JP2014016185A - Imaging radar apparatus and signal processing apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress spurious images of synthesized images formed of a plurality of SAR images observed in mutually different frequency bands and thereby improve image quality.SOLUTION: An imaging radar apparatus that reproduces images on the basis of a plurality of signals received by a plurality of sensors in wave motions of a plurality of mutually different frequency bands comprises phase compensating amount calculating means that synthesizes a plurality of images reproduced on the basis of the plurality of received signals and calculates such phase compensating amounts as to minimize 1-norm of these synthesized images; phase compensating means that compensates the phases of the plurality of images reproduced on the basis of the plurality of received signals by using the phase compensating amounts calculated by the phase compensating amount calculating means; and image synthesizing means that synthesizes the plurality of images whose phases have been compensated by the phase compensating means.

Description

  The present invention relates to an image radar device and a signal processing device.

  Synthetic Aperture Radar, a so-called SAR (Synthetic Aperture Radar), radiates a transmission signal from the antenna mounted on a satellite or an aircraft to the ground as a radio wave, and reproduces an image (hereinafter referred to as a SAR image) from the reflected signal from the ground. An image radar device.

  Since the distance resolution of the SAR is proportional to the reciprocal of the frequency band of the transmission signal, if the frequency band of the transmission signal is widened to improve the distance resolution, the frequency band of the receiver is also widened. / D converter etc. are required.

  Therefore, in order to improve the distance resolution without expanding the frequency band of the transmission signal, for example, Patent Document 1 proposes a method for improving the distance resolution by correcting and synthesizing the phases of SAR images observed in different frequency bands. Has been.

JP 2007-225442 A

  However, in the conventional image radar device disclosed in Patent Document 1, as will be described later, a signal model represented by the sum of a plurality of signals (exponential functions) is estimated, and the phase correction amount is calculated using this signal model. When estimating, there is a problem in that the estimation of the number of signals fails and a correct phase correction amount is not estimated, so that a false image is generated in the combined SAR image and the image quality is deteriorated.

  The present invention has been made to solve the above-described problems. When a plurality of SAR images observed in different frequency bands are synthesized, the amount of phase correction is not dependent on the number of signals and the estimation of the signal model. Is intended to suppress the false image of the combined SAR image and improve the image quality as the SAR image.

  An image radar apparatus according to the present invention is an image radar apparatus that reproduces an image based on a plurality of reception signals respectively received by a plurality of sensors in waves of a plurality of different frequency bands, and is based on the plurality of reception signals. And a phase correction amount calculating means for calculating a phase correction amount for minimizing a 1-norm of the combined images, and a phase calculated by the phase correction amount calculating means. Phase correction means for correcting the phases of the plurality of images reproduced based on the plurality of received signals using the correction amount, and image composition means for combining the plurality of images whose phases are corrected by the phase correction means. And.

  According to the present invention, the image radar apparatus can suppress the false image of the combined image and improve the image quality.

1 is a block diagram showing an image radar apparatus according to Embodiment 1 of the present invention. 1 is a block diagram showing an image radar apparatus according to Embodiment 1 of the present invention. 1 is a block diagram showing an image radar apparatus according to Embodiment 1 of the present invention. Explanatory drawing for demonstrating the image radar apparatus by Embodiment 1 of this invention Configuration diagram showing an image radar apparatus according to Embodiment 2 of the present invention

Embodiment 1 FIG.
1 to 3 are block diagrams showing an image radar apparatus according to Embodiment 1 of the present invention. In each figure, the same numerals indicate the same or corresponding parts. In FIG. 1, 1a is a transmitter / receiver, 1b is a transmitter / receiver, 2a is an antenna, 2b is an antenna, 3a SAR (Synthetic Aperture Radar) image reproducing means, 3b is SAR image reproducing means, 4 is alignment means, and 5 is a phase correction amount. Range direction phase correction amount calculation means as a calculation means, 6 a range direction phase correction means as a phase correction means, 7 an image composition means, and 8 a display means.

  In FIG. 1, here, assuming two different frequency bands fa and fb, the information of the SAR image observed in the frequency band fa is called the first image, and the information of the SAR image observed in the frequency band fb. Is called a second image. Note that the number of frequency bands different from each other is not limited to two, and the configuration can be expanded starting with providing three or more antennas and SAR image reproducing means even when there are three or more frequency bands.

  FIG. 2 is a configuration diagram showing the positioning means 4 in detail. 2, 400a is an absolute value calculating means, 400b is an absolute value calculating means, 401a is a two-dimensional Fourier transform means, 401b is a two-dimensional Fourier transform means, 402 is a multiplication means, 403 is a two-dimensional inverse Fourier transform means, and 404 is Maximum value detection means, 405 is a range / azimuth correction amount calculation means, and 406 is a range / azimuth correction means.

