WO2021234906A1

WO2021234906A1 - Radar image processing device, radar image processing method, and radar image processing program

Info

Publication number: WO2021234906A1
Application number: PCT/JP2020/020111
Authority: WO
Inventors: 亮吾堀内; 昇大石
Original assignee: 三菱電機株式会社
Priority date: 2020-05-21
Filing date: 2020-05-21
Publication date: 2021-11-25
Also published as: JP7143550B2; JPWO2021234906A1

Abstract

A radar image processing device (2) is configured so as to comprise: a virtual image suppression unit (11) which suppresses a virtual image included in a radar image by performing compression sensing processing on the radar image; and an image identification unit (12) that performs a Fourier transform on the radar image, for which the virtual image has been suppressed by the virtual suppression unit (11), in the azimuth axis direction of the radar image to thereby calculate a signal spectrum of a true image candidate included in the virtual image-suppressed radar image, and if the angle of the signal spectrum with respect to the Doppler frequency axis in the virtual image-suppressed radar image is less than a first threshold, the image identification unit determines that the true image candidate is a true image, and if the angle of the signal spectrum is the first threshold or greater, the image identification unit determines that the true image candidate is a virtual image.

Description

Radar image processing device, radar image processing method and radar image processing program

This disclosure relates to a radar image processing device, a radar image processing method, and a radar image processing program.

The radar image obtained from the synthetic aperture radar (Synthetic Aperture Radar) mounted on an artificial satellite or a moving object such as an aircraft is orthogonal to the range axis representing the distance from the synthetic aperture radar to the observation target. It is an image showing the space with the azimuth axis which is the direction. The synthetic aperture radar can obtain a radar image in a wider range as the repetition frequency (PRF: Pulse repetition frequency) of the transmission pulse is lower. However, low PRF can cause aliasing. By the occurrence of aliasing, a virtual image called azimuth ambiguity is generated on the radar image. When a virtual image is generated on the radar image, the detection accuracy of the target reflected in the radar image is deteriorated.

The following Non-Patent Document 1 discloses a radar image processing device that calculates a signal spectrum of a true image candidate included in a radar image by Fourier transforming the radar image in the azimuth axis direction. The radar image processing device identifies whether the true image candidate is a true image indicating a target or a virtual image based on the shape of the signal spectrum.

In the radar image processing apparatus disclosed in Non-Patent Document 1, when a radar image contains a noise component, the shape of the calculated signal spectrum may differ from the correct shape due to the influence of the noise component. .. There is a problem that if the shape of the signal spectrum is different from the correct shape, an error may occur in the discrimination result of whether the true image candidate is a true image or a virtual image.

The present disclosure has been made to solve the above-mentioned problems, and it is possible to identify whether the true image candidate is a true image or a virtual image even if the radar image contains a noise component. The purpose is to obtain a radar image processing device, a radar image processing method, and a radar image processing program that can be used.

The radar image processing apparatus according to the present disclosure captures a virtual image suppression unit that suppresses a virtual image contained in the radar image by performing compression sensing processing on the radar image, and a radar image after the virtual image suppression by the virtual image suppression unit. By Fourier transforming the radar image in the azimuth axis direction, the signal spectrum of the true image candidate included in the radar image after virtual image suppression is calculated, and the angle of the signal spectrum with respect to the Doppler frequency axis in the radar image after virtual image suppression. Is smaller than the first threshold value, it is determined that the true image candidate is a true image, and if the angle of the signal spectrum is equal to or larger than the first threshold value, the true image candidate is determined to be a virtual image. It is equipped with.

According to the present disclosure, even if the radar image contains a noise component, it is possible to identify whether the true image candidate is a true image or a virtual image.

It is a block diagram which shows the radar image processing apparatus which concerns on Embodiment 1. FIG. It is a hardware block diagram which shows the hardware of the radar image processing apparatus which concerns on Embodiment 1. FIG. It is a hardware block diagram of the computer when the radar image processing apparatus 2 is realized by software, firmware and the like. It is a flowchart which shows the radar image processing method which is the processing procedure of the radar image processing apparatus 2 which concerns on Embodiment 1. FIG. It is a flowchart which shows the processing procedure of the image identification processing unit 16. An explanatory diagram showing a radar image stored by the radar image storage unit 1, FIG. 6B is an explanatory diagram showing a radar image after the virtual image suppression by the virtual image suppression unit 11. It is explanatory drawing which shows the discard process of the erroneous detection image by the tentative true image candidate selection unit 14. FIG. 8A is an explanatory diagram showing a binarized image including four pseudo-true image candidates (1) to (4), and FIG. 8B is an explanatory diagram showing four pseudo-true image candidates (1) to (4), respectively. An explanatory diagram showing a pixel having the largest power among the plurality of pixels included in the above, FIG. 8C is a pseudo-true image candidate (1) to remaining without being discarded by the pseudo-true image candidate selection unit 14. It is explanatory drawing which shows (3). FIG. 9A is an explanatory diagram showing an example of a true image included in the radar image after the virtual image suppression by the virtual image suppression unit 11, and FIG. 9B is a virtual image included in the radar image after the virtual image suppression by the virtual image suppression unit 11. It is explanatory drawing which shows an example. FIG. 10A is an explanatory diagram showing an example of the signal spectrum of the true image calculated by Fourier transforming the true image in the azimuth axis direction, and FIG. 10B is calculated by Fourier transforming the virtual image in the azimuth axis direction. It is explanatory drawing which shows an example of the signal spectrum of a virtual image. FIG. 11A is an explanatory diagram showing a true image included in the radar image stored by the radar image storage unit 1, and FIG. 11B is a virtual image included in the radar image stored by the radar image storage unit 1. 11C is an explanatory diagram showing a true image included in the radar image after the virtual image suppression by the virtual image suppression unit 11, and FIG. 11D is included in the radar image after the virtual image suppression by the virtual image suppression unit 11. It is explanatory drawing which shows the virtual image. FIG. 12A is an explanatory diagram showing a signal spectrum of a true image included in a radar image (radar image shown in FIG. 11A) in which the virtual image suppression process is not performed by the virtual image suppression unit 11, and FIG. 12B is a virtual image suppression unit. An explanatory diagram showing the signal spectrum of the virtual image included in the radar image (radar image shown in FIG. 11B) in which the virtual image suppression process by No. 11 is not performed, FIG. 12C is a radar image after the virtual image suppression by the virtual image suppression unit 11. An explanatory diagram showing the signal spectrum of the true image included in (the radar image shown in FIG. 11C), FIG. 12D is included in the radar image (radar image shown in FIG. 11D) after the virtual image is suppressed by the virtual image suppressing unit 11. It is explanatory drawing which shows the signal spectrum of a virtual image. It is a block diagram which shows the radar image processing apparatus which concerns on Embodiment 2. It is a hardware block diagram which shows the hardware of the radar image processing apparatus which concerns on Embodiment 2. FIG.

