspect.A neural network is preferably utilized in the signal processorto aid in detection. The neuralnetwork is trained to recognize backscattered signals from weaponry prior to field use.Preferably, the weapons detector provides a result within approximately one second oftransmitter output. The weapons detector can indicate presence and absence of weaponry.Possible indicators are: audible signals. silent signals, tactile signals. visual signals, mechanicalsignals, and displayed messages. The weapons detector is useful for detecting weaponry such ashandguns, rifles, shotguns, and pipe bombs. The weapons detector is also useful for discerningweaponry from objects such as belt buckles, bracelets, wristwatches, tape recorders, soft drink cans,coins, calculators, lipstick holders, calculators, campaign buttons, cellular telephones, key rings,keys and the like. Weaponry can be detected in or under clothing or accessories such as purses,belts, holsters, pants, briefcases, coats and shirts.Principal objects of the present invention are to detect concealed weapons with a highaccuracy (i.e., 98%), a one second response time or better, portability, an effective operating range(e.g., four to 15 yards, equivalent to the distance of a typical traffic stop, or more (up to fifty yards)),limited operational complexity, and high durability.Another object of the invention is to provide a door model able to cover an areaapproximately three yards by five yards or better around an entrance.Other objects, advantages and novel features, and further scope of applicability of thepresent invention will be set forth in part in the detailed description to follow, taken in conjunctionwith the accompanying drawings, and in part will become apparent to those skilled in the art uponexamination of the following, or may be learned by practice of the invention. The objects and?1015202530CA 02265457 l999-03- 10WO 98/12573 PCT/US97/16944.5-advantages of the invention may be realized and attained by means of the instrumentalities andcombinations particularly pointed out in the appended claims.BRIEF DESCRIPTION OF THE DRAWINGSThe accompanying drawings, which are incorporated into and form a part of the speci?cation,illustrate several embodiments of the present invention and, together with the description, serve toexplain the principles of the invention. The drawings are only for the purpose of illustrating apreferred embodiment of the invention and are not to be construed as limiting the invention. In thedrawings:Figure 1 depicts the operation of one embodiment of the hand-held embodiment of theinvention in use by a law enforcement officer to survey a suspect standing outside of a stoppedvehicle;Figures 2A and 2B illustrates the hand-held concealed weapons detector of the invention ofFig. 1;Figure 3 is a block diagram of the system components of the weapons detector of theinvention;Figures 4A and 4B are graphical comparisons of the time-domain signatures of a humanbody with and without a firearm;Figure 5 provides the graphical difference of the time-domain signatures in Figures 4A and4B;Figures 6A and 6B are graphical comparisons of the Fourier transform of the time-domainsignatures of a human body with and without a firearm in Figures 4A and 4B;Figure 7 is a top-level block diagram of the invention;?1015202530CA 02265457 l999-03- 10WO 98/12573 PCT/US97/16944-7.Figure 8 is a block diagram of the radar section of the invention;Figure 9 is a second block diagram of the radar section of the invention;Figure 10 is flow diagram of the preferred rangefinding method;Figure 11 is an outline diagram of the preferred radar microwave section of the invention;Figure 12 is a left lower rear perspective exploded view of the preferred embodiment of theinvention;Figure 13 is an right upper front perspective exploded view of Fig. 12 embodiment;Figure 14 is a left lower rear perspective partial section view of the Fig. 12 embodiment;Figure 15 is a right upper front perspective partial section view of Fig. 12 embodiment;Figure 16 is a plot of a test ofthe artificial neural network of the invention;Figure 17 is a chart of the raw data of the test of Figure 16;Figure 18 is a plot showing the spectral difference and normalized difference (frontside)between a man with a weapon under the arm and a man without a weapon;Figure 19 is a plot showing the spectral difference and normalized difference (backside)between a man with a weapon under the arm and a man without a weapon;Figure 20 is a plot of test results employing a 3 GHz horn antenna; andFigure 21 is a plot of trial averages employing a 3 GHz patch antenna vertically polarized.?1015202530CA 02265457 l999-03- 10W0 98/ 12573 PCT/U S97/ 16944-3-DESCRIPTION OF THE PREFERRED EMBODIMENTS(BEST MOQES FOR CARRYING QUT THE mygu I lgmThe weapons detection system of the present invention uses a short-pulse radar approachthat is stepped over a series of frequencies. The system preferably comprises a trigger, pulser,broad band antenna, amplifier and signal processor. The radar first enters the range finder-mode todetermine the distance to the target being surveyed. The range information is used to set the gainof the amplifier so that the output is normalized to range. This normalization permits the waveformreturned from the target to be analyzed independent of range. The use of the range gate eliminatesbackground clutter or scatter from overriding the return from the target. The system uses a lowvoltage pulserto produce a series of microwave pulses transmitted on a compact, broad bandantenna. The reflected waveform is returned from the target through the same antenna. Thereceived signal is range-gated and converted to an IF signal. The signal is amplified in the IFamplifier and is then envelop-detected. The detected signal is then amplified in a video amplifierand widened in time using a sample and hold circuit. The stretched signal is sampled by an analog-to-digital converter. The digitized output of the sampler is processed by the signal processor,preferably by a neural network. The signal processor makes a detection decision based uponwhether the waveform it has received is similar to waveforms it has been trained on when a weaponis present and when there is no weapon present. If the output of the logic processor results in apositive detection decision, indicating a high probability that a weapon is present, a signal, such as