GB2421386A - Dual circular polarization radar - Google Patents

Abstract

An area of interest is scanned with a left-handed circular polarization radar signal (102), LR and LL return signals are received from the area of interest. The combining of the LR and LL return signals are processed to create cartesian LR and LL images of different colours, which are superimposed to create a multi-color image. Alternately, at least one polarimetric signature value may be determined base on the LR and LL return signals in an LR/LL radar cross-section space. In further embodiments, the combined LR and LL return signals may be processed to reduce a false alarm rate. The system may be installed in a vehicle such as a missile, and may be used to control it.

Description

METHODS AND SYSTEMS FOR DUAL CIRCULAR POLARIZATION RADAR

DETECTION

FIELD OF THE INVENTION

This invention relates to radar detection, and, more specifically, to methods and systems for dual circular polarization radar detection of targets.

BACKGROUND OF THE INVENTION

Radar can be used to detect ground mobile targets from guided missiles. Unfortunately, current radar systems and methods suffer from certain disadvantages. For example, some current radar systems gather insufficient information to detect targets in realtime with manageable false alarm rates. Other systems, such as Synthetic Aperture Radar (SAR) systems, are costly. Other sensor types, including infrared and visual systems, lack the all-weather capability and area coverage rates needed for wide-area search. Without the ability to automatically detect ground mobile targets on board in real-time, missiles must fly to Global Positioning System (GPS) coordinates and hope that the target has not moved, and that GPS has not been jammed.Therefore, a need exists for target detection systems that allow a missile to detect ground mobile targets better and in real-time.

SUMMARY OF THE INVENTION

The present invention is directed to methods and systems for dual circular polarization radar detection of targets. Embodiments of methods and systems in accordance with the present invention may advantageously detect ground mobile targets in real-time onboard a sensing platform (e.g. a missile) using current processor technology.

In one embodiment, a method of detecting a target includes scanning an area of interest with a dual circular polarization radar signal, receiving an LR (left circular transmit/right circular receive) return signal from the area of interest, receiving an LL (left circular transmit/left circular receive) return signal from the area of interest, combining the LR and LL return signals, and analyzing the combined LR and LL return signals to detect the target. The combining of the LR and LL return signals may include assigning first and second color scales to the LR and LL return signals, and superimposing the first and second color scales to create a multi-color image. Alternately, the combining of the LR and LL return signals may include determining at least one polarimetnc signature value based on the LR and LL return signals in an LR/LL radar cross-section space.In further embodiments, the combined LR and LL return signals may be processed to reduce a false alarm rate.

BRIEF DESCRIPTION OF THE DRAWINGS

The file of this patent contains at least one drawing executed in color. Copies of this patent with color drawing(s) will be provided by the Patent and Trademark Office upon request and payment of the necessary fee. Preferred and alternate embodiments of the present invention are described in detail below with reference to the following drawings.

FIGURE 1 is a flowchart of a method for detecting targets using dual circular polarization radar in accordance with an embodiment of the invention; [0007] FIGURE 2 is an LR radar cross-section image of an area of interest that contains a target signature; [0008] FIGURE 3 is an LL radar cross-section image of the area of interest of FIGURE 2; [0009] FIGURE 4 is a combination of the LR/LL radar cross-section images of FIGURES 2 and 3, respectively; [0010] FIGURE 5 is a flowchart of a method for detecting targets using dual circular polarization radar in accordance with an alternate embodiment of the invention; [0011] FIGURE 6 is a graph of a radar cross-section image plotted in LR/LL crosssectional space in accordance with another embodiment of the invention; [0012] FIGURE 7 is an initial detection image in accordance with a further embodiment of the invention;

FIGURE 8 is a final detection image in accordance with yet another embodiment of the invention; [0014] FIGURE 9 shows the final detection image of FIGURE 8 overlaid on the combined LR/LL image of FIGURE 4; [0015] FIGURE 10 illustrates an aircraft including a radar system in accordance with another embodiment of the present invention; and [0016] FIGURE 11 is a schematic view of a radar system in accordance with yet another embodiment of the invention.

DETAILED DESCRIPTION

The present invention relates to methods and systems for dual circular polarization radar detection of targets. Many specific details of certain embodiments of the invention are set forth in the following description and in FIGURES 1-11. One skilled in the art, however, will understand that the present invention may have additional embodiments, or that the present invention may be practiced without several of the details described in the following description.

