JP2021047019A - Synthetic aperture radar device - Google Patents

Abstract

To provide a synthetic aperture radar device that improves the visibility of a target in an obtained image.SOLUTION: A synthetic aperture radar device of the embodiment has an object axis extraction unit and an image correction unit. The synthetic aperture radar device performs synthetic aperture processing to image reflected wave of a signal transmitted toward an imaging range. The object axis extraction unit extracts a direction along a shape of an object shown in an image obtained by converting two-dimensional data of the range-Doppler axis obtained from the reflected wave into brightness values, from the image. The image correction unit adjusts the brightness value in a range determined in the direction along the shape of the object including the object.SELECTED DRAWING: Figure 2

Description

Embodiments of the present invention relate to synthetic aperture radar devices.

In the image obtained by Synthetic Aperture Radar (SAR) or Inverse Synthetic Aperture Radar (ISAR), it is a reflection point that reflects the radar wave transmitted periodically and shows the target shape. Of the reflection points, the brightness of the reflection point far from the radar may be lower than the brightness of the reflection point near the radar. For example, in an image obtained by ISAR for observing a moving ship, it may be difficult to confirm the shape and overall length of the ship from the image because the brightness of the reflection point far from the radar among the reflection points in the ship is low. ..

In addition, 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 with reference to the high brightness, the visibility of the reflection point with low brightness may decrease. Further, if the brightness of each reflection point in the image is adjusted with reference to the low brightness, the brightness may be saturated and the visibility of the shape and the total length of the ship may be lowered.

The spread in the Doppler axis direction in the image obtained by ISAR is proportional to the amount of rotation of each axis of the ship's yaw, pitch, and roll with respect to the radar during the transmission period of the radar wave. 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.

Japanese Patent No. 4908291

Kazuo Ouchi, "Basics of Synthetic Aperture Radar for Remote Sensing", (5.4 Range Direction Image Generation Using Pulse Compression Technology), Tokyo Denki University Press, pp. 131-149, 2004 Kazuo Ouchi, "Basics of Synthetic Aperture Radar for Remote Sensing", (Synthetic Aperture Processing in 6.3 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

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

The synthetic aperture radar device of the embodiment has a target axis extraction unit and an image correction unit. The synthetic aperture radar device performs synthetic aperture processing to image the reflected wave of the signal transmitted toward the imaging range. The target axis extraction unit extracts from the image the direction along the target shape shown in the image obtained by converting the two-dimensional data of the range-Doppler axis obtained from the reflected wave into the brightness value. The image correction unit adjusts the brightness value in a range determined in the direction along the shape of the target including the target.

The figure which shows the outline of the spotlight mapping method in the prior art. The figure which shows the structural example of the synthetic aperture radar apparatus in 1st Embodiment. The figure which shows an example of the image obtained in 1st Embodiment. The flowchart which shows the processing by the target axis extraction part and the image correction part in 1st Embodiment. The figure which shows the outline of the process which extracts the target axis by the target axis extraction part in 1st Embodiment. The figure which shows an example of the process which the target axis extraction part detects the center of the target image in 1st Embodiment. The flowchart which shows the processing by the target axis extraction part and the image correction part in the 2nd Embodiment. The figure which shows an example of the luminance adjustment range determined by the image correction part in 2nd Embodiment. The flowchart which shows the processing by the target axis extraction part and the image correction part in 3rd Embodiment. The flowchart which shows the processing by the target axis extraction part and the image correction part in 4th Embodiment. The figure explaining the correction matrix in 4th Embodiment. The figure which shows an example of the correction in 4th Embodiment. The flowchart which shows the processing by the image correction part in 5th Embodiment. The graph which shows the outline of the conversion in 5th Embodiment. The flowchart which shows the processing by the image correction part in 6th Embodiment. The graph which shows an example of conversion in 6th Embodiment. The flowchart which shows the processing by the image correction part in 7th Embodiment. The figure which shows an example of the process in 7th Embodiment. The flowchart which shows the processing by the image correction part in 8th Embodiment. The graph which shows an example of conversion in 8th Embodiment. The flowchart which shows the processing by the image correction part in 9th Embodiment. The graph which shows an example of conversion in 9th Embodiment.

