JP7346188B2 - Synthetic aperture radar device - Google Patents

Description

Embodiments of the present invention relate to a synthetic aperture radar device.

In images obtained by synthetic aperture radar (SAR) or inverse synthetic aperture radar (ISAR), a reflection point that reflects periodically transmitted radar waves and indicates the shape of the target. Among the reflection points, the brightness of a reflection point far from the radar may be lower than the brightness of a reflection point close to the radar. For example, in images obtained by ISAR that observes moving ships, the brightness of the reflection points on the ship that are far from the radar is low, making it sometimes difficult to confirm the shape and overall length of the ship from the image. .

Further, the brightness of each reflection point may vary depending on the shape of the ship and the direction in which the ship moves. In such a case, if the brightness of each reflection point in the image is adjusted based on the high brightness, the visibility of the reflection points with low brightness may decrease. Furthermore, if the brightness of each reflection point in the image is adjusted based on low brightness, the brightness may become saturated and the visibility of the shape and overall length of the ship may deteriorate.

The spread in the Doppler axis direction of an image obtained by ISAR is proportional to the amount of rotation of the ship in the yaw, pitch, and roll axes with respect to the radar during the radar wave transmission period. Therefore, when the amount of rotation of the ship during the transmission period is small, the spread in the Doppler axial direction becomes small, and the visibility of the shape and overall length of the ship may deteriorate.

Patent No. 4908291

Kazuo Ouchi, "Fundamentals of synthetic aperture radar for remote sensing" (5.4 Image generation in the range direction using pulse compression technology), Tokyo Denki University Press, pp. 131-149, 2004 Kazuo Ouchi, "Basics of synthetic aperture radar for remote sensing" (6.3 Synthetic aperture processing in the time domain), Tokyo Denki University Press, pp. 171-176, 2004 Supervised by Takashi Yoshida, “Revised Radar Technology” (11.3.1 Synthetic Aperture Radar), Institute of Electronics, Information and Communication Engineers, pp. 280-283, 1996 D. E. WAHL et al., "Phase Gradient Autofocus - A Robust Tool for High Resolution SAR Phase Correction", IEEE Transaction on Aerospace and Electronic systems, Vol. 30, No. 3, pp.827-835, July 1994

The problem to be solved by the present invention is to provide a synthetic aperture radar device that improves the visibility of a target in an obtained image.

The synthetic aperture radar device of the embodiment includes a target axis extraction section and an image correction section. A synthetic aperture radar device performs synthetic aperture processing on reflected waves of signals transmitted toward an imaging range to create an image. The target axis extraction unit extracts from the image a direction along the shape of the target shown in the image obtained by converting the two-dimensional data of the range-Doppler axis obtained from the reflected wave into a luminance value. The image correction unit adjusts the brightness value in a range including the target and determined in a direction along the shape of the target. The image correction unit enlarges the image of the target along the Doppler axis direction in the image by multiplying the image by a correction matrix determined according to a direction along the shape of the target.

FIG. 2 is a diagram showing an overview of a spotlight mapping method in the prior art. FIG. 1 is a diagram illustrating a configuration example of a synthetic aperture radar device in a first embodiment. The figure which shows an example of the image obtained in 1st Embodiment. 5 is a flowchart showing processing by a target axis extraction unit and an image correction unit in the first embodiment. FIG. 3 is a diagram showing an overview of processing for extracting a target axis by a target axis extraction unit in the first embodiment. FIG. 7 is a diagram showing an example of a process in which the target axis extraction unit detects the center of the target image in the first embodiment. 7 is a flowchart showing processing by a target axis extraction unit and an image correction unit in the second embodiment. FIG. 7 is a diagram illustrating an example of a brightness adjustment range determined by an image correction unit in the second embodiment. 12 is a flowchart showing processing by a target axis extraction section and an image correction section in the third embodiment. 12 is a flowchart showing processing by a target axis extraction section and an image correction section in the fourth embodiment. FIG. 7 is a diagram illustrating a correction matrix in a fourth embodiment. FIG. 7 is a diagram showing an example of correction in the fourth embodiment. 12 is a flowchart showing processing by an image correction unit in the fifth embodiment. 7 is a graph showing an overview of conversion in the fifth embodiment. 10 is a flowchart showing processing by an image correction unit in a sixth embodiment. 7 is a graph showing an example of conversion in the sixth embodiment. 12 is a flowchart showing processing by an image correction unit in the seventh embodiment. FIG. 7 is a diagram illustrating an example of processing in the seventh embodiment. 10 is a flowchart showing processing by an image correction unit in the eighth embodiment. A graph showing an example of conversion in the eighth embodiment. 12 is a flowchart showing processing by an image correction unit in the ninth embodiment. A graph showing an example of conversion in the ninth embodiment.

Hereinafter, a synthetic aperture radar device according to an embodiment will be described with reference to the drawings. In the following embodiments, components with the same reference numerals perform similar operations, and redundant explanations will be omitted as appropriate. The SAR method used in the synthetic aperture radar device of this embodiment will be described. FIG. 1 is a diagram showing an overview of the spotlight mapping method (Non-Patent Document 3). In the spotlight mapping method, a radar device mounted on an aircraft or the like controls a real aperture beam so that the real aperture beam is always directed toward an imaging target area. Data is acquired for each range cell within the PRI for each pulse transmitted at PRI (Pulse Repetition Interval) intervals during the synthetic aperture time (one cycle). A synthetic aperture radar device acquires an image by performing SAR processing using the acquired data.

Although a spotlight mapping method in which a real aperture beam is always directed toward the imaging target range has been illustrated here, the synthetic aperture radar device of this embodiment can also use other methods ( A strip map method, a squint mode mapping method, etc.) may also be used. In addition, although FIG. 1 is illustrated as a plane (two-dimensional) to simplify the description, the synthetic aperture radar device of this embodiment has an observation range that covers a space centered on an arbitrary point in three-dimensional space. Detect and observe targets. Further, the synthetic aperture radar device of this embodiment may observe a moving target in the imaging target range without moving itself. That is, the synthetic aperture radar device may operate as a reverse synthetic aperture radar that utilizes the movement or attitude change of the target to be observed.

