Image playback device, image playback method, and program

Abstract

This image reproduction device performs range compression on a reception signal obtained as a result of object observation using an inverse synthetic aperture radar, decomposes the range-compressed signal into a plurality of sub-apertures in a time direction such that adjacent sub-apertures partially overlap, performs azimuth compression on each of the sub-apertures, adds a secondary phase thereto, and performs de-rotation, and thereafter reproduces an image by synthesizing the plurality of sub-apertures and performing secondary azimuth compression using the secondary phase.

Description

Inverse synthetic aperture radar image reproducing device, image reproducing method, and recording medium

[0001] The present disclosure relates to an image reproducing device, an image reproducing method, and a recording medium for an inverse synthetic aperture radar.

[0002] Synthetic aperture radar (SAR) is a microwave sensor mounted on a satellite or aircraft that observes the Earth's surface while moving. It combines small antenna apertures to virtually form a larger aperture (radar diameter), and then repeatedly transmits and receives radio waves, combining them while taking the Doppler effect into account to improve resolution. Inverse synthetic aperture radar (ISAR) is also known, which improves resolution by utilizing the movement and attitude change of a target (object) rather than the movement of the radar antenna. It compresses the range and azimuth of the object and images the target on the range-azimuth axis. Meanwhile, space debris, such as used satellites and rocket fragments, orbits the Earth at high speeds. To protect satellites and astronauts from the threat of space debris, attempts are being made to accurately track the orbits of space debris through space situation awareness (SSA). SSA requires not only the location information of space objects but also image information to improve classification accuracy.

[0003] Various techniques for ISAR image generation have been developed. Patent Document 1 discloses range alignment and rotation correction for high-resolution ISAR images, receiving full-aperture data samples and decomposing them into multiple subapertures, generating a primitive image for each subaperture while selecting prominent points, performing range alignment correction and rotation correction for each primitive image, correcting phase errors using a phase gradient autofocus algorithm, and combining the primitive images to generate a single high-resolution image. Patent Document 2 discloses an ISAR radar system that tracks targets acquired from received signals on both the range and Doppler frequency axes to obtain the target image center, and then images the target on the range and Doppler frequency axes using range compression and azimuth compression. The synthetic aperture length is decomposed into a time series to generate multiple ISAR images, extracting the principal axes of the multiple target images and observing the time change in the rotation of the principal axes as rotation speed. The cross-range image dimensions are corrected based on the rotation speed to obtain absolute position information for the target image.

[0004] US Patent No. 6,255,981 JP 2017-3494 A

[0005] In ISAR image reconstruction processing, high-resolution images can be obtained by performing range compression, azimuth compression, and various correction processes on received signals. While ISAR technology can image radar signals, radars that are not designed for ISAR processing, such as SSA radars, cannot obtain high-resolution images using simple synthetic aperture processing when the pulse repetition frequency (PRF) is low due to the distance to the target or hardware limitations. Therefore, there is a need to obtain high-resolution ISAR images using only software ingenuity, even with low-PRF radars, without modifying the hardware.

[0006] To improve the azimuth resolution of ISAR images, it is necessary to increase the synthetic aperture time. In doing so, the Doppler bandwidth of the signal from the target also increases, so to represent the signal from the target, observations must be made at a PRF greater than the increased Doppler bandwidth due to the Nyquist criterion. However, because the radar PRF depends on the hardware and the distance to the observation target, it is difficult to arbitrarily increase it.

[0007] One possible method is to simulate a received signal observed at a high PRF by upsampling and frequency-shifting a received signal observed at a low PRF during image reconstruction processing. However, to accurately simulate a high-PRF target signal, highly accurate observation information of the target is required. Furthermore, upsampling to the required PRF increases the data volume of the received signal, which increases the load on image processing.