Hereinafter, the synthetic aperture radar device of the embodiment will be described with reference to the drawings. In the following embodiments, the components with the same reference numerals perform the same operation, and duplicate description will be omitted as appropriate. The SAR method used in the synthetic aperture radar device of the present embodiment will be described. FIG. 1 is a diagram showing an outline of a spotlight mapping method (Non-Patent Document 3). In the spotlight mapping method, the radar device mounted on an aircraft or the like controls the actual aperture beam so that the actual aperture beam is always emitted toward the imaging target range. Data is acquired in units of range cells in the PRI for each pulse transmitted at PRI (Pulse Repetition Interval) intervals in the synthesis opening time (1 cycle). The synthetic aperture radar device acquires an image by performing SAR processing using the acquired data.

Here, the spotlight mapping method in which the actual aperture beam is always irradiated toward the imaging target range has been illustrated, but the synthetic aperture radar device of the present embodiment may be another method (if an image can be obtained by SAR processing). A strip map method, a squint mode mapping method, etc.) may be used. Further, although the description is illustrated in a plane (two-dimensional) in FIG. 1 for the sake of simplicity, the synthetic aperture radar device of the present embodiment has an observation range in a space centered on an image of an arbitrary point in the three-dimensional space. To detect and observe the target. Further, the synthetic aperture radar device of the present embodiment may observe a moving target in the imaging target range without the own device moving. 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 according to the first embodiment. The synthetic aperture radar device 100 includes a transmitter 1, a circulator 2, an antenna 3, a beam control unit 4, a frequency converter 5, an AD converter 6, a range compression unit 7, a cross range compression unit 8, an autofocus processing unit 9, and a target. The axis extraction unit 10 and the image correction unit 11 are provided. The transmission unit 1 supplies the generated transmission signal to the antenna 3 via the circulator 2. The transmitted signal is transmitted as a radar wave from the antenna 3 in the beam direction directed to the imaging range which is the observation range. For the generation of the transmission signal, code modulation using a predetermined code sequence, linear frequency modulation in which the frequency is constantly changed in a predetermined frequency range, or the like is used. For example, a code-modulated pulse chirp signal is used as the transmission signal.

The beam direction of the antenna 3 is controlled by the beam control unit 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 so as to direct the beam direction to the observation range. When a parabolic antenna is used as the antenna 3, the mechanical mechanism of the pedestal is controlled so that the directivity direction of the parabolic antenna is directed to the observation range. Any antenna that can direct the beam direction to the observation range may be used as the antenna 3.

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

The autofocus processing unit 9 uses a known technique, for example, the 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 obtain 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 an 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 an image to the display device to display the image, or may supply the image to the storage device to store the image.

Hereinafter, a method for generating a SAR image will be described, but other methods may be used. The range compression unit 7 performs correlation processing for calculating 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". When the processing when the correlation processing is performed in the frequency domain is formulated, it can be expressed by 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 a Fourier transform with respect to the t-axis (fast-time axis). Sin (ω, u) represents the result of the t-axis Fourier transform on the input signal sig (t, u). Sref (ω, u) represents the result of the t-axis Fourier transform on the reference signal. f represents the frequency of the transmitted signal, and μ represents the frequency gradient (B / τ) in linear frequency modulation when the transmitted signal is generated. B and τ represent the frequency bandwidth and pulse width of the transmitted signal, respectively. t represents the sampling interval in the AD converter 6. u represents a sample point of synthetic aperture length. The "*" on the right shoulder of ^{A * represents the complex conjugate.}

In order to return the result of the correlation processing s (ω, u) to the signal in the time domain, an inverse Fourier transform may be performed on the 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 transfers the signals s (ω, u) on the ω-axis and u-axis to the cross-range compression unit 8. Supply. The cross-range compression unit 8 generates a cross-range reference signal fs0. The reference signal fs0 is obtained by the 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 positions (coordinates) of the imaging center, which is the center of the observation range. u represents a sample point of synthetic aperture length. k (= 2π / λ) and λ represent the frequency and wavelength of the transmitted signal, respectively. j is an imaginary unit.

