Image generation device, synthetic aperture radar system, image processing method, and program

Abstract

Improve image quality while reducing processing load. [Solution] The image generation device comprises a division unit that divides the received signal obtained by squint observation in the time direction to generate sub-apertures, a synthesis unit that synthesizes each sub-aperture after predetermined signal processing without omission to generate a synthesized signal, and an image signal generation unit that generates an image signal based on the synthesized signal. The division unit determines the signal size in the time direction of the sub-apertures to suppress aliasing due to the squint observation, and determines the Fourier transform size in the azimuth direction based on the amount of shift in the time direction due to the signal processing.

Description

[Technical Field]

[0001] This disclosure relates to an image generation apparatus, a synthetic aperture radar system, an image processing method, and a program. [Background technology]

[0002] Synthetic Aperture Radar (SAR) is a type of radar mounted on moving objects such as aircraft and satellites to observe surfaces such as the Earth's surface. In recent years, to improve object identification capabilities through increased information acquisition and to enhance agility, techniques such as squint observations have been developed, where the SAR mounted on a moving object is swung forward or backward to conduct observations.

[0003] There is a technique that generates high-resolution images without image degradation without reducing processing efficiency, even when observing at large squint angles or with large range bandwidths, by using a long synthetic aperture time (see, for example, Patent Document 1). In the technique disclosed in Patent Document 1, the received signal is divided in the azimuth direction based on the movement of the radar device and the range bandwidth of the transmitted wave, a Fourier transform is applied to the divided received signals in the azimuth direction, pulse compression is applied to the Fourier-transformed received signals in the range direction, an azimuth-direction phase compensation signal is generated by multiplying the range-direction compressed received signal by an azimuth-direction reference function, scaling is applied to the azimuth-direction phase compensation signal in the azimuth direction, an inverse Fourier transform is applied to the azimuth-direction scaled received signal, and the received signals with the inverse Fourier transform applied in the azimuth direction are synthesized to generate a high-resolution image. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2012-093257 [Overview of the project] [Problems that the invention aims to solve]

[0005] The following analysis was performed by the inventors of this disclosure.

[0006] The technology disclosed in Patent Document 1 does not address squint observation in the first place. Furthermore, the regions are not always smoothly connected when the divided regions (sub-apertures) are combined. As a result, it is not possible to completely prevent signal leakage from the sub-apertures due to the effects of squint observation, and the signals are not combined without loss. Therefore, an improvement in image quality cannot be expected.

[0007] This disclosure is made in view of the above circumstances and aims to provide an image processing technology that contributes to improving image quality while reducing processing load. [Means for solving the problem]

[0008] According to the first perspective of this disclosure, A division unit that divides the received signal obtained by squint observation in the time direction to generate sub-apertures, A combining unit that generates a combined signal by combining each sub-aperture after predetermined signal processing without any omissions, The system includes an image signal generation unit that generates an image signal based on the aforementioned composite signal, The provided image generation device includes a segmentation unit that determines the time-direction signal size of the sub-aperture to suppress aliasing due to the squint observation, and determines the azimuth Fourier transform size based on the time-direction shift amount due to the signal processing.

[0009] According to the second perspective of this disclosure, The above-mentioned image generation device, A synthetic aperture radar device that collects the received signal by the aforementioned squint observation, A synthetic aperture radar system is provided, comprising an output device that generates and outputs image data from the aforementioned image signal.

[0010] According to the third perspective of this disclosure, An image processing method executed by a computer of an image generating apparatus, comprising: a dividing step of dividing a received signal obtained by squint observation in a time direction to generate sub-apertures; a combining step of combining all of the respective sub-apertures after performing predetermined signal processing without omission to generate a combined signal; an image signal generating step of generating an image signal based on the combined signal; in the dividing step, the signal size of the sub-apertures in the time direction is determined so as to suppress aliasing caused by the squint observation, and the Fourier transform size in the azimuth direction is determined based on the shift amount in the time direction caused by the signal processing. An image processing method is provided.

[0011] According to a fourth aspect of the present disclosure, there is provided a program for causing a computer of an image generating apparatus to execute the image processing method described above.