[0008] In Patent Document 1, data samples are decomposed into multiple sub-apertures, and then various corrections are made to the resulting coarse images, which are then combined to obtain a single high-resolution image. However, this does not alleviate the increased image processing load. In Patent Document 2, the center of a target image obtained from a received signal is compressed, imaged, and decomposed into a time series of synthetic aperture lengths. The image dimensions are corrected based on the rotation speed of the main axis of multiple ISAR images, and the absolute position of the target image is calculated. This allows for highly accurate observation of the target position, but does not result in a high-resolution image from received signals observed at a low PRF without increasing the image processing load. For this reason, there has been a need for the development of a technology for reconstructing high-resolution ISAR images from received signals observed at a low PRF without increasing the computational cost of image reconstruction processing.

[0009] An object of the present disclosure is to provide an image reproducing device, an image reproducing method, and a recording medium for an inverse synthetic aperture radar that solves the above-mentioned problems.

[0010] A first aspect of the present disclosure is an image reproduction device comprising: a range compression processing unit that performs range compression on a received signal obtained by observing an object with an inverse synthetic aperture radar; and an image correction processing unit that decomposes the range-compressed signal into a plurality of subapertures in the time direction so that adjacent subapertures partially overlap, performs azimuth compression, adds a secondary phase, and derotates for each subaperture, and then combines the plurality of subapertures and performs secondary azimuth compression using the secondary phase to reproduce an image.

[0011] A second aspect of the present disclosure is an image reproduction method that performs range compression on a received signal obtained by observing an object with an inverse synthetic aperture radar, decomposes the range-compressed signal into multiple subapertures in the time direction so that adjacent subapertures partially overlap, performs azimuth compression, adds a secondary phase, and derotates for each subaperture, combines the multiple subapertures, and performs secondary azimuth compression using the secondary phase to reproduce an image.

[0012] A third aspect of the present disclosure is a recording medium storing a program for causing a computer of an image reproduction device to perform the following operations: range compression of a received signal obtained by observing an object with an inverse synthetic aperture radar; decomposing the range-compressed signal into a plurality of subapertures in the time direction so that adjacent subapertures partially overlap; performing azimuth compression, adding a secondary phase, and derotation for each subaperture; and combining the plurality of subapertures to perform secondary azimuth compression using the secondary phase, thereby reproducing an image.

[0013] According to the present disclosure, it is possible to reconstruct a high-resolution ISRA image from a received signal observed at a low PRF by an ISAR radar without increasing the image processing load.

[0014] Fig. 5 is a block diagram of an inverse synthetic aperture image reconstruction system according to the present disclosure. Fig. 6 is a graph illustrating a method of dividing received data relating to an inverse synthetic aperture image into a plurality of sub-apertures according to the present disclosure. Fig. 7 is a block diagram of an example configuration of an ISAR image reconstruction device according to the configuration of the present disclosure. Fig. 8 is a block diagram showing details of image correction processing of the ISAR image reconstruction device shown in Fig. 3. Fig. 9 is a flowchart for explaining an overview of ISAR image reconstruction processing according to the present disclosure. Fig. 10 is a flowchart for explaining details of image correction processing of the ISAR image reconstruction processing shown in Fig. 5.

[0015] An image reproducing device, an image reproducing method, and a program for an inverse synthetic aperture radar (ISAR) that transmits microwaves to a target, receives radio waves reflected from the target, and generates an image will be described in detail with reference to the accompanying drawings, along with embodiments thereof.

[0016] 1 shows the configuration of an inverse synthetic aperture image reconstruction system according to the present disclosure, and is composed of an input device 10, an output device 20, and an ISAR image reconstruction device 100. The input device 10 reads data obtained by ISAR observation, for example, by reading radio waves (reflected waves) reflected from a target when an ISAR satellite or the like transmits microwaves to a target as a received signal. The ISAR image reconstruction device 100 performs ISAR image processing (range compression processing, synthetic aperture processing, etc.) on the received signal from the input device 10 to reconstruct an ISAR image. The output device 20 outputs the ISAR image reconstructed by the ISAR image reconstruction device to an external device.