(First embodiment)
FIG. 2 is a diagram showing a configuration example of the synthetic aperture radar device 100 in the first embodiment. The synthetic aperture radar device 100 includes a transmitter 1, a circulator 2, an antenna 3, a beam controller 4, a frequency converter 5, an AD converter 6, a range compressor 7, a cross range compressor 8, an autofocus processor 9, and a target. It includes an axis extraction section 10 and an image correction section 11. The transmitter 1 supplies the generated transmission signal to the antenna 3 via the circulator 2. The transmission signal is sent out as a radar wave from the antenna 3 in a beam direction directed toward an imaging range, which is an observation range. Code modulation using a predetermined code sequence, linear frequency modulation in which the frequency is constantly changed within a predetermined frequency range, and the like are used to generate the transmission signal. For example, a code-modulated pulse chirp signal is used as the transmission signal.

The beam direction of the antenna 3 is controlled by a beam controller 4. When an array antenna is used as the antenna 3, the beam control unit 4 supplies the array antenna with a control signal indicating the amount of phase shift determined to direct the beam direction to the observation range. Furthermore, when a parabolic antenna is used as the antenna 3, the mechanical mechanism of the pedestal is controlled to direct the parabolic antenna toward the observation range. As the antenna 3, any antenna that can direct the beam direction to the observation range may be used.

A received signal received by antenna 3 is supplied to frequency converter 5 via circulator 2 . When a target to be observed is located within the observation range, the radar wave includes a reflected wave component that is reflected by the target. The frequency converter 5 down-converts the received signal supplied via the circulator 2 to a baseband signal, and supplies the baseband signal obtained by the down-conversion to the AD converter 6. The AD converter 6 converts the baseband signal into a digital signal and supplies it to the range compression section 7. The sampling frequency in the AD converter 6 is determined according to the bandwidth of the transmission signal. The range compression unit 7 performs pulse compression on the digital signal using a reference signal corresponding to the transmission signal. The range compression section 7 supplies the signal obtained by pulse compression to the cross range compression section 8. The cross-range compression section 8 performs correlation processing on the signal supplied from the range compression section 7 using a reference signal based on the synthetic aperture length. The cross-range compression unit 8 converts the signal obtained by the correlation processing into a SAR image and supplies the SAR image to the autofocus processing unit 9.

The autofocus processing unit 9 uses a known technique, for example, PGA (Phase Gradient Autofocus) described in Non-Patent Document 4 or the inverse filter method for each range cell described in Patent Document 1, to improve the image quality of the SAR image. Improve. The autofocus processing unit 9 supplies the SAR image with improved image quality to the target axis extraction unit 10. The target axis extraction unit 10 extracts an axis along the shape of the target included in the SAR image (hereinafter also simply referred to as "image"). The target axis extraction unit 10 supplies information indicating the extracted target axis and the image to the image correction unit 11. The image correction unit 11 uses the supplied information and the image to improve the visibility of the target in the image and improve the image quality of the image. The image correction unit 11 outputs an image with improved image quality. For example, the image correction unit 11 may supply the image to a display device to display the image, or may supply the image to a storage device to store the image.

A method of generating a SAR image will be described below, but other methods may be used. The range compression unit 7 performs correlation processing to calculate the correlation between the digital signal supplied from the AD conversion unit and the reference signal for range compression (Non-Patent Document 1). The digital signal is hereinafter referred to as an "input signal." The processing when correlation processing is performed in the frequency domain can be formulated as Equations (1-1) to (1-3).

式（１－１）～（１－３）において、ｓｉｇ（ｔ，ｕ）は、入力信号を表し、ＦＦＴｘ［・］はｔ軸（ｆａｓｔ－ｔｉｍｅ軸）に対するフーリエ変換を表す。Ｓｉｎ（ω，ｕ）は、入力信号ｓｉｇ（ｔ，ｕ）に対するｔ軸のフーリエ変換の結果を表す。Ｓｒｅｆ（ω，ｕ）は、参照信号に対するｔ軸のフーリエ変換の結果を表す。ｆは、送信信号の周波数を表し、μは、送信信号を生成するときの線形周波数変調における周波数勾配（Ｂ／τ）を表す。Ｂ及びτは、それぞれ送信信号の周波数帯域幅及びパルス幅を表す。ｔは、ＡＤ変換器６におけるサンプリング間隔を表す。ｕは、合成開口長のサンプル点を表す。Ａ^＊の右肩上付き「＊」は、複素共役を表す。

In equations (1-1) to (1-3), sig(t, u) represents an input signal, and FFTx[·] represents Fourier transform with respect to the t-axis (fast-time axis). Sin(ω,u) represents the result of t-axis Fourier transform for the input signal sig(t,u). Sref (ω, u) represents the result of t-axis Fourier transform for the reference signal. f represents the frequency of the transmission signal, and μ represents the frequency gradient (B/τ) in linear frequency modulation when generating the transmission signal. B and τ represent the frequency bandwidth and pulse width of the transmission signal, respectively. t represents the sampling interval in the AD converter 6. u represents the sample point of the synthetic aperture length. The superscript “*” in A ^* represents a complex conjugate.

In order to return the correlation processing result s(ω, u) to a time domain signal, it is sufficient to perform an inverse Fourier transform on s(ω, u) with respect to the ω axis (range axis). However, in order to perform cross-range compression (non-patent document 2) in the cross-range compression unit 8, the range compression unit 7 directly transmits the signal s(ω, u) on the ω-axis and the u-axis to the cross-range compression unit 8. supply The cross-range compression unit 8 generates a cross-range reference signal fs0. Reference signal fs0 is obtained by equations (2-1) and (2-2).

式（２－１）及び（２－２）において、Ｘｃ及びＹｃは、観測範囲の中心である画像化中心の位置（座標）を表す。ｕは、合成開口長のサンプル点を表す。ｋ（＝２π／λ）及びλは、それぞれ送信信号の周波数及び波長を表す。ｊは、虚数単位である。

In equations (2-1) and (2-2), Xc and Yc represent the position (coordinates) of the imaging center, which is the center of the observation range. u represents the sample point of the synthetic aperture length. k (=2π/λ) and λ represent the frequency and wavelength of the transmission signal, respectively. j is an imaginary unit.