[0012] These programs can be recorded on a computer-readable storage medium. The storage medium may be a non-transitory medium such as a semiconductor memory, a hard disk, a magnetic recording medium, or an optical recording medium. The present disclosure can also be embodied as a computer program product. Effects of the Invention

[0013] According to the present disclosure, it is possible to provide an image processing technique that contributes to improving image quality while suppressing processing load. Brief Description of the Drawings

[0014] [Figure 1] (a) is an overall configuration diagram of an example of a synthetic aperture radar system according to an embodiment of the present disclosure, and (b) is a functional block diagram of an example of an image generating apparatus according to an embodiment of the present disclosure. [Figure 2] (a) and (b) are diagrams for explaining an outline of non-squint observation and squint observation, respectively. [Figure 3] This is a flowchart of the image generation process in one embodiment of the present disclosure. [Figure 4] This is a functional block diagram of an example of an image generation device according to one embodiment of the present disclosure. [Figure 5] (a) and (b) are diagrams illustrating a conventional sub-aperture and a sub-aperture of one embodiment of the present disclosure, respectively. [Figure 6] This figure illustrates the processing of sub-aperture division and overlap portion in one embodiment of the present disclosure. [Figure 7] This is a diagram illustrating the processing during sub-aperture synthesis in one embodiment of the present disclosure. [Figure 8] This is a flowchart of the image generation process in one embodiment of the present disclosure. [Figure 9] (a) and (b) are example images obtained using the conventional method and the method of this disclosure, respectively. [Figure 10] This is a hardware configuration diagram of an image generation device according to one embodiment of the present disclosure. [Modes for carrying out the invention]

[0015] Hereinafter, an outline of one embodiment of this disclosure (hereinafter referred to as "this embodiment") will be described with reference to the drawings. The reference numerals in the drawings are added to each element for convenience as an example to aid understanding, and are not intended to limit this disclosure to the illustrated embodiments. In addition, the connecting lines between blocks in the drawings and other references referred to in the following description include both bidirectional and unidirectional lines. Unidirectional arrows schematically indicate the flow of the main signal (data) and do not exclude bidirectionality.

[0016] Furthermore, although there are ports and interfaces at the input / output connection points of each block in the diagram, they are omitted from the illustration. Also, in the following explanation, "A and / or B" means either A or B, or A and B.

[0017] <<First Embodiment>> Hereinafter, one embodiment of the present disclosure will be described with reference to the drawings. Figure 1(a) is an overall configuration diagram of the synthetic aperture radar (SAR) system 100 of this embodiment.

[0018] The SAR system 100 of this embodiment includes a radar device 110, an image generation device 200, and an output device 120.

[0019] The radar system 110 is mounted on a flying object (for example, an artificial satellite or an airplane) and transmits and receives electromagnetic waves. For this purpose, it comprises a transmitter 111 and a receiver 112. The transmitter 111 irradiates the observation area 194 with electromagnetic wave pulses (pulse signals) as transmission waves at predetermined time intervals. The receiver 112 receives the reflected signals of the transmission waves and transmits them as received signals to the image generation device 200.

[0020] The image generation device 200 performs various signal processing on the received signal to generate an image signal and transmits it to the output device 120. The process of generating an image signal from a received signal is also referred to as image reproduction. The image generation device 200 may also be mounted on the flying object.

[0021] The output device 120 generates image data from the image signal and outputs it.

[0022] [Squint observation] In this embodiment, we will explain using the case of performing squint observation as an example. Before describing the details of the image generation device 200, we will explain non-squint observation and squint observation.

[0023] As described above, the radar device 110 is mounted on a mobile object such as an aircraft or satellite, and is used to observe, for example, the Earth's surface. Here, we will explain using the case where the radar device 110 is mounted on satellite 190 as an example.

[0024] In a typical, non-squint observation, as shown in Figure 2(a), the field of view of the radar device 110 moves as the satellite 190 moves (direction of movement 193), and the observation area 194 also moves.

[0025] Squint observation is an observation method in which observations are performed by swinging the radar device 110 mounted on satellite 190, or the satellite 190 itself equipped with the radar device 110, forward or backward, as shown in Figure 2(b). The angle between the direction perpendicular to the direction of travel 193 and the central direction of the illumination direction is called the squint angle 192.

[0026] The received signal obtained by the radar device 110 cannot be seen directly, so it is transmitted to the image generation device 200, where signal processing is performed and the image is reconstructed. However, conventional image processing algorithms for reconstructing images from the received signal of the radar device 110 are based on the assumption that the received signal is obtained from non-squint observations (squint angle 192 = 0°). Therefore, they cannot be directly applied to image processing of received signals obtained from squint observations.

[0027] Furthermore, in Spotlight observations with a narrower observation area, the Doppler frequency in the azimuth direction (193° in the direction of propagation) changes over time. As a result, the azimuth bandwidth of the received signal becomes larger than the pulse repetition frequency (PRF). In this case, in image processing, the received signal data acquired at a PRF below the Nyquist frequency is time-divided. Then, various signal processing operations are performed on each of the divided received signal data (hereinafter referred to as sub-apertures), and finally all sub-apertures are combined. Examples of signal processing include synthetic aperture processing, secondary chirp addition (secondary phase addition), and de-rotation processing.

[0028] However, the time-division (sub-aperture division) and synthesis (sub-aperture synthesis) processes of the received signal described above cannot be directly applied to the received signal observed using squint observation.