ared-colored light emitting diode (LED) is illuminated on the system's display panel and a series ofaudible tones are also engaged (or other appropriate alarm(s)). If the output of the signal processorreaches a negative decision, indicating a high probability that no weapon is present, a signal, suchas a green-colored LED is illuminated on the system's display panel (or other signal(s) given). Ifneither decision is reached, a signal, such as a yellow-colored LED is illuminated on the system'sdisplay panel (or other signal(s)), indicating the operator should use caution and ask the subject toturn (e.g., at a series of 90-degree angles) until a detection decision is achieved.The present invention provides for the detection of concealed weapons on the human body,under clothing or garments, and inside handbags. The invention provides for the discrimination offirearms from other metallic scatterers, such as belt buckles, jewelry, watches, metal buttons, andcoins. In one embodiment, the system design is optimized for the requirements of a hand-held ?1015202530CA 02265457 l999-03- 10W0 98/ 12573 PCT/US97/16944-9-device used by law enforcement and security personnel to detect concealed weapons at rangesbetween approximately three yards and fifty yards. Specifically, the system design includes lowpower circuitry used to produce a microwave pulse that is transmitted on a compact, broad bandantenna. The same antenna receives a backscattered signal from the surveyed target and theclutter environment. A signal processing unit implements a detection algorithm that is used toprocess the backscattered signal to discriminate and identify the signature of a firearm or a largeknife blade against the clutter background of the human body and a random assortment of small,metallic scattering objects such asjewelry, belt buckles, coins. etc.The portable, hand-held embodiment of the invention (see Figures 1-2) is designed to belightweight. lt preferably employs batteries (e.g., 4 C-cell batteries) as its power supply. Theportable embodiment may have both the LED and LCD displays for its information output. A backlitLCD panel can display a wide variety of information, including battery condition and distance totarget as well as when a decision has been made.An alternative embodiment is a door unit using, preferably, phased antenna arrays to surveyindividuals as they approach a door from approximately three to five yards away. if a handgun isdetected, a barrier can activate, such as an electronic lock which is activated to keep the armedindividual from entering an area. Security personnel can also be alerted through a discrete pagingsystem or by an audible or visual or vibrating alarm. A camera (optional) can also be installed totake photos or videos of individuals who are detected carrying concealed weapons.For a clearer understanding of the operation of the portable embodiment of the presentinvention, attention is initially directed to Figure 1. Specifically, the law enforcement officer 11points the concealed weapons detector (CWD) 12 at the suspect 13. The CWD emits microwaveenergy 14 at the suspect. The backscattered signal 15 that contains the information about thesuspect is then received by the CWD. The CWD then notifies the law enforcement officer as to thepresence or absence of a concealed weapon 16 on the body of the suspect.Figures 2A and 2B include a handle-grip 17, trigger 18, transmit and receive antenna 19, anda light-emitting diode display 20, which indicates the presence 21, absence 22, or indeterminate?1015202530CA 02265457 l999-03- 10W0 98/12573 PCT/US97ll6944-10-detection of a weapon 23. The unit is portable. The internal power supply battery may berecharged through an alternating current adapter 24 or off a motor vehicle power system adapter 25.The system includes a transmitter which sends microwave energy toward the suspect to besurveyed. The transmitter output includes the frequencies at which metal parts of a firearm will self-resonate. The microwave energy backscattered by the human body and the objects surrounding itis received by the receiver. The characteristics of the received signal determine whether or not afirearm is present on the suspect. The characteristics of the received signal may include a relativeincrease or decrease in amplitude and shifts in phase as a function of frequency. The systemincludes a processor which determines whether or not the characteristics of the received signal areconsistent with the presence of a firearm.Various transmitter and receiver configurations may be utilized. The transmitter output maybe stepped or swept-chirped in frequency.The transmitter output may be broad band, or it may be swept across the range of possibleresonant frequencies. Similarly, the receiver may be wide band or may be swept across the set ofpossible resonant frequencies. The transmitter and receiver cannot be broad band in the samesystem because this precludes distinguishing among the various resonant frequencies. If bothtransmitter and receiver are being swept, they must be operated in synchronism.Figure 3 is a block diagram of the system components of the preferred embodiment. TheCWD includes a power supply 26 which provides power to the other system components. Thetransmitter includes a pulser 27 that is coupled to a broad band antenna 28. Typically, the outputpulse is approximately 10 nanoseconds wide and has rise and fall times on the order of 1nanosecond. The receiver 29 is preferably a conventional superheterodyne receiver with a verywide IF bandwidth, in the order of 400 MHZ. This is to preserve the pulse rise time. The output ofthe receiver is preferably connected to a signal processor 30.In the preferred embodiment, the output from the range finder 31 is used to normalize theamplitude of the received signal. The range finder may be implemented as an acoustic, optical or?1015202530CA 02265457 l999-03- 10WO 98/12573 PCT/US97/16944-11-microwave subsystem. The output from the range finder may be used to set the gain of the linearamplifier 32 in the receiver. Alternatively. the automatic gain control may be realized by adjustingthe power of the transmitted signal, or by using the signal processor to scale the output of thereceiver.The signal processor may use one or both of two possible methods for the detection of aconcealed weapon. The first method utilizes the specular backscatter from firearms