[0017] The ISAR image reproduction device 100 is configured with an information processing device such as a personal computer (PC), and implements desired functions by having a processor execute software (programs related to ISAR image reproduction processing) stored in memory. The processor may include a CPU (Central Processing Unit) that controls the entire device, a GPU (Graphics Processing Unit) that performs image analysis, a GPGPU (General-Purpose GPU), and a TPU (Tensor Processing Unit) that performs data processing. The ISAR image reproduction device 100 implements multiple functional units 110 to 170 by having the processor execute programs stored in memory. Specifically, the ISAR image reproduction device 100 implements a range compression processing unit 110, a sub-aperture division processing unit 120, a synthetic aperture processing unit 130, a secondary phase addition processing unit 140, a de-rotation processing unit 150, a sub-aperture synthesis processing unit 160, and a secondary azimuth compression processing unit 170.

[0018] In ISAR image processing, reflected waves from a target that are spread two-dimensionally in the range and azimuth directions are compressed into a single point using pulse compression technology (compression in the range direction) and synthetic aperture technology (compression in the azimuth direction), enabling clear imaging even if the target is moving. The range compression processor 110 performs range compression on the received signal, compressing the received signal, which is reflected waves from a target that are spread two-dimensionally in two orthogonal directions, the direction of travel of the target relative to the ISAR antenna (azimuth) and the direction in front of the antenna (range), in the range direction. The range compression processor 110 generates a range-compressed signal.

[0019] The sub-aperture division processing unit 120 time-divides the range-compressed signal through sub-aperture division processing. Specifically, the sub-aperture division processing unit 120 divides the range-compressed signal so that adjacent sub-apertures partially overlap, and multiplies the overlapping portions by the first half or second half of a window function (Hanning window) so that adjacent sub-apertures are smoothly connected during sub-aperture synthesis processing. The synthetic aperture processing unit 130 performs azimuth compression for each sub-aperture of the range-compressed signal through synthetic aperture processing. The azimuth compression is performed using compression parameters that are set according to the relative movement between the ISAR and the target, but the compression parameters may also be set in advance based on ISAR observation. The synthetic aperture processing unit 130 azimuth-compresses the range-compressed signal for each sub-aperture to generate a range-azimuth compressed signal.

[0020] The secondary phase adding processor 140 applies secondary phase adding processing to the range-azimuth compression signal for secondary azimuth compression, adding a phase that results in a continuous linear chirp between multiple sub-apertures. The linear chirp linearly changes the instantaneous frequency over time, adding an integer multiple of the starting frequency as time elapses (chirp rate), and linearly increasing the frequency over time between multiple sub-apertures. The derotation processor 150 applies derotation processing to the range-azimuth compression signal that has undergone secondary phase adding processing. This frequency shifts the Doppler frequency (Doppler rate) of the target signal, thereby bringing the derotated data frequency within the pulse repetition frequency (PRF) at the time of receiving the target signal.

[0021] The subaperture synthesis processor 160 combines all subapertures of the range-azimuth compression signal after derotation into one subaperture through the subaperture synthesis process. In the subaperture synthesis process, the effect of the window function multiplied to the overlapping portions of adjacent subapertures in the subaperture division process allows for smooth subaperture synthesis by adding multiple subapertures. Unlike the azimuth compression performed by the synthetic aperture processor 130 for each subaperture of the received data, the secondary azimuth compression processor 170 performs secondary azimuth compression using the secondary phase added by the secondary phase addition processor 140 on the range-azimuth compression signal obtained by combining multiple subapertures to obtain a high-resolution ISAR image. In this way, the ISAR image reconstruction device 100 generates a reconstructed ISAR image from the received signal obtained by ISAR observation. The output device 20 outputs the reconstructed ISAR image to an external device. For example, the reconstructed ISAR image may be output to a subsequent information processing device or an external display device.