The cross-range compression unit 8 calculates a signal cs(ω, u) by multiplying the signal s(ω, u) and the reference signal fs0. The signal cs(ω, u) is expressed by equation (3).

The cross-range compression unit 8 calculates the signal fcs(ω, ku) by performing FFT on the u-axis (slow-time axis) of the signal cs(ω, u). The signal fcs(ω, ku) is expressed by equation (4).

The SAR image fp(t, ku) as an output is calculated by inverse FFT regarding the ω axis of the signal fcs(ω, ku). Image fp(t, ku) is expressed by equation (5). The amplitude at the position (t, ku) of the image fp(t, ku) may be used as the brightness value, or the brightness value may be calculated from the amplitude using a predetermined conversion formula.

式（５）において、ＩＦＦＴｘ［・］は、ω軸（レンジ軸）に関する逆ＦＦＴを表す。

In Equation (5), IFFTx[•] represents an inverse FFT regarding the ω axis (range axis).

In the synthetic aperture radar device 100, after cross-range compression (azimuth compression) is performed, autofocus processing is performed to further improve the image quality of the SAR image. The autofocus processing unit 9 performs PGA (non-patent document 4) or the inverse filter method (patent document 1) on the image fp(t, ku) supplied from the cross-range compression unit 8 to improve the image quality.

FIG. 3 is a diagram showing an example of an image obtained in the first embodiment. Range compression and cross-range compression produce a range-Doppler axis image that includes an image of the target. Among the reflection points representing the shape of a target such as a ship or an aircraft, the intensity of the reflected wave reflected at the reflection point on the side far from the synthetic aperture radar device 100 is low, so the image brightness of the reflection point on the far side may be low. be. The synthetic aperture radar device 100 improves the visibility of the target even in images in such a case. In the synthetic aperture radar device 100, the target axis extraction unit 10 extracts an axis along the shape of the target from the target image included in the image, and the image correction unit 11 calculates the reflection point of the target based on the extracted axis. Improve target visibility by adjusting image brightness. Hereinafter, the axis along the target shape will be referred to as the "target axis." For example, the longitudinal direction of a target such as a ship or an aircraft serves as the target axis.

The processing performed by the target axis extraction section 10 and the image correction section 11 will be explained using FIGS. 4 and 5. FIG. 4 is a flowchart showing processing by the target axis extraction section 10 and the image correction section 11 in the first embodiment. FIG. 5 is a diagram illustrating an overview of the process of extracting a target axis by the target axis extraction unit 10 in the first embodiment.

When the target axis extraction unit 10 receives the image from the autofocus processing unit 9, it performs target centering to move (shift) the image of the target included in the image to the center of the image (step S11). In the target centering process, the target axis extraction unit 10 detects the center of the target image in the image. FIG. 6 is a diagram illustrating an example of a process in which the target axis extraction unit 10 detects the center of the target image in the first embodiment. As shown in FIG. 6, the target axis extraction unit 10 projects the amplitude onto each of the range axis (X-axis) and the Doppler axis (Y-axis), and makes sure that the projected amplitude exceeds a predetermined amplitude threshold. Extract the exceeded range as the target range. The target axis extraction unit 10 calculates the center of the target range as the center of the target image (Xtc, Ytc). The target axis extraction unit 10 moves the target image in the image so that the center (Xtc, Ytc) of the target image becomes the center (Xc, Yc) of the image.

Note that the amplitude may be projected onto the range axis and the Doppler axis so that the maximum amplitude value is obtained, or the sum of the projected amplitudes may be obtained. Additionally, the amplitude threshold may be determined based on the amplitude of noise in the image. For example, it may be determined using the average or variance of noise. Also, different amplitude thresholds may be used for the range and Doppler axes.

The target axis extraction unit 10 extracts the target axis and width of the target included in the image by using a plurality of predetermined template images for the image obtained by target centering in step S11. The width of the target is the length of the target in the direction perpendicular to the target axis. For example, as shown in FIG. 5, each of the plurality of template images has a band-shaped region having a width Wt, and a region whose center line passes through the center of the image is defined. The combination of the region width Wt and the center line inclination θt is different for each template image. An amplitude of "1" is assigned to a band-shaped region in the template image, and an amplitude of "0" is assigned to other regions. The target axis extraction unit 10 multiplies the image obtained by target centering and the template image, and adds each amplitude (step S12). The target axis extraction unit 10 stores the addition result in association with the template image (step S13). The target axis extraction unit 10 repeatedly performs the operations of steps S12 and S13 for each template image (steps S14 and S15).

When the target axis extraction unit 10 obtains the addition results for all template images, it selects the template image with the largest addition result (step S16). The band-shaped area in the selected template image has the widest area that overlaps with the target image, and the area in which the difference in inclination and width between the center line of the band-shaped area and the target axis is the smallest. By selecting the template image in this way, the target axis extraction unit 10 extracts the target axis and the width of the target in the image (step S17).

The image correction unit 11 adjusts the brightness value (amplitude value) in the image obtained by target centering based on the target axis and width extracted by the target axis extraction unit 10 (step S18), and ends the process. . Since the image correction unit 11 adjusts the brightness value even when the brightness value of the reflection point corresponding to the edge of the target is low in the region along the target axis, the visibility of the target can be improved. Specifically, the image correction unit 11 sets the target axis range specified by the target axis and width as the brightness adjustment range, increases the brightness value within the brightness adjustment range by a predetermined magnification, or increases the brightness value within the brightness adjustment range by a predetermined magnification. The brightness value is adjusted by adding a predetermined value to the brightness value within the adjustment range. In this way, the synthetic aperture radar device 100 improves the visibility of the target in the image and improves the image quality by increasing the brightness value within the brightness adjustment range without changing the amplitude of noise outside the brightness adjustment range. can.

Note that the width Wt of the strip-shaped area in the plurality of template images may be constant. That is, the target axis extraction unit 10 may extract the target axis using a plurality of template images in which the center line of the band-shaped region has a different slope θt. In this case, a template image having a width Wt determined depending on the target to be detected or observed may be used. That is, the target axis extraction unit 10 may extract at least the target axis from the image, and the image correction unit 11 may adjust the brightness of a brightness adjustment range including the target axis to improve the visibility of the target.