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

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

The SAR image fp (t, ku) as an output is calculated by the inverse FFT with respect to the ω axis of the signal fcs (ω, ku). The image fp (t, ku) is represented by the equation (5). The amplitude at the position (t, ku) of the image fp (t, ku) may be used as the luminance value, or the luminance value may be calculated from the amplitude by 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 with respect to the ω axis (range axis).

In the synthetic aperture radar device 100, after cross-range compression (azimuth compression) is performed, an autofocus process for further improving the image quality of the SAR image is performed. The autofocus processing unit 9 applies PGA (Non-Patent Document 4) or an inverse filter method (Patent Document 1) to 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 an image of the range-Doppler axis containing the target image. Of the reflection points that represent the shape of a target such as a ship or aircraft, the intensity of the reflected wave reflected at the reflection point far from the composite aperture radar device 100 is low, so the image brightness of the reflection point on the far side may be low. is there. The synthetic aperture radar device 100 improves the visibility of the target even if the image is in such a case. In the synthetic aperture radar device 100, the target axis extraction unit 10 extracts an axis along the target shape from the target image included in the image, and the image correction unit 11 determines the target reflection point based on the extracted axis. By adjusting the image brightness, the visibility of the target is improved. Hereinafter, the axis along the shape of the target is referred to as a "target axis". For example, the longitudinal direction of a target such as a ship or an aircraft is the target axis.

The processing performed by the target axis extraction unit 10 and the image correction unit 11 will be described with reference to FIGS. 4 and 5. FIG. 4 is a flowchart showing processing by the target axis extraction unit 10 and the image correction unit 11 in the first embodiment. FIG. 5 is a diagram showing an outline of a process of extracting a target axis by the target axis extraction unit 10 in the first embodiment.

When the image is supplied from the autofocus processing unit 9, the target axis extraction unit 10 performs target centering that moves (shifts) the target image 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 showing an example of a process in which the target axis extraction unit 10 in the first embodiment detects the center of the target image. As shown in FIG. 6, the target axis extraction unit 10 projects an amplitude with respect to each of the range axis (X axis) and the Doppler axis (Y axis), and the projected amplitude determines 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 of the target image (Xtc, Ytc) becomes the center of the image (Xc, Yc).

The amplitude projected on the range axis and the Doppler axis may be such that the maximum amplitude value can be obtained or the sum of the projected amplitudes can be obtained. Further, 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 dispersion of noise. Further, different amplitude thresholds may be used for each of the range axis and the Doppler axis.

The target axis extraction unit 10 extracts the target axis width and width of the target included in the image by using a plurality of predetermined template images with respect to the image obtained by the target centering in step S11. The width of the target is the length of the target in the direction orthogonal to the target axis. In the plurality of template images, for example, as shown in FIG. 5, a band-shaped region having a width Wt and a region in which the center line of the region passes through the center of the image is defined. The combination of the width Wt of the region and the slope θt of the center line is different for each template image. An amplitude "1" is assigned to the band-shaped region in the template image, and an amplitude "0" is assigned to the 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 saves the addition result in association with the template image (step S13). The target axis extraction unit 10 repeats 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 the template images, the target axis extraction unit 10 selects the template image having the maximum addition result (step S16). The band-shaped area in the selected template image has the widest area overlapping with the target image, and the area having the smallest difference in inclination and width between the center line of the band-shaped area and the target axis. 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 end 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 the width as the brightness adjustment range, and increases the brightness value within the brightness adjustment range by a predetermined magnification, or increases the brightness. 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 raising the brightness value within the brightness adjustment range without changing the amplitude of the noise outside the brightness adjustment range. it can.