Hereinafter, in order to explain the present disclosure in more detail, a mode for carrying out the present disclosure will be described in accordance with the attached drawings.

Embodiment 1.
FIG. 1 is a configuration diagram showing a radar image processing device 2 according to the first embodiment.
FIG. 2 is a hardware configuration diagram showing the hardware of the radar image processing device 2 according to the first embodiment.
The radar image storage unit 1 is realized by, for example, a first storage circuit.
The radar image storage unit 1 stores the radar image output from the synthetic aperture radar.
In the radar image processing device 2 shown in FIG. 1, the radar image storage unit 1 is provided outside the radar image processing device 2. However, this is only an example, and the radar image storage unit 1 may be provided inside the radar image processing device 2.
In the radar image processing device 2 shown in FIG. 1, a synthetic aperture radar is mounted on a moving body, and a radar image storage unit 1, a radar image processing device 2, and a processed image storage unit 3 are mounted on a ground station for synthesis. It is assumed that the radar image is transferred from the aperture radar to the radar image storage unit 1. However, this is only an example, and the radar image storage unit 1 and the radar image processing device 2 are also mounted on the moving body, and the radar image after removing the virtual image from the radar image processing device 2 is the processed image storage unit 3. It may be transferred to.

The radar image processing device 2 includes a virtual image suppression unit 11 and an image identification unit 12.
The radar image processing device 2 identifies whether the image reflected in the radar image stored by the radar image storage unit 1 is a true image or a virtual image.
The radar image processing device 2 removes a virtual image reflected in the radar image, and outputs the radar image after removing the virtual image to the processed image storage unit 3.

The processed image storage unit 3 is realized by, for example, a second storage circuit.
The processed image storage unit 3 stores the radar image after removing the virtual image output from the radar image processing device 2.
In the radar image processing device 2 shown in FIG. 1, the processed image storage unit 3 is provided outside the radar image processing device 2. However, this is only an example, and the post-processing image storage unit 3 may be provided inside the radar image processing device 2.

The first storage circuit and the second storage circuit are, for example, RAM (Random Access Memory), ROM (Read Only Memory), flash memory, EPROM (Erasable Project Online Memory), EPROM (Electric) Non-volatile or volatile semiconductor memory such as, magnetic disk, flexible disk, optical disk, compact disk, mini disk, or DVD (Digital Versaille Disc) corresponds to this.

The virtual image suppression unit 11 is realized by, for example, the virtual image suppression circuit 21 shown in FIG.
The virtual image suppression unit 11 acquires a radar image stored by the radar image storage unit 1.
The virtual image suppression unit 11 suppresses the virtual image contained in the radar image by performing compressed sensing processing on the radar image.
The virtual image suppression unit 11 outputs the radar image after the virtual image suppression to the image identification unit 12.

The image identification unit 12 includes an image conversion unit 13, a tentative true image candidate selection unit 14, a true image candidate extraction unit 15, and an image identification processing unit 16.
The image identification unit 12 Fourier transforms the radar image after the virtual image suppression by the virtual image suppression unit 11 in the azimuth axis direction of the radar image, so that the signal spectrum of the true image candidate included in the radar image after the virtual image suppression Is calculated.
If the angle of the signal spectrum with respect to the Doppler frequency axis in the radar image after suppressing the virtual image is smaller than the first threshold value, the image identification unit 12 determines that the true image candidate is a true image.
If the angle of the signal spectrum is equal to or greater than the first threshold value, the image identification unit 12 determines that the true image candidate is a virtual image.

The image conversion unit 13 is realized by, for example, the image conversion circuit 22 shown in FIG.
The image conversion unit 13 acquires a radar image after the virtual image is suppressed by the virtual image suppression unit 11.
The image conversion unit 13 converts the radar image after suppressing the virtual image into a binarized image, and outputs the binarized image to the pseudo-true image candidate selection unit 14.

The pseudo-true image candidate selection unit 14 is realized by, for example, the pseudo-true image candidate selection circuit 23 shown in FIG.
The pseudo-true image candidate selection unit 14 selects each of one or more pixel groups in which pixels having a pixel value of 1 are gathered in the binarized image output from the image conversion unit 13. Specify as.
The pseudo-true image candidate selection unit 14 discards the pseudo-true image candidates whose number of pixels included is smaller than the second threshold value among the identified pseudo-true image candidates. The second threshold value may be stored in the internal memory of the pseudo-true image candidate selection unit 14, or may be given from the outside of the radar image processing device 2.
The tentative true image candidate selection unit 14 outputs the tentative true image candidates that remain without being discarded to the true image candidate extraction unit 15.

The true image candidate extraction unit 15 is realized by, for example, the true image candidate extraction circuit 24 shown in FIG.
The true image candidate extraction unit 15 acquires a radar image after the virtual image is suppressed by the virtual image suppression unit 11.
The true image candidate extraction unit 15 extracts from the radar image after the virtual image suppression a pixel group having the same pixel position as the false true image candidate that remains without being discarded by the pseudo true image candidate selection unit 14 as a true image candidate.

