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l_Derive inverse transform 490 Transform Determine
Capture reference weights and Sum and
reference image to phase shifts transform to Re Sample Image from frequency for frequency time domain and upload
Retrieve Transmit Receive P
transform scene 7 Returns
1 2 MIRROR IMAGE TARGET DETECTION AND FIG. 4 is a flow chart of a process for detecting targets. RECOGNITION FIG. 5 is a flow chart of a process for detecting targets. NOTICE OF COPYRIGHTS AND TRADE DRESS DETAiLED DESCRiPTiON
A portion of the disclosure of this patent document contains material which is subject to copyright protection. This patent document may show and/ or describe matter which is or may become trade dress of the owner. The copyright and trade dress owner has no objection to the facsimile reproduction by anyone of the patent disclosure as it appears in the Patent and Trademark Oflice patent files or records, but otherwise reserves all copyright and trade dress rights whatsoever.
This disclosure relates to detecting and identifying targets using a radar system.
2. Description of the Related Art
Radar systems typically transmit an RF signal and receive a portion of the RF signal retumed or reflected from targets and other objects. While radars may transmit a wide variety of signals, a common fonn of radar transmits a narrow beam of RF energy, typically within the microwave or millimeter wave portions of the electromagnetic spectrum. A target is typically detected by monitoring the amplitude of a return signal reflected or scattered from the target. Some radar systems detennine the velocity of a target from the frequency difference between the transmitted RF signal and the return signal due to the Doppler effect. Other radar systems transmit a pulsed RF signal and determine the distance to a target from the elapsed time between a transmitted pulse and a retumed pulse.
The retum signal received from a typical target, such as a weapon or a vehicle, may differ substantially, in both the time domain and the frequency domain, from the transmitted RF signal. Some conventional radar systems attempt to recognize or identify the type of a target from the time-frequency characteristics of the returned pulses. A typical radar system with automatic target recognition converts the return signal to digital form and then extracts time-frequency infonnation. For example, the frequency spectrum, as a function of time, of the returned pulses may be calculated using a windowed Fourier transfonn or other computational technique. The time-frequency characteristics of the return signal may then be matched with the time-frequency characteristics of a plurality of known objects. Numerical comparison, vector distance calculation, correlation, and/or other analytical techniques may be used to determine a degree of similarity between the extracted time-frequency information and the time-frequency characteristics of the known objects. A target may be considered to be identified if the degree of similarity with a known object exceeds a predetennined threshold. Such conventional automatic target recognition requires substantial and costly processing capability in the radar receiver.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a system for identifying targets.
FIG. 2A is a representation of an exemplary radar signal in the time domain.
FIG. 2B is a representation of the magnitude of an exemplary radar signal in the frequency domain.
FIG. 3 is a block diagram of a radar system for identifying targets.
Description of Apparatus
Referring now to FIG. 1, a system for identifying targets 100 using mirror image detection and recognition may include a radar system 1 10. The radar system 1 1 0 may include a transmitter 120 to transmit a radio-frequency signal 122 and a receiver 130 to receive a retum signal 132 scattered, or reflected, from a scene which may contain a target 105. The transmitter 120 and receiver 130 may be coupled to separate antemras or a common anterma, which is not shown in FIG. 1.
The transmitted radio frequency signal 122 may be modulated by an inverse transform data set 155 from an inverse transfonn library 150. The function of the inverse transfonn data set may be understood by briefly considering the radar signal waveforms shown in FIG. 2.
FIG. 2A shows a pulsed radio-frequency wavefonn which may be representative of the wavefonn transmitted by a conventional radar system. The pulsed radio-frequency waveform may be comprised of periodic pulses of RF energy having a fundamental frequency fo. The fundamental frequency fo may be, for example, in the microwave or millimeter wave portion of the electromagnetic spectrum. Each pulse may have a pulse width A and the period between the start of successive pulses may be T. The pulses may also be defined by a pulsed repetition rate (PRF) which is the inverse of the period T.
FIG. 2B shows the spectral content, or spectrum, of the pulsed radio-frequency wavefonn of FIG. 2A, which may be determined by taking a Fourier transform of the wavefonn shown in FIG. 2A. The spectrum of the pulsed radio-frequency wavefonn may have a complex structure consisting of a plurality of narrow frequency components such as frequency component 230 bounded by an envelope including a primary “lobe” 210 and a plurality of “side lobes” 220. The shape of the lobes 210, 220 and the spacing of the frequency components may be determined by the fundamental frequency fo, the pulse width A and the period T of the pulsed radio -frequency waveform.