  FIG. 3 is a configuration diagram showing the range direction phase correction amount calculating means 5 in detail. 3 (a) and 3 (b), 500 is a frequency correction means, 501 is a 1-norm minimizing means in FIG. 3 (a), and 501a is the p-norm maximum in FIG. 3 (b). Means.

  The image radar apparatus according to the first embodiment of the present invention is mounted on a platform (not shown) such as a satellite or an aircraft moving along the ground surface. Alternatively, the antennas 2a and 2b and the transceivers 1a and 1b may be mounted on the platform, and the signal processing device including the SAR image reproducing means 3a and 3b to the display means 8 may be arranged on the ground. In addition, a part of the signal processing device may be mounted on the platform including the SAR image reproducing means 3a and 3b. At this time, information on the image processed by the signal processing device may be notified by communication means between the platform and the ground.

  Next, the operation will be described. In FIG. 1, transceivers 1a and 1b output signals of different frequency bands fa and fb to antennas 2a and 2b, respectively. The antennas 2a and 2b transmit the input signals as radio waves, respectively, and the transmitted signals are reflected from the target, for example, the ground surface, and received by the antennas 2a and 2b as reflected signals in the frequency bands fa and fb, respectively. The

  Although the first embodiment of the present invention has been described as a monostatic radar that also uses the antennas 2a and 2b for transmission and reception, it may of course be configured as a multistatic radar with another antenna for transmission and reception. . In addition, the transmitted and received waves are not limited to radio waves, and can be expanded to waves such as light waves and sound waves. At this time, a sensor corresponding to the waves may be used instead of the antenna.

  In FIG. 1, the reflected signals of the frequency bands fa and fb received by the antennas 2a and 2b are output to the transceivers 1a and 1b, respectively, and the transceivers 1a and 1b convert the input signals into signal amplification, detection, Processing such as filtering is performed, and the result is output to the SAR image reproducing means 3a and 3b.

  The SAR image reproduction means 3a and 3b reproduce the first image and the second image from the received signals in the input frequency bands fa and fb, respectively. At this time, as a SAR image reproduction algorithm, for example, “Range Doppler algorithm” disclosed in “Ian G. Cumming, et al.,“ Digital processing of synthetic radar data, ”pages 146 to 157, Artech House, 2005.” ”,“ Chirp scaling algorithm ”or“ Omega-K algorithm ”.

  Here, even in a SAR image in which the same region is observed, there may be a positional shift in the range direction and the azimuth direction between the first image and the second image due to the imaging time or the shift of the trajectory. Therefore, in order to align the positions of the first image and the second image, the first image and the second image respectively reproduced by the SAR image reproducing means 3 a and 3 b are output to the alignment means 4. Of course, if there is no positional deviation between the first image and the second image, the positioning means 4 may be omitted.

  In FIG. 2, the alignment means 4 performs a process of calculating the correlation between the first image and the second image and estimating the positional deviation in the range direction and the positional deviation in the azimuth direction from the position where the cross-correlation value is maximized. ing. Hereinafter, detailed processing in the positioning means 4 will be described.

  The absolute value calculation means 400a and 400b calculate the absolute value of the first image and the absolute value of the second image, respectively. The two-dimensional Fourier transform units 401a and 401b perform two-dimensional Fourier transform on the first image and the second image output from the absolute value calculation units 400a and 400b, respectively, and convert them into the frequency domain.

  The multiplying unit 402 multiplies the first image and the second image that have been two-dimensional Fourier transformed in the frequency domain. The two-dimensional inverse Fourier transform unit 403 performs a two-dimensional inverse Fourier transform on the image multiplied by the multiplication unit 402. The maximum value detecting means 404 searches for and detects the maximum value of the amplitude of the image after the two-dimensional inverse Fourier transform. The range / azimuth correction amount calculation unit 405 calculates a correction amount for the positional deviation in the range direction and a correction amount for the positional deviation in the azimuth direction from the detected position of the maximum value. The range / azimuth correction unit 406 corrects the positional deviation in the range direction and the positional deviation in the azimuth direction of the second image using the calculated positional deviation correction amount in the range direction and the positional deviation correction amount in the azimuth direction. .

  The accuracy of the alignment of the first image and the second image by the processing of the alignment means 4 as described above is only about the resolution of one SAR image, and the distance resolution is improved by synthesizing a plurality of SAR images. Therefore, it is desirable to perform alignment with a phase order with higher accuracy. Therefore, the range direction phase correction amount calculation unit 5 calculates the phase correction amounts of the first image and the second image. Hereinafter, detailed processing in the range direction phase correction amount calculation means 5 will be described.