The image identification processing unit 16 is realized by, for example, the image identification processing circuit 25 shown in FIG.
The image identification processing unit 16 calculates the signal spectrum of the true image candidate by Fourier transforming the true image candidate extracted by the true image candidate extraction unit 15 in the azimuth axis direction.
The image identification processing unit 16 calculates the angle of the signal spectrum with respect to the Doppler frequency axis in the radar image after the virtual image is suppressed by Hough transforming the signal spectrum of the true image candidate.
If the angle of the signal spectrum is smaller than the first threshold value, the image identification processing unit 16 determines that the true image candidate is a true image.
If the angle of the signal spectrum is equal to or greater than the first threshold value, the image identification processing unit 16 determines that the true image candidate is a virtual image. The first threshold value may be stored in the internal memory of the image identification processing unit 16 or may be given from the outside of the radar image processing device 2.
The image identification processing unit 16 removes the virtual image reflected in the radar image after suppressing the virtual image, and outputs the radar image after removing the virtual image to the processed image storage unit 3.

In FIG. 1, each of the virtual image suppression unit 11, the image conversion unit 13, the pseudo-true image candidate selection unit 14, the true image candidate extraction unit 15, and the image identification processing unit 16, which are the components of the radar image processing device 2, is shown in FIG. It is assumed that it will be realized by dedicated hardware as shown in. That is, it is assumed that the radar image processing device 2 is realized by the virtual image suppression circuit 21, the image conversion circuit 22, the pseudo-true image candidate selection circuit 23, the true image candidate extraction circuit 24, and the image identification processing circuit 25.
Each of the imaginary image suppression circuit 21, the image conversion circuit 22, the pseudo-true image candidate selection circuit 23, the true image candidate extraction circuit 24, and the image identification processing circuit 25 is, for example, a single circuit, a composite circuit, a programmed processor, or a parallel program. An ASIC (Application Specific Integrated Circuit), an FPGA (Field-Programmable Gate Array), or a combination thereof is applicable.

The components of the radar image processing device 2 are not limited to those realized by dedicated hardware, but the radar image processing device 2 is realized by software, firmware, or a combination of software and firmware. There may be.
The software or firmware is stored as a program in the memory of the computer. A computer means hardware that executes a program, and corresponds to, for example, a CPU (Central Processing Unit), a central processing unit, a processing unit, a computing device, a microprocessor, a microcomputer, a processor, or a DSP (Digital Signal Processor). do.

FIG. 3 is a hardware configuration diagram of a computer when the radar image processing device 2 is realized by software, firmware, or the like.
When the radar image processing device 2 is realized by software, firmware, or the like, each of the virtual image suppressing unit 11, the image conversion unit 13, the pseudo-true image candidate selection unit 14, the true image candidate extraction unit 15, and the image identification processing unit 16. A program for causing the computer to execute the processing procedure is stored in the memory 31. Then, the processor 32 of the computer executes the program stored in the memory 31.

Further, FIG. 2 shows an example in which each of the components of the radar image processing device 2 is realized by dedicated hardware, and FIG. 3 shows an example in which the radar image processing device 2 is realized by software, firmware, or the like. ing. However, this is only an example, and some components in the radar image processing device 2 may be realized by dedicated hardware, and the remaining components may be realized by software, firmware, or the like.

Next, the operation of the radar image processing device 2 shown in FIG. 1 will be described.
FIG. 4 is a flowchart showing a radar image processing method which is a processing procedure of the radar image processing device 2 according to the first embodiment.
FIG. 5 is a flowchart showing a processing procedure of the image identification processing unit 16.

The virtual image suppression unit 11 acquires a radar image stored by the radar image storage unit 1.
As shown in FIG. 6A, the radar image stored by the radar image storage unit 1 shows a true image of an in-focus target and a defocused virtual image.
FIG. 6A is an explanatory diagram showing a radar image stored by the radar image storage unit 1.
The radar image shown in FIG. 6A is an image showing the space between the azimuth axis and the range axis.

The virtual image suppression unit 11 suppresses the virtual image contained in the radar image by performing a compressed sensing process on the radar image shown in FIG. 6A (step ST1 in FIG. 4).
The compressed sensing process assumes that the radar image is sparse, and restores the received signal of the synthetic aperture radar when it is imaged with a normal PRF from the received signal of the synthetic aperture radar when it is imaged with a low PRF. Is. The radar image calculated from the received signal of the synthetic aperture radar restored by the compressed sensing process corresponds to the radar image after the virtual image is suppressed.
A low PRF is a frequency lower than the target Doppler frequency, and a normal PRF is a frequency higher than the target Doppler frequency. A sparse radar image means that the radar image contains pixels with zero power.
The virtual image suppression unit 11 outputs the radar image after the virtual image suppression to the image identification unit 12.
FIG. 6B is an explanatory diagram showing a radar image after the virtual image is suppressed by the virtual image suppressing unit 11.
As shown in FIG. 6B, the radar image after the virtual image suppression shows the virtual image in which the power is suppressed. The amount of suppression for each virtual image varies, and as shown in FIG. 6B, in the radar image after the virtual image is suppressed, in addition to the virtual image in which the power is sufficiently suppressed, the power remains unsuppressed. There is a virtual image.
The noise component included in the radar image is also suppressed by the virtual image suppression process by the virtual image suppression unit 11.