The width Wt of the band-shaped region in the plurality of template images may be constant. That is, the target axis extraction unit 10 may extract the target axis by using a plurality of template images having different inclinations θt of the center lines of the band-shaped region. In this case, a template image having a width Wt determined according to 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 the brightness adjustment range including the target axis to improve the visibility of the target.

(Second embodiment)
In the first embodiment, a method of extracting a band-shaped region including a target in an image and adjusting the brightness value in the region has been described. This method can adjust the brightness value by suppressing the influence of noise outside the brightness adjustment range, but cannot suppress the influence of noise within the brightness adjustment range. In the second embodiment, a method of suppressing the influence of noise within the brightness adjustment range will be described. Since the synthetic aperture radar device according to the second embodiment has the same configuration as the synthetic aperture radar device 100 according to the first embodiment, the description thereof will be omitted and the description will be described with reference to the configuration and reference numerals shown in FIG. ..

FIG. 7 is a flowchart showing processing by the target axis extraction unit 10 and the image correction unit 11 in the second embodiment. When the image is supplied from the autofocus processing unit 9, the target axis extraction unit 10 extracts the target axis and the width of the target (step S21). The operation of step S21 is the same as the operation of 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 the width on 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 a 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, among the band-shaped regions, a predetermined region 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 contacts the target range in the target axis range, and determines a region determined by the length La and the width Wa as the brightness adjustment range. In the example shown in FIG. 8, of 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 the reflection point far from the synthetic aperture radar device 100 tends to decrease. The image correction unit 11 may determine both regions in contact with 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 adjustment of the luminance value in step S25 is the same as the adjustment of the luminance value in the first embodiment (step S18).

The synthetic aperture radar device 100 of the second embodiment determines a part of the target axis range as the brightness adjustment range, thereby suppressing the influence of noise in the target axis range and reflecting the reflection corresponding to the end of the target. The brightness value of the point can be adjusted, and the visibility of the target can be improved.

(Third Embodiment)
In the second embodiment, a method of suppressing the influence of noise in the target axis range by further limiting the area for adjusting the brightness in the target axis range determined by the target axis and the width has been described. However, when there is a lot of noise in the area within the target axis range and outside the target range, the visibility of the target may not be improved even if the brightness value is adjusted. In the third embodiment, a method capable of improving the visibility of the target will be described even in such a case. Since the synthetic aperture radar device according to the third embodiment has the same configuration as the synthetic aperture radar device 100 according to the first embodiment, the description thereof will be omitted and the description will be described with reference to the configuration and reference numerals shown in FIG. ..

FIG. 9 is a flowchart showing processing by the target axis extraction unit 10 and the image correction unit 11 in the third embodiment. When the image is supplied from the autofocus processing unit 9, the target axis extraction unit 10 extracts the target axis and the width of the target (step S31). The operation of step S31 is the same as the operation of 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 the width on 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 target range (step S33) as the range in which the amplitude projected on the range axis (X-axis) or the Doppler axis (Y-axis) exceeds the amplitude threshold, and sets the target end positions (x, y). Detect (step S34).

The image correction unit 11 weights and adds the detected position (x, y) and the position of the target end (x, y) in the image obtained in the previous cycle using the forgetting coefficient as a weight (step S35). .. The weighted addition in step S35 is represented by the 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 the equation (6), Xs (.) Represents the smooth 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 this 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 with each CPI (Coherent Pulse Interval).