[0022] Next, the operation of the inverse synthetic aperture image reconstruction system shown in Figure 1 will be described in detail. An input device 10 inputs a received signal acquired during ISAR observation and supplies it to an ISAR image reconstruction device 100. In the ISAR image reconstruction device 100, a range compression processing unit 110 range-compresses the received signal related to ISAR observation to generate a range-compressed signal. A subaperture division processing unit 120 performs subaperture division processing on the range-compressed signal so that the frequency band of a target signal included in each subaperture is equal to or less than the pulse repetition frequency (PRF), generating multiple subapertures in the time domain. Figure 2 shows a method for dividing multiple subapertures, in which adjacent subapertures are divided so that they overlap, and the overlapping portions are multiplied by a window function (Hanning window) to smoothly connect adjacent subapertures in the subaperture synthesis processing. FIG. 2 shows three subapertures (Sub1 to Sub3) adjacent to each other in the azimuth direction (Az direction) corresponding to the observation time. For example, in the overlapping portion between the rear portion of subaperture Sub2 and the front portion of subaperture Sub3, the rear portion of subaperture Sub2 is multiplied by a Hanning window (second half), and the front portion of subaperture Sub3 is multiplied by a Hanning window (first half).

[0023] For example, when observing a space object such as space debris using ISAR, it can be assumed that the difference in Doppler frequency of the reflected waves from each point on the object at each time is small, and the spread of the Doppler band of the received signal at each time is narrow. Therefore, the Doppler frequency of the received signal at each time is estimated from the orbital information of the object, and the size of the subaperture is determined so that the bandwidth of the received signal contained in the subaperture is equal to or less than the pulse repetition frequency (PRF). The subapertures Sub1 to Sub3 shown in Figure 2 have sizes determined in this way and partially overlap in the time domain.

[0024] The synthetic aperture processing unit 130 performs azimuth compression by synthetic aperture processing on the range compression signal in sub-aperture units to generate a range-azimuth compression signal. Note that various synthetic aperture processing methods have been developed, and there are no particular restrictions on the synthetic aperture processing method applied in this embodiment. The secondary phase addition processing unit 140 applies a linear chirp K to the range-azimuth compression signal (hereinafter referred to as signal S) in sub-aperture units. scl The secondary phase addition process is performed in the frequency domain, and the signal S' after the secondary phase addition is expressed by the following formula 1. In formula 1, symbol S' indicates the signal after the secondary phase addition, symbol S indicates the azimuth-compressed signal (range-azimuth compressed signal), and symbol K scl denotes a linear chirp, and the symbol f η indicates the Doppler frequency.

[0025]

[0026] In the derotation processing unit 150, in order to fit the amount of change in the Doppler frequency of the received signal within the pulse repetition frequency (PRF) during ISAR observation, the chirp K rot The derotation process is performed in the time domain, and the derotated signal S rot The following equation 2 generates the chirp K rot and the derotated signal S rot In Equation 2, symbol S' denotes a signal after adding a secondary phase, and symbol S rot denotes the signal after derotation, and the symbol v tar denotes the velocity of the target, the symbol λ denotes the wavelength of the carrier wave, and the symbol R ZD indicates the target distance at zero Doppler, and the symbol f DC ​denotes the Doppler frequency at the observation center time, symbol η denotes time based on zero Doppler, and symbol α denotes a derotation parameter. The derotation parameter α is set as follows: α=1−PRF / BW (BW denotes the bandwidth of the target signal throughout the observation). In some embodiments of the present disclosure, the derotation parameter α assumes that the target is a point, but in practice it is desirable to add a margin of several percent to several tens of percent to the bandwidth of the target signal.

[0027]

[0028] The sub-aperture synthesis processing unit 160 performs sub-aperture synthesis processing by adding all sub-apertures in the time domain. The secondary azimuth compression processing unit 170 performs a fast Fourier transform (FFT) on the signal after the sub-aperture synthesis to obtain the chirp K eff The reference signal S ref Finally, an ISAR image is generated by performing an inverse FFT. eff and the reference signal S ref and the symbol f η denotes the Doppler frequency, and the symbol K scl denotes a linear chirp, and the symbol K rot indicates the chirp used in the de-rotation. The output device 20 outputs the ISAR image reconstructed by the ISAR image reconstruction device 100.