The image conversion unit 13 of the image identification unit 12 acquires a radar image after the virtual image is suppressed by the virtual image suppression unit 11.
The image conversion unit 13 converts the radar image after suppressing the virtual image into a binarized image, and outputs the binarized image to the pseudo-true image candidate selection unit 14 (step ST2 in FIG. 4).
That is, the image conversion unit 13 calculates a threshold matrix having a matrix element corresponding to each pixel included in the radar image by performing CFAR (Constant False Allarm Rate) processing on the radar image after the virtual image is suppressed. do.
The value of each matrix element included in the threshold matrix is 1, if the power of the pixel corresponding to the matrix element among the plurality of pixels included in the radar image is larger than the power threshold value 1. If it is below the threshold value, it is 0. Since the CFAR treatment itself is a known technique, detailed description thereof will be omitted.
The image conversion unit 13 outputs the threshold matrix as a binarized image to the pseudo-true image candidate selection unit 14. The binarized image is also an image showing the space between the azimuth axis and the range axis, similar to the radar image.

The virtual image included in the radar image is suppressed by the virtual image suppressing unit 11. However, due to the virtual image suppression process by the virtual image suppression unit 11, an image different from each of the true image and the virtual image (hereinafter, referred to as “false detection image”) may be generated. Due to the occurrence of the erroneous detection image, the erroneous detection image is also included in the binarized image. The size of the false positive image is smaller than the size of the true image.
As shown below, the pseudo-true image candidate selection unit 14 performs a process of removing the false detection image included in the binarized image.

The pseudo-true image candidate selection unit 14 selects each of one or more pixel groups in which pixels having a pixel value of 1 are gathered in the binarized image output from the image conversion unit 13. (Step ST3 in FIG. 4).

The pseudo-true image candidate selection unit 14 counts the number of pixels included in each pseudo-true image candidate.
As shown in FIG. 7, if the number of pixels included in the pseudo-true image candidate is less than the second threshold value, the pseudo-true image candidate selection unit 14 determines that the pseudo-true image candidate is a false detection image. Judgment is made, and the pseudo-true image candidate is discarded (step ST4 in FIG. 4).
As shown in FIG. 7, if the number of pixels included in the pseudo-true image candidate is equal to or greater than the second threshold value, the pseudo-true image candidate selection unit 14 does not indicate that the pseudo-true image candidate is a false detection image. Judgment is made and the pseudo-true image candidate is not discarded.
As the second threshold value, for example, the number of pixels included in the true image of the minimum assumed target among the targets that may be reflected in the radar image is set.
FIG. 7 is an explanatory diagram showing a process of discarding an erroneous detection image by the false true image candidate selection unit 14.
In FIG. 7, the binarized image on the left side is the binarized image before the false detection image is discarded, and the binarized image on the right side is the binarized image after the false positive image is discarded. It is a binarized image.

The pseudo-true image candidates that remain without being discarded by the pseudo-true image candidate selection unit 14 may include two pseudo-true image candidates for one target. That is, one tentative image candidate originally appears in one goal. However, due to the CFAR processing in the image conversion unit 13, the pseudo-true image candidate for one target may be divided into two. Therefore, it is necessary to discard one of the pseudo-true image candidates that has been divided into two.
Of the remaining pseudo-true image candidates, if the distance between the two pseudo-true image candidates adjacent to each other is short, the two pseudo-true image candidates adjacent to each other have a pseudo-true image candidate for one target. It is highly possible that it is divided into two parts.
The pseudo-true image candidate selection unit 14 calculates the distance between two pseudo-true image candidates that are adjacent to each other among the remaining pseudo-true image candidates. Since the process itself for calculating the distance between the two pseudo-true image candidates is a known technique, detailed description thereof will be omitted.

If the distance L between the two adjacent pseudo-true image candidates among the remaining pseudo-true image candidates is shorter than the third threshold value, the pseudo-true image candidate selection unit 14 determines the two pseudo-true image candidates. Of these, one of the pseudo-true image candidates is discarded. The third threshold value may be stored in the internal memory of the pseudo-true image candidate selection unit 14, or may be given from the outside of the radar image processing device 2.
Hereinafter, the process of discarding any of the pseudo-true image candidates by the pseudo-true image candidate selection unit 14 will be specifically described.

The pseudo-true image candidate selection unit 14 acquires a radar image after the virtual image is suppressed by the virtual image suppression unit 11.
The pseudo-true image candidate selection unit 14 extracts a pixel group having the same pixel position as one of the two pseudo-true image candidates from the radar image after the virtual image is suppressed.
The pseudo-true image candidate selection unit 14 identifies the largest power P _{1, max} _{by comparing the power P 1 of a plurality of pixels constituting the extracted pixel group with each other.}
Further, the pseudo-true image candidate selection unit 14 extracts a pixel group having the same pixel position as the other pseudo-true image candidate among the two pseudo-true image candidates from the radar image after the virtual image is suppressed.

The pseudo-true image candidate selection unit 14 identifies the largest power P _{2, max} _{by comparing the power P 2 of a plurality of pixels constituting the extracted pixel group with each other.}
The tentative image candidate selection unit 14 compares the specified power P _{1, max} with the specified power P _{2, max,} and if the power P _{1, max} is the power P _{2, max} or more, one of the tentative truths. The other pseudo-true image candidate is discarded, leaving the image candidate.
If the power P _{1, max} is smaller than the power P _{2, max} , the pseudo-true image candidate selection unit 14 leaves the other pseudo-true image candidate and discards one pseudo-true image candidate.
The tentative true image candidate selection unit 14 outputs the tentative true image candidates that remain without being discarded to the true image candidate extraction unit 15.

FIG. 8 is an explanatory diagram showing a process of discarding any of the pseudo-true image candidates by the pseudo-true image candidate selection unit 14.
FIG. 8A shows a binarized image including four pseudo-true image candidates (1) to (4).
In the binarized image shown in FIG. 8A, the distance between the pseudo-true image candidate (1) and the pseudo-true image candidate (2) is L _1-2 , and the pseudo-true image candidate (1) and the pseudo-true image candidate (3). The distance is L _1-3 , and the distance between the pseudo-true image candidate (1) and the pseudo-true image candidate (4) is L _1-4 .
The distance between the pseudo-true image candidate (2) and the pseudo-true image candidate (3) is L _2-3 , and the distance between the pseudo-true image candidate (2) and the pseudo-true image candidate (4) is L _2-4 . The distance between the pseudo-true image candidate (3) and the pseudo-true image candidate (4) is L _3-4 .
In the binarized image shown in FIG. 8A, all of the distance L _1-2 , the distance L _1-3, and the distance L _1-4 are equal to or higher than the third threshold value. Further, both the distance L _2-3 and the distance L _2-4 are equal to or higher than the third threshold value. The distance L _3-4 is shorter than the third threshold.