The image correction unit 11 determines a predetermined region in contact with the position Xs (n) calculated by the equation (6) as the brightness adjustment range (step S36), and adjusts the brightness value of the brightness adjustment range (step S37). The adjustment of the luminance value in step S37 is the same as the adjustment of the luminance value in the first embodiment (step S18). The image correction unit 11 determines whether or not the cycle for generating the SAR image has been completed (step S38), proceeds to step S39 if it continues, and ends the process if it ends. In step S39, the target axis extraction unit 10 is supplied with a new SAR image from the autofocus processing unit 9. After that, 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 end even when the accuracy of extracting the target end is low, and the position of the extracted target end is smoothed by smoothing. It changes smoothly. As a result, it is possible to prevent the position of the brightness adjustment range from changing significantly between cycles and improve the visibility of the target in images that are continuous in the time direction.

The process of adjusting the brightness of the third embodiment can be combined with the process of adjusting the brightness of the second embodiment. By adjusting the brightness of the third embodiment after the brightness adjustment of 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 of improving the visibility of the target by adjusting the brightness value at the edge of the target has been described. In the fourth embodiment, a method for dealing with factors other than the luminance value that deteriorate the visibility of the target will be described. The spread in the Doppler axis direction in the SAR image is proportional to the amount of rotation of each axis of the target yaw, pitch, and roll at the synthetic aperture time when observing the target. Therefore, if 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. The amount of rotation of each axis is the amount of rotation relative to the synthetic aperture radar device. The synthetic aperture radar device according to the fourth embodiment describes a method capable of improving the visibility of the target even in such a case. Since the synthetic aperture radar device according to the fourth embodiment has the same configuration as the synthetic aperture radar device 100 according to the first embodiment, the description thereof will be omitted and the description will be described with reference to the configuration and reference numerals shown in FIG. ..

FIG. 10 is a flowchart showing processing by the target axis extraction unit 10 and the image correction unit 11 in the fourth embodiment. When the image is supplied from the autofocus processing unit 9, the target axis extraction unit 10 extracts the target axis and the width of the target (step S41). The operation of step 41 is the same as the operation of steps S11 to S17 of the process shown in FIG. 4 described in the first embodiment. The image correction unit 11 performs an inverse FFT on the image obtained by target centering with respect to the Doppler axis (Y axis), and calculates the data of the range-Doppler axis (step S42). The image correction unit 11 calculates the correction coefficient of the range-Doppler axis according to the inclination (inclination θt) of the target axis (step S43). The image correction unit 11 multiplies the correction coefficient by the data on the range-Doppler axis to perform correction (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.

Hereinafter, the operations of steps S42 to S46 will be described in more detail. The calculation of the range-Doppler axis data in step S42 is expressed by the 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 the formula (7), Ximage represents the image fp (t, ku) represented by the formula (5). Xmat represents the range-Doppler axis data (hereinafter referred to as “RD data”) before correction. The RD data is data in which compression processing is performed on the range axis and compression processing is not performed on the Doppler axis (Y axis). IFFTy [·] represents an inverse FFT with respect to the Doppler axis (Y axis).

Next, the correction matrix used for the correction is formulated. FIG. 11 is a diagram illustrating a correction matrix according to 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 of the X-axis (fast-time). The phase gradient is determined so that the phase gradient increases according to 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 the correction matrix Cal according to any one of the formulas (8) to (10) according to the inclination of the target axis.

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

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

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

[Start: Step: End] representing vectorx and vector in the formulas (8) and (9) represents a vector of Step intervals in the range from Start to End. Vectorx and Vector represent vectors on the X-axis (fast-time) and the Y-axis (slow-time), respectively. Cal is a matrix of size number × number. numx and number represent the number of pixels on the X-axis and the Y-axis in the image, respectively. ratio represents the multiplication factor and i represents the imaginary unit. The ^{t on} the right shoulder of the vector represents the transpose of the vector.

For example, when the inclination of the target axis is positive and number = 2 and number = 2, vector = [-1,0,1] and vector = [-1,0,1], and the equation (8) The equation (11) is obtained by substituting it into the equation expressing the gradient of the phase component of the correction matrix Cal of.