[0029]

[0030] ​​An exemplary configuration of the present disclosure will be described with reference to FIGS. 3 and 4. FIG. 3 is a block diagram showing the functions of an exemplary configuration of an ISAR image reconstruction device 200 according to the present disclosure, which executes a range compression process 210 and an image correction process 220. The range compression process 210 performs range compression on the received signal obtained by ISAR observation. The image correction process 220, which is the core of several embodiments of the present disclosure, performs image correction on the received signal after range compression. The image correction includes azimuth compression for each subaperture, quadratic phase addition, and derotation. After all subapertures are combined, quadratic azimuth compression using the quadratic phase is performed. This enables high-resolution ISAR images to be reconstructed even with radar observations at low PRF.

[0031] 4 is a block diagram showing detailed functions of the image correction processing 220. The received signal after range compression is divided into multiple subapertures in the time direction by subaperture division 230. Then, synthetic aperture processing (azimuth compression) 240, quadratic phase addition 250, and de-rotation 260 are performed for each subaperture, and all subapertures are combined into one by subaperture combination 270. The received signal after subaperture combination is subjected to quadratic azimuth compression 280 using the quadratic phase, and an ISAR image is generated.

[0032] FIG. 5 is a flowchart illustrating an example of a function according to the present disclosure, corresponding to the configuration example of FIG. 3 . The ISAR image reproduction device 200 executes a range compression process (S10) and an image correction process (S20). FIG. 6 is a flowchart illustrating details of the image correction process (S20), which is a core function of some embodiments of the present disclosure. The range-compressed received signal is decomposed into multiple sub-apertures in the time direction (S21), and then image correction is performed (S22). In the image correction, azimuth compression, quadratic phase addition, and de-rotation are performed for each sub-aperture. The multiple sub-apertures are then combined into one (S23), and quadratic azimuth compression using a quadratic phase is performed (S24). The present disclosure is characterized in that azimuth compression, quadratic phase addition, and de-rotation are performed for each sub-aperture on the range-compressed received signal. In the quadratic phase addition, a linear chirp K is applied to the azimuth-compressed signal S for each sub-aperture, as expressed in Equation 1. scl In the de-rotation, the chirp K is added as shown in Equation 2. rot and converts the signal S' into the de-rotated signal S rot In de-rotation, the de-rotation parameter α is set to fit the target signal after de-rotation within the PRF. In second-order azimuth compression, as shown in Equation 3, the chirp K eff =K scl -K rot A reference signal S ref Second-order azimuth compression is performed using

[0033] In the present disclosure, ISAR image reproduction device 200 implements the above-described functions by having a processor (CPU, GPU, etc.) execute a range compression program, an image correction program, etc. However, the computer program may be stored in a storage medium (semiconductor memory such as ROM or RAM) or may be distributed from another device. Furthermore, the computer program may be a program for realizing part of the above-described functions, or may be a so-called difference file (or difference program) that can realize the above-described functions in combination with a program already stored in the computer system.

[0034] The present disclosure features an inverse synthetic aperture radar (ISAR) image processing technique that divides a received signal acquired at a pulse repetition frequency (PRF) lower than the Nyquist frequency in the time direction to generate multiple subapertures, performs synthetic aperture processing (azimuth compression), quadratic phase addition (linear chirp addition), and derotation on each subaperture, and finally combines all the subapertures. This makes it possible to reconstruct high-resolution ISAR images without using highly accurate target trajectory information, even for low-PRF observation data such as that from radar for space situational awareness. Note that the Nyquist frequency refers to half the sampling frequency in the sampling theorem, but is not limited thereto. It is a frequency that satisfies the Nyquist criterion for reproducing the original signal when a specific signal group is sampled and reconstructed.