[0029] The sub-aperture division and sub-aperture synthesis processes of received signals are performed in the time domain. In the case of received signals obtained by non-squint observation, the signal within the time range extracted by sub-aperture division is subject to sub-aperture synthesis. However, in the case of received signals obtained by squint observation, when second-order phase addition is performed, the signal is time-shifted in the azimuth direction with respect to the slant range frequency. Therefore, if sub-aperture synthesis is performed in the same way as for received signals obtained by non-squint observation, it becomes impossible to synthesize all of the signal. As a result, some of the signal is lost, and unwanted virtual images are generated in the azimuth direction.

[0030] Furthermore, one method to mitigate the effects of squint observations is to employ a back projection algorithm (time-domain processing) in the synthetic aperture algorithm. However, this method increases the computational load compared to conventional frequency-domain processing algorithms.

[0031] To solve this problem, the image generation device 200 of this embodiment incorporates the following improvements in the sub-aperture division process and the sub-aperture synthesis process.

[0032] [Sub-aperture splitting process] 1) When determining the time-direction signal size of the signal data to be included in the sub-aperture, consider the effect of squint. 2) Furthermore, in determining the size of the Fourier transform size in the azimuth direction of the sub-aperture (hereinafter referred to as the azimuth FFT (Fast Fourier Transform) size), the amount of azimuth shift of the signal during second-order phase addition due to the effect of squint observation is taken into consideration. Note that zeros are used to pad the portion outside the signal data. [Sub-aperture compositing process] 3) The signals in the sub-aperture are shifted in the positive and negative directions in the time domain due to the addition of a second-order phase. During synthesis, this should be taken into consideration, and all signals should be synthesized through the sub-aperture without any omissions.

[0033] [Image generation device] Figure 1(b) shows the functional configuration of the image generation device 200 of this embodiment, which realizes this. As shown in this figure, the image generation device 200 of this embodiment includes a splitting unit 210, a signal processing unit 220, a combining unit 230, and an image signal generation unit 240.

[0034] The splitting unit 210 generates sub-apertures by splitting the received signal obtained by squint observation in the time direction. At this time, the splitting unit 210 determines the time-direction signal size of the sub-apertures to be split in order to suppress aliasing caused by squint observation. The splitting unit 210 also determines the azimuth FFT size based on the amount of time-direction shift caused by signal processing. In squint observation, the frequency of the transmitted wave has a width. The FFT size is determined taking into account the width of the Doppler center frequency shift caused by this.

[0035] The signal processing unit 220 performs signal processing on each sub-aperture to cancel out the azimuth chirp during observation and to ensure that the bandwidth after sub-aperture synthesis with added secondary phase components falls within the Nyquist range of the pulse repetition frequency. The signal processing performed here is basically the same as the signal processing performed on sub-apertures obtained by dividing the received signal obtained in non-squint observation. Specifically, the signal processing unit 220 performs, for example, azimuth compression, addition of secondary chirp (phase), and phase rotation.

[0036] The combining unit 230 generates a combined signal by combining all the sub-apertures after signal processing. The combining unit 230 combines all the sub-apertures by adding them in the time domain. For example, it combines all signals, including those that have been azimuth-shifted in the negative direction in the time domain, without any omissions.

[0037] The image signal generation unit 240 generates an image signal from the composite signal. The image signal generation unit 240 performs an azimuth FFT on the composite signal, performs azimuth compression using the phase difference of the added chirp and phase rotation, and finally performs an inverse FFT to generate the image signal as a reconstructed image. The image signal generation unit 240 outputs the generated image signal to the output device 120.

[0038] [Image generation processing] The image generation process flow of the image generation device 200 of this embodiment will be explained. Figure 3 shows the processing flow of the image generation process of this embodiment. This process is started when a received signal is received.

[0039] The splitting unit 210 performs a sub-aperture splitting process to split the received signal into sub-apertures (step S1101).

[0040] The signal processing unit 220 performs predetermined signal processing on each sub-aperture (step S1102).

[0041] The combining unit 230 performs a sub-aperture combining process to combine each sub-aperture after signal processing and generate a combined signal (step S1103).

[0042] Finally, the image signal generation unit 240 generates an image signal from the composite signal (step S1104), outputs it, and terminates the process.

[0043] As described above, according to this embodiment, the signal size of each sub-aperture is appropriately set. That is, the Doppler bandwidth of the target signal is divided so that it does not exceed the pulse repetition frequency (PRF) during observation. Therefore, signal aliasing of the PRF within the sub-aperture due to the influence of squint observation can be reduced.

[0044] Furthermore, according to this embodiment, the FFT size in the azimuth direction of each sub-aperture is appropriately set. This reduces signal leakage due to the sub-aperture signal leaking outside the extraction time range during signal processing due to the influence of squint observation.