(Second embodiment)
In the first embodiment, a method has been described in which a band-shaped region including a target is extracted from an image and the brightness value within the region is adjusted. Although this method can adjust the brightness value by suppressing the influence of noise outside the brightness adjustment range, it cannot suppress the influence of noise within the brightness adjustment range. In the second embodiment, a method for suppressing the influence of noise within the brightness adjustment range will be described. Since the synthetic aperture radar device in the second embodiment has the same configuration as the synthetic aperture radar device 100 in the first embodiment, the description thereof will be omitted and will be explained with reference to the configuration and symbols shown in FIG. .

FIG. 7 is a flowchart showing processing by the target axis extraction section 10 and the image correction section 11 in the second embodiment. When the target axis extraction unit 10 receives the image from the autofocus processing unit 9, it extracts the target axis and the width of the target (step S21). The operation in step S21 is the same as the operation in steps S11 to S17 of the process shown in FIG. 4 described in the first embodiment. The image correction unit 11 projects the amplitude within the target axis range specified by the target axis and width onto the range axis (X axis) or the Doppler axis (Y axis) in the image obtained by target centering (step S22). The image correction unit 11 extracts the range in which the amplitude projected on the range axis (X-axis) or the Doppler axis (Y-axis) exceeds the amplitude threshold as a target range (step S23).

The image correction unit 11 determines a predetermined area of the band-shaped area that is in contact with the target range along the target axis as the brightness adjustment range (step S24). FIG. 8 is a diagram showing an example of the brightness adjustment range determined by the image correction unit 11 in the second embodiment. The image correction unit 11 determines an area in the target axis range that is in contact with the target range and determined by the length La and the width Wa as the brightness adjustment range. In the example shown in FIG. 8, among the regions in contact with the target range, the region on the far side in the range axis direction is determined as the brightness adjustment range. This is because, as described above, the brightness of reflection points far from the synthetic aperture radar device 100 tends to decrease. Note that the image correction unit 11 may determine both areas adjacent to the target range as the brightness adjustment range.

The image correction unit 11 adjusts the brightness value in the brightness adjustment range (step S25), and ends the process. The brightness value adjustment in step S25 is similar to the brightness value adjustment (step S18) in the first embodiment.

By determining a part of the target axis range as the brightness adjustment range, the synthetic aperture radar device 100 of the second embodiment suppresses the influence of noise within the target axis range, and also suppresses the reflection corresponding to the edge of the target. The brightness value of the point can be adjusted to improve the visibility of the target.

(Third embodiment)
In the second embodiment, a method has been described in which the area in which the brightness is adjusted is further limited in the target axis range defined by the target axis and the width to suppress the influence of noise within the target axis range. However, if there is a lot of noise in a region within the target axis range but outside the target range, the visibility of the target may not improve even if the brightness value is adjusted. In the third embodiment, a method that can improve the visibility of the target even in such a case will be described. Since the synthetic aperture radar device in the third embodiment has the same configuration as the synthetic aperture radar device 100 in the first embodiment, the description thereof will be omitted and will be explained with reference to the configuration and symbols shown in FIG. 2. .

FIG. 9 is a flowchart showing processing by the target axis extraction section 10 and image correction section 11 in the third embodiment. When the target axis extraction unit 10 receives the image from the autofocus processing unit 9, it extracts the target axis and the width of the target (step S31). The operation in step S31 is the same as the operation in steps S11 to S17 of the process shown in FIG. 4 described in the first embodiment. The image correction unit 11 projects the amplitude within the target axis range specified by the target axis and width onto the range axis (X axis) or the Doppler axis (Y axis) in the image obtained by target centering (step S32). The image correction unit 11 sets the range in which the amplitude projected on the range axis (X axis) or the Doppler axis (Y axis) exceeds the amplitude threshold as a target range (step S33), and determines the position (x, y) of the end of the target. Detected (step S34).

The image correction unit 11 adds the detected position (x, y) and the position (x, y) of the end of the target in the image obtained in the previous cycle in a weighted manner using the forgetting coefficient as a weight (step S35). . The weighted addition in step S35 is expressed by equation (6).

式（６）において、Ｘｓ（・）は、座標ベクトルの平滑値（ｘ，ｙ）を表す。Ｘｍ（・）は、今回のサイクルで検出された目標の端の位置を示す座標ベクトル（ｘ，ｙ）を表す。ｇは、忘却係数を表す。ｎは、ＳＡＲ画像が生成されるサイクル数を表す。サイクルは、ＣＰＩ（Coherent Pulse Interval）ごとに増加する値である。

In equation (6), Xs(·) represents the smoothed value (x, y) of the coordinate vector. Xm(·) represents a coordinate vector (x, y) indicating the position of the end of the target detected in the current cycle. g represents the forgetting coefficient. n represents the number of cycles in which the SAR image is generated. The cycle is a value that increases every CPI (Coherent Pulse Interval).

The image correction unit 11 determines a predetermined area adjacent to the position Xs(n) calculated by equation (6) as a brightness adjustment range (step S36), and adjusts the brightness value of the brightness adjustment range (step S37). The brightness value adjustment in step S37 is similar to the brightness value adjustment (step S18) in the first embodiment. The image correction unit 11 determines whether the cycle of generating the SAR image has ended (step S38), and if it continues, the process proceeds to step S39, and if it ends, the process ends. In step S39, the target axis extraction section 10 is supplied with a new SAR image from the autofocus processing section 9. Thereafter, the operations of steps S31 to S38 are repeated until the SAR image generation cycle is completed.

The synthetic aperture radar device 100 of the third embodiment improves the detection accuracy of the target edge even when the accuracy of extracting the target edge is low, and the position of the extracted target edge is smoothed by smoothing processing. Changes smoothly. This prevents the position of the brightness adjustment range from changing significantly between cycles, and improves the visibility of the target in images that are continuous in the time direction.

The process of adjusting brightness in the third embodiment can be combined with the process of adjusting brightness in the second embodiment. By performing the brightness adjustment according to the third embodiment after the brightness adjustment according to the second embodiment, the detection accuracy of the edge of the target can be improved and the visibility of the target can be further improved.

