JP6178245B2 - Synthetic aperture radar apparatus and image processing method thereof - Google Patents

Description

  The present embodiment relates to a synthetic aperture radar apparatus and an image processing method thereof.

  In a conventional synthetic aperture radar (SAR) device, in the case of a SAR process, a fixed target has a fixed position within the synthetic aperture time. Thus, it is possible to combine the phases so that the maximum vector is obtained. Therefore, the target target in the specified imaging region can be imaged at a correct position by polar format conversion (see Non-Patent Documents 1, 2, and 3).

  However, in polar format conversion, there are restrictions on the imaging area, and if there are target targets beyond the imaging area, fixed targets outside that area cannot be imaged. In order to widen the imaging region, it has been studied to set a frequency such as PRF (Pulse Repetition Frequency) high. However, the amount of acquired data increases, and there are cases where it cannot be realized due to processing scale limitations.

SAR method (range compression): Ouchi, “Basics of Synthetic Aperture Radar for Remote Sensing”, Tokyo Denki University Press, pp.131-149 (2003) SAR method (Az compression): Ouchi, “Basics of Synthetic Aperture Radar for Remote Sensing”, Tokyo Denki University Press, pp.171-178 (2003) SAR method (large aperture array synthesis, spotlight SAR): Yoshida, 'Revised radar technology', IEICE, pp.280-283 (1996) SAR processing method (polar format conversion reconstruction processing): MEHRDAD SOUMEKH, “Synthetic Aperture Radar Signal Processing”, JOHN WILEY & SONS, INC., Pp.319-325 (1999) Affine transformation rev.a, Tamura, 'Computer image processing', Ohmsha, pp288-290 (2002)

  As described above, in the conventional synthetic aperture radar apparatus, there is a case where the fixed target cannot be imaged due to the limitation of the imaging region.

  The present embodiment has been made in view of the above problems, and can increase the imaging area of the fixed target while suppressing an increase in processing scale, thereby realizing imaging of the fixed target that has been subjected to the area limitation. An object of the present invention is to provide a synthetic aperture radar apparatus and an image processing method thereof.

In order to solve the above problems, the synthetic aperture radar apparatus according to the present embodiment performs synthetic aperture processing on the beam output obtained by directing the radar wave toward a specific imaging range along with the movement of the mounted mobile body. The image center is changed in M (M is a natural number of 2 or more) for each of the N (N is a natural number of 2 or more) center positions of the region to be imaged, and the cross range axis is changed. The reference signal and the beam output acquisition data are used to generate an SAR (Synthetic Aperture Radar) process to generate an N × M SAR image, and the N × M SAR image The shift is corrected, and the N × M SAR images in which the shift is corrected are integrated to generate a SAR integrated image.

The block diagram which shows the structure of the synthetic aperture radar apparatus which concerns on 1st Embodiment. The flowchart which shows the flow of a process of a signal processing part in the apparatus shown in FIG. The conceptual diagram for demonstrating the imaging range in a synthetic aperture process in the apparatus shown in FIG. The conceptual diagram which shows the positional relationship of an airframe flight axis (flight path | route) and the imaging range in the apparatus shown in FIG. The flowchart which shows the flow of the process which extracts the production | generation method of a FFT image, and the value of the fixed target in a FFT image in the apparatus shown in FIG. The conceptual diagram for demonstrating the M offset of a FFT image in the apparatus shown in FIG. The conceptual diagram which shows the processing content of polar format conversion in the apparatus shown in FIG. The conceptual diagram for demonstrating the processing content of an affine transformation in the apparatus shown in FIG. The conceptual diagram for demonstrating the case of the compound conversion in the apparatus shown in FIG. The conceptual diagram which shows the integration process by the amplitude addition of the several image by which polar format conversion was carried out in the apparatus shown in FIG.

  Hereinafter, embodiments will be described with reference to the drawings.

(First embodiment)
With reference to FIG. 1 thru | or FIG. 10, the synthetic aperture radar apparatus mounted in the aircraft which concerns on 1st Embodiment is demonstrated.