[0045] Furthermore, according to this embodiment, during the sub-aperture synthesis process, signals that have undergone azimuth shift in the time domain are also synthesized without omission. This reduces signal loss in subsequent processing and suppresses the generation of unwanted virtual images.

[0046] Furthermore, to mitigate the effects of squint observations, the back projection algorithm (time-domain processing) is not used. Therefore, the computational load is reduced compared to methods that employ time-domain processing.

[0047] Based on the above, this embodiment makes it possible to improve image quality while suppressing the processing load.

[0048] <<Second Embodiment>> Next, a second embodiment of the present disclosure will be described. This embodiment is a more detailed embodiment of the first embodiment. In this embodiment, components with the same names as in the first embodiment basically have the same function as in the first embodiment. The following description of this embodiment will focus on the differences from the first embodiment.

[0049] The overall configuration of the SAR system 100 in this embodiment is the same as that of the first embodiment shown in Figure 1(a). However, instead of the image generation device 200, an image generation device 200a is provided. In the image generation device 200a of this embodiment, in the sub-aperture division process, a window function such as a Hanning window is multiplied by the overlap portion between each sub-aperture, for example, in order to ensure that the signals between sub-apertures are smoothly connected by simple addition during synthesis.

[0050] [Image generation device] Figure 4 shows the functional configuration of the image generation device 200a of this embodiment, which realizes this. As shown in this figure, the image generation device 200a of this embodiment includes a range compression unit 250, a sub-aperture division unit 210a, a composite aperture processing unit 260, a secondary phase addition unit 270, a derotation unit 280, a sub-aperture synthesis unit 230a, and a secondary azimuth compression unit 240a.

[0051] The range compression unit 250 performs range compression of the received signal. Range compression is a process to improve the resolution of the SAR in the distance direction (range direction). Here, the pulse width of the received signal pulse is narrowed by performing a predetermined cross-correlation process (a process that evaluates how closely the two sequential signals are interdependent or similar) on the shape of the transmitted signal and the shape of the received signal.

[0052] The sub-aperture division unit 210a corresponds to the division unit 210 of the first embodiment. The sub-aperture division unit 210a time-divides the received signal data and generates sub-apertures. At this time, as in the first embodiment, the signal size and azimuth direction FFT size of each sub-aperture are determined. Furthermore, in this embodiment, adjacent sub-apertures are divided so that they overlap. In addition, the overlapping portion is multiplied by the first or second half of a window function (Hanning window) so that it connects smoothly during the subsequent synthesis process.

[0053] Here, each sub-aperture will be explained using Figures 5(a) and 5(b). A conventional sub-aperture 310, as shown in Figure 5(a), includes a region where the extracted signal is stored (signal region 311) and a zero-filled data region (zero-filled region 312) for performing an FFT in the azimuth direction.

[0054] As shown in Figure 5(b), the sub-aperture 310a of this embodiment comprises a signal region 311, two additional signal regions 313, and a zero-padding region 312. The azimuth FFT size is the sum of these regions.

[0055] The sub-aperture division section 210a has a frequency band of PRF(f PRF The signal size of sub-aperture 310a is calculated so that it is equal to or less than the calculated value. Then, the received signal data is divided in the time domain using the calculated signal size. The signal size is the size (time width) in the time direction of the signal domain 311.

[0056] The sub-aperture division unit 210a takes into account the influence of squint observations when calculating the signal size of the sub-aperture 310a. The signal size (extracted time width) of sub-aperture 310 is calculated by the following equation (1).

number

[0057] Furthermore, in non-squint observations, the FFT size is usually the smallest power of 2 greater than or equal to the signal size. The azimuth FFT size is the azimuth buffer size of sub-apertures 310 and 310a. On the other hand, for received signals obtained in squint observations, it is necessary to consider the azimuth time shift after quadratic phase addition when calculating the azimuth FFT size.

[0058] Here, the azimuth FFT size N of sub-aperture 310a sub,FFT This is calculated using the following formula (2).

number

[0059] In the present embodiment, consideration is given to ΔShift Az which is unnecessary in non-squint observation. That is, this portion is added as the additional signal region 313. Accordingly, as shown in FIG. 5(b), the azimuth FFT size of the sub-aperture 310a can be set to a size that does not cause aliasing even when the signal is azimuth time-shifted.

[0060] Further, as described above, the sub-aperture dividing unit 210a performs division such that the signal regions 311 of adjacent sub-apertures 310a overlap. This state is shown in FIG. 6. Furthermore, the sub-aperture dividing unit 210a multiplies the overlapping portion by a Hanning window. This is to allow adjacent sub-apertures 310a to be connected smoothly in the sub-aperture combining process described later. Accordingly, even when an azimuth time shift of a signal occurs during quadratic phase addition, combining can be performed by simple signal addition during sub-aperture combining.