(Fourth embodiment)
In the second and third embodiments, a method has been described in which the visibility of the target is improved by adjusting the brightness value at the edge of the target. In the fourth embodiment, a method for dealing with factors other than brightness values that degrade target visibility will be described. The spread in the Doppler axis direction in the SAR image is proportional to the amount of rotation of the target in each of the yaw, pitch, and roll axes during the synthetic aperture time when observing the target. Therefore, when the amount of rotation is small, the spread in the Doppler axis direction in the SAR image becomes small, and the visibility of the target may deteriorate. Note that the amount of rotation of each axis is the amount of rotation relative to the synthetic aperture radar device. Regarding the synthetic aperture radar device in the fourth embodiment, a method that can improve target visibility even in such a case will be described. The synthetic aperture radar device in the fourth embodiment has the same configuration as the synthetic aperture radar device 100 in the first embodiment, so the description thereof will be omitted and will be explained with reference to the configuration and symbols shown in FIG. 2. .

FIG. 10 is a flowchart showing processing by the target axis extraction section 10 and the image correction section 11 in the fourth embodiment. When the target axis extraction unit 10 receives the image from the autofocus processing unit 9, it extracts the target axis and the width of the target (step S41). The operation in step 41 is the same as the operation in steps S11 to S17 of the process shown in FIG. 4 described in the first embodiment. The image correction unit 11 performs inverse FFT regarding the Doppler axis (Y axis) on the image obtained by target centering, and calculates range-Doppler axis data (step S42). The image correction unit 11 calculates a range-Doppler axis correction coefficient according to the inclination (inclination θt) of the target axis (step S43). The image correction unit 11 performs correction by multiplying the range-Doppler axis data by a correction coefficient (step S44). The image correction unit 11 performs cross-range compression on the corrected range-Doppler axis data (step S45), outputs the corrected image (step S46), and ends the process.

The operations of steps S42 to S46 will be explained in more detail below. Calculation of range-Doppler axis data in step S42 is expressed by equation (7).

式（７）において、Ｘｉｍａｇｅは、式（５）で表される画像ｆｐ（ｔ，ｋｕ）を表す。Ｘｍａｔは、補正前のレンジ－ドップラ軸のデータ（以下、「ＲＤデータ」という。）を表す。ＲＤデータは、レンジ軸において圧縮処理が行われ、ドップラ軸（Ｙ軸）において圧縮処理が行われていないデータである。ＩＦＦＴｙ［・］は、ドップラ軸（Ｙ軸）に関する逆ＦＦＴを表す。

In equation (7), Ximage represents the image fp(t, ku) expressed by equation (5). Xmat represents range-Doppler axis data (hereinafter referred to as "RD data") before correction. RD data is data that is compressed on the range axis and not compressed on the Doppler axis (Y axis). IFFTy[·] represents an inverse FFT regarding the Doppler axis (Y axis).

Next, a correction matrix used for correction will be formulated. FIG. 11 is a diagram illustrating a correction matrix in the fourth embodiment. As shown in FIG. 11, the correction matrix corresponds to setting a phase gradient with respect to the Y-axis (slow-time) for each range cell on the X-axis (fast-time). The phase gradient is determined so that the phase gradient increases depending on the position of the range cell. Since the phase gradient needs to be changed according to the inclination of the target axis, the image correction unit 11 selects one of the correction matrices Cal from equations (8) to (10) according to the inclination of the target axis.

When the tilt of the target axis is positive, that is, when the image is divided into four quadrants, and the target axis is in the first and third quadrants:

When the slope of the target axis is negative, i.e. when the target axis is in the second and fourth quadrants:

When the tilt of the target axis is 0, that is, when the target axis overlaps the Y axis:

[Start:Step:End] representing vectx and vecty in equations (8) and (9) represents a vector of Step intervals in the range from Start to End. vectx and Vecty represent vectors on the X-axis (fast-time) and Y-axis (slow-time), respectively. Cal is a matrix with size numx×numy. numx and numy represent the number of pixels on the X-axis and Y-axis in the image, respectively. ratio represents a multiplication coefficient, and i represents an imaginary unit. The superscript t in vect ^t represents the transposition of vect.

For example, if the inclination of the target axis is positive, numx = 2, numy = 2, vectx = [-1, 0, 1], vecty = [-1, 0, 1], and equation (8) By substituting the gradient of the phase component of the correction matrix Cal into the equation, equation (11) is obtained.

Each column component in the correction matrix Cal is the product of the gradient on the X-axis (fast-time) multiplied by the component of vecty on the Y-axis (slow-time), and represents that the phase gradient changes for each range cell. There is. FIG. 11 shows an example of the phase gradient represented by the correction matrix Cal. The phase gradient changes for each range cell, and the polarity of the slope of the phase gradient is opposite on the left and right sides of the center of the Y-axis. By correcting the image using a correction matrix including such a phase gradient, the image of the target can be enlarged along the Doppler axis. Since the sign of the phase gradient used for correction needs to be changed depending on the inclination of the target axis in the SAR image, the above-mentioned case classification is necessary. Image correction using the correction matrix Cal is expressed by equation (12).

式（１２）において、Ｘｍａｔｃａｌは、補正後のＲＤデータを表し、Ｘｍａｔは、補正前のＲＤデータを表す。

In equation (12), Xmatcal represents RD data after correction, and Xmat represents RD data before correction.

The corrected SAR image Ximage can be calculated using equation (13).

式（１３）において、ＦＦＴｙ［・］は、ドップラ軸（Ｙ軸）に関するＦＦＴを表す。

In Equation (13), FFTy[·] represents FFT regarding the Doppler axis (Y axis).

FIG. 12 is a diagram showing an example of correction in the fourth embodiment. As shown in FIG. 12, since the image of the target is enlarged along the Doppler axis, the visibility of the target can be improved even if the spread in the Doppler axis direction is small.

Note that the inclination of the target axis required for correction only needs to be known as to which of the three types it corresponds to as described above, so it is not necessary to obtain it using a method other than the method using the template image described in the first embodiment. It's okay. For example, the target axis extraction unit divides the image before correction shown in FIG. The inclination of the target axis may be obtained by comparing the sum of the sums of the fourth quadrant and the sum of the sums of the fourth quadrant. When the sum of the sums of the first and third quadrants is large, the inclination of the target axis is positive, and when the sum of the sums of the second and fourth quadrants is large, the inclination of the target axis is negative. If the difference between the respective sums is 0 or less than a certain value, the inclination of the target axis may be determined to be 0.