located on thehuman body. In this method. the backscattered signal is higher in amplitude when a firearm ispresent. The second method utilizes the self-resonant scattering from the metal parts of thefirearm.A specific example of the output of the sampler is depicted in Figures 4A and 4B. Figures 4Aand 4B are a graphical comparison of the time-domain signature of a human body with and withoutthe presence of a firearm. The time-domain waveform in Figure 4B depicts a higher magnitudeecho due to the presence of the firearm. Figure 5 is the graphical difference of the time-domainsignatures in Figures 4A and 48.Figures 6A and 6B are a graphical comparison of the fourier transform of the time-domainwaveforms of Figures 4A and 4B. The metallic parts of a firearm resonate at approximately one-half wavelength of the physical dimensions, independent of the orientation to the incidentmicrowave energy. When a firearm is present, the backscattered waveform has higher frequencycontent. A comparison of Figures 6A and 6B show that a successful discrimination of the presenceof firearms can be based upon the presence of the higher frequency content in the backscatteredsignal.The signal processing unit may reach decisions using either or both of the detectionapproaches described above. In the preferred embodiment, a multilayer artificial neural networkperforms the signal processing task. The frequency-domain data from the receiver (which may beprocessed by fast Fourier transform) is presented to the input layer of the artificial neural network.The output layer of the artificial neural network drives the processor I/O circuitry which is connectedto the light emitting diode display. The artificial neural network is trained to distinguish between the?1015202530CA 02265457 l999-03- 10WO 98/12573 PCTIUS97ll6944-12-applied patterns at the input layer and then produces the desired response to the output layer. Thearti?cial neural network may be simulated on a conventional microprocessor subsystem, orimplemented in specialized integrated circuits or hardware accelerators.Referring to the top level block diagram on Figure 7, the central control for the system is inthe main processor/controller. That processor determines the actions required to ful?ll the functionof the device. Upon initiation by the user by pressing the trigger, the processor begins themeasurement process. While initializing, the processor first illuminates all the LED indicators on thedisplay. This serves as a check that the display is operating properly. Shortly after internal checks,the system enters the rangefinder mode. in this mode, the distance to the target is measured and a“range gate" is positioned so that only reflections at that range are measured. Reflections at otherranges are ignored, thereby diminishing returns from other objects, called clutter. After deciding onthe range to the target, a test sequence is initiated which measures the radar returns from a target.Those returns are a function of frequency, which are read as a spectral pattern. That pattern is thencompared to those learned by an artificial neural network processor and a decision is made as towhether or not a weapon is present in the field of interest. The entire process can be completed inless than one second.industrial Applicability:The invention is further illustrated by the following non-limiting examples.Bangefinder. The following is an example of the preferred operation of the presentinvention. Rangefinding accomplished by the method of the present invention (see Figures 8 and 9,which are alternative embodiments). The transmitter and local oscillators are tuned to the center ofthe test band and are normally held there forthe duration of this process. A pulse (e.g., 10nanoseconds) is then transmitted and the transmit/receive (T/R) and range gate switches arecommanded to receive a pulse in the first range bin. Each range bin is one yard. The first rangebin is the minimum distance at which the CWD is to operate (e.g., 4 yards). For the rangefinderfunction each range bin is set to a width (e.g., 2 yards) which corresponds to a receive pulse width(e.g., 12 nanoseconds). The first range bin is set to the distance (e.g., 4 yards), which requires adelay (e.g., 24 nanoseconds) between the transmit pulse and the range gate pulse. Radar range is?1015202530CA 02265457 1999-03-10WO 98/12573 PCT/U S97/ 16944-13-6 nanoseconds per yard. Thus, forthis example, the transmitted pulse is 10 nanoseconds wide andthe T/R/range gate control pulse is 12 nanoseconds wide and is delayed by 24 nanoseconds fromthe transmit pulse.The return is digitized and stored in a memory location. A second pulse (e.g., onemillisecond later) is transmitted but now the delay for the range gate is increased (e.g., by 6nanoseconds), thereby sliding the range gate back one range bin. The return is measured andstored in the next memory location. This process is repeated until all twelve range bins have beenmeasured. This takes approximately 12 milliseconds. The memory locations are checked to see ifat least one has a signal greaterthan a predetermined threshold. If so, the entire process isrepeated four more times so that a total of, preferably, 60 measurements has been made.If, afterthe first pass through all the range bins, there is no signal above the threshold, thetransmitter frequency is decreased by 50 MHz and the first pass is repeated. if there is still nosignal over threshold, the frequency is increased by 100 MHz and the first pass is repeated. Thepurpose of the frequency shifts is to negate the effects of multipath cancellation which is a likelycause for insufficient radar return.After a series of, e.g., five passes has been made (representing 60 measurements and datarecordings), the range bins are polled. The time required for the passes (e.g., five passes) ispreferably less than 100 milliseconds or 0.1 seconds. The polling process determines how manyover threshold hits occurred in each range bin. The closest range bin with at least three hits is thenselected as the range to target and an LED is illuminated to indicate that target lock has beenachieved. The display then indicates the range to the selected target. This serves as a check forthe operator that the desired target is being examined.In this example, the processor then performs the following functions:1. The range gate is reduced in width (e.g., 6 nanoseconds). This