[0061] The synthetic aperture processing unit 260 performs azimuth compression by synthetic aperture processing for each sub-aperture 310a. Azimuth compression is processing for increasing the resolution in the azimuth direction (traveling direction 193) of SAR.

[0062] The secondary phase addition unit 270 adds a phase that is a continuous linear chirp between sub-apertures 310a to the azimuth-compressed data in preparation for secondary azimuth compression processing. Specifically, for the signal S that has been azimuth-compressed in units of sub-apertures 310a, a linear chirp signal K is added. scl The following is performed: This process is carried out in the frequency domain, and the signal S' after the addition of the second phase is expressed by the following equation (3).

number

[0063] The de-rotation unit 280 performs frequency shifting. Frequency shifting cancels out the change in the Doppler frequency (Doppler rate) of the target signal. This keeps the target signal within the PRF (Pressure Frequency Range) at the time of reception.

[0064] Specifically, the de-rotation unit 280 uses a chirp signal K in order to keep the amount of change in the Doppler frequency of the received signal within the PRF at the time of observation. rot The phase rotation is performed. This process is carried out in the time domain. Note that the signal S after de-rotation is... rot This can be expressed by the following equation (4).

number

[0065] The above-described composite aperture processing unit 260, secondary phase addition unit 270, and derotation unit 280 correspond to the signal processing unit 220 of the first embodiment.

[0066] The sub-aperture combining unit 230a corresponds to the combining unit 230 of the first embodiment. Similar to the combining unit 230, the sub-aperture combining unit 230a combines all sub-apertures 310a into one. In this embodiment, the sub-apertures 310a can be combined by adding each sub-aperture 310a due to the effect of the Hanning window multiplied by the overlap portion of the sub-apertures 310a. At this time, the received signal that has been shifted in the azimuth direction by the addition of the second phase is also combined without omission. For example, if the signal from time t_0 to time t_1 is extracted by sub-aperture division, conventionally, the signal from t_0 to t_1 would be the target of combination. However, in this embodiment, if the amount of time shift of the signal in the negative direction due to the addition of the second phase is Δt_m and the amount of time shift of the signal in the positive direction due to the addition of the second phase is Δt_p, the sub-aperture combining unit 230a will combine the signal from t_0-Δt_m to t_1+Δt_p.

[0067] The sub-aperture combining unit 230a adds the signals of all sub-apertures 310a in the time domain. At this time, as shown in Figure 7, in addition to the original signal domain 311, the signals of the added signal domain 314, which has shifted to the negative side from the sub-aperture start time due to the effect of second-order phase addition, and the added signal domain 315, which has shifted to the positive side, are also added together.

[0068] The sub-aperture combining unit 230a adds the azimuth time positive side signal (additional signal region 315) from 0 to (azimuth FFT size - signal size of sub-aperture 310a) / 2 to the buffer size (azimuth FFT size) of the sub-aperture 310a. It also adds the azimuth time negative side signal (additional signal region 314) from (azimuth FFT size - signal size of sub-aperture 310a) / 2 to the azimuth FFT size.

[0069] In squint observations, the signal can shift depending on the frequency range when a second phase is added, and it can also wrap around depending on the bandwidth. Therefore, if the edges are cut off during synthesis, information that should be added will not be added, resulting in gaps. To prevent this, the sub-aperture synthesis unit 230a in this embodiment adds all overlapping parts. In this embodiment, a Hanning window is applied during sub-aperture division, so the edges of the data are inherently dark. Therefore, simply adding them together results in a natural connection.

[0070] The secondary azimuth compression unit 240a corresponds to the image signal generation unit 240 of the first embodiment. In this embodiment, the secondary azimuth compression unit 240a performs azimuth compression processing on the signal synthesized with the sub-aperture 310a using the added secondary phase. This results in an ISAR (Inverse Synthetic Aperture Radar) image signal with high resolution.

[0071] Specifically, the secondary azimuth compression unit 240a first performs an FFT in the azimuth direction. Then, as shown in equation (5) below, chirp K eff =K scl -K rot Azimuth compression is performed using the reference signal. Finally, an inverse FFT is performed to obtain the reconstructed image data ISAR.

number

[0072] [Image generation processing] The image generation process flow of the image generation device 200a in this embodiment will be explained. Figure 8 shows the processing flow of the image generation process in this embodiment. This process is performed each time a received signal is received from the receiving device 112.

[0073] First, the range compression unit 250 performs range compression processing using the method described above (step S2101).

[0074] The sub-aperture division unit 210a divides the received signal after range compression into sub-apertures 310a (step S2102). Prior to division, the sub-aperture division unit 210a determines the signal size and azimuth FFT size. It also multiplies the overlap portion of the signal region 311 by a window function.

[0075] The synthetic aperture processing unit 260 performs synthetic aperture processing as described above (step S2103). Here, azimuth compression is performed.