The method of enlarging the target along the Doppler axis in the fourth embodiment may be used in combination with the brightness value adjustment in the first to third embodiments. When combining these techniques, magnification of the target along the Doppler axis is performed before adjusting the brightness values.

(Fifth embodiment)
In the first to fourth embodiments, the visibility of the target is improved by extracting the target axis from the image, adjusting the brightness value near the edge of the target, and enlarging the target image along the Doppler axis. We explained the method to improve In the fifth embodiment, in addition to these methods, a method will be described in which the visibility of the target is improved by further converting the luminance value. Since the synthetic aperture radar device in the fifth embodiment has the same configuration as the synthetic aperture radar device 100 in the first embodiment, the description thereof will be omitted and will be explained with reference to the configuration and symbols shown in FIG. 2. .

FIG. 13 is a flowchart showing processing by the image correction unit 11 in the fifth embodiment. The image correction unit 11 normalizes the amplitude value in the image after adjusting the brightness value (amplitude value) according to the first to third embodiments or after enlarging the target image according to the fourth embodiment. (step S51). The image correction unit 11 logarithmically transforms the normalized amplitude value (step S52), and calculates the average value m and standard deviation σ of noise included in the image on the logarithmic axis (step S53). The image correction unit 11 divides the amplitude value included in the image by (noise average value m+standard deviation σ) (step S54), and sets the amplitude value for which the division result is 1 or less to 0 (step S55). The image correction unit 11 normalizes each amplitude value obtained by the division (step S56), and ends the process. When the above-mentioned processing is formulated, it is expressed by equations (14) and (15).

式（１４）において、Ｘｉｍａｇｅｎは変換後の画像を表し、Ｘｉｍａｇｅは変換前の画像を表す。ｍはノイズの平均値を表し、σはノイズの標準偏差を表し、Ｎは係数を表す。Ｎの値として、例えば、１、２、３の何れかが用いられ、画像に含まれるノイズの程度に応じて選択される。ｌｏｇｎ［・］は対数変換を表す。なお、第５の実施形態の対数変換では、変換結果が負の場合、変換結果を０とする。なお、振幅値を（ｍ＋Ｎ・σ）で除算せずに、Ｎ・σで除算してもよい。すなわち、平均値ｍを用いずともよい。

In equation (14), Ximagen represents the image after conversion, and Ximage represents the image before conversion. m represents the average value of noise, σ represents the standard deviation of noise, and N represents the coefficient. For example, one of 1, 2, and 3 is used as the value of N, and is selected depending on the degree of noise included in the image. logn[·] represents logarithmic transformation. Note that in the logarithmic transformation of the fifth embodiment, when the transformation result is negative, the transformation result is set to 0. Note that the amplitude value may be divided by N·σ instead of (m+N·σ). That is, the average value m may not be used.

式（１５）はステップＳ５６の正規化を表し、ｍａｘ［・］は最大値を取得する演算を表す。すなわち、正規化後の値は０から１の範囲の値となる。なお、ステップＳ５６における正規化は、式（１５）以外の演算を用いて行ってもよい。

Equation (15) represents normalization in step S56, and max[.] represents an operation to obtain the maximum value. That is, the value after normalization is a value in the range of 0 to 1. Note that the normalization in step S56 may be performed using calculations other than equation (15).

FIG. 14 is a graph showing an overview of conversion in the fifth embodiment. In FIG. 14, the horizontal axis represents the amplitude before conversion, and the vertical axis represents the amplitude after conversion. As shown in the graph of FIG. 14, the amplitude value after conversion is flattened. In such a logarithmic transformation, high brightness becomes low and low brightness becomes high. By using the logarithmic transformation of the fifth embodiment, even if the target is likely to have shading in brightness, variations in brightness values at each reflection point of the target can be suppressed, and the visibility of the target can be further improved.

(Sixth embodiment)
In the sixth embodiment, similarly to the fifth embodiment, a method of improving the visibility of the target by further converting the luminance value will be described. The synthetic aperture radar device in the sixth embodiment has the same configuration as the synthetic aperture radar device 100 in the first embodiment, so the description thereof will be omitted and will be explained with reference to the configuration and symbols shown in FIG. 2. .

FIG. 15 is a flowchart showing processing by the image correction unit 11 in the sixth embodiment. The image correction unit 11 normalizes the amplitude value in the image after adjusting the brightness value (amplitude value) according to the first to third embodiments or after enlarging the target image according to the fourth embodiment. (step S61). The image correction unit 11 limits the normalized amplitude value to a predetermined amplitude value (limit value) (step S62). Specifically, a limit process is performed in which a value exceeding a predetermined amplitude value is replaced with a predetermined amplitude value. The image correction unit 11 normalizes the amplitude value in the image after the limit processing (step S63), and ends the process. When the above-mentioned processing is formulated, it is expressed by equations (16) to (18).

式（１６）はステップＳ６１の正規化を表し、ｍａｘ［・］は最大値を取得する演算を表す。

Equation (16) represents normalization in step S61, and max[.] represents an operation to obtain the maximum value.

式（１７）はステップＳ６２のリミット処理を表し、ｌｉｍ［・］は所定値を超える値を所定値に置き換える演算を表す。リミット処理における所定値（制限値）は、例えば、振幅値が大きい上位１０％の度数が制限されるように選択される。所定値は、他の選択方法を用いて決めてもよく、振幅の分布に応じて決めてもよい。

Equation (17) represents the limit process in step S62, and lim[.] represents an operation for replacing a value exceeding a predetermined value with a predetermined value. The predetermined value (limit value) in the limit process is selected such that, for example, the frequencies of the top 10% of the largest amplitude values are limited. The predetermined value may be determined using another selection method or may be determined according to the amplitude distribution.

式（１８）はステップＳ６３の正規化を表す。ステップＳ６１及び６３における正規化は、式（１６）及び（１８）以外の演算を用いて行ってもよい。

Equation (18) represents the normalization in step S63. The normalization in steps S61 and 63 may be performed using calculations other than equations (16) and (18).