FIG. 2A and FIG. 2B illustrate a basic pulsed radar waveform. More complex wavefonns may be used. For example, the amplitude, phase, and/or frequency of the signal may be varied during each pulse.
In a conventional radar system, the transmitter may transmit a pulsed radio-frequency signal, such as that shown in FIG. 2A, and the receiver may receive a return signal scattered, or reflected, from a target. Each of the frequency components of the transmitted signal, as shown in FIG. 2B, has a different wavelength. Some of the frequency components of the transmitted signal may be strongly reflected due to resonances within the target. For example, a target including a conductive element with a length L may strongly reflect a signal having a wavelength of 4L due to resonance of the conductive element. Reflections from resonant elements within a target may be relatively independent of the aspect angle at which the radar signal impinges upon the target.
In addition, each of the frequency components of the transmitted signal may reflect from multiple features of the target. Multiple reflections of each frequency component will sum coherently, such that some frequency components of the received signal may have a different relative amplitude and phase compared to the corresponding component of the transmitted signal. The coherent sum of multiple reflected signal
components may be highly dependent on aspect angle at which the radar signal impinges upon the target.
Thus a target may effectively transfonn a transmitted pulsed radio-frequency signal into a retum signal that contains infonnation indicative of both the structure and orientation of the target.
The relationship between the transmitted and returned signals may be described as
where P(t) is a transmitted pulsed radio-frequency signal, R(t) is a received retum signal, and The is a function describing the transfonn perfonned as the transmitted signal is reflected from the target. The transform function Tk,@ may be unique to a specific target k and a specific aspect angle G. The transform function Tk,@ may not be definable by a closed mathematical expression, but may be detennined from reference images for the target k at aspect angle G.
An inverse transform, or mirror-image, signal T‘1k,@(t) is defined such that the following relationship holds:
Thus an inverse transfonn signal is a time-varying waveform that, when transmitted from a radar system, is transformed upon reflection from a specific target at a specific aspect angle into a retum signal that is similar to a pulse wavefonn. More specifically, an inverse transform signal is a time-varying wavefonn that, when transmitted from a radar system and reflected from a specific target at a specific aspect angle, forms a retum signal that is an autocorrelation function with a single detectable correlation peak.
Referring back to FIG. 1, each inverse transform data set stored in the inverse transform library 150 may be adapted to modulate a carrier wave signal to produce an associated inverse transfonn signal. The inverse transform library may store a plurality of inverse transform data sets representing a multiplicity of reference targets at various aspect angles. For example, a radar system intended for detecting weapons concealed upon a person might store a total of 1800 inverse transfonn data sets representing 50 different potential targets, each at 36 aspect angles. The transmitter 120 may transmit a signal modulated by each of the inverse transfonn data sets in sequence. To improve the performance of the receiver 130 and detector 140, the transmitter 120 may transmit the signal modulated by each of the inverse transfonn data sets multiple times, for example 10 times or 20 times, in succession, allowing the receiver 130 and/or the detector 140 to average the multiple return signals for improved signal -to-noise ratio. For further example, the 1800 different inverse transfonn signals may each be transmitted 20 times in succession for a total of 36,000 transmitted pulses for each cycle through the content of the transfonn library 150. In this example, a transmitter having a PRF of 36 kHz may cycle through the content of the transfonn library in 1 second. Another transmitter with a higher PRF of 100 kHz may cycle through the content of transfonn library nearly 3 times per second.
The number of inverse transfonn data sets may be optimized by applying known analytical techniques, such as disclosed in U.S. Pat. No. 5,947,413 for selecting maximum average correlation height filters, to select transfonn data sets particularly suited to distinguishing between a plurality of different targets.
The use of polarization on transmit and receive may enhance the capability of the system for identifying targets 100 to identify targets and may reduce the number of false recognitions. Thus the inverse transform library 150 may include inverse transfonn data intended to be transmitted in a
specific polarization state, and the transmitter 120 and/or receiver 130 may include means (not shown in FIG. 1) to select the polarization state of the transmitted and received signals.