  FIG. 1 is a block diagram showing a system configuration of the synthetic aperture radar apparatus (hereinafter referred to as SAR), and FIG. 2 is a flowchart showing a specific processing flow thereof. In FIG. 1, an antenna 1 is a phased array antenna in which a plurality of antenna elements are arranged to form a large aperture array. A PRF (Pulse Repetition Frequency) signal) is transmitted in a designated direction and the reflected wave is received. The transmission / reception unit 21 performs phase control on the signals received by the plurality of antenna elements of the antenna 1 in accordance with instructions from the beam control unit 22 and combines them to form a reception beam in the range to be imaged (FIG. 2: step). S11). A reception signal obtained by the reception beam is sent to the signal processor 3.

  The signal processor 3 includes an AD (Analog-Digital) conversion unit 31, a range compression unit 32, an offset correction unit 33, an offset amount control unit 34, an Az compression unit 35, an imaging unit 36, an image correction unit 37, and an image integration. The unit 38 is provided.

  The AD conversion unit 31 converts the PRF reception signal supplied from the transceiver 2 into a digital signal (FIG. 2: step S12), and the conversion result is a range compression unit 32, an offset correction unit 33, an offset amount control. Sent to the SAR processing unit by the unit 34 and the Az compression unit 35.

  In the SAR processing unit, the range compression unit 32 pulse-compresses the digitized PRF reception signal of the aperture array in the range (distance) direction for each reception cycle of the synthetic aperture (FIG. 2: step S13). Pulse compression is a correlation process between an input signal and a reference signal for pulse compression, and this is performed on the frequency axis (see cited document 1). Specifically, on the frequency axis, the FFT (Fast Fourier Transform) result of the input signal and the FFT result of the reference signal are multiplied to obtain an FFT image. The range-compressed FFT image is sent to the offset correction unit 33. The offset correction unit 33 is a process for adjusting the center (image center) of the imaging region to be cross-range compressed (the Az compression at the subsequent stage) with N amounts in accordance with the control of the offset amount control unit 34. (FIG. 2: Step S14). The FFT image subjected to the offset correction is sent to the Az compression unit 35, subjected to pulse compression in the azimuth angle (cross range) direction (FIG. 2: step S15), and sent to the imaging unit 36.

  The imaging unit 36 outputs a two-dimensional FFT image by sequentially performing polar format conversion on the N types of input FFT images (FIG. 2: step S16). The image correcting unit 37 performs affine transformation on the input image in order to correct the positional deviation and distortion generated in the N two-dimensional FFT images obtained by the imaging unit 36 (FIG. 2: step). S17, S18). The image integration unit 38 integrates N two-dimensional FFT images subjected to affine transformation (FIG. 2: Step S19).

  In the synthetic aperture radar apparatus having the above-described configuration, the SAR scheme will be schematically described with reference to FIG. FIG. 3 shows the case of the spotlight SAR (cited document 3). In this method, the beam is directed so as to always irradiate the imaging area with the actual aperture beam by the on-board radar, and within the synthetic aperture time (one cycle). For each pulse transmitted at a PRI (Pulse Repetition Interval) interval, data is acquired in units of range cells in the PRI. SAR processing is performed using the acquired data to obtain a SAR image.

  FIG. 3 is a diagram in the case of the spotlight SAR, but other methods such as a strip map SAR for observing the side may be used as long as the SAR image can be obtained. FIG. 4 shows the definition of the coordinate system. In FIG. 4, the X axis is the range, the Y axis is the cross range, the flight direction speed of the mounted machine is the cross range direction, the coordinate point of the movement target is (X, Y) from the position of the mounted machine, and the speed vector is V.

Next, a method for generating an FFT image and a method for extracting a fixed target value in the FFT image will be described with reference to a processing flow shown in FIG.
First, when the data of the sampling point t of the sampling time t and the synthetic aperture length is acquired (step S21), range compression is executed (see cited document 1: steps S22 to S23). Here, the range compression is a correlation process between an input signal and a range compression signal, and a case where this is performed in the frequency domain is formulated as follows.