Each column component in the correction matrix Cal is obtained by multiplying the gradient of the X-axis (fast-time) by the component of the vector of the Y-axis (slow-time), indicating 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 positive and negative of the slope of the phase gradient is opposite to the left and right with respect to the center of the Y axis. By correcting the image using the correction matrix including such a phase gradient, the target image can be enlarged along the Doppler axis. Since it is necessary to change the sign of the phase gradient used for the correction according to the inclination of the target axis in the SAR image, it is necessary to classify the cases as described above. The image correction using the correction matrix Cal is represented by the 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 the formula (12), Xmatcal represents the RD data after the correction, and Xmat represents the RD data before the correction.

The corrected SAR image Ximage can be calculated by the 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 an FFT with respect to 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 when the spread in the Doppler axis direction is small.

Since it is only necessary to know which of the three types the inclination of the target axis required for correction corresponds to as described above, it is acquired by using a method other than the method using the template image described in the first embodiment. You may. For example, the target axis extraction unit divides the uncorrected image shown in FIG. 12 into four quadrants, calculates the sum of the amplitudes for each quadrant, and sums the sums of the first and third quadrants and the second quadrant. And the sum of the sums of the fourth quadrants may be compared to obtain the inclination of the target axis. 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. When the difference between the sums is 0 or less than a certain value, it may be determined that the slope of the target axis is 0.

The method of expanding the target along the Doppler axis in the fourth embodiment may be used in combination with the adjustment of the brightness value in the first to third embodiments. When these methods are combined, the target is magnified along the Doppler axis before adjusting the brightness value.

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

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 performs logarithmic conversion of the normalized amplitude value (step S52), and calculates the average value m of noise included in the image on the logarithmic axis and the standard deviation σ (step S53). The image correction unit 11 divides the amplitude value included in the image by (noise mean value m + standard deviation σ) (step S54), and sets the amplitude value at which the division result is 1 or less to 0 (step S55). The image correction unit 11 normalizes each amplitude value obtained by division (step S56), and ends the process. When the above processing is formulated, it is represented 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 the formula (14), Ximage represents the image after conversion, and Ximage represents the image before conversion. m represents the mean value of noise, σ represents the standard deviation of noise, and N represents the coefficient. As the value of N, for example, any one of 1, 2, and 3 is used, and it is selected according to the degree of noise contained in the image. logn [・] represents logarithmic conversion. In the logarithmic conversion of the fifth embodiment, when the conversion result is negative, the conversion result is set to 0. The amplitude value may be divided by N · σ without dividing by (m + N · σ). That is, it is not necessary to use the average value m.

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 the normalization of step S56, and max [·] represents the operation of acquiring the maximum value. That is, the value after normalization is in the range of 0 to 1. The normalization in step S56 may be performed by using an operation other than the equation (15).

FIG. 14 is a graph showing an outline of the conversion according to 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 converted amplitude value is flattened. In such a logarithmic conversion, the high luminance is low and the low luminance is high. By using the logarithmic conversion of the fifth embodiment, even if the target is likely to have light and shade in brightness, the variation in the brightness value at each reflection point of the target is suppressed, and the visibility of the target is further improved.

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

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 by a predetermined amplitude value (limit value) (step S62). Specifically, limit processing is performed in which a value exceeding a predetermined amplitude value is replaced with a predetermined amplitude value. The image correction unit 11 standardizes the amplitude value in the image after the limit processing (step S63), and ends the processing. When the above processing is formulated, it is represented by the formulas (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 the normalization of step S61, and max [·] represents the operation of acquiring 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 processing in step S62, and lim [·] represents an operation of replacing a value exceeding a predetermined value with a predetermined value. The predetermined value (limit value) in the limit processing is selected so that, for example, the frequency of the upper 10% having a large amplitude value is limited. The predetermined value may be determined by using another selection method or may be determined according to the distribution of amplitude.