The inverse transform library 150 may store a large plurality of inverse transfonn data sets, each representing a specific reference target at a specific aspect angle. The number of reference targets may be more or less than 50, and the number of aspect angles per reference targets may be more or less than 36. The number of target types, the number of aspect angles per target type, and the radar frequency, pulse width, and PRF may be tailored to the application of the radar system. For example, a radar system to detect concealed personal weapons at close range may need sub-second response and thus may limit the number of inverse transform data sets or require a high PRF transmitter. Conversely, a radar system for detecting vehicles at a long distance may allow several seconds for target recognition and thus accommodate more inverse transform data sets.
The radar frequency may be selected such that the wavelength of the transmitted signal is comparable to the feature size of the targets of interest. For example, a radar system to detect concealed personal weapons may transmit a signal in the Ka band (approximately ll mm wavelength) or W band (approximately 3 mm wavelength) portions of the electromagnetic spectrum. Conversely, a radar system for detecting vehicles may transmit a lower frequency, longer wavelength signal which may be, for example, in the X band portion of the electromagnetic spectrum (approximately 3 cm wavelength).
The inverse transfonn library 150 may be organized as a tiered or tree structure. For example, the radar system 110 may first transmit a first plurality of inverse transfonn signals based on a first group of higher level inverse transform data sets. The signals retumed in response to the first plurality of transmitted signals may be used to select a second group of one or more lower-level inverse transfonn data sets. The radar system 110 may then transmit a second plurality of inverse transfonn signals based the second group of lower level inverse transform data sets. The final decision on the presence and/ or type of target may be based on the signal retumed from the second plurality of transmitted signals. For further example, the higher level inverse transform data sets may represent targets at coarsely-spaced aspect angles, such as 90-degree intervals, and the lower level inverse transfonn data sets may represent the targets at aspect angles at finer angular intervals. The inverse transfonn library may be organized in a structure having more than two tiers.
Each inverse transform data set stored in the inverse transform library 150 may be detennined from a reference image of a known reference target at a known aspect angle. A reference image library 160 may include reference images for all of the anticipated targets at a plurality of aspect angles. Each reference image in the reference image library may be essentially a retum signal received from the reference target when the reference target is illuminated by a known transmitted pulse signal at a known aspect angle under controlled experimental conditions. The controlled conditions may ensure, for example, that the reference image is free of clutter and noise.
In the case of relatively simple target objects, a reference image may also be obtained through simulation of the reference target object and the transmitted pulse signal. For example, a numerical model of the reference target may be created and a simulation technique such a finite difference time domain analysis may be used to compute a reference image signal that would be reflected when the target is illuminated by a known pulse signal.
Each inverse transfonn data set may be derived from the associated reference image by a transform extractor 165. The transfonn extractor 165 may, for example, transfonn the reference image into the frequency domain. The transfonn extractor 165 may then compare the amplitude and phase of the frequency components of the transformed reference image with the frequency components of the transmitted pulse signal and calculate weighting coeflicients and phase shifts for some or all of the frequency components of the transmitted pulse signal. The weighted/phase-shifted frequency components may then be summed and transformed back into the time domain to provide the inverse transfonn signal. Other techniques, such as known techniques for developing target recognition filters for conventional radar systems in which the filters are applied to analyze a retum signal, may be adapted for use in the transfonn extractor 165 to derive inverse transform data sets for modulating the transmitted signal in the system 100.
The reference image library 160 and the transfonn extractor 165 may be separate from or a portion of the radar system 110. The reference image library 160 may be populated with reference images off-line to the radar system 110, and the inverse transform data sets extracted by the transfonn extractor 165 may be up-loaded into the transform library 150 within the radar system 110.
The detector 140 may receive a received signal 135 from the receiver 130. The detector 140 may determine if a target has been detected. The detector 140 may average or otherwise combine the received signal 135 for multiple transmission of the same inverse transform signal. The detector 140 may consider the amplitude, pulse width, and other characteristics of the received signal 140 or the averaged received signals to determine if a target corresponding to the reference target corresponding to the inverse transfonn signal being transmitted has been detected within the scene. In this description, the term “detected” has the meaning of “judged to be sufficiently the same according to at least one detection criteria”. The detector 140 may use one or more fixed detection criteria or one or more adaptive detection criteria to detennine the similarity between the target in the scene and the reference target. For example the detector 140 may be a constant false alann rate detector or other adaptive detector.