[0076] The secondary phase addition unit 270 performs a secondary phase addition process to add phase to the data after azimuth compression in preparation for secondary azimuth compression (step S2104).

[0077] The de-rotation unit 280 performs de-rotation processing (step S2105). Here, the chirp signal K rot Perform a phase rotation.

[0078] The sub-aperture combining unit 230a performs a sub-aperture combining process to combine each of the sub-apertures 310a after the signal processing (step S2106). Here, each of the processed sub-apertures 310a is added together.

[0079] The secondary azimuth compression unit 240a performs secondary azimuth compression processing (step S2107), generates an image signal, outputs the generated image signal, and terminates the process.

[0080] As described above, the image generation device 200a of this embodiment has the same configuration as the image generation device 200 of the first embodiment. Therefore, this embodiment provides the same effects as the first embodiment.

[0081] Furthermore, in this embodiment, after splitting, a window function is multiplied by the overlap portion between each sub-aperture 310a. As a result, during synthesis, the signals between sub-apertures 310a are smoothly connected by simple addition. In other words, during sub-aperture synthesis, by simply adding the overlapping portion of the signals even to the azimuth-shifted signals after quadratic phase addition, it becomes possible to perform synthesis processing that suppresses discontinuities between sub-apertures 310a.

[0082] Based on the above, this embodiment makes it possible to improve image quality while suppressing the processing load.

[0083] <Examples> Here, an example of an image 420 obtained from the simulation data of squint observation, which is generated by the image generation devices 200 and 200a (hereinafter referred to as image generation device 200) of each embodiment described above, is shown in Figure 9(b). As a comparative example, an example of an image 410 generated from the simulation data of squint observation, which is generated when a conventional method is used to divide into sub-apertures 310 and 310a, is shown in Figure 9(a).

[0084] This simulation is an example of observations performed at a squint of -15 degrees relative to a point target. In the figure, contour level lines at -3dB, -15dB, -30dB, and -45dB have been added.

[0085] As shown in Figure 9(a), when image reconstruction processing is performed using the conventional method, data loss occurs in the signal, and as a result, a virtual image is generated in the azimuth direction in image 410.

[0086] On the other hand, as shown in Figure 9(b), the image generation device 200 does not generate a virtual image in the azimuth direction, and it is possible to obtain an image 420 that is appropriately compressed in both the range direction and the azimuth direction.

[0087] <Variation> In the second embodiment, the synthetic aperture processing is explained using azimuth compression as an example, but the processing algorithm for synthetic aperture processing is not limited to this.

[0088] In the second embodiment, the overlapping portion of each sub-aperture 310a is multiplied by a Hanning window, but the window function to be multiplied is not limited to a Hanning window. When composing the sub-apertures 310a, it is sufficient if the overlapping portions can be smoothly connected. For example, a Hamming window, a Blackman window, etc., may also be used.

[0089] Furthermore, this disclosure is applicable regardless of the number of sub-apertures 310, 310a (hereinafter referred to as sub-aperture 310).

[0090] Furthermore, the signal size only needs to be such that signal interference in subsequent processing is avoided. Therefore, a smaller size can be set if computational resources are not a consideration. However, the signal size must be at least large enough to guarantee the determinism of the FFT processing.

[0091] The azimuth FFT size set for sub-aperture 310 should be sufficient to prevent signal interference after the addition of the second phase. Therefore, a larger size can be set if computational resources are not a consideration.

[0092] Furthermore, in the sub-aperture synthesis process, for the sake of simplifying the process, the signal feedback of the entire sub-aperture 310 is taken into consideration during addition. However, this is not the only method. For example, the azimuth shift of the signal may be taken into consideration, and only the signal portion may be added.

[0093] [Hardware configuration] The image generation apparatus 200 in each of the above embodiments may be implemented, for example, as an integrated circuit (IC) dedicated to each process, an application-specific integrated circuit (ASIC), a system-on-a-chip (SOC), a field-programmable gate array (FPGA), or the like. Alternatively, it may be implemented as a so-called general-purpose information processing device (computer).

[0094] When the image generation device 200 is implemented as a general-purpose information processing device, as shown in Figure 10, it includes, for example, a CPU (Central Processing Unit) 291, a main memory (memory) 292, an auxiliary storage device 293, an input / output I / F 294, and an expansion I / F 295, all interconnected by an internal bus.

[0095] The CPU 291 implements the above functions, for example, by loading a program stored in the auxiliary storage device 293 into the main memory device 292 and executing it, and also comprehensively controls the entire image generation device 200. Alternatively, one or more processors, such as an MPU (Micro Processing Unit), may be used instead of the CPU 291.

[0096] The main memory 292 is a type of memory such as RAM (Random Access Memory). The main memory 292 is the work area where the CPU 291 processes programs executed by the image generation device 200.