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 of step S63. The normalization in steps S61 and 63 may be performed by using an operation other than the 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, in which the horizontal axis indicates the amplitude and the vertical axis indicates the frequency of each amplitude value. By performing the limit processing, the brightness of the reflection point having a high brightness value is suppressed and the brightness of the reflection point having a low brightness value becomes relatively high even for a target in which the brightness tends to be biased. Further, since the distribution of the histogram is flattened, the same effect as the contrast enhancement in the image can be obtained, and the visibility of the target is further improved.

(7th Embodiment)
In the seventh embodiment, as in the fifth and sixth embodiments, a method for improving the visibility of the target by further converting the luminance value will be described. Since the synthetic aperture radar device according to the seventh embodiment has the same configuration as the synthetic aperture radar device 100 according to the first embodiment, the description thereof will be omitted and the description will be described with reference to the configuration and reference numerals 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 exceeds a predetermined amplitude threshold 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. Each reflection point is extracted (step S71). The image correction unit 11 performs weighting addition using the amplitude at the same position in the image obtained in the previous cycle and the forgetting coefficient for each extracted reflection point (step S72). The image correction unit 11 determines whether or not the cycle for generating the SAR image has been completed (step S73), proceeds to step S74 if it continues, and ends the process if it ends. In step S74, a SAR image of a new cycle is acquired. After that, the operations of steps S71 to S73 are repeated until the SAR image generation cycle is completed. When the operation of step S72 is formulated, it is represented by the 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 the formula (19), As (.) Represents the weighted addition result of the amplitude value. Am (・) represents the amplitude value of the reflection point extracted in this 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 with each CPI (Coherent Pulse Interval).

FIG. 18 is a diagram showing an example of processing in the seventh embodiment. As shown in FIG. 18, by performing weighting 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 an afterimage in the output image. By this processing, the reflection point of the target becomes clearer in the image, so that the edge of the target can be easily visually recognized even when the brightness value is small, and the visibility of the target is improved.

(8th Embodiment)
In the eighth embodiment, as in the fifth to seventh embodiments, a method for improving the visibility of the target by further converting the luminance value will be described. Since the synthetic aperture radar device according to the eighth embodiment has the same configuration as the synthetic aperture radar device 100 according to the first embodiment, the description thereof will be omitted and the description will be described with reference to the configuration and reference numerals shown in FIG. ..

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 value of each section so as to superimpose and align the amplitude value 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, in which the horizontal axis indicates the 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 be taken is equally divided into five sections, and the amplitude of each section is converted so as to be aligned with the amplitude of section # 1. That is, each of the amplitude values in the intervals # 2 to # 5 is replaced with the amplitude value in the interval # 1.

By converting the amplitude value (luminance value) in this way, the brightness value of the reflection point having a large amplitude can be suppressed and the brightness value of the reflection point having a small amplitude can be suppressed even if the target tends to have light and shade in the brightness. It is emphasized and the brightness value is flattened. This flattening makes it easier to see the edge of the target and improves the visibility of the target.

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

(9th Embodiment)
In the ninth embodiment, as in the fifth to eighth embodiments, a method for improving the visibility of the target by further converting the luminance value will be described. Since the synthetic aperture radar device according to the ninth embodiment has the same configuration as the synthetic aperture radar device 100 according to the first embodiment, the description thereof will be omitted and the description will be described with reference to the configuration and reference numerals shown in FIG. ..

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 the 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. In the graph shown in FIG. 22, the correspondence between the amplitude values before and after the conversion is shown, the horizontal axis represents the amplitude before the conversion, and the vertical axis represents the amplitude after the conversion. In the example shown in FIG. 22, the increasing gradient of the brightness in the range where the brightness value before conversion is in the range of a1 to a2 becomes large, and the brightness in the range where the brightness value before conversion is in the range of a2 to a3 and the range from a3 to a4. The increasing gradient of is set to be small.