Embodiments of methods and systems to automatically detect targets using dual polarization real beam data, including dual circular polarization SAR data, are disclosed. The underlying algorithms are simple enough to operate in real-time onboard a sensing platform using current processor technology. More specifically, the systems and methods preferably may be used to detect ground mobile targets (e.g. military vehicles) using sensors and data-processing capabilities on board a missile without the need for target position information from extraneous sources.

In brief, a dual circular polarization radar is a radar that transmits one circular polarization state (either left-handed circular (L) or right-hand circular (R)) and simultaneously receives both circular polarization states. For example, consider a dual circular polarization radar that transmits left-hand circular polarization and simultaneously receives left and right-hand circular polarizations. This radar gathers two channels of information. One channel is LR radar cross-section (left circular transmit/right circular receive) and the other is LL radar cross-section (left circular transmit/left circular receive). Thus, a dual circular polarization radar generates two radar cross-section images for each region scanned, one for LR radar cross-section and one for LL radar cross-section.

FIGURE 1 is a flowchart of a method 100 for detecting targets using a dual circular polarization radar in accordance with an embodiment of the invention. In this embodiment, the method 100 includes scanning an area of interest with the dual circular polarization radar at a block 102. At a block 104, the LR radar range-angle information is mapped onto a Cartesian grid. The LL radar range-angle information is mapped onto a Cartesian grid at a block 108. FIGURES 2 and 3 show LR and LL radar cross-section images 200, 300, respectively, of the area of interest, which contains a target signature. The two images 200, 300 correspond to the same area of interest scanned by the radar but show differences in brightness. These differences can be used to extract targets from the background clutter.

In one embodiment, the LR and LL radar cross-section images 200, 300 (FIGURES 2 and 3) may be displayed as color images. For example, in one particular embodiment, the LR radar cross-section image (FIGURE 2) is displayed as shades of green, and the corresponding LL radar cross-section image (FIGURE 3) is displayed as shades of red.

Comparison of the images 200, 300 shows that some objects in the area of interest may preferentially return LR or LL polarizations. It may, however, be difficult to track subtle changes by visually comparing these two images 200, 300. A more convenient way of displaying the dual circular polarization imagery is to display both polarizations simultaneously in the same image. Referring again to FIGURE 1, the method 100 includes displaying both LR and LL radar cross-section images simultaneously at a block 110. In one embodiment, this is accomplished by using the LR image 200 as the green channel of an RGB image and using the LL image 300 as the red channel. The result of combining the two images 200, 300 into one dual circular polarization image 400 is shown in FIGURE 4.At a block 112, one or more target locations are then determined more readily from the combined LR and LL image 400.

When the LR image 200 is used as the green channel of an RGB image and the LL image 300 is used as the red channel, in the resulting combined image 400 (FIGURE 4), red regions indicate that the LL radar cross-section dominates, green regions indicate that the LR radar cross-section dominates, and yellow regions indicate that the LL and LR returns are balanced. In this example, dual circular polarization RGB images 400 tend to have a greenish tint because LR radar cross-section tends to dominate natural clutter. On the other hand, geometrically simple man-made objects tend to be either strongly LR (green) or strongly LL (red). An example of a type of object with this type of return is a polarimetric calibration reflector. Similarly, trihedral comer reflectors typically give a return that is nearly all LR radar cross-section (e.g. green in dual circular polarization RGB image).Also, an ideal trihedral illuminated by an ideal dual circular polarization radar would have all energy returned in the LR channel of the radar and none in the LL channel. Dihedral corner reflector signatures are typically dominated by LL radar cross-section (e.g. red in dual circular polarization RGB image). An ideal dihedral illuminated by an ideal dual circular polarization radar would have all energy returned in the LL channel of the radar and none in the LR channel. Military vehicles typically have complex polarimetric responses that contain significant contributions from both polarization channels, LR and LL. Thus, in the abovereferenced embodiment, signatures from military vehicles would typically be colored yellow in dual circular polarization RGB images 400. As shown in FIGURE 4, a target outline 402 is provided which represents a location of a military vehicle signature.The target outline 402 may be determined by visual analysis of the image 400, or alternately, may be determined by automated data processing algorithms (illustrated in FIGURES 5 though 9) operating on board the sensor platform (e.g. the missile) which analyze the image 400 in a real-time manner.