FIG. 8B shows the pixel having the largest power among the plurality of pixels included in each of the four pseudo-true image candidates (1) to (4).
FIG. 8C shows the pseudo-true image candidates (1) to (3) that remain without being discarded by the pseudo-true image candidate selection unit 14.
In the example of FIG. 8C, the distance L _3-4 is shorter than the third threshold value, and the maximum power of the pixel possessed by the pseudo-true image candidate (4) is larger than the maximum power of the pixel possessed by the pseudo-true image candidate (3). Due to its small size, the tentative true image candidate (4) is discarded.

The true image candidate extraction unit 15 acquires a radar image after the virtual image is suppressed by the virtual image suppression unit 11.
The true image candidate extraction unit 15 extracts from the radar image after the virtual image suppression a pixel group having the same pixel position as the false true image candidate that remains without being discarded by the pseudo true image candidate selection unit 14 as a true image candidate (FIG. FIG. Step 4 ST5).
In the radar image processing device 2 shown in FIG. 1, the true image candidate extraction unit 15 extracts a pixel group having the same pixel position as the false true image candidate as a true image candidate from the radar image after the virtual image is suppressed. However, this is only an example, and the true image candidate extraction unit 15 identifies the pixel having the maximum power among the plurality of pixels included in the pseudo true image candidate, and from the radar image after the virtual image is suppressed. , A pixel group having the same pixel position as the peripheral region including the specified pixel may be extracted as a true image candidate. However, the peripheral area including the pixel is assumed to be an area including all of the pseudo-true image candidates.

The image identification processing unit 16 calculates the signal spectrum of the true image candidate by Fourier transforming the true image candidate extracted by the true image candidate extraction unit 15 in the azimuth axis direction (step ST11 in FIG. 5).
Synthetic aperture radars generally integrate the received signal to increase directional resolution. If the range migration R, which is a change in the distance between the radar device and the target, exceeds the distance resolution of the synthetic aperture radar within the integration time of the received signal, the radar image is deteriorated. Therefore, the radar image processing device 2 performs signal processing for correcting the range migration R, for example, when reproducing the radar image.
The range migration R is expressed by the following equation (1).

In equation (1), R ₀ is the distance between the target existing in the observation area of the synthetic aperture radar and the synthetic aperture radar at the time of closest approach.
λ is the wavelength of the radio wave is a radar signal emitted from a synthetic aperture radar, is f _eta, Doppler frequency, V _r is the effective speed of the synthetic aperture radar body.

The true image is corrected for range migration R (R ₀ , f _η). Therefore, in the space obtained by Fourier transforming the radar image in the azimuth axis direction, that is, in the space represented by the Doppler frequency axis and the range axis, the signal spectra of the true image are arranged in the same range bin.
On the other hand, in the virtual image, even if the range migration R (R ₀ , f _η ) is corrected, the range migration R _residual (f _η ) shown in the following equation (2) remains in the virtual image.

In equation (2), _Ra is the range migration of the virtual image itself.
R _'0 is the distance at the time of closest approach between the reflection point and the synthetic aperture radar according to the observation area outside the virtual image of the synthetic aperture radar.
n is the number of virtual image aliasings, and f _PRF is PRF.

The distance R _'0, can not be calculated accurately, it can be approximated by the following expression (3).

In equation (3), θ _n is the angle formed by the true image and the virtual image of the target centered on the main body of the synthetic aperture radar, and is based on the relationship between the Doppler frequency corresponding to the observation region width of the synthetic aperture radar and the PRF. I want it.

By correcting the range migration R (R ₀ , f _η ), for example, the range migration R _residual (f _η ) represented by the equation (2) does not remain in the true image shown in FIG. 9A. Therefore, the signal spectrum of the true image shown in FIG. 9A is parallel to the Doppler frequency axis as shown in FIG. 10A.
Even after the correction of the range migration R (R ₀ , f _η ), for example, the range migration R _residual (f _η ) represented by the equation (2) remains in the virtual image shown in FIG. 9B. Therefore, the signal spectrum of the virtual image shown in FIG. 9B becomes slanted with respect to the Doppler frequency axis as shown in FIG. 10B.

FIG. 9A is an explanatory diagram showing an example of a true image included in the radar image after the virtual image is suppressed by the virtual image suppressing unit 11.
FIG. 9B is an explanatory diagram showing an example of a virtual image included in the radar image after the virtual image is suppressed by the virtual image suppressing unit 11.
FIG. 10A is an explanatory diagram showing an example of the signal spectrum of the true image calculated by Fourier transforming the true image in the azimuth axis direction.
FIG. 10B is an explanatory diagram showing an example of the signal spectrum of the virtual image calculated by Fourier transforming the virtual image in the azimuth axis direction.
As is clear from FIGS. 10A and 10B, the shape of the signal spectrum of the true image and the shape of the signal spectrum of the virtual image are different.
Even if the virtual image suppression unit 11 performs suppression processing of the virtual image contained in the radar image captured at a low PRF, as shown in FIG. 10A, the shape characteristics related to the signal spectrum of the true image are not lost. .. Further, as shown in FIG. 10B, the shape characteristics of the virtual image with respect to the signal spectrum are not lost.