  In order to convert the signal s on the time axis, the signal s may be subjected to inverse Fourier transform. However, since the cross range compression (Az compression, cited reference 2) is performed thereafter, the signal s remains on the (ω, u) axis. And

Next, a cross-range reference signal fs0 is generated.

  The offset values (Xcalfft, Ycalfft) are necessary for generating the FFT image. The effect of the offset will be described with reference to FIG.

First, the signal cs is obtained by multiplying the above-mentioned s by fs0 for cross-range compression.

Using this, the signal fcs (ω, ku) is obtained by performing FFT on the u-axis.

The FFT image output can be calculated by inverse FFT with respect to the ω-axis of fcs.

The offset of the FFT image will be described with reference to FIG. In the FFT image, the X-axis corresponds to the range by pulse compression (proportional to the round trip time t to the target, t-axis), and the Y-axis corresponds to the cross range by FFT (proportional to the Doppler component, ku-axis). If the image center of the fixed target is (Xc, Yc), the imaging range of the fixed target is (Xc ± X0, Yc ± Y0). Fixed target, since in some cases the above image generation range, in order to extensively synthesized, as described below, the image center to generate an image of M Street is offset (in FIG. 6 M = 9) After the image generation, the entire image is integrated.

Next, with reference to FIG. 5, a SAR image processing method by polar format conversion will be described (Cited document 4). Up to the pulse compression output s is the same as described above. First, using this s, Fourier transform is performed with respect to the u-axis (step S24).

Next, M offset values (Xoff, Yoff) are used to widen the image generation range, offset correction is performed to generate an image centering on the moving target image by shifting the center of the fixed target image, and Az Multiply the reference signal for compression (step S25). This offset amount may be determined by dividing the image range to be generated. In this case, it goes without saying that a method of dividing by an orthogonal coordinate (X, Y) axis, a method of dividing by a polar coordinate (R, θ), or the like may be used, but here, the case of an orthogonal coordinate will be described.

Further, kx and ky can be obtained by the following equations.

Using this cross range reference signal, fsm is calculated by the following equation.

Polar format conversion shown in FIG. 7 is performed using fsm, and lattice point data F (kx, ky) is generated on the kx and ky axes (step S26). Polar format conversion is a method of calculating the data of grid points on the (kx, ky) axis using the acquired data using an interpolation method or the like, and details are omitted as in the cited reference. Using this fsm, an image is output by two-dimensional inverse FFT (step S27).

Each of the images generated using M different offset amounts has a different image center, and therefore a positional shift or distortion due to a squint angle (an angle of the beam directing direction with respect to the flight axis) occurs. In order to correct this, affine transformation represented by the following equation is performed (Cited Document 5).

For example, the following equation is obtained only for enlargement / reduction and parallel movement.

The affine transformation will be described with reference to FIGS.
Generally expressed as a matrix, it is as follows.

(A) In the case of enlargement / reduction (see FIG. 8 (a))

(B) In the case of parallel movement (see FIG. 8 (b))

(C) In the case of rotational movement (see FIG. 8 (c))

(D) In the case of skew (θx) (see FIG. 8D)

(E) In the case of skew (θy) (see FIG. 8E)

In the case of complex conversion, conversion may be performed in the form of matrix multiplication in the order of conversion. For example, in the case of rotation → translation, conversion is performed as shown in FIGS. This composite transformation is expressed by the following equation.

  As is clear from the above description, in the above processing, each coefficient in the expression (13) generates a fixed target of the reference position (Prefx, Prefy), and the position of the fixed target in the output of the expression (12) is true. Decide to correct the position. Then, M correction images having different offset amounts are added in amplitude, and N (different of the whole image is divided into N) images having different center positions are combined to obtain an integrated image as a whole.

  Here, in the polar format conversion, for example, as shown in FIG. 10A, when there is a target target beyond the set image area, an image omission occurs. In contrast, in the present embodiment, as shown in FIG. 10B, a plurality of images having different image centers are integrated by amplitude addition, so that an appropriate image can be obtained without causing image omission. It is done.