FIG. 16 is a graph showing an example of conversion in the sixth embodiment. The graph shown in FIG. 16 is a histogram, where the horizontal axis indicates amplitude and the vertical axis indicates the frequency of each amplitude value. By performing the limit processing, even if the target is a target where brightness tends to be biased, the brightness of reflection points with high brightness values is suppressed, and the brightness of reflection points with low brightness values becomes relatively high. Furthermore, since the distribution of the histogram is flattened, an effect similar to contrast enhancement in an image can be obtained, and the visibility of the target is further improved.

(Seventh embodiment)
In the seventh embodiment, similarly to the fifth and sixth embodiments, a method of improving the visibility of the target by further converting the luminance value will be described. Since the synthetic aperture radar device in the seventh embodiment has the same configuration as the synthetic aperture radar device 100 in the first embodiment, the description thereof will be omitted and will be explained with reference to the configuration and symbols shown in FIG. .

FIG. 17 is a flowchart showing processing by the image correction unit 11 in the seventh embodiment. The image correction unit 11 adjusts the brightness value (amplitude value) according to the first to third embodiments, or after enlarging the target image according to the fourth embodiment, the image correction unit 11 adjusts the brightness value (amplitude value) to exceed a predetermined amplitude threshold. Each reflection point is extracted (step S71). The image correction unit 11 performs weighted addition for each extracted reflection point using the amplitude and forgetting coefficient at the same position in the image obtained in the previous cycle (step S72). The image correction unit 11 determines whether the cycle of generating the SAR image has ended (step S73), and if it continues, the process proceeds to step S74, and if it ends, the process ends. In step S74, a new cycle of SAR images is acquired. Thereafter, the operations of steps S71 to S73 are repeated until the SAR image generation cycle is completed. The operation of step S72 can be formulated as Equation (19).

式（１９）において、Ａｓ（・）は、振幅値の重み付け加算結果を表す。Ａｍ（・）は、今回のサイクルで抽出された反射点の振幅値を表す。ｇは、忘却係数を表す。ｎは、ＳＡＲ画像が生成されるサイクル数を表す。サイクルは、ＣＰＩ（Coherent Pulse Interval）ごとに増加する値である。

In equation (19), As(·) represents the weighted addition result of the amplitude values. Am(·) represents the amplitude value of the reflection point extracted in the current cycle. g represents the forgetting coefficient. n represents the number of cycles in which the SAR image is generated. The cycle is a value that increases every CPI (Coherent Pulse Interval).

FIG. 18 is a diagram illustrating an example of processing in the seventh embodiment. As shown in FIG. 18, by performing weighted addition using the processing result of the previous cycle and the forgetting coefficient for each cycle, the reflection points that appeared in the image in the previous cycle appear as afterimages in the output image. Through this processing, the reflection point of the target becomes clearer in the image, so even if the brightness value is small, the edge of the target becomes easier to see, improving the visibility of the target.

(Eighth embodiment)
In the eighth embodiment, similarly to the fifth to seventh embodiments, a method of improving the visibility of the target by further converting the luminance value will be described. The synthetic aperture radar device in the eighth embodiment has the same configuration as the synthetic aperture radar device 100 in the first embodiment, so the description thereof will be omitted and will be explained with reference to the configuration and symbols shown in FIG. 2. .

FIG. 19 is a flowchart showing processing by the image correction unit 11 in the eighth embodiment. The image correction unit 11 normalizes the amplitude value in the image after adjusting the brightness value (amplitude value) according to the first to third embodiments or after enlarging the target image according to the fourth embodiment. (step S81). The image correction unit 11 divides the amplitude value into N sections (step S82). The image correction unit 11 converts the amplitude values of each section so as to overlap and align the amplitude values of any one section (step S83), and ends the process.

FIG. 20 is a graph showing an example of conversion in the eighth embodiment. The graph shown in FIG. 20 is a histogram, where the horizontal axis indicates amplitude and the vertical axis indicates the frequency of each amplitude value. In the example shown in FIG. 20, the range in which the amplitude of the reflection point can take is equally divided into five sections, and the amplitude of each section is converted to be equal to the amplitude of section #1. That is, each of the amplitude values in sections #2 to #5 is replaced with the amplitude value in section #1.

By converting amplitude values (brightness values) in this way, even if the target is a target that tends to have shading in brightness, the brightness values of reflection points with large amplitudes can be suppressed, and the brightness values of reflection points with small amplitudes can be reduced. It is emphasized and the brightness value is flattened. This flattening makes it easier to see the edges of the target and improves the visibility of the target.

In the example shown in FIG. 20, the case is shown in which it is divided into five sections and the amplitude values of the other sections are made equal to the amplitude value of section #1, but the present invention is not limited to this. The number of divisions and the range in which the amplitude values should be aligned may be determined depending on the noise included in the image, the amplitude distribution at the target reflection point, and the like.

(Ninth embodiment)
In the ninth embodiment, similarly to the fifth to eighth embodiments, a method of improving the visibility of the target by further converting the luminance value will be described. Since the synthetic aperture radar device in the ninth embodiment has the same configuration as the synthetic aperture radar device 100 in the first embodiment, the description thereof will be omitted and will be explained with reference to the configuration and symbols shown in FIG. 2. .

FIG. 21 is a flowchart showing processing by the image correction unit 11 in the ninth embodiment. The image correction unit 11 normalizes the amplitude value in the image after adjusting the brightness value (amplitude value) according to the first to third embodiments or after enlarging the target image according to the fourth embodiment. (step S91). The image correction unit 11 divides the amplitude value into a plurality of sections (step S92). The image correction unit 11 sets the converted amplitude value for each section (step S93), and determines a linear conversion formula for the amplitude value for each section (step S94). The image correction unit 11 converts the amplitude value of each section using the determined linear conversion formula (step S95), and ends the process.

FIG. 22 is a graph showing an example of conversion in the ninth embodiment. The graph shown in FIG. 22 shows the correspondence between amplitude values before and after conversion, with the horizontal axis representing the amplitude before conversion, and the vertical axis representing the amplitude after conversion. In the example shown in FIG. 22, the gradient of increase in brightness is large in the range where the brightness values before conversion are from a1 to a2, and the brightness increases in the range where the brightness values before conversion are from a2 to a3 and from a3 to a4. is set so that the increasing slope of is small.