limits the range ofmeasurement to one yard. One yard is considered sufficient to contain a person and yet narrowenough to reject clutter in other range bins.?1015202530CA 02265457 l999-03- 10W0 98/12573 PCT/US97/16944-14-2. The IF gain is adjusted for the range by "looking up" the bias required to achieve thedesired gain. Data for the lookup table is determined by calculation and empirical testing during themanufacturing process.3. The range gate delay is set for the selected range bin.4. The transmit and local oscillators are set to their respective starting frequencies.5. The weapon detection process is initiated.The processor then commands the pulse and timing board to initiate a measurement sequence.That sequence consists of:1. Open the transmitter modulator to transmit a pulse (e.g., 10 nanoseconds).2. Count down to the time corresponding to the delay for the selected range gate.3. Switch the T/R switch into the receive mode.4. Simultaneously open the range gate switch.5. Count down to the delay time for the analog to digital converter (A/D) timing.6. Trigger the A/D to digitize the return signal and store the results in memory.7. Command the oscillators to proceed to the next frequency in the test sequence.8. Check and increment a counter.9. If the counter value is less than a predetermined number (eg, 50), wait for a periodof time (e.g., one millisecond) and repeat the above steps 1 through 8.?1015202530CA 02265457 l999-03- 10W0 98/12573 PCT IUS97/1 6944' -15-10. if the counter value is the predetermined number (e.g., 50), reset the oscillators totheir starting frequencies and repeat the entire sequence.11. After the sequence is repeated (e.g., five times), average the results at each of thefrequencies (e.g., 50 frequencies).12. Feed the averaged data to the neural net processor for pattern matching.13. Depending on whether or not a pattern is recognized, and its degree of certainty,illuminate the appropriate LED.14. Keep repeating the measurement process as long as the trigger switch is depressedor otherwise activated.15. Each time the frequency sweep is completed, the new data is placed in a shiftregister so that the newest data replaces the oldest data. This results in a continuous runningaverage for a group of five frequency sweeps.16. The new averaged pattern is fed to the neural net after each frequency sweep forpattern matching.17. At any point where the pattern match decision changes, a new LED is illuminated.Each time a new subject is to be scanned, the trigger or other activation mechanism should bereleased and re-depressed or deactivated and reactivated so that the rangefinder measurement isrepeated, assuring that the new subject will be scanned.Alternative Embodiments.The invention preferably operates by using a short pulse radar as the sensor to safelyexamine a subject. it generates a radar return pattern where the frequency of the interrogationsignal is the independent variable. There are a variety of techniques which can be used to obtain?10152025CA 02265457 l999-03- 10WO 98/12573 PCTIUS97/16944-15-such information. As described above, the preferred technique is to use a stepped frequencymeasurement. That is, the frequency band of interest is divided (e.g., into 50 points) and ameasurement is made at each of the points. The data is then assembled to offer a pattern ofamplitude versus frequency.Other methods that can be employed in the present invention to acquire the sameinformation are:1. Chirged signal. A chirped signal is one where each pulse covers all or a portion ofthe frequency band by continuously changing the frequency of the carrier during each pulse. Sincethe pulses required for this application are very narrow, there are only a small number of rf or carriercycles occurring during the pulse, and thus accurate measurement is critical.2. FM-CW radar. A FM-CW radar technique is commonly employed for radaraltimeters where precise distance measurements are desired. in this type of sensor the signalfrequency is continuously varied in a linear fashion and the frequency change at the time of return ismeasured. A variation of this can be employed in the present invention where two signals aretracked at a frequency separation corresponding to the range to target but the amplitude variationsof the return are sampled and measured.3. Impulse radar. Impulse radar generates a very narrow pulse in the order or tens orsometimes hundreds of picoseconds (e.g., 10“ seconds). Such narrow pulses contain energy overa large portion of the frequency spectrum. The spectral content for a rectangular pulse is a sin(x)/xpattern and the first null is at a frequency corresponding to 1/7. Therefore, if it is desired to havesignificant energy existing at a frequency of 3 GHz, then a pulse width of less than, e.g., 300picoseconds is required. This means that all the data collection must occur in under, e.g., 300picoseconds because the signal only lasts that long. This requires a system that is very expensiveand difficult to realize.?1015202530CA 02265457 l999-03- 10WO 98/12573 PCT/US97/16944-17-4. Subcarrier radar. Subcarrier radar can use a higher frequency main carriermodulated with a microwave subcarrier in the e.g., 3 GHz range. The main carrier can be anywherefrom the IR (e.g., 10 micron wavelength) through an X-ray range for clothing penetration.Decision Making Qriteria. The neural net processor of the invention makes a decision as towhether or not a weapon is present by comparing the pattern of signal returns in the band abovee.g. 2 GHZ to stored patterns. A weapon will tend to resonate in the 2 to 4 GHz frequency band andcreate a stronger return than other smaller objects. The criteria is that a signal exists which isgreater in amplitude than the background but smaller than a calibrated large radar reflector.Outputs ang Indicators. Upon making a decision, the CWD can transmit its information in avery large variety of ways. One of the simplest methods is to illuminate a visual indicator, such asan LED. Equally simple are audible or tactile indicators. However, there will be many situationswhere neither of these signals is appropriate. In those cases, a signal can be transmitted e.g., overthe air, via fiber optics, on electrical conductors, etc. Those signals can be used to activate aremote pager or receiver with e.g., an audible, visual or a mechanical movement. This can be usedto alert selected individuals