[0035] The present disclosure is not limited to the above-described embodiment, and the configuration and functions of the ISAR image reconstruction device are not limited to those shown in FIGS. 1 to 6 . The present disclosure does not limit synthetic aperture processing and is applicable regardless of the synthetic aperture processing method. The window function multiplied by multiple subapertures in the subaperture division process is not limited to a Hanning window. Any window function can be applied that can smoothly connect overlapping portions of adjacent subapertures when combining the subapertures. Furthermore, although this may lead to an increase in azimuth ambiguity (azimuth ambiguity components that generate phase differences when aligning images), multiple subapertures may be combined by using half of the overlapping portions of each subaperture without multiplying the multiple subapertures by a window function.

[0036] The following describes the effects of an ISAR image reconstruction device (method) that takes into account the means for solving the problems of the present disclosure. (1) In the present disclosure, a received signal (e.g., a reflected wave received by irradiating a target, such as a space object or an object on the Earth's surface, with microwaves) acquired at a low PRF during ISAR observation is divided into multiple subapertures in the time direction (azimuth direction), and synthetic aperture processing is performed for each subaperture. The multiple subapertures are then combined to reconstruct a high-resolution ISAR image. (2) The division into multiple subapertures in the time direction is performed so that the Doppler frequency of the target signal (e.g., a reflected wave from the target) does not exceed the PRF during observation. If the accuracy of the target's orbit information is low, the image reconstruction process can be continued by reducing the amount of data to be included in each subaperture (or reducing the size of the subaperture) depending on the target's uncertainty. (3) Even when high-precision trajectory information of the target being observed cannot be obtained during observation using a low-PRF radar, high-resolution ISAR reconstructed images can be obtained without modifying the hardware by performing synthetic aperture processing on a subaperture basis and azimuth compression processing using a second-order phase (linear chirp) that is independent of the trajectory accuracy of the target. (4) This disclosure does not include upsampling of the received signal (increasing the sampling frequency), and image reconstruction processing can be performed using the low-PRF received signal as is, thereby avoiding an increase in the calculation load due to upsampling and preventing an increase in the calculation cost of the image reconstruction processing.

[0037] The technical features of the image reproduction device according to the present disclosure can be summarized as Supplementary Notes 1 to 5. (Supplementary Note 1) The image reproduction device includes a range compression processing unit that performs range compression on a received signal obtained by observing a target using an inverse synthetic aperture radar, and an image correction processing unit that decomposes the range-compressed signal into multiple subapertures in the time direction so that adjacent subapertures partially overlap, performs azimuth compression, adds a quadratic phase, and derotates each subaperture, and then combines the multiple subapertures to perform quadratic azimuth compression using the quadratic phase to reproduce an image. (Supplementary Note 2) The image correction processing unit may apply a window function to overlapping portions of adjacent subapertures in the multiple subapertures. (Supplementary Note 3) The image correction processing unit may add a first chirp to the signal after azimuth compression by adding the quadratic phase. (Supplementary Note 4) The image correction processing unit may perform phase rotation of a second chirp on the signal after adding the quadratic phase by derotation. (Supplementary Note 5) The image correction processing unit may perform secondary azimuth compression on the signal obtained by combining the multiple sub-apertures using a reference signal including a third chirp corresponding to the difference between the first chirp and the second chirp.

[0038] The present disclosure also includes an image reproduction method and a recording medium storing a program thereon, as described in Supplementary Note 6 and Supplementary Note 7. (Supplementary Note 6) The image reproduction method performs range compression on a received signal obtained by observing an object with an inverse synthetic aperture radar, decomposes the range-compressed signal into multiple subapertures in the time direction so that adjacent subapertures partially overlap, performs azimuth compression, adds a secondary phase, and derotates for each subaperture, combines the multiple subapertures, and performs secondary azimuth compression using the secondary phase to reproduce an image. (Supplementary Note 7) The recording medium stores a program for causing a computer of an image reproduction device to perform range compression on a received signal obtained by observing an object with an inverse synthetic aperture radar, decomposes the range-compressed signal into multiple subapertures in the time direction so that adjacent subapertures partially overlap, performs azimuth compression, adds a secondary phase, and derotates for each subaperture, combines the multiple subapertures, and performs secondary azimuth compression using the secondary phase to reproduce an image.