The output 145 of the detector 140 may be an absolute (yes/ no) target detection indicating that a target that is at least similar to the reference target corresponding to a particular inverse transform data set has or has not been detected within the scene. The output 145 of the detector 140 may be a quantitative value, or score, indicating how similar a target within the scene is to the reference target corresponding to an inverse transform data set that is currently being transmitted rather than an ab solute detection. The detector 140 may accumulate scores while a plurality of inverse transform waveforms are transmitted and then provide at output in the fonn of a ranked list of reference targets in order of similarity to a potential target within the scene.
The output 145 of the detector 140 may be used by the controller 170 to select which inverse transfonn data set or data sets will be transmitted. The output 145 of the detector 140 may also be displayed or otherwise communicated to an operator.
FIG. 3 is a simplified block diagram of an exemplary radar system 310 which may be suitable for use as the radar system 110. An inverse transform library 350 stores, in digital form, a plurality of inverse transform data sets. The inverse transform library may be one or more high speed static random access memory devices, read-only memory devices, or other memory devices. Each inverse transform data set may be read
from the inverse transfonn library 350 in response to address and control signals provided by a controller 370. A digitalto-analo g (D/A) converter 3 12 may convert the data read from the inverse transfonn library into an analog signal. The D/A converter 312 may operate at a high conversion rate, such as 1 GHz or 4 GHz or some other conversion rate. Each inverse transfonn data set may include a plurality of data values representing the amplitude of an inverse transform signal at regular time increments corresponding to the conversion rate of the D/A converter 312. The output of the D/A converter may be an analog signal representing the amplitude envelope of an inverse transform signal.
A first mixer 314 may multiply an RF carrier signal 323 from a local oscillator 322 and the output signal from the D/A converter 312 to fonn a pulsed radio frequency signal 315. The pulsed radio frequency signal 315 may be amplified by a transmit amplifier 316 and coupled to an antenna 318 through a duplexer 324. The duplexer 324 may be a circulator, a transmit/receive switch, or other circuitry to isolate the transmit signal 317 from the receive signal 325.
The return signal reflected from a target 305 may be received at the antenna 318 and coupled to a receive amplifier 326 via the duplexer 324. A second mixer 328 may multiply the amplified received signal 327 and the signal 323 from the local oscillator 322. The output 329 of the second mixer 328 may be applied to a detector 340 that detennines if a target has been detected. The detector 340 may consider the amplitude, pulse width, and other characteristics of the signal 329 to determine if a target is present. The detector 340 may be a constant false alarm rate detector or other adaptive detector circuit. The output 345 of the detector 340 may be an absolute detection, a quantitative value, a score, or a ranked list.
The output 345 of the detector 340 may be used by the controller 370 to select which inverse transfonn data set or data sets will be transmitted. The output 345 of the detector 340 may also be displayed or otherwise communicated to an operator.
Description of Processes
FIG. 4 is a flow chart of an exemplary process 400 for detecting targets using a radar system. At 410, a first inverse transfonn data set may be retrieved from an inverse transfonn library. The first inverse transform data set may correspond to a first specific target viewed at a first aspect angle. The first inverse transfonn data set may be selected from a predetermined list of inverse transform data sets, may be selected randomly from a plurality of inverse transform data sets, or may be selected in some other manner. At 415, the first inverse transfonn data set may be used to modulate a wavefonn transmitted from the radar system. At 420, the radar system may receive a return signal reflected from a scene including at least one potential target. The process 400 may loop from 425 back to 415 such that the waveform modulated by the first inverse transform data set may be transmitted a predetermined number of times. At 430, the return signals from the repeated transmissions of the wavefonn modulated by the first inverse transfonn data set may be average or otherwise combined and evaluated. The output from 430 may be, for example, a quantitative value or score indicating the similarly between a target in the scene and the first target viewed at the first aspect angle.
At 435, a determination may be made if the inverse transform library 450 contains more inverse transform data sets to be transmitted. The process 400 may repeat from 410 through 435, with a different inverse transform data set, corresponding to a different target and/ or a different aspect angle, selected at 430 during each repetition. The process 400 may repeat from 410 through 435 until all of the inverse transfonn