or to automatically energize a barrier (e.g., locking or other protectivemechanism), or activate a camera to take a close-up photograph of the subject.Different versions of the CWD can be produced to suit a variety of applications. Forexample, the hand held unit allows a police officer to examine a subject at a traffic stop, from a safedistance. Another embodiment is a unit that is fixed or mounted on a wall or above a portal. Thisembodiment is generally set to a continuous scan mode at a fixed range so that all individualsapproaching the portal are screened. in a light traffic area, a passive detector, such as infra-red,can be used to triggerthe CWD when someone approaches it.In this situation, where a pager is activated, the screened individual does not know that he orshe is suspected of having a concealed weapon. A guard who has been alerted by a pager can thenassume a strategic position to prevent the detected individual from conducting any criminalactivities. If an erroneous reading is obtained or if the individual has no intent of committing acriminal act, there is no harm done and no one has been accused of any activity. This type of?1015202530CA 02265457 l999-03- 10W0 98/ 12573 PCT/U S97/ 16944-18-operation is most useful in institutions such as banks, race tracks, jewelry stores, or places wherelarge amounts of cash or valuables are concentrated.in other situations, where an intentional indication is desired, the approached door can belocked. an audible alarm (either recorded or synthesized voice or buzzer) can sound, or a light canflash, etc. This type of response may be most desirable for 24 hour convenience stores, airports,court houses, bars, tourist attractions. etc.Microwave/Radar. Refer to the block diagram, Figure 10, to assist in understanding themicrowave/radar section.Transmitter. The transmission signal is derived from the Tx voltage controlled oscillator(VCO), 01. That signal is fed to a modulator to change the CW waveform into a series of pulses(e.g., 10 nanoseconds). The resulting Tx signal is a pulse train that is tuned over a certain range(e.g. 2.90 to 3.15 Ghz). The pulses occur at a e.g., 1 Khz rate and the frequency is stepped anincrement of e.g., 5 Mhz for each pulse. Each measurement consists of 51 readings, each at a newfrequency (incremented 5 MHz from the previous one). Thus, a complete measurement spans arange of e.g. 250 MHz. The processor averages a group of e.g., 50 measurements to make adecision. The tuning voltage to O1 is generated in the timing circuit and is a staircase waveform.This is adjustable in amplitude, offset and linearity. Each step occurs right after each reading toprovide maximum time for O1 to tune and stabilize at the next frequency.The transmission signal is modulated in switch, S1. The signal from O1 is CW and themodulation signal is generated in the timing circuit. The modulator is followed by a high pass filter,F1, to block the video modulating signal energy that will leak out of the modulator output connectorfrom arriving at the output power amplifier. The pulse (10 nanoseconds) modulated signal is thenamplified to e.g., about +23 dBm and is applied to the transmit/receive (T/R) switch, 82. This switchis normally held in the transmit position and is switched to the receive position at a precisely delayedtime afterthe modulation pulse is applied to 81. This delay is in the range of e.g., 24 to 90nanoseconds. It is held in the receive position for a duration of either e.g., 6 or 12 nanosecondsthereby providing some range gating function. 82 is followed by a second range gate switch, S3,?1015202530CA 02265457 1999-03-10W0 98/12573 PCT/US97/16944-1 9.that operates in unison with S2. 83 may be eliminated if sufficient isolation can be achieved on S2to reduce zero time leakage to an acceptable level. S3 and S2 will pass a signal to the low noiseRF amplifier, A2, for the duration ofthe time that it is desired to receive a radar return.The common port of the T/R switch, S2, is connected to the antenna port. A preselectorfilter, F2, is placed in this path. That filter attenuates spurious signals and harmonics created by thetransmitter oscillator/modulator/amplifier combination and prevents out-of-band extraneous signalsfrom entering the receiver. That filter has a e.g., 1 dB pass band of e.g., 2.85 to 3.25 GHz. Theantenna port is an SMA female connector.Receiver. A high pass filter with a cutoff of e.g., approximately 1.7 GHz is included to blockvideo pulse leakage from the T/R and Range Gate switches from reaching the low noiseamplifier, A2. The return signal is amplified in RF amplifier, A2, that has a gain of e.g., 20 dB. Theamplified signal is fed to a double balanced mixer, M1.The local oscillator for the mixer is derived from a second VCO, 02. This oscillator is offsetfrom 01 by e.g., 700 MHz. That is, it tunes from e.g., 2.20 to 2.45 GHz. It is synchronously tunedwith O1 to maintain an IF output centered at e.g., 700 MHz. 02 is stepped in e.g., 5 MHzincrements along with 01. It's tuning voltage is also generated in the timing circuitry. The IF signalis filtered in a bandpass filter, F4, centered at e.g., 700 MHz and has a bandwidth of e.g., 400 MHz.Its passband is e.g., 500 to 900 MHz. This bandwidth was selected in order to maintain the qualityof the fast rise, narrow pulses required to retain range resolution. The filtered IF signal is thenamplified in a variable gain IF amplifier, A3. The gain control is an analog e.g., 0 to 5 volt signalwith maximum gain occurring at e.g., 0 volts. The gain control is generated by the processor and isderived from the range to target information. That is, as the range gate delay is increased, the IFgain is increased. This provides radar sensitivity time control (STC) and compensates for the factthat a target at longer range will result in a weaker return. The gain of the IF amplifier can be variedfrom e.g., about 15 to 65 dB. A change in return due to a range change of e.g., 4 to 15 meters, for atarget of constant radar cross section, is e.g., 23 dB. The change over e.g., 3 to 15 meters ise.g., 28 dB. Thus, a range of e.g., 50 dB has sufficient margin to accommodate a wide range of?1015202530CA 02265457 1999-03-10W0 98/ 12573 PCT/US97/16944-20-calibrations and not be the limiting factor in achieving the desired range. This margin may bereduced if a cost savings can be realized.The IF pulsed signals are detected