[0097] The auxiliary storage device 293 is, for example, a ROM (Read Only Memory), an HDD (Hard Disk Drive), or an SSD (Solid State Drive). The auxiliary storage device 293 stores various programs executed by the image generation device 200. The auxiliary storage device 293 may also include storage media such as a flexible disk, hard disk, optical disk, CD-ROM, CD-R, magnetic tape, non-volatile memory card, or DVD.

[0098] Furthermore, programs stored in the auxiliary storage device 293 can be provided as program products recorded on a non-transitory computer-readable storage medium. The auxiliary storage device 293 can be used to store various programs recorded on non-transitory computer-readable storage media for medium to long term.

[0099] The input / output interface 294 is an interface for inputting and outputting signals and data via wired or wireless means. In this embodiment, the image generation device 200 receives received signals from the receiving device 112 via this input / output interface 294. Alternatively, the image signal may be output to the output device 120 as a calculation result.

[0100] The Expansion I / F295 is an interface for connecting display devices, input devices, etc. Display devices include, for example, LCD monitors. Input devices are, for example, devices that accept user input such as keyboards and mice.

[0101] Each of the above functions of the image generation device 200 in this embodiment is realized by the CPU 291 loading a program stored in the auxiliary storage device 293 into the main storage device 292 and executing it.

[0102] Furthermore, the data used by the image generation device 200 for processing may be pre-stored in the auxiliary storage device 293. In addition, data generated during processing is stored in the main storage device 292 or the auxiliary storage device 293.

[0103] The hardware configuration of the image generation device 200 is not limited to this. It may include hardware not shown in the diagram.

[0104] In the flowchart used in the above explanation, multiple steps (processes) are listed in order, but the execution order of each step is not limited to that order. For example, the order of the diagrammed steps can be changed to the extent that it does not affect the content, such as by executing each process in parallel.

[0105] While the embodiments of this disclosure have been described above, this disclosure is not limited to the embodiments described above, and further modifications, substitutions, and adjustments can be made without departing from the basic technical concept of this disclosure. For example, the network configurations and element configurations shown in each drawing are examples to aid in understanding this disclosure and are not limited to the configurations shown in these drawings.

[0106] Finally, we summarize the preferred forms of this disclosure. Some or all of the embodiments described above may also be described as follows, but are not limited to the following: (Note 1) The image generation device includes a division unit that divides the received signal obtained by squint observation in the time direction to generate sub-apertures, A combining unit that generates a combined signal by combining each sub-aperture after predetermined signal processing without any omissions, The system includes an image signal generation unit that generates an image signal based on the aforementioned composite signal, The division unit determines the time-direction signal size of the sub-aperture to suppress aliasing due to the squint observation, and determines the azimuth-direction Fourier transform size based on the time-direction shift amount due to the signal processing. (Note 2) In the image generation apparatus described in Appendix 1, The signal size is preferably determined such that the Doppler frequency bandwidth falls within the Nyquist range of the pulse repetition frequency of the transmitted signal. (Note 3) In the image generation apparatus described in Appendix 1 or 2, It is desirable that the Fourier transform size be determined taking into account the amount of azimuth time shift due to the second-order phase addition included in the signal processing. (Note 4) In the image generation device described in any of the appendices 1 to 3, It is desirable that the division portion multiplies the overlap portion of the sub-aperture with adjacent sub-apertures by a window function. (Note 5) In the image generation device described in any of the appendices 1 to 4, It is preferable that the synthesis unit synthesizes each sub-aperture after the signal processing by adding it in the time direction over the entire Fourier transform size, including the signal that has wrapped around to the negative side in the time axis direction. (Note 6) In the image generation device described in any of the appendices 1 to 5, Before generating the sub-aperture, a range compression unit performs range compression on the received signal, For each of the aforementioned sub-apertures, a synthetic aperture processing unit performs azimuth compression processing as the signal processing, A secondary phase addition unit adds a linear chirp as a signal processing to each of the sub-apertures after the azimuth compression process, The system further comprises a de-rotation unit that performs phase rotation as signal processing on each of the sub-apers after the linear chirp has been added, It is desirable that the image signal generation unit performs a Fourier transform in the azimuth direction on the composite signal, performs azimuth compression, and performs an inverse Fourier transform on the composite signal to generate the image signal. (Note 7) Synthetic aperture radar systems are An image generation device as described in any of Appendix 1 to 6, A synthetic aperture radar device that collects the received signal by the aforementioned squint observation, The system includes an output device that generates and outputs image data from the aforementioned image signal. (Note 8) The image processing method performed by the computer of the image generation device is: A division step that divides the received signal obtained by squint observation in the time direction to generate sub-apertures, A synthesis step that generates a composite signal by combining each sub-aperture after predetermined signal processing without any omissions, The system includes an image signal generation step of generating an image signal based on the aforementioned composite signal, In the division step, the time-direction signal size of the sub-aperture is determined to suppress aliasing due to the squint observation, and the azimuth Fourier transform size is determined based on the time-direction shift amount due to the signal processing. (Note 9) The program causes the computer of the image generation device to execute the image processing method described in Appendix 8. Furthermore, the forms described in Appendices 8 and 9 can be expanded into the forms described in Appendices 2-6, similar to Appendice 1.