By scanning an area of interest with a dual circular polarization radar, targets within the area of interest may be determined by combining the resultant LL and LR radar cross-section images into a combined image, and applying assumptions about the characteristics of target radar returns versus that of natural environmental objects. Algorithms containing such assumptions may be programmed into the data processing system of the sensor platform, such as a missile, which may then analyze and interpret the combined LL/LR image to determine the location of the target. In this way, embodiments of the present invention may be used to detect ground mobile targets (e.g. military vehicles) simply, quickly, and efficiently using sensors and data-processing capabilities that are resident on board the missile without the need for target position information from extraneous sources.In the event that the target has moved, or that GPS transmissions are being jammed, embodiments of the present invention may independently detect target locations and improve overall mission performance.

Alternate embodiments of methods and systems in accordance with the present invention may be conceived. The invention is not limited to the particular embodiments shown in FIGURES 1-4. For example, alternate combinations of colors (or other intensity scales) may be used for the LR image 200, the LL image 300, and the resulting LR/LL image 400 that differ from the particular red/green/yellow embodiment. More specifically, in one alternate embodiment, the LR image 200 may be assigned a yellow scale, the LL image 300 may be assigned a blue scale, and the resulting LR/LL image 400 may include regions of yellow, blue, and green. Furthermore, if these images are not going to be analyzed visually, it is not necessary to assign colors to the LR image 200, the LL image 300, and the combined LL/LR image 400 at all.In addition, various portions of the method 100 shown in FIGURE 1 may be combined or performed simultaneously, including, for example, the scanning of the area of interest with the LR radar (block 102) and the LL radar (block 106), and the mapping of the LR data (block 104) and the mapping of the LL data (block 108). Still other variations of the method 100 may be conceived without departing from the spirit and scope of the present invention. [0026J For example, FIGURE 5 is a flowchart of a method 500 for detecting targets using dual circular polarization radar in accordance with an alternate embodiment of the invention. In this embodiment, the method 500 includes scanning an area of interest with LR and LL radar at a block 502. At a block 504, a polarimetric signature of each pixel is displayed in LR/LL radar cross-section space.FIGURE 6 shows a notional depiction of the combined LR/LL image 400 of FIGURE 4 in LR/LL radar cross-section space. As shown in FIGURE 6, by discarding the spatial relationships between the pixels of the LR/LL image 400, the polarimetric signature of each pixel can be plotted in a two dimensional LR/LL radar cross-section space and analyzed to determine target locations. For example, pixels belonging to natural clutter regions (represented as circles in FIGURE 6) tend to have low total radar cross-section, referred to as a Natural Clutter Region 602 in FIGURE 6, with a few atypical values in other regions. Geometrically simple man-made objects typically have responses that have larger magnitude and are dominated by either LR or LL radar crosssection.The LR dominated man-made object returns (represented as diamonds in FIGURE 6) mainly lie in an LR Man-Made Region 604 (FIGURE 6), with a few atypical values crossing into adjacent regions. Similarly, the LL dominated man-made object returns (represented as squares in FIGURE 6) mainly lie in an LL Man-Made Region 606, with a few atypical values crossing into adjacent regions. Military vehicles tend to be composed of many facets and are thus relatively geometrically complex. The returns of military vehicles (represented as darkened triangles in FIGURE 6) tend to be concentrated in a Military Vehicle Region 608 (FIGURE 6), with a few atypical values crossing into adjacent regions.

Each pixel in FIGURE 4 can be viewed as a vector containing one LR value and one LL value. As shown in FIGURE 6, these vectors (LR, LL) (or at least the endpoints thereof) can be plotted on a graph with axes: LR, LL. This allows analysis of how returns from different objects/clutter types cluster.In the event that it may be desirable to assign color values to the data points shown in FIGURE 6, in the above-referenced embodiment that employs a green scale for the LR image and a red scale for the LL image, the notional depiction of LR/LL radar cross-section space shown in FIGURE 6 may provide green points typical of natural clutter returns in the Natural Clutter Region 602 (in place of the circles shown in FIGURE 6), cyan points typical of LR returns from man-made objects in the LR Man-Made Region 604 (in place of the diamonds shown in FIGURE 6), red points typical of LL returns from simple man-made objects in the LL Man-Made Region 606 (in place of squares shown in FIGURE 6), and yellow points typical of military vehicles (near equal returns from LR and LL) in the Military Vehicle Region 608 (in place of the darkened triangles shown in FIGURE 6).