in a high speed detector, D1. The current instrumentuses a back diode and is a combination of detector, low pass filter and matching pad. The detectorassembly cuts off at a frequency between e.g., 300 and 500 MHz to attenuate the IF carrier farbelow the pulse envelope, The pulse envelope is then amplified in a video amplifier, A4. The videoamplifier has a gain of e.g., 40 dB to bring the detected video signal which typically has a peakvalue of some tens of millivolts, (e.g., 40-50 mv) up to a level of e.g., 4 or 5 volts. These pulses arethen stretched in an asychronous sample and hold circuit to increase their pulse width from e.g., 10nanoseconds to approximately 50 microseconds. At about 15 microseconds into the pulse, thedigitizer (analog to digital converter) reads the level and produces a digital representation of thesignal amplitude. That output is a coaxial connector to facilitate the use of a shielded cable, therebypreventing extraneous pickup and noise from influencing the measurements.Test Points. Test points are shown and are to be included for monitoring and troubleshooting the circuit after it is integrated into a one-piece assembly. Troubleshooting is nearimpossible once the connectors are eliminated and the components connected via printedtransmission lines. The test points are derived from directional couplers placed at strategic points inthe circuit. The first point, TP1, is placed at the output of 01. This will permit observation of thebase oscillator CW signal, priorto any applied modulation. Such observations will permit easymeasurements of the frequency of each step. the power level and the Tx signal stability. Thesecond test point, TP2, is placed after the Tx output amplifier. This location will permit observationof the modulation timing, the transmitted signal waveshape, output peak power and pulse width.The third test point, TP3, is placed at the output of the T/R switch. Signals will only be observableat this test point when the target is very close to the radar. It will however, serve as an accuratetiming reference for a base line range gate from which other ranges can be calibrated. It will alsopermit observation of all reflected signals from mismatches and close in clutter. The forth test point,TP4 is at the output of the local oscillator. This will permit observation of the output of 02 forchecking linearity and tracking to 01, along with the mixer drive levels. The fifth test point, TP5, isplaced at the output of the IF amplifier. This location will provide information on the conversion loss?1015202530CA 02265457 1999-03-10WO 98/12573 PCT/US97l16944-21-of the mixer and the gain of the IF amplifier to ascertain linear performance of the receiver. A valueof e.g., 15 dB is chosen for the test point couplers to minimize the insertion loss of each device.Mechanical. The outline of the preferred microwave assembly is shown in Figure 11. Theoverall package preferably fit into a footprint of, e.g., 8x8 inches and is less than, e.g., 1 inch high.The antenna port is preferably an SMA female connector and is cabled to the antenna input.However, it is highly desirable to have the antenna blind mate to the RF unit at final assembly. Thehigh speed connectors which input the modulation and T/R signals are preferably coaxial, such asSMCs. The five test ports are preferably SMA female. The low frequency inputs which include theTx and LO tuning controls and the IF gain control are preferably integrated into a multipinconnector. A second multipin connector is used for the power supply dc inputs.gummam. A summary of the preferred specifications described herein is shown in thefollowing tables.Table 1a. Summary of Preferred Operational SpecificationsParameter Specification:.._._..:_....j____.__._.__.___2.90 to 3.25 GHZ Transmission Frequency RangePeak Power Output (to antenna) +20 dBmPulse Width 10 nanoseconds nominalPulse Rise/Fall Times 1 nanosecondPulse Repetition Frequency 1 kHz, capable of 10 kHzSpurious Outputs -30 dBcReceiver Noise Figure 6 dB max3rd Order Intercept +20 dBmReceiver Noise Floor -83 dBmIF Gain Voltage variable from 15 to 60 dBPreselection 2.8 to 3.4 GHZ. 6 section ButterworthIF Frequency and Bandwidth 700 MHz center, 400 MHz bandwidthSizeApprox 8x8x1 inches, exact TBD?101520CA 02265457 l999-03- 10W0 98/ 12573-22-PCT/U S97/ 16944Weight1 lb max.Antenna beam width27 x 27 degreesAntenna frequency range2.80 to 3.30 GHZAntenna gain16 dB/minAntenna sizeApproximately 8x8x.75 inchesTable 1b. inputsRange Gate DelayParameter ‘ Specification 18 to 90 nanosecondsRange Gate Width6 or 12 nanosecondsRange Gate Rise/Fall Times<1.5 nanosecondsModulation and Range Gate Control LevelsTTLTuning Voltage to VCOsapprox 3 volts pk-pk, centered between 3 and 8volts.IF Gain Control 0 to 5 volts dcConnectors Modulator and Range Gate: SMC femaleOscillator Tuning and IF Gain: Multipin, TBSTable 10. OutputsParameterPulse Output to Digitizer SpecificationStretched pulses, SopsecondsOutput Level5 volts peak for -45 dBm input at antenna portwith IF Gain=2O dBOutput Impedance<50QAntenna Port Impedance500, VSWR 2:1 max.?10152025CA 02265457 l999-03- 10W0 98/12573 PCTIUS97/16944-23-Connectors Antenna Port: SMA femalePulse Output: SMC femaleTest Ports: SMA femaleZero Time Leakage Level <5 millivoltsTable 1d. Environmental RequirementsParameter I Specification General Rugged Handling, Field UseOperating Temp Range -20 to +45oCStorage Temp -40 to +70oCShock 50 g, 11 millisec half sineVibration 2 to 2000 Hz, .05" displacementHumidity 0 to 98% RHAltitude O to 15K feetThe preferred casing for the portable embodiment of the invention is illustrated inFigures 12-15. Components comprise transmitter/receiver, RF module 108, antenna 107, displaycontrol board 111, CPU and logic control board 109, display panel 112, screws 130,131,132,133, I/Oconnector 110, trigger switch assembly 125, trigger switch, switch frames, interface connector 120,display window 113, grip vibrator 116, "C" batteries 119, interface connector cover 105, powersupply cover 104, display bezel 103, case 102, rear grip 118. front bezel 101, indicator LED 115,and ceramic buzzer 114.This invention is particularly useful for detecting firearms and other weaponry, including butnot limited to the following: .22 caliber revolver, .38 caliber revolver, .357 caliber revolver, .22caliber sawed-off rifle, .380 automatic, .9 mm automatic, .22 caliber automatic, and pipe