JPS61194570A - Focusing system of synthetic aperture radar image - Google Patents

Abstract

PURPOSE:To eliminate haze in range direction by providing a means to obtain multilook image data in the range direction and correcting the shift amount of image in the range direction amongst the plural range look image data obtained by said means. CONSTITUTION:The received image data 1 is Fourier transformed through the fast Fourier transform means 2, initial pulse chirp rate storage device 3, point image function preparation means 4 and multiplied by the multiplication means 5. Then, the frequency band cutting out means 6 splits the range direction frequency image after multiplication into two per each line, of which from the higher frequency, the first range look image is obtained via the reverse fast Fourier transform means 7, transposition means 8 and azimuth direction compression system 9. Next, from the lower frequency image obtained after dividing the frequency image into two, the second range look image is obtained via the means 6 and system 9, etc. The range direction/position shift is detected by the position shift detector 15 and in this manner, range direction haze can be eliminated through the range direction point image correction means 16, etc.

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to a synthetic aperture radar mounted on an artificial satellite or an aircraft.
The present invention relates to an image focusing method in a digital processing system for reproducing an image representing a ground slam from imaging data obtained by dar (hereinafter referred to as "5ARJ"), that is, a radar hologram.

[Background of the invention]

In the field of remote sensing using artificial satellites or aircraft, SA is used as a sensor to image the ground surface and can obtain high-resolution images in the microwave band that penetrates clouds.
R is attracting attention.

Figure 1 shows the overall system of 8AR. The 8AR, which has a radar sensor and an antenna An, is mounted on an artificial satellite and images the ground surface while moving in the direction of the arrow on the flight path P. Imaging data from 8AR is received by ground station L8 and processed by data processor D to create video film IP, data storage magnetic tape MT, etc. In addition, C is the decomposition cell, R1 is the range direction on the ground surface of the data collected with 8A, A2 is the same azimuth direction, AB is the antenna beam, and Cw
indicates the cutting width.

In a received SAR image, one point on the original image is distributed with the spread of a point spread pattern h(X*y), which cannot be understood by humans.

Here, X is the microwave egg, and y is the ajima.

direction. The information spread in the received image is first compressed in the range direction and then in the azimuth direction. The compression process is performed by correlating each line of image data with point image pattern data. However, if you run the correlation process as is,
Since it takes a considerable amount of processing time, fast Fourier transform (hereinafter referred to as "FFT"), complex multiplication, and fast inverse Fourier transform (hereinafter referred to as "IFFTj") are used to speed up the process.

In order to improve the image quality of the image data after the compression process, the first step is to accurately obtain point image pattern data.
is important. Conventionally, a method has been used in which the Doppler rate of change (chirp rate), which is a parameter of a point spread pattern in the azimuth direction, is estimated with high accuracy from the amount of positional shift between look images. The look image refers to the I
This is a compressed image obtained by dividing a frequency band in the frequency space immediately before PFT and obtaining each of the divided bands.

Since the point image is a linear frequency modulation signal expressed by the following equation, if an error or difference δk is added to the Doppler change rate k used for reproduction processing, a positional deviation d between look images will occur in the azimuth direction as shown in the following equation, and the positional deviation will be This is because the error δk can be estimated from d.

""k k+δk(2) In equations (1) and (2), α and β are constants,
ω is the frequency difference between the look processing band centers.

The above automatic focusing method in the azimuth direction can be applied directly to the range direction because the point image h(x) in the range direction is the same linear frequency modulation signal as in equation (1), and the true pulse chirp rate can be estimated. It has been thought that it can be used for

On the other hand, in the azimuth compression processing in the frequency space, which is normally performed in the image processing of synthetic aperture radar, blurring in the range direction caused by so-called range walk, which is the diagonal spread of the signal, has been pointed out in the following literature. There is.
"SAR correlation algorithm suitable for large areas by M. Y. Jin and Shi Wu" (IEE Transactions on Science and Remote Sensing GB-22, No. 6, November 1984) (M, Y.
, J in and C, WU :A 8ARCorr
Elatio'n Algorithm which
Accommodates Large -Range
Migration”(IE”Trans, on
Geoscience aM Re'moteSen
sing, volGB-22, no, 6, No
In this document, it is shown that the range direction blur caused by range walking can be theoretically determined and corrected and removed by changing the range chirp rate by a small amount from the true value based on the shape of the incense. □As mentioned above, in order to correct blur in the range direction of synthetic aperture radar, conventionally, the blur factor is calculated by dividing the pulse chirp rate error (S
F! In the case of the A8AT-I, there were two types of blur: 0.21') and range walk blur caused by azimuth compression processing in the frequency space.Autofocus is used for the former, and Dotsupra is used for the latter. A method was used in which correct pulse chirp rate correction amounts were determined for each by theoretical calculations from the center frequency, and the values were added to determine a new range chirp rate, which had the problem of being unsophisticated.