Referring again to FIGURE 5, the method 500 further includes classifying the pixels in LR/LL space at a block 506. In other words, the data shown in FIGURE 6 may be used to classify pixels in a dual circular polarization image. In one embodiment, the LL and LR returns from each pixel are used to index a two dimensional Look-Up Table (LUT). The 2D LUT may be patterned after FIGURE 6 with target responses declared for pixels falling within the Military Vehicle Region 608, and for all other responses a non-target response may be indicated. In some embodiments, the 2D LUT may be performed as generally disclosed, for example, in U.S. Patent No. 6,750,805 issued to Cameron, and in U.S. Patent No. 6,756,935 issued to Cameron et al., which patents are incorporated herein by reference.

For example, in one particular embodiment, a 2D LUT may be constructed and used to map each color pixel in FIGURE 4 to a binary valued pixel in an Initial Detection Image 700 shown in FIGURE 7. The Initial Detection Image 700 has the same spatial dimensions as FIGURE 4. In this embodiment, white pixels indicate target-like pixels and black pixels represent non-target-like pixels. [0030J As shown in FIGURE 7, a large concentration of target pixels occur at approximately the same location as the target outline 402 in FIGURE 4, and a number of target-like responses occur scattered about the Initial Detection Image (Fig. 7) in non-target or clutter regions. Using only the dual circular polarization 2D LUT for detection of targets might therefore result in many clutter false alarms.Therefore, the method 500 (FIGURE 5) further includes a processing of the Initial Detection Image 700 to reduce false alarms at a block 508. In one embodiment, the clutter false alarms that occur in the Initial Detection Image 700 (FIGURE 7) may be reduced or removed using morphological processing. More specifically, in one particular embodiment, the clutter false alarms may be reduced or eliminated by performing a morphological erode function using a 3x3 structuring element. The morphological erode function may eliminate, for example, any detects that span less than a 3x3 pixel area.

As shown in FIGURE 8, a resulting Final Detection Image 800 is formed using the morphological erode on the Initial Detection Image 700. In the Final Detection Image 800 formed by morphological processing of the Initial Detection Image 700, white pixels indicate target responses and black pixels indicate non-target responses. The method 500 then analyzes the Final Detection Image 800 to detect the presence of targets at a block 510. As shown in FIGURE 9, for comparison purposes, the Final Detection Image 800 (FIGURE 8) is overlaid onto the dual circular polarization LR/LL image 400 (FIGURE 4) with the target outline 902 and final detected pixels displayed in white. By employing the method 500 (FIGURE 5), the target may be detected with little or no clutter false alarms.

Embodiments of methods and systems in accordance with the present invention may advantageously enable target detection in a relatively simple, efficient, and robust manner. The processing of the LR and LL image data in radar cross-section space lends itself to the speedy determination of targets using a 2D LUT. Furthermore, the morphological processing of the resultant Initial Detection Image favorably reduces or eliminates false alarms due to clutter. In this way, embodiments of the present invention may be used to detect ground mobile targets (e.g. military vehicles) accurately and in real time using sensors and data-processing capabilities that are resident on board the missile without the need for target position information from extraneous sources.

Embodiments of methods of detecting targets using dual circular polarization radar in accordance with the present invention may be implemented in a variety of sensor platforms. For example, referring now to FIGURE 10, an aircraft 1000 has an avionics system 1010 that includes a radar system 1012 adapted to perform target detection in accordance with the present invention. The aircraft 1000 includes a variety of known components, including a fuselage 1002, lift generating surfaces 1004 such as a pair of wings, a propulsion system 1006, and a host of other systems and subsystems that enable proper operation of the aircraft 1000.