bombs orother cylinders. The invention is capable of discerning weaponry from clutter objects, including butnot limited to: belt buckles, bracelets, wristwatches, tape recorders, soft drink cans, coins,?1015202530CA 02265457 l999-03- 10WO 98112573 PCT/US97/16944-24-calculations, lipstick holders, campaign buttons, cellular telephones, key rings and keys. Theinvention is also capable of detecting the weaponry under various clothing or inside the following(including but not limited to): purses, belts (front. side back), shoulder holsters, ankle holster underslacks, briefcase. coats (including leather, overcoats, wind breakers, and polo shirts).The following examples are tests of the present invention.Figure 16 is a plot of an artificial neural net test, and Figure 17 shows the raw data for thetest. It shows five weapons being tested (.9mm automatic, .380 automatic, .38 revolver, a .357revolver and a .22 caliber starter pistol ) from data taken from a system according to the presentinvention. Each of the 19 plots was averaged over 50 points over 250 MHz of bandwidth at 2.2 Ghz.On the left of the plot are numbers ranging from 0 to 1. When a weapon is present, 1 is the targetnumber. When a weapon is not present, 0 is the target number. For actual operation, a result ofgreaterthan .8 would cause the weapons detector to turn on a red light to indicate a weapon ispresent. A result of less than .2 would cause the weapons detector to turn on a green light indicatingno weapon is present. Any result between .2 and .8 would cause the yellow light to the illuminated,indicating that a second test should be taken as the suspect is turning (allowing better resolution ona potential weapon). The white bar represents the target output. The black bar represents the actual’output. The first three tests involved a .9mm handgun. The first test was for a .9mm under a man'sarm. The second test was a .9mm placed barrel down in a holster along the man's side. The suspecthad his hands up. The third test was a man with a .9mm placed in a belt in the man's back. Thefourth test was a .380 caliber automatic in the man's front belt. The fifth test was a .380 automatichandgun on the side with the suspect's arms up. The sixth test was a .380 in the belt at the man'sback. The seventh test was a .38 caliber revolver located in the belt at the suspect's side with hisarms down. The eight test was a .38 caliber revolved placed in the belt at the man's back. The ninthtest was a .9mm automatic placed in a suspect's front belt in a cluttered environment (a largewestern-style beltbuckle, a large key ring in the suspect's pocket, a calculator on the suspect's belt,a calculator in the front shirt pocket, and a large number of coins in the suspect's pocket). The tenthtest was a .9mm automatic on the man's left side with his arms down in a similar clutteredenvironment. The eleventh test was a .9mm automatic located in the back belt on a cluttered man.The twelfth test was a .357 caliber handgun located in a holster on the suspect's side with his arms ?1015202530CA 02265457 l999-03- 10WO 98/12573 PCT /U S97/ 16944-25-up and with the same type of clutter environment. The thirteenth test was a .357 caliber handgun inthe man's belt at the back with the same type of clutter environment. The fourteenth test was a .38caliber handgun in a holster on the side in the same type of clutter environment. The fifteenth testwas a .38 caliber revolver under a man's arm with the same type of clutter environment. Thesixteenth test was a .38 caliber revolver stuck in a belt on a man's side in a cluttered environment.The seventeenth test was a .38 caliber revolved located in the belt on the suspect's back in acluttered environment. The eighteenth test was a .22 caliber starter pistol (sp in the log) located inthe front belt of the suspect in a cluttered environment. The nineteenth test was a starter pistol in theback belt in a cluttered environment. In all cases, the trained neural net was able to successfullydetect the presence of the handgun.Figure 18 shows the spectral difference and the normalized difference between a man with aweapon under his arm and the same man without a weapon. The solid lines represent the differencein waveform when a weapon is present and the dotted line shows the normalized difference when aweapon is present. Of significance is the higher resolution in the 2 to 3 GHz range. This representsthe standard phenomena we discovered from the ringing of the return from the weapon atapproximately 3 GHz. This is repeated consistently no matter what weapon is tested. Note also the.002 volt difference in waveforms at 2 GHz. This is the frontside of the weapon, which in this casewas a .9mm automatic.Figure 19 again this shows the spectral difference and the normalized difference between aman with a weapon and a man without a weapon with the gun located in the belt on the back oftheman. The solid lines represent the difference when a weapon is present and the dotted line showsthe normalized difference when a weapon is present. At 1.5 GHz the difference is approximately.0025 volts and at 3 GHz it is about .0015 volts. The waveform here shows the same phenomena ofan increased resolution at both 1.5 GHz and at 3 GHz. These differences are easily discernible witha neural net pattern recognition program.The tests of Figure 21 used a .38 caliber revolver located on the side of a man. The dottedlines show the return from a man without a weapon. The heavy dotted line is the average of 50calculations of a man without a weapon. The solid lines are the waveforms from a man with a?10CA 02265457 2005-04-22-25-weapon. The heaviest solid line (located right in the middle of all the solid lines) is the averageof all the calculations of a man with a weapon. The difference in the waveforms is an average ofmore than 1.475v.Figure 21 shows the averages of 50 calculations with a man (dotted line) and a man witha gun (solid line). The gun was a .9mm automatic. Again the difference is signi?cant (about .5 to1.0v for the majority of the calculations).Although the invention has been described in detail with particular reference to thesepreferred embodiments, other embodiments can achieve the same results. Variations andmodifications of the present invention will be obvious to those skilled in the art and it is intendedto cover in the appended claims all such modifications and equivallents.