The linear transformation formula for each interval is determined so that the distribution of luminance values is flattened based on the amplitude histogram in the image before conversion. For example, the image correction unit 11 calculates the cumulative power for each section in the amplitude histogram, and uses the reciprocal of the calculated cumulative power as the correction coefficient for the increasing gradient to determine the linear conversion formula for each section. By performing such a piecewise linear conversion on the amplitude, the amplitude (luminance) in a predetermined range can be set to an arbitrary value even for a target in which the brightness tends to be shaded. By making such a setting, the edge of the target becomes easy to see, and the visibility of the target is further improved.

The conversion of the luminance value (amplitude value) described in the fifth to ninth embodiments may be used in combination. The order in which the transformations are combined may be arbitrary.

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

According to at least one embodiment described above, the SAR image shows the target axis, which is the direction along the target shape shown in the SAR image obtained by converting the two-dimensional data of the range-Doppler axis obtained from the reflected wave into the luminance value. By having a target axis extraction unit 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 brightness value is not changed in the SAR image except in the vicinity of the target image. The visibility of the target can be improved by changing the brightness value representing the image.

Although some embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the gist of the invention. These embodiments and modifications thereof are included in the scope and gist of the invention, as well as in the scope of the invention described in the claims and the equivalent scope thereof.

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

Claims (10)

In a synthetic aperture radar device that performs synthetic aperture processing to image the reflected wave of a signal transmitted toward the imaging range.
A target axis extraction unit that extracts a direction along a target shape shown in an image obtained by converting two-dimensional data of a range-Doppler axis obtained from the reflected wave into a brightness value from the image, and a target axis extraction unit.
An image correction unit that adjusts the brightness value in a range determined in the direction along the shape of the target including the target, and an image correction unit.
Synthetic aperture radar device. The target axis extraction unit extracts the width of the target in a direction orthogonal to the direction along the shape of the target from the image.
The image correction unit adjusts the brightness value in the direction along the shape of the target including the target and in the range determined by the width of the target.
The synthetic aperture radar device according to claim 1. The image correction unit extracts the end of the target from the image, and the brightness value in the region in contact with the end of the target in the range determined by the direction along the shape of the target and the width of the target including the target. To adjust,
The synthetic aperture radar device according to claim 1 or 2. The image correction unit extracts the position of the end of the target in the image obtained for each cycle, and weights and adds the extracted position and the position of the end of the target in the previous cycle using an oblivion coefficient. , Calculate the position of the end of the target in the current cycle, and adjust the brightness value in the area in contact with the calculated end of the target.
The synthetic aperture radar device according to any one of claims 1 to 3. The image correction unit expands the image of the target along the Doppler axis direction in the image by multiplying the image by a correction matrix determined according to the direction along the shape of the target.
The synthetic aperture radar device according to any one of claims 1 to 4. The image correction unit normalizes each luminance value of the image with the standard deviation of noise in the image, and logarithmically transforms each normalized luminance value.
The synthetic aperture radar device according to any one of claims 1 to 5. The image correction unit limits the luminance value exceeding a predetermined value in the image to the predetermined value.
The 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 for each cycle, and sets the brightness value of the extracted reflection point and the brightness value of the reflection point in the previous cycle. On the other hand, weighting addition using the oblivion coefficient is performed to generate an output image in the current cycle.
The synthetic aperture radar device according to any one of claims 1 to 7. The image correction unit divides the range of the brightness values that can be taken in the image into a plurality of sections, and performs a conversion that aligns the brightness values in any one section of the plurality of sections with the brightness values in the other sections.
The synthetic aperture radar device according to any one of claims 1 to 8. The image correction unit divides a range of luminance values that can be taken in the image into a plurality of sections, determines a linear conversion formula for converting the luminance value for each section, and uses the linear conversion formula to determine each of the sections. Convert the brightness value,
The synthetic aperture radar device according to any one of claims 1 to 9.

Images