In addition, a plurality of missiles 1050 are attached to the aircraft 1000. Each missile 1050 includes a radar system 1062 adapted to perform target detection in accordance with the present invention, and a variety of other known components, including a fuselage 1052, lift generating surfaces 1054, a propulsion system 1056, and other systems and subsystems that enable proper operation of the missile 1050. Preferably, the radar system 1062 is operatively coupled to one or more other components of the missile 1050, such as the propulsion system 1056, the lift generating surfaces 1054, or other systems and subsystems involved in the guidance and control of the missile 1050, and is adapted to provide targeting course corrections during flight to direct the missile 1050 to the intended target.

In one embodiment, the aircraft 1000 may be a fighter aircraft, such as, for example, an F/A-18E Super Hornet manufactured by The Boeing Company of Chicago, Illinois. However, radar systems in accordance with the present invention may be included in any other suitable aircraft. For example, in alternate embodiments, the aircraft may be a fighter aircraft, a rotary aircraft, a bomber aircraft, or any other suitable type of manned or unmanned aircraft, including those described, for example, in The Illustrated Encyclopedia of Military Aircraft by Enzo Angelucci, published by Book Sales Publishers, September 2001, and in Jane's All the World's Aircraft published by Jane's Information Group of Coulsdon, Surrey, United Kingdom, which texts are incorporated herein by reference.Similarly, the missiles 1050 may be any suitable type of missiles that include a radar system (or other suitable navigational system), including Harpoons, HARMs, Sparrows, AMRAAMs, or any other suitable missiles, including those described in the above-referenced texts.

The radar systems 1012, 1062 of the aircraft 1000 and of the missiles 1050 may be any suitable type of dual circular polarization radar systems. For example, FIGURE 11 is a schematic view of a radar system 1100 in accordance with an alternate embodiment of the invention. The radar system 1100 comprises a radar data processor 1102, a power supply 1106, an analog signal converter 1103, a transmitter 1105, a receiver/exciter 1104 and an antenna array 1107. In operation, the radar system 1100 uses a high voltage power supply 1113 to electronically steer the antenna array 1107 in the E-Plane, (perpendicular to the direction of the stubs). A beam steering computer 1111 is used to set the phase-shifters 1114 to steer the beam in the H-Plane to a desired pointing angle.The processor 1102 may be suitably adapted to perform one or more embodiments of methods of detecting targets in accordance with the present invention.

The radar system 1100 may be any suitable dual circular polarization radar, and may be a mechanically-scanned radar system, similar to an APG-65 radar system manufactured by the Hughes Aircraft Co. of Los Angeles, California, or may be an electronically-scanned radar system, similar to those electronically-scanned radar systems generally disclosed, for example, in U.S. Patent No. 5,469,165 issued to Milroy, and in U.S. Patent No. 5,038,147 issued to Cerro et al., which patents are incorporated herein by reference. Of course, the radar system 1100 may be any other suitable dual circular polarization radar system.

While preferred and alternate embodiments of the invention have been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of these preferred and alternate embodiments. Instead, the invention should be determined entirely by reference to the claims that follow. What is claimed is: 1. A method of detecting a target, comprising: combining an LR (left circular transmit/right circular receive) return signal and an LL (left circular transmit/left circular receive) return signal; and analyzing the combined LR and LL return signals to detect the target.

Claims (1)

1. 2. The method of Claim 1, further comprising: scanning an area of interest with a dual circular polarization radar signal; receiving the LR return signal from the area of interest; and receiving the LL return signal from the area of interest.

   3. The method of Claim 1, wherein combining the LR and LL return signals includes assigning a first color scale to the LR return signal, assigning a second color scale to the LL return signal, and superimposing the first and second color scales to create a multi-color image.

   4. The method of Claim 1, wherein combining the LR and LL return signals includes mapping the LR return signal onto a space, and mapping the LL return signal onto the space. 5. The method of Claim 1, wherein combining the LR and LL return signals includes determining at least one polarimetric signature value based on the LR and LL return signals in an LR/LL radar cross-section space.

   6. The method of Claim 1, further comprising processing the combined LR and LL return signals to reduce a false alarm rate.