In FIGS. 3A and 3B, the first image and the second image after the alignment by the alignment unit 4 are represented as I ₁ [l, m] and I ₂ [l, m], respectively. However, I ₁ [l, m] and I ₂ [l, m] are complex numbers, l and m indicate sample points in the range direction and azimuth direction, respectively, l is 1 to L point, and m is 1 to M Let it be a point. L is a natural number and M is a natural number.

3A and 3B, the second image I ₂ [l, m] is observed in a frequency band different from that of the first image I ₁ [l, m]. Therefore, the frequency correction unit 500 corrects the frequency of the second image I ₂ [l, m]. The second image whose frequency is corrected is represented as I ′ ₂ [l, m].

Next, in FIG. 3A, the 1-norm minimizing means 501 searches for the phase φ as shown in the equation (1), and performs the first image I ₁ [l, m] and the second image I. The phase correction amount α is determined so that the 1-norm of the composite image with ₂ [l, m] is minimized. Thereby, it is possible to obtain a phase correction amount that best matches the first image I ₁ [l, m] and the second image I ′ ₂ [l, m].

Alternatively, as shown in FIG. 3B, the range direction phase correction amount calculation unit 5 may be configured to include a p-norm maximization unit 501a instead of the 1-norm minimization unit 501. . At this time, the p-norm maximization means 501a generates a composite image of the first image I ₁ [l, m] and the second image I ′ ₂ [l, m] as shown in the equation (2). A phase correction amount α that maximizes the p-norm is obtained. However, p is a real number larger than 2.

  It should be noted that by using Expression (2), the larger the value of p, the more accurately the phase correction amount can be obtained when the phase correction amount is small, for example, compared to the case of using Expression (1). Become.

In FIGS. 3A and 3B, the range direction phase correction unit 6 calculates the phase correction amount obtained from the equations (1) and (2) for the second image I ′ ₂ [l, m]. The phase is corrected using α.

Next, in FIG. 1, the image synthesizing unit 7 combines the first image I ₁ [l, m] and the second image I ′ ₂ [l, m] phase-corrected by the range direction phase correcting unit 6. Compositing and high-resolution resolution to obtain a high-resolution SAR image. The display means 8 displays a high-resolution SAR image after the synthesis on the screen.

  FIG. 4 is an explanatory diagram for explaining the image radar apparatus according to Embodiment 1 of the present invention. FIG. 4 (a) shows the problem of the conventional image radar apparatus disclosed in Patent Document 1. In FIG. FIG. 4B is a diagram for explaining the operational effects of the image radar apparatus according to the first embodiment of the present invention. In the figure, SAR images observed in different frequency bands fa and fb are shown on the frequency axis, and a one-dimensional frequency spectrum is shown for simplicity of explanation.

  In FIG. 4A, the conventional image radar device disclosed in Patent Document 1 performs eigenvalue analysis on the SAR images observed in the frequency bands fa and fb, estimates the number of signals, and The signal model is estimated using the number of signals. Thereafter, the conventional image radar apparatus estimates the amount of phase correction between signals in the frequency bands fa and fb using the nonlinear least square method in the frequency bands fa and fb, and the SAR image observed in the frequency bands fa and fb. And finally, inverse Fourier transform is performed to obtain a high-resolution SAR image. However, in the conventional image radar apparatus, as shown in FIG. 4A, if the estimation of the number of signals fails, the signal model is incorrect and the correct phase correction amount cannot be estimated. There is a problem in that a false image due to an unnecessary peak occurs in the synthesized SAR image, and image quality may be deteriorated.

  On the other hand, in FIG. 4B, the image radar apparatus according to the first embodiment of the present invention estimates the phase correction amount by using 1-norm minimization or p-norm maximization, thereby obtaining the number of signals. It is possible to calculate the phase correction amount without using the estimation of the signal model, and the correct phase correction amount can be estimated, synthesized without shifting in the range direction, and high range resolution without generating false images due to unnecessary peaks Can be realized.

  As described above, in the image radar apparatus according to the first embodiment of the present invention, when a plurality of SAR images observed in different frequency bands are combined to achieve high distance resolution, the 1-norm of the combined image is minimized. Thus, a phase correction amount or a phase correction amount that maximizes the p-norm of the composite image is obtained, and a SAR image whose phase is corrected by the obtained phase correction amount is synthesized. As a result, it is possible to suppress the spurious image after combining a plurality of SAR images observed in different frequency bands and to improve the image quality as a SAR image.

Embodiment 2. FIG.
5 is a block diagram showing an image radar apparatus according to Embodiment 2 of the present invention. In each figure, the same numerals indicate the same or corresponding parts. In FIG. 5, reference numeral 9 denotes a projection plane correction unit, and the other configuration is the same as that of the image radar apparatus according to the first embodiment shown in FIG.