Next, the image identification processing unit 16 smoothes the signal spectrum by moving averaging the signal spectra of the true image candidates.
In order to accommodate the signal spectrum of the true image candidate in the same image, the image identification processing unit 16 detects the cell in the azimuth axis direction having the lowest power in the smoothed signal spectrum. Then, the image identification processing unit 16 cyclically shifts the pixels included in the smoothed signal spectrum in the azimuth direction in order to move the detected cell to the position at the end of the signal spectrum (step ST12 in FIG. 5). ).
The image identification processing unit 16 cyclically shifts the pixels of the smoothed signal spectrum in the azimuth direction, thereby reducing the influence of the target shape, spatial distribution, and the like.

Next, the image identification processing unit 16 calculates the angle α of the signal spectrum with respect to the Doppler frequency axis of the radar image by Hough transforming the signal spectrum of the true image candidate after the circular shift (step ST13 in FIG. 5). Since the process of calculating the angle α by Hough transforming the signal spectrum itself is a known technique, detailed description thereof will be omitted.
Since the noise component contained in the radar image is also suppressed by the virtual image suppression processing by the virtual image suppression unit 11, the image identification processing unit is compared with the case where the virtual image suppression processing by the virtual image suppression unit 11 is not performed. The accuracy of calculating the angle of the signal spectrum according to 16 is improved.
Further, since the pixels of the signal spectrum are cyclically shifted, the angle calculation accuracy of the signal spectrum by the image identification processing unit 16 is improved as compared with the case where the cyclic shift is not performed.

FIG. 11A is an explanatory diagram showing a true image included in the radar image stored by the radar image storage unit 1.
FIG. 11B is an explanatory diagram showing a virtual image included in the radar image stored by the radar image storage unit 1.
The radar image shown in FIG. 11A contains a noise component in addition to the true image. Further, the radar image shown in FIG. 11B contains a noise component in addition to the virtual image.
FIG. 11C is an explanatory diagram showing a true image included in the radar image after the virtual image is suppressed by the virtual image suppressing unit 11.
FIG. 11D is an explanatory diagram showing a virtual image included in the radar image after the virtual image is suppressed by the virtual image suppressing unit 11.
Since the noise component contained in the radar image is also suppressed by the virtual image suppression process by the virtual image suppression unit 11, the radar image shown in FIG. 11C has a noise component contained in the radar image as compared with the radar image shown in FIG. 11A. Is decreasing. Further, the radar image shown in FIG. 11D has a reduced noise component contained in the radar image as compared with the radar image shown in FIG. 11B.

FIG. 12A is an explanatory diagram showing a signal spectrum of a true image included in a radar image (radar image shown in FIG. 11A) in which the virtual image suppression process is not performed by the virtual image suppression unit 11.
FIG. 12B is an explanatory diagram showing a signal spectrum of a virtual image included in a radar image (radar image shown in FIG. 11B) in which the virtual image suppression process is not performed by the virtual image suppression unit 11.
FIG. 12C is an explanatory diagram showing a signal spectrum of a true image included in a radar image (radar image shown in FIG. 11C) after the virtual image is suppressed by the virtual image suppressing unit 11.
FIG. 12D is an explanatory diagram showing a signal spectrum of a virtual image included in a radar image (radar image shown in FIG. 11D) after the virtual image is suppressed by the virtual image suppression unit 11.
Since the noise component contained in the radar image is also suppressed by the virtual image suppression process by the virtual image suppression unit 11, the signal spectrum of the true image included in the radar image shown in FIG. 12C is shown in FIG. 12A. The shape of the signal spectrum is calculated with higher accuracy than the signal spectrum of the true image included in the radar image. Further, the signal spectrum of the virtual image included in the radar image shown in FIG. 12D is calculated with higher accuracy than the signal spectrum of the virtual image included in the radar image shown in FIG. 12B. There is.

Next, the image identification processing unit 16 compares the angle α of the signal spectrum with the first threshold value Th _1.
If the angle α of the signal spectrum _{is smaller than the first threshold value Th 1} (in the case of step ST14: YES in FIG. 5), the image identification processing unit 16 determines that the true image candidate is a true image (FIG. 5). Step ST15).
If the angle α of the signal spectrum is _{equal to or greater than the first threshold value Th 1} (step ST14 in FIG. 5: NO), the image identification processing unit 16 determines that the true image candidate is a virtual image (step 5 in FIG. 5). ST16).
By repeating the processes of steps ST11 to ST16 in FIG. 5, the image identification processing unit 16 discriminates whether all the true image candidates extracted by the true image candidate extraction unit 15 are true images or virtual images. (Step ST6 in FIG. 4).
The image identification processing unit 16 removes the identified virtual image from the radar image after the virtual image suppression by the virtual image suppression unit 11, and outputs the radar image after the virtual image removal to the processed image storage unit 3.

In the above-described first embodiment, the virtual image suppression unit 11 that suppresses the virtual image contained in the radar image by performing the compression sensing process on the radar image, and the radar image after the virtual image suppression by the virtual image suppression unit 11 are obtained. By Fourier transforming the radar image in the azimuth axis direction, the signal spectrum of the true image candidate included in the radar image after virtual image suppression is calculated, and the angle of the signal spectrum with respect to the Doppler frequency axis in the radar image after virtual image suppression. Is smaller than the first threshold value, it is determined that the true image candidate is a true image, and if the angle of the signal spectrum is equal to or larger than the first threshold value, the true image candidate is determined to be a virtual image. The radar image processing device 2 is configured to include the above. Therefore, the radar image processing device 2 can identify whether the true image candidate is a true image or a virtual image even if the radar image contains a noise component.

Embodiment 2.
In the second embodiment, even if the angle α of the _{signal spectrum is smaller than the first threshold value Th 1} , the image identification unit 12 has a true image if the length Len of the signal spectrum _{is shorter than the fourth threshold value Th 4.} The radar image processing device 2 that determines that the candidate is a virtual image will be described.