   7. The method of Claim 6, wherein processing the combined LR and LL return signals to reduce a false alarm rate includes performing a morphological processing on the combined LR and LL return signals. 8. A method of detecting a target, comprising: scanning an area of interest with a dual circular polarization radar signal; receiving an LR return signal from the area of interest; receiving an LL return signal from the area of interest; mapping the LR return signal onto a first Cartesian grid; mapping the LL return signal onto a second Cartesian grid; assigning a first color scale to the LR return signal; assigning a second color scale to the LL return signal; superimposing the first and second color scales to create a multi-color image; and analyzing the multi-color image to determine a location of a target color.

   9. The method of Claim 8, wherein assigning a first color scale to the LR return signal includes assigning a green scale to the LR return signal, and wherein assigning a second color scale to the LL return signal includes assigning a red scale to the LL return signal. 10. The method of Claim 8, further comprising processing the LR and LL return signals to reduce a false alarm rate.

   11. A method of detecting a target, comprising: scanning an area of interest with a dual circular polarization radar signal; receiving an LR return signal from the area of interest; receiving an LL return signal from the area of interest; combining the LR and LL return signals including determining at least one polarimetric signature value based on the LR and LL return signals in an LR/LL radar crosssection space; and determining whether the polarimetric signature value lies within an assumed target region in the LR/LL radar cross-section space.

   12. The method of Claim 11, further comprising displaying the polarimetric signature value in the LR/LL radar cross-section space. 13. The method of Claim 11, further comprising processing the combined LR and LL return signals to reduce a false alarm rate.

   14. A method of altering a course of a vehicle, comprising: combining an LR (left circular transmit/right circular receive) return signal and an LL (left circular transmit/left circular receive) return signal; and varying a direction of travel of the vehicle based on the combined LR and LL return signals.

   15. The method of Claim 14, further comprising: scanning an area of interest with a dual circular polarization radar signal; receiving the LR return signal from the area of interest; and receiving the LL return signal from the area of interest.

   16. The method of Claim 14, wherein varying a direction of travel of the vehicle includes transmitting a control signal to a guidance system of the vehicle. 17. The method of Claim 14, wherein combining the LR and LL return signals includes mapping the LR return signal onto a space, and mapping the LL return signal onto the space.

   18. The method of Claim 14, wherein combining the LR and LL return signals includes determining at least one polarimetric signature value based on the LR and LL return signals in an LR/LL radar cross-section space. 19. A radar system, comprising: at least one antenna; a transmitter component coupled to the antenna and adapted to transmit a dual circular polarization radar signal; a receiver component coupled to the antenna and adapted to receive an LR return signal and an LL return signal; and a signal processor coupled to the receiver component and adapted to detect a target by a method comprising: scanning an area of interest with the dual circular polarization radar signal; receiving the LR return signal from the area of interest; receiving the LL return signal from the area of interest; combining the LR and LL return signals; and analyzing the combined LR and LL return signals to detect the target. 20. The radar system of Claim 19, wherein combining the LR and LL return signals includes mapping the LR return signal onto a first Cartesian grid, mapping the LL return signal onto a second Cartesian grid, and superimposing the first and second Cartesian grids.

   21. The radar system of Claim 19, wherein combining the LR and LL return signals includes determining at least one polarimetric signature value based on the LR and LL return signals in an LR/LL radar cross-section space.

   22. The radar system of Claim 19, further comprising processing the combined LR and LL return signals to reduce a false alarm rate including performing a morphological processing on the combined LR and LL return signals. 23. A vehicle, comprising: a fuselage; and a detecting system at least partially disposed within the fuselage, including: at least one antenna; a receiver component coupled to the antenna and adapted to receive an LR return signal and an LL return signal; and a signal processor coupled to the receiver component and adapted to detect a target by a method comprising combining an LR return signal and an LL return signal, and analyzing the combined LR and LL return signals to detect the target. 24. The vehicle of claim 23, wherein the detecting system includes: a transmitter component coupled to the antenna and adapted to transmit a dual circular polarization radar signal; and wherein the method of detecting the target further includes: scanning an area of interest with the dual circular polarization radar signal; receiving the LR return signal from the area of interest; and receiving the LL return signal from the area of interest.

   25. The vehicle of claim 23, wherein the detecting system is operatively coupled to at least a portion of a guidance system of the missile, and is further adapted to provide targeting course corrections to the guidance system. 26 A radar system as hereinbefore described and with reference to and as shown in the accompanying drawings.