Claims (33)

CLAIMS:

1. A weapon detection system comprising:
a transmitter for transmitting signals at a plurality of different frequencies;
a receiver for receiving backscattered signals at each of the transmitted frequencies; and processing means for comparing the received signals to a stored signature representative of backscattered signals from a weapon and for determining the presence of a weapon when the received signals substantially match the stored signature.

2. The weapon detection system of claim 1, wherein the processing means is operable to compare the received signals to a plurality of stored signatures representative of backscattered signals from a plurality of different weapons and to determine the presence of a weapon when the received signals substantially match one of the stored signatures.

3. The weapon detection system of claim 1 or 2, wherein the transmitter is operable to transmit signals at frequencies including frequencies at which metal parts of a firearm self-resonate.

4. The weapon detection system of any one of claims 1 to 3, wherein the transmitter is operable to transmit a plurality of frequencies within a 250 MHz band.

5. The weapon detection system of any one of claims 1 to 4, wherein the transmitter is operable to transmit a plurality of frequencies between 1 and 10 GHz.

6. The weapon detection system of any one of claims 1 to 5, wherein the transmitter is operable to transmit low intensity short pulse radar signals.

7. The weapon detection system of any one of claims 1 to 6, wherein the processing means is operable to detect an amplitude change in the presence of a weapon and to compare said amplitude change to one or more stored signatures of amplitude change representative of one or more weapons.

8. The weapon detection system of claim 7, wherein the processing means is operable to detect specular backscatter in the received signals.

9. The weapon detection system of any one of claims 1 to 8, wherein the processing means is operable to detect a phase shift in the received signals in the presence of a weapon and to compare said phase shift to one or more stored signatures of phase shift representative of one or more weapons.

10. The weapon detection system of claim 9, wherein the processing means is operable to detect self-resonant scattering from metal parts of a weapon in the received signals.

11. The weapon detection system of any one of claims 1 to 10, wherein the processing means is operable to normalize received signals for distance.

12. The weapon detection system of any one of claims 1 to 11, wherein the receiver is operable to receive signals only after a time delay following transmission by the transmitter.

13. The weapon detection system of any one of claims 1 to 12, wherein the processing means is operable to select signals representative of a predetermined distance range from the transmitter.

14. The weapon detection system of any one of claims 1 to 13, wherein the transmitter is operable to transmit a signal varying continuously in frequency.

15. The weapon detection system of any one of claims 1 to 13, wherein the transmitter is operable to transmit 50 signals of different frequency.

16. The weapon detection system of any one of claims 1 to 15, wherein the processing means is operable to obtain an average of the received signals and to compare the average to one or more stored signatures representative of backscattered signals from one or more weapons.

17. The weapon detection system of any one of claims 1 to 16, wherein the detection system has at least one characteristic selected from a list including:
hand held;
portable; and battery powered.

18. A method of detecting a weapon, comprising the steps of:
transmitting signals at a plurality of different frequencies;
receiving backscattered signals at each of the transmitted frequencies;
comparing the received signals to a stored signature representative of backscattered signals from a weapon; and determining the presence of a weapon when the received signals substantially match the stored signature.

19. The method of claim 18, wherein the comparing step compares the received signals to a plurality of stored signatures representative of backscattered signals from a plurality of different weapons; the determining step determines the presence of a weapon when the received signals substantially match one of the stored signatures.

20. The method of claim 18 or 19, wherein the signals are transmitted at frequencies including frequencies at which metal parts of a firearm self-resonate.

21. The method of any one of claims 18 to 20, wherein the signals are transmitted at a plurality of frequencies within a 250 MHz band.

22. The method of any one of claims 18 to 21, wherein the signals are transmitted at a plurality of frequencies between 1 and 10 GHz.

23. The method of any one of claims 18 to 22, wherein the signals are transmitted as low intensity short pulse radar signals.

24. The method of any one of claims 18 to 23, including the step of detecting an amplitude change in the received signals in the presence of a weapon and the comparing step compares said amplitude change to one or more stored signatures of amplitude change representative of one or more weapons.

25. The method of claim 24, wherein the detecting step detects specular backscatter in the received signals.

26. The method of any one of claims 18 to 25, including the step of detecting a phase shift in the received signals in the presence of a weapon and the comparing step compares said phase shift to one or more stored signatures of phase shift representative of one or more weapons.

27. The method of claim 26, wherein the detecting step detects self-resonant scattering from metal parts of a weapon in the received signals.

28. The method of any one of claims 18 to 27, including the step of normalising received signals for distance.

29. The method of any one of claims 18 to 28, including the step of receiving signals only after a time delay following transmission.

30. The method of any one of claims 18 to 29, including the step of selecting signals representative of a predetermined distance range.

31. The method of any one of claims 18 to 30, wherein a signal varying continuously in frequency is transmitted.

32. The method of any one of claims 18 to 30, wherein 50 signals of different frequency are transmitted.

33. The method of any one of claims 18 to 32, including the steps of:
obtaining an average of the received signals; and comparing the average to one or more stored signatures representative of backscattered signals from one or more weapons.