FIG. 13 is a configuration diagram showing a radar image processing device 2 according to the second embodiment. In FIG. 13, the same reference numerals as those in FIG. 1 indicate the same or corresponding parts, and thus the description thereof will be omitted.
FIG. 14 is a hardware configuration diagram showing the hardware of the radar image processing device 2 according to the second embodiment. In FIG. 14, the same reference numerals as those in FIG. 2 indicate the same or corresponding parts, and thus the description thereof will be omitted.

The image identification unit 17 includes an image conversion unit 13, a tentative true image candidate selection unit 14, a true image candidate extraction unit 15, and an image identification processing unit 18.
The image identification unit 17 Fourier-converts the radar image after the virtual image suppression by the virtual image suppression unit 11 in the azimuth axis direction of the radar image, so that the signal spectrum of the true image candidate included in the radar image after the virtual image suppression Is calculated.
In the image identification unit 17, the angle α of the signal spectrum with respect to the Doppler frequency axis in the radar image after suppressing the virtual image _{is smaller than the first threshold value Th 1} , and the length Len of the signal spectrum is ₄ or more the fourth threshold value Th 4. If so, it is determined that the true image candidate is a true image.
If the angle α of the signal spectrum is equal to or greater than _{the first threshold value Th 1} _{or the length Len of the signal spectrum is shorter than the fourth threshold value Th 4} , the image identification unit 17 determines that the true image candidate is a virtual image. ..

The image identification processing unit 18 is realized by, for example, the image identification processing circuit 26 shown in FIG.
Similar to the image identification processing unit 16 shown in FIG. 1, the image identification processing unit 18 Fourier transforms the true image candidate extracted by the true image candidate extraction unit 15 in the azimuth axis direction to obtain a signal spectrum of the true image candidate. Is calculated.
The image identification processing unit 18 calculates the angle α of the signal spectrum with respect to the Doppler frequency axis and the length Len of the signal spectrum in the radar image after the virtual image suppression by Hough transforming the signal spectrum of the true image candidate.
In the image identification processing unit 18, if the angle α of the signal spectrum _{is smaller than the first threshold value Th 1} and the length Len of the signal spectrum is ₄ or more of the fourth threshold value Th 4, the true image candidate is a true image. Judge that there is.
If the angle α of the signal spectrum is equal to or greater than _{the first threshold value Th 1} _{or the length Len of the signal spectrum is shorter than the fourth threshold value Th 4, the} image identification processing unit 18 determines that the true image candidate is a virtual image. do. Each of the first threshold value Th ₁ and the fourth threshold value Th ₄ may be stored in the internal memory of the image identification processing unit 18 or may be given from the outside of the radar image processing device 2. ..
Similar to the image identification processing unit 16 shown in FIG. 1, the image identification processing unit 18 removes the virtual image reflected in the radar image after suppressing the virtual image, and outputs the radar image after removing the virtual image to the processed image storage unit 3. do.

In FIG. 13, each of the virtual image suppression unit 11, the image conversion unit 13, the pseudo-true image candidate selection unit 14, the true image candidate extraction unit 15, and the image identification processing unit 18, which are the components of the radar image processing device 2, is shown in FIG. It is assumed that it will be realized by dedicated hardware as shown in. That is, it is assumed that the radar image processing device 2 is realized by the virtual image suppression circuit 21, the image conversion circuit 22, the pseudo-true image candidate selection circuit 23, the true image candidate extraction circuit 24, and the image identification processing circuit 26.
Each of the imaginary image suppression circuit 21, the image conversion circuit 22, the pseudo-true image candidate selection circuit 23, the true image candidate extraction circuit 24, and the image identification processing circuit 26 is, for example, a single circuit, a composite circuit, a programmed processor, or a parallel program. The processor, ASIC, FPGA, or a combination thereof is applicable.

The components of the radar image processing device 2 are not limited to those realized by dedicated hardware, but the radar image processing device 2 is realized by software, firmware, or a combination of software and firmware. There may be.
When the radar image processing device 2 is realized by software, firmware, or the like, each of the virtual image suppressing unit 11, the image conversion unit 13, the pseudo-true image candidate selection unit 14, the true image candidate extraction unit 15, and the image identification processing unit 18. A program for causing the computer to execute the processing procedure is stored in the memory 31. Then, the processor 32 of the computer executes the program stored in the memory 31.

Further, FIG. 14 shows an example in which each of the components of the radar image processing device 2 is realized by dedicated hardware, and FIG. 3 shows an example in which the radar image processing device 2 is realized by software, firmware, or the like. ing. However, this is only an example, and some components in the radar image processing device 2 may be realized by dedicated hardware, and the remaining components may be realized by software, firmware, or the like.

Next, the operation of the radar image processing device 2 shown in FIG. 13 will be described. However, since the parts other than the image identification processing unit 18 are the same as the radar image processing device 2 shown in FIG. 1, only the operation of the image identification processing unit 18 will be described here.
When the image identification processing unit 18 receives a true image candidate from the true image candidate extraction unit 15, the image identification processing unit 18 performs a Fourier transform on the true image candidate in the azimuth axis direction in the same manner as the image identification processing unit 16 shown in FIG. Calculate the candidate signal spectrum.

Next, the image identification processing unit 18 calculates the angle α of the signal spectrum with respect to the Doppler frequency axis and the length Len of the signal spectrum in the radar image after the virtual image suppression by Hough transforming the signal spectrum of the true image candidate. do. Since the angle α calculation process and the length Len calculation process itself by Hough transforming the signal spectrum are known techniques, detailed description thereof will be omitted.

Next, the image identification processing unit 18 compares the angle α of the signal spectrum with the first threshold value Th ₁ , and compares the length Len of the signal spectrum with the fourth threshold value Th ₄ . The fourth threshold Th ₄ is set to a length between the assumed true image signal spectrum length Len and the assumed virtual image signal spectrum length Len.
In the image identification processing unit 18, if the angle α of the signal spectrum _{is smaller than the first threshold value Th 1} and the length Len of the signal spectrum is ₄ or more of the fourth threshold value Th 4, the true image candidate is a true image. Judge that there is.
If the angle α of the signal spectrum is equal to or greater than _{the first threshold value Th 1} _{or the length Len of the signal spectrum is shorter than the fourth threshold value Th 4, the} image identification processing unit 18 determines that the true image candidate is a virtual image. do.
The image identification processing unit 18 removes the identified virtual image from the radar image after the virtual image is suppressed, and outputs the radar image after removing the virtual image to the processed image storage unit 3.