(Second Embodiment)
In the first embodiment, amplitude addition is used when combining images to obtain an integrated image. At this time, if there is a deviation due to a correction error in a plurality of images, the images may be synthesized with a slightly shifted amplitude and may appear as a blurred image. In order to suppress this influence, a superimposition (overwrite) process is performed on a portion that overlaps the plurality of images when the plurality of images are integrated. However, in the case of the superimposition process, the target may be suppressed by overwriting when a part that overlaps between a plurality of images is imaged on the one hand but not imaged on the other hand.

  Since this process only replaces the internal processing of the image integration unit 38 in the processing diagram of FIG. 1 and the processing flow of FIG. 2 in the first embodiment from superposition processing to amplitude processing, the system diagram and processing flow are Omit.

  Here, as to which one of the first embodiment and the second embodiment is adopted, it may be properly used depending on the operation, such as whether to display the image quality or the target reliably.

  In addition, as for the synthetic aperture processing method of the above-described embodiment, the polar reconstruction conversion image reconstruction processing (Cited document 4) has been described, but it goes without saying that other synthetic aperture processing methods may be used.

  Note that the present embodiment is not limited to the above-described embodiment as it is, and can be embodied by modifying constituent elements without departing from the scope of the invention in the implementation stage. In addition, various inventions can be formed by appropriately combining a plurality of components disclosed in the embodiment. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, constituent elements over different embodiments may be appropriately combined.

  DESCRIPTION OF SYMBOLS 1 ... Antenna, 2 ... Transmitter / receiver, 21 ... Transmitter / receiver, 22 ... Beam control part, 3 ... Signal processor, 31 ... AD (Analog-Digital) conversion part, 32 ... Range compression part, 33 ... Offset correction part, 34 ... offset amount control unit, 35 ... Az compression unit, 36 ... imaging unit, 37 ... image correction unit, 38 ... image integration unit.

Claims (10)

In a synthetic aperture radar apparatus that images a beam output obtained by directing a radar wave toward a specific imaging range along with the movement of a mounted mobile body,
For each of the N (N is a natural number of 2 or more) center positions of the region to be imaged , the image center is changed to M (M is a natural number of 2 or more) to generate a reference signal in the cross-range axis , Image generating means for generating an N × M SAR image by performing SAR (Synthetic Aperture Radar) processing using the reference signal and the acquired beam output data ;
Correction means for correcting a shift between the N × M SAR images;
A synthetic aperture radar apparatus comprising: integration means for generating an SAR integrated image by integrating the N × M SAR images in which the shift is corrected.   The synthetic aperture radar apparatus according to claim 1, wherein the image generation unit generates the SAR image by polar format conversion.   The synthetic aperture radar apparatus according to claim 1, wherein the correction unit corrects the shift of the SAR image by affine transformation.   The synthetic aperture radar apparatus according to claim 1, wherein the integration unit integrates the plurality of SAR images by amplitude addition.   The synthetic aperture radar apparatus according to claim 1, wherein the integration unit integrates the plurality of SAR images by superposition synthesis. In the image processing method of the synthetic aperture radar apparatus for performing image formation by performing synthetic aperture processing on the beam output obtained by directing the radar wave toward a specific imaging range along with the movement of the mounted mobile body,
For each of the N (N is a natural number of 2 or more) center positions of the region to be imaged , the image center is changed to M (M is a natural number of 2 or more) to generate a reference signal in the cross-range axis ,
SAR (Synthetic Aperture Radar) processing is performed using the reference signal and the acquisition data of the beam output to generate an N × M SAR image,
Correcting the shift of the N × M SAR images,
An image processing method of a synthetic aperture radar device that integrates N × M SAR images in which the shift is corrected to generate a SAR integrated image.   The synthetic aperture radar apparatus image processing method according to claim 6, wherein in the image generation, the SAR image is generated by polar format conversion.   The synthetic aperture radar apparatus image processing method according to claim 6, wherein the correction of the shift includes correcting the shift of the SAT image by affine transformation.   The synthetic aperture radar apparatus image processing method according to claim 6, wherein the integration of the plurality of SAR images is performed by integrating the plurality of SAR images by amplitude addition.   7. The synthetic aperture radar apparatus image processing method according to claim 6, wherein the plurality of SAR images are integrated by superimposing the plurality of SAR images.

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