  Next, the operation will be described. Even in the SAR image in which the same region is observed as described in the first embodiment, in the first image and the second image, the positional deviation in the range direction and the position in the azimuth direction are caused by the imaging time or the orbital deviation. Deviation may occur. Further, there may be a case where the projection plane is shifted due to the imaging time or the orbital shift.

  Therefore, in FIG. 5, the projection plane correction unit 9 performs a process of matching the projection planes of the first image and the second image respectively reproduced by the SAR image reproduction units 3a and 3b. As this processing method, for example, a processing method performed by a projection plane compensation unit described in JP-A-2009-52960 is used. Next, the alignment unit 4 performs a process of correcting the positional deviation in the range direction and the positional deviation in the azimuth direction between the first image and the second image that have been subjected to the process of matching the projection planes.

  As described above, in the image radar apparatus according to the second embodiment of the present invention, in addition to the same configuration as that of the first embodiment, the projection plane correcting means is provided so that the projection planes of the first image and the second image coincide with each other. I try to let them. As a result, even when a projection plane shift occurs due to an imaging time or a trajectory shift, as in the first embodiment, a false image after combining a plurality of SAR images observed in different frequency bands is suppressed. There is an effect that image quality improvement as a SAR image can be performed.

  In the radar apparatus according to the above-described first and second embodiments, the example in the case of synthesizing two SAR images observed in two different frequency bands has been described. However, the present invention is not limited to this. N is a natural number greater than or equal to 2 and can be expanded when N SAR images are combined. For example, with respect to the image after the phase correction and synthesis between the first image and the second image, the range direction phase correction amount calculation unit 5 and the range direction phase correction unit 6 perform phase correction and synthesis with the third image. If this processing is repeated until the Nth image, the same effects are obtained.

  1a, 1b transceiver, 2a, 2b antenna, 3a, 3b SAR (Synthetic Aperture Radar) image reproduction means, 4 alignment means, 5 range direction phase correction amount calculation means, 6 range direction phase correction means, 7 image composition means, 8 display means, 9 projection plane correction means, 500 frequency correction means, 501 1-norm minimization means, 501a p-norm maximization means.

Claims (6)

An image radar device that reproduces an image based on a plurality of reception signals respectively received by a plurality of sensors in a plurality of different frequency band waves,
A phase correction amount calculating means for combining a plurality of images respectively reproduced based on the plurality of received signals and calculating a phase correction amount that minimizes the 1-norm of the combined images;
Using the phase correction amount calculated by the phase correction amount calculation means, phase correction means for correcting the phases of a plurality of images respectively reproduced based on the plurality of received signals;
Image synthesizing means for synthesizing a plurality of images whose phases are corrected by the phase correcting means;
An image radar apparatus comprising: An image radar device that reproduces an image based on a plurality of reception signals respectively received by a plurality of sensors in a plurality of different frequency band waves,
A plurality of images respectively reproduced based on the plurality of received signals are combined, and when p is a real number larger than 2, a phase correction amount is calculated so as to maximize the p-norm of the combined image. A phase correction amount calculating means;
Using the phase correction amount calculated by the phase correction amount calculation means, phase correction means for correcting the phases of a plurality of images respectively reproduced based on the plurality of received signals;
Image synthesizing means for synthesizing a plurality of images whose phases are corrected by the phase correcting means;
An image radar apparatus comprising: Alignment means for performing alignment in the range direction and the azimuth direction of a plurality of images respectively reproduced based on the plurality of received signals;
The image radar apparatus according to claim 1, further comprising: Projection plane correction means for correcting a shift of the projection plane between the plurality of images respectively reproduced based on the plurality of received signals;
The image radar apparatus according to claim 1, further comprising: A signal processing device that reproduces an image based on a plurality of received signals that respectively receive waves in a plurality of different frequency bands,
A phase correction amount calculating means for combining a plurality of images respectively reproduced based on the plurality of received signals and calculating a phase correction amount that minimizes the 1-norm of the combined images;
Using the phase correction amount calculated by the phase correction amount calculation means, phase correction means for correcting the phases of a plurality of images respectively reproduced based on the plurality of received signals;
Image synthesizing means for synthesizing a plurality of images whose phases are corrected by the phase correcting means;
A signal processing apparatus comprising: A signal processing device that reproduces an image from a plurality of received signals that respectively receive waves of a plurality of different frequency bands,
A plurality of images respectively reproduced based on the plurality of received signals are combined, and when p is a real number larger than 2, a phase correction amount is calculated so as to maximize the p-norm of the combined image. A phase correction amount calculating means;
Using the phase correction amount calculated by the phase correction amount calculation means, phase correction means for correcting the phases of a plurality of images respectively reproduced based on the plurality of received signals;
Image synthesizing means for synthesizing a plurality of images whose phases are corrected by the phase correcting means;
A signal processing apparatus comprising:

Images