The linear conversion formula for each section is determined based on the amplitude histogram in the image before conversion so that the distribution of brightness values is flattened. For example, the image correction unit 11 calculates the cumulative frequency for each section in the amplitude histogram, and determines the linear transformation formula for each section by using the reciprocal of the calculated cumulative frequency as a correction coefficient for the increasing slope. By performing such piecewise linear conversion on the amplitude, the amplitude (luminance) within a predetermined range can be set to an arbitrary value even for a target where brightness tends to vary. By making such a setting, the edge of the target becomes easier to visually recognize, and the visibility of the target is further improved.

The luminance value (amplitude value) conversions described in the fifth to ninth embodiments may be used in combination. The order in which the transformations are combined may be determined arbitrarily.

The synthetic aperture radar device in the above embodiment includes a CPU (Central Processing Unit), a memory, an auxiliary storage device, etc. connected via a bus, and when the CPU executes a program, the range compression unit 7, the cross-range compression unit 8. It may operate as the autofocus processing section 9, the target axis extraction section 10, and the image correction section 11. The CPU may perform some or all of the operations in the synthetic aperture radar device by executing programs stored in the auxiliary storage device. Further, all or part of the operations in the synthetic aperture radar device may be realized using hardware such as an ASIC (Application Specific Integrated Circuit), a PLD (Programmable Logic Device), or an FPGA (Field Programmable Gate Array). The program may be recorded on a computer-readable recording medium. The computer-readable recording medium is a non-temporary recording medium such as a flexible disk, a magneto-optical disk, a ROM, a CD-ROM, or other portable medium, or a hard disk or other storage device built into a computer system. The program may be transmitted via a telecommunications line.

According to at least one embodiment described above, the target axis, which is a direction along the shape of the target shown in the SAR image obtained by converting the two-dimensional data of the range-Doppler axis obtained from the reflected wave into a brightness value, is set in the SAR image. By having a target axis extractor that extracts from the target axis and an image correction unit that adjusts the brightness value in the brightness adjustment range determined by the target axis, the target axis can be extracted without changing the brightness values of areas other than the vicinity of the target image in the SAR image. The visibility of the target can be improved by changing the brightness value representing the image.

Although several embodiments of the invention have been described, these embodiments are presented by way of example and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, substitutions, and changes can be made without departing from the gist of the invention. These embodiments and their modifications are included within the scope and gist of the invention as well as within the scope of the invention described in the claims and its equivalents.

DESCRIPTION OF SYMBOLS 1... Transmission part, 2... Circulator, 3... Antenna, 4... Beam control part, 5... Frequency converter, 6... AD converter, 7... Range compression part, 8... Cross range compression part, 9... Auto focus processing part , 10...Target axis extraction unit, 11...Image correction unit, 100...Synthetic aperture radar device

Claims (9)

In a synthetic aperture radar device that performs synthetic aperture processing on the reflected waves of signals transmitted toward an imaging range and creates images,
a target axis extraction unit that extracts from the image a direction along the shape of the target shown in an image obtained by converting two-dimensional range-Doppler axis data obtained from the reflected wave into a brightness value;
an image correction unit that adjusts a brightness value in a range that includes the target and is determined in a direction along the shape of the target;
Equipped with
The image correction unit enlarges the image of the target in the image along the Doppler axis direction by multiplying the image by a correction matrix determined according to a direction along the shape of the target.
Synthetic aperture radar equipment. In a synthetic aperture radar device that performs synthetic aperture processing on the reflected waves of signals transmitted toward an imaging range and creates images,
a target axis extraction unit that extracts from the image a direction along the shape of the target shown in an image obtained by converting two-dimensional range-Doppler axis data obtained from the reflected wave into a brightness value;
an image correction unit that adjusts a brightness value in a range that includes the target and is determined in a direction along the shape of the target;
Equipped with
The image correction unit normalizes each brightness value of the image using a standard deviation of noise in the image, and logarithmically transforms each normalized brightness value.
Synthetic aperture radar equipment. In a synthetic aperture radar device that performs synthetic aperture processing on the reflected waves of signals transmitted toward an imaging range and creates images,
a target axis extraction unit that extracts from the image a direction along the shape of the target shown in an image obtained by converting two-dimensional range-Doppler axis data obtained from the reflected wave into a brightness value;
an image correction unit that adjusts a brightness value in a range that includes the target and is determined in a direction along the shape of the target;
Equipped with
The image correction unit divides a range of brightness values that can be taken in the image into a plurality of sections, and performs conversion to align the brightness value in any one of the plurality of sections with the brightness value in another section.
Synthetic aperture radar equipment. The target axis extraction unit extracts a width of the target in a direction perpendicular to a direction along the shape of the target from the image,
The image correction unit adjusts a brightness value in a range including the target and determined by a direction along the shape of the target and a width of the target.
A synthetic aperture radar device according to any one of claims 1 to 3 . The image correction unit extracts an edge of the target from the image, and calculates a brightness value in a region that includes the target and is in contact with the edge of the target, within a range defined by the direction along the shape of the target and the width of the target. adjust the
A synthetic aperture radar device according to any one of claims 1 to 4 . The image correction unit extracts the position of the end of the target in the image obtained for each cycle, and performs weighted addition using a forgetting coefficient on the extracted position and the position of the end of the target in the previous cycle. , calculating the position of the edge of the target in the current cycle, and adjusting the brightness value in a region touching the calculated edge of the target;
A synthetic aperture radar device according to any one of claims 1 to 5 . The image correction unit limits a luminance value exceeding a predetermined value in the image to the predetermined value.
A synthetic aperture radar device according to any one of claims 1 to 6. The image correction unit extracts a reflection point having a brightness value exceeding a predetermined threshold in the image obtained in each cycle, and calculates a brightness value between the brightness value of the extracted reflection point and the brightness value of the reflection point in the previous cycle. Perform weighted addition using a forgetting coefficient to generate an output image in the current cycle.
A synthetic aperture radar device according to any one of claims 1 to 7. The image correction unit divides the range of brightness values that can be taken in the image into a plurality of sections, determines a linear conversion formula for converting the brightness value for each section, and converts the brightness value of each section using the linear conversion formula. Convert brightness values,
A synthetic aperture radar device according to any one of claims 1 to 8 .

Images