[0107] Furthermore, the disclosures in the above-mentioned patent documents, etc., are incorporated into this book by reference. Within the framework of the entire disclosure (including the claims), further modifications and adjustments to the embodiments or variations are possible based on the basic technical concept. Also, within the framework of this disclosure, various combinations or selections of various disclosed elements (including each element of each claim, each element of each embodiment or variation, each element of each drawing, etc.) are possible. In other words, this disclosure naturally includes the entire disclosure, including the claims, and various modifications and alterations that a person skilled in the art could make in accordance with the technical concept. In particular, with respect to the numerical ranges described in this book, any numerical value or sub-range included within that range should be interpreted as being specifically described, even if not otherwise stated. [Explanation of Symbols]

[0108] 100: SAR system, 110: radar device, 111: transmitter, 112: receiver, 120: output device, 190: satellite, 192: squint angle, 193: direction of travel, 194: observation area 200: Image generation device, 200a: Image generation device, 210: Splitting unit, 210a: Sub-aperture splitting unit, 220: Signal processing unit, 230: Synthesis unit, 230a: Sub-aperture synthesis unit, 240: Image signal generation unit, 240a: Secondary azimuth compression unit, 250: Range compression unit, 260: Synthesis aperture processing unit, 270: Secondary phase addition unit, 280: Derotation unit, 291: CPU, 292: Main memory, 293: Auxiliary memory, 294: Input / Output I / F, 295: Expansion I / F, 310: Sub-aperture, 310a: Sub-aperture, 311: Signal area, 312: Zero-fill area, 313: Additional signal area, 313d: Additional signal area width, 314: Additional signal area (negative signal), 315: Additional signal area (positive signal) 410: Image, 420: Image

Claims

1. A division unit that divides the received signal obtained by squint observation in the time direction to generate sub-apertures, A combining unit that generates a combined signal by combining each sub-aperture after predetermined signal processing without any omissions, The system includes an image signal generation unit that generates an image signal based on the aforementioned composite signal, The division unit determines the time-direction signal size of the sub-aperture to suppress aliasing due to the squint observation, and determines the azimuth Fourier transform size based on the time-direction shift amount due to the signal processing, in this image generation device.

2. An image generation apparatus according to claim 1, The signal size is determined such that the Doppler frequency bandwidth falls within the Nyquist range of the pulse repetition frequency of the transmitted signal in this image generating device.

3. An image generation apparatus according to claim 1, The Fourier transform size is determined by taking into account the amount of azimuth time shift due to the addition of a second-order phase included in the signal processing, in an image generation device.

4. An image generation apparatus according to claim 1, The division section is an image generation device that multiplies the overlap portion of the sub-aperture with adjacent sub-apertures by a window function.

5. An image generation apparatus according to claim 1, The image generation device combines the sub-apertures after the signal processing by adding them in the time direction over the entire Fourier transform size, including the signals that have wrapped around to the negative side in the time axis direction.

6. An image generation apparatus according to claim 1, Before generating the sub-aperture, a range compression unit performs range compression on the received signal, For each of the aforementioned sub-apertures, a synthetic aperture processing unit performs azimuth compression processing as the signal processing, A secondary phase addition unit adds a linear chirp as a signal processing to each of the sub-apertures after the azimuth compression process, The system further comprises a de-rotation unit that performs phase rotation as signal processing on each of the sub-apers after the linear chirp has been added, The image signal generation unit is an image generation device that generates the image signal by performing a Fourier transform in the azimuth direction on the composite signal, performing azimuth compression, and performing an inverse Fourier transform on the composite signal.

7. The image generation apparatus according to claim 1, A synthetic aperture radar device that collects the received signal by the aforementioned squint observation, A synthetic aperture radar system comprising an output device that generates and outputs image data from the aforementioned image signal.

8. An image processing method performed by a computer in an image generation device, A division step that divides the received signal obtained by squint observation in the time direction to generate sub-apertures, A synthesis step that generates a composite signal by combining each sub-aperture after predetermined signal processing without any omissions, The system includes an image signal generation step of generating an image signal based on the aforementioned composite signal, An image processing method comprising: in the division step, determining the time-direction signal size of the sub-aperture to suppress aliasing due to the squint observation, and determining the azimuth Fourier transform size based on the time-direction shift amount due to the signal processing.

9. A program that causes the computer of an image generation device to execute the image processing method described in claim 8.

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