[Purpose of the invention]

The present invention has been made in order to eliminate the above-mentioned drawbacks, and its purpose is to solve the sum of the error in the initial pulse chirp rate and the fine adjustment of the pulse chirp rate for correcting range walk blur accompanying frequency space processing. [Summary of the Invention] In order to achieve the above object, the present invention provides a focusing method for synthetic aperture radar images that estimates and removes range direction blur.
A means for obtaining multi-look image data in a direction (perpendicular to the trajectory) is provided, and a deviation in the range direction of the image is measured between the plurality of lunge-look image data obtained by the means, and blurring in the range direction due to the above two different factors is simultaneously measured. The feature is that the range chirp rate correction weight necessary for removing the range chirp rate is determined, and the synthetic aperture radar image is reproduced using the corrected range chirp rate.

[Embodiments of the invention]

Embodiments of the present invention will be described below based on the drawings. Second
The figure is a diagram showing an outline of a synthetic aperture radar image reproduction processing system according to the present invention. The received image data 1 stored in M'l'I is Fourier transformed in the range direction by the FF'T means 2. Point spread function creating means 4 from the initial pulse chirp rate from the initial pulse chirp rate storage device 3
creates a linear chirp signal and further Fourier transforms it.

The multiplier 5 multiplies the Fourier-transformed point spread function and the F'FT-processed received image for each line of the image in frequency space. The frequency band cutting means 6 divides the multiplied range direction frequency image into two parts per line, and first sends the high frequency image to the IFFT means 7.

The transposing means 8 converts the range compressed image from the JF'FT means 7 (
However, the image (for one look) is transposed vertically and horizontally and sent to the azimuth direction compression system 9. The azimuthal compression system 9 is
It consists of FFT means 10, range curvature correction means 11, Fourier transform point image multiplication means 12, and IPFT means 13. The first range look image after azimuthal compression (
The image is temporarily stored in the image storage device 14. Next, the frequency band extraction means 6 converts the remaining low frequency image divided into two into a second range look image image 5 by using the IF'FT tenth stage 7, the speed stage 7 transposition means 8, and the azimuth direction compression system 9.
The positional deviation in the range direction from the first look image in the image storage device 14 is determined by the positional deviation detection device [15]. The range direction point image correction means 16 is stored in the positional deviation 11 and the storage device 3. A corrected pulse chirp rate is determined from the initial pulse chirp rate. Point spread function creation means 17 creates a corrected point spread function and sends it to multiplication means 5. The received image data is again subjected to range pressure line, transposition, and azimuth compression processing using a modified point spread function.
MT becomes reproduced image data 18 without range direction blur.
2 is output.

〔Effect of the invention〕

According to the present invention, the error t in the initial pulse chirp rate is estimated from the difference between specially created range look images of only range pressure lines, and li! The pulse chirp rate correction required to correct the range walk blur associated with IL wavenumber space processing is not necessary, as it is theoretically calculated from the same wavenumber at the Doppler center and a new pulse chirp rate is calculated by taking both into consideration. This has the effect of being able to focus in the range direction all at once from positional deviations between azimuth-compressed range-look images.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the overall system configuration of a synthetic aperture radar, and FIG. 2 is a block diagram of a synthetic aperture radar image reproduction processing system showing an embodiment of the present invention. Explanation of codes 1... Received image data, 2... FFT cow stage, 3...
- Initial pulse chirp rate storage device, 4... Point spread function creating means, 5... Multiplying means, 6... Frequency band extraction means 7... I F F T means, 8... Transposing means,
9...Azimuth direction compression system, 10...Ii'
F T means, 11... range curvature sleeve correction means,
12... Multiplication means, 13... IPFT means, 14.
... Image storage device, 15 ... Positional deviation detection device, 16
. . . Range direction point spread correction means, 17 . . . Point spread function creation means, 18 . . . Reproduction image data.

Claims (1)

[Claims] In an image processing system that reproduces an image from imaging data obtained by a synthetic aperture radar, a means for obtaining multi-look image data in the range (orbit perpendicular) direction is provided, and an image is divided between the plurality of range-look image data obtained by the means. Measure the amount of deviation in the range direction, and use the measured amount of deviation to find the sum of the error in the initial pulse chirp rate and the pulse chirp rate deviation for removing image blur in the range direction caused by range walk blur in frequency space processing. A method for focusing a synthetic aperture radar image, which is characterized in that the initial pulse chirp rate is corrected.