[0039] Although the present disclosure has been described above with reference to the embodiments, the present disclosure is not limited to the above-described embodiments. Various modifications that can be understood by those skilled in the art can be made to the configuration and details of the present disclosure within the scope of the present disclosure. Furthermore, each embodiment can be combined with other embodiments as appropriate.

[0040] This application claims priority based on Japanese Patent Application No. 2023-095429, filed on June 9, 2023, the disclosure of which is incorporated herein in its entirety by reference.

[0041] The present disclosure relates to receiving reflected waves of microwaves irradiated onto an object by an Inverse Synthetic Aperture Radar (ISAR) and performing image correction processing such as range compression and azimuth compression to reconstruct ISAR images, but the scope of application is not limited to observing the space situation of space objects (such as space debris), and can also be applied to observing surface structures (such as oil tanks), ships, observing subsidence in specific areas on the ground, and observing environmental destruction such as water source pollution and deforestation.

[0042] 10 Input device 20 Output device 100, 200 ISAR image reproduction device 110 Range compression processing unit 120 Sub-aperture division processing unit 130 Synthetic aperture processing unit 140 Secondary phase addition processing unit 150 De-rotation processing unit 160 Sub-aperture synthesis processing unit 170 Secondary azimuth compression processing unit 210 Range compression processing 220 Image correction processing 230 Sub-aperture division 240 Synthetic aperture processing (azimuth compression) 250 Secondary phase addition 260 De-rotation 270 Sub-aperture synthesis 280 Secondary azimuth compression

Claims

1. a range compression processing unit that performs range compression on a received signal obtained by observing an object using the inverse synthetic aperture radar; an image correction processing unit that decomposes the range-compressed signal into a plurality of subapertures in a time direction so that adjacent subapertures partially overlap, performs azimuth compression, adds a secondary phase, and derotates each subaperture, and then combines the plurality of subapertures and performs secondary azimuth compression using the secondary phase to reconstruct an image; An image reproduction device comprising:

2. 2. The image reproducing apparatus according to claim 1, wherein said image correction processing section applies a window function to overlapping portions of adjacent sub-apertures among said plurality of sub-apertures.

3. 2. The image reproducing apparatus according to claim 1, wherein the image correction processing unit adds a first chirp to the signal after the azimuth compression in the addition of the secondary phase.

4. 4. The image reproduction device according to claim 3, wherein the image correction processing unit performs the de-rotation by performing a phase rotation of a second chirp on the signal to which the quadratic phase has been added.

5. 5. The image reproduction device according to claim 4, wherein the image correction processing unit performs the second-order azimuth compression on the signal obtained by combining the plurality of sub-apertures using a reference signal including a third chirp corresponding to a difference between the first chirp and the second chirp.

6. Range compression is performed on the received signal obtained by observing the target using the inverse synthetic aperture radar, decomposing the range-compressed signal into a plurality of sub-apertures in a time direction such that adjacent sub-apertures partially overlap each other; Azimuth compression, secondary phase addition, and de-rotation are performed for each sub-aperture. a second-order azimuth compression process using the second-order phase is performed by combining the plurality of sub-apertures to reconstruct an image; Image playback method.

7. The image reproduction method according to claim 6, wherein a window function is applied to overlapping portions of adjacent subapertures in the plurality of subapertures.

8. An image reproduction method as described in Claim 6, wherein the addition of the secondary phase adds a first chirp to the signal after the azimuth compression.

9. An image reproduction method as described in Claim 8, wherein the derotation involves performing a phase rotation of a second chirp on the signal after the addition of the quadratic phase.

10. The computer of the image reproduction device Range compression is performed on the received signal obtained by observing the target using the inverse synthetic aperture radar, decomposing the range-compressed signal into a plurality of sub-apertures in a time direction such that adjacent sub-apertures partially overlap each other; Azimuth compression, secondary phase addition, and de-rotation are performed for each sub-aperture. a second-order azimuth compression process using the second-order phase is performed by combining the plurality of sub-apertures to reconstruct an image; A program to make it happen.