In the second embodiment described above, even if the angle of the signal spectrum is smaller than the first threshold value, if the length of the signal spectrum is shorter than the fourth threshold value, the true image candidate is a virtual image. The radar image processing device 2 shown in FIG. 13 was configured so as to determine that there is. Therefore, in the radar image processing device 2 shown in FIG. 13, similarly to the radar image processing device 2 shown in FIG. 1, even if the radar image contains a noise component, whether the true image candidate is a true image or a virtual image. It is possible to identify whether it is. Further, the radar image processing device 2 shown in FIG. 13 has improved discrimination accuracy between a true image and a virtual image as compared with the radar image processing device 2 shown in FIG.

In the present disclosure, any combination of the embodiments can be freely combined, any component of the embodiment can be modified, or any component can be omitted in each embodiment.

This disclosure is suitable for radar image processing devices, radar image processing methods, and radar image processing programs.

1 Radar image storage unit, 2 Radar image processing device, 3 Post-processing image storage unit, 11 Virtual image suppression unit, 12, 17 Image identification unit, 13 Image conversion unit, 14 Pseudo-true image candidate selection unit, 15 True image candidate extraction unit , 16, 18 image identification processing unit, 21 virtual image suppression circuit, 22 image conversion circuit, 23 pseudo-true image candidate selection circuit, 24 true image candidate extraction circuit, 25, 26 image identification processing circuit, 31 memory, 32 processor.

Claims

A virtual image suppression unit that suppresses the virtual image contained in the radar image by performing compressed sensing processing on the radar image,
By Fourier-converting the radar image after the virtual image suppression by the virtual image suppression unit in the azimuth axis direction of the radar image, the signal spectrum of the true image candidate included in the radar image after the virtual image suppression is calculated and the virtual image is suppressed. If the angle of the signal spectrum with respect to the Doppler frequency axis in the later radar image is smaller than the first threshold value, it is determined that the true image candidate is a true image, and the angle of the signal spectrum is equal to or larger than the first threshold value. If there is, a radar image processing device including an image identification unit that determines that the true image candidate is a virtual image.
The image identification unit is
An image conversion unit that converts the radar image after the virtual image suppression by the virtual image suppression unit into a binarized image and outputs the binarized image, and the image conversion unit.
In the binarized image output from the image conversion unit, each of one or more pixel groups in which pixels having a pixel value of 1 are gathered is specified as a pseudo-true image candidate, and the specified pseudo-true image is specified. Among the candidates, a pseudo-true image candidate selection unit that discards pseudo-true image candidates whose number of pixels included is less than the second threshold value, and
A true image candidate extraction unit that extracts a pixel group having the same pixel position as a false true image candidate that remains without being discarded by the false true image candidate selection unit from the radar image after the virtual image suppression by the virtual image suppression unit. When,
The signal spectrum of the true image candidate is calculated by Fourier transforming the true image candidate extracted by the true image candidate extraction unit in the direction of the azimuth axis, and the signal spectrum with respect to the Doppler frequency axis in the radar image after suppressing the virtual image. If the angle of is smaller than the first threshold value, it is determined that the true image candidate is a true image, and if the angle of the signal spectrum is equal to or larger than the first threshold value, the true image candidate is a virtual image. The radar image processing apparatus according to claim 1, further comprising an image identification processing unit for determining.
If the distance between the two adjacent pseudo-true image candidates among the remaining pseudo-true image candidates is shorter than the third threshold value, the pseudo-true image candidate selection unit may use the two pseudo-true image candidates. The radar image processing apparatus according to claim 2, wherein any one of the pseudo-true image candidates is discarded.
The radar image processing device according to claim 2, wherein the image identification processing unit calculates an angle of the signal spectrum by Hough transforming the signal spectrum of the true image candidate.
The image identification unit determines that the true image candidate is a virtual image if the angle of the signal spectrum is smaller than the first threshold value but the length of the signal spectrum is shorter than the fourth threshold value. The radar image processing apparatus according to claim 1, wherein the radar image processing apparatus is characterized by the above.
The virtual image suppression unit suppresses the virtual image contained in the radar image by performing compressed sensing processing on the radar image.
The image identification unit Fourier-converts the radar image after the virtual image suppression by the virtual image suppression unit in the azimuth axis direction of the radar image to obtain the signal spectrum of the true image candidate included in the radar image after the virtual image suppression. If the angle of the signal spectrum with respect to the Doppler frequency axis in the radar image after suppressing the virtual image is smaller than the first threshold value, it is determined that the true image candidate is a true image, and the angle of the signal spectrum is the first. A radar image processing method for determining that the true image candidate is a virtual image if it is equal to or higher than the threshold value of 1.
A processing procedure for suppressing a virtual image contained in the radar image by performing a compressed sensing process on the radar image by the virtual image suppressing unit, and a processing procedure for suppressing the virtual image included in the radar image.
The image identification unit Fourier-converts the radar image after the virtual image suppression by the virtual image suppression unit in the azimuth axis direction of the radar image to obtain the signal spectrum of the true image candidate included in the radar image after the virtual image suppression. If the angle of the signal spectrum with respect to the Doppler frequency axis in the radar image after suppressing the virtual image is smaller than the first threshold value, it is determined that the true image candidate is a true image, and the angle of the signal spectrum is the first. A radar image processing program for causing a computer to execute a processing procedure for determining that the true image candidate is a virtual image if it is equal to or higher than a threshold value of 1.