JPH0682386B2 - Focusing method for synthetic aperture radar images - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Application of the Invention The present invention relates to a synthetic aperture radar (Synthetic Aperture Radar.
Say. 2) image pickup data, that is, an image focusing method in a digital processing system for reproducing an image representing a ground pattern from a radar hologram.

[Background of the Invention]

In the field of remote sensing using artificial satellites or aircraft, SAR that can obtain high-resolution images in the microwave band through clouds as a sensor for imaging the ground surface
Is attracting attention.

Figure 1 shows the overall SAR system. Radar sensor R
The SAR having the antenna s and the antenna An is mounted on an artificial satellite and moves along the flight path Fp in the direction of the arrow A to image the ground surface. The image data from the SAR is received by the ground station Ls and processed by the data processor Dp to create a video film IF and a data storage magnetic tape MT. In addition, c is a decomposition cell, Ra is the range direction on the ground surface of the data collected by SAR, A _Z is the same azimuth direction, A _B is the antenna beam, and C _W is the cutting width. .

In the SAR received image, one point on the original image is distributed with the spread of the point image pattern h (x, y), which cannot be understood by humans. Here, x indicates the range direction and y indicates the azimuth direction. The information spread in the received image is first compressed in the range direction and then in the azimuth direction. The compression processing is performed by the correlation processing with the point image pattern data for each line of the image data. However, if the correlation process is executed as it is, it will take an enormous amount of processing time.
Fast Fourier transform (hereinafter referred to as "FFT") complex multiplication and fast inverse Fourier transform (hereinafter referred to as "IFFT") are used to achieve high speed.

In order to improve the image quality of the image data after the compression processing, it is first necessary to accurately obtain the point image pattern data.
Is important to. Conventionally, a method has been used in which the Doppler change rate (chiap rate), which is a parameter of the point image pattern in the azimuth direction, is estimated with high accuracy from the amount of positional deviation between the look images. The look image is the IF when the azimuth compression is performed.
It is a compressed image obtained by dividing the frequency band in the frequency space immediately before the FT and obtaining each of the divided bands. Since the point image is a linear frequency modulation signal of the following equation, When an error δk is added to the Doppler change rate k used in the reproduction process, a positional deviation d between the look images is generated in the azimuth direction as in the following equation, and the error δk can be estimated from the positional deviation d.

In equations (1) and (2), α and β are constants, and ω is the difference in frequency between the band centers of the look processing.

Since the point image h (x) in the range direction is the same linear frequency modulation signal as in the equation (1), the automatic focusing method in the azimuth direction can be applied to the range direction as it is, and the true pulse chirp rate can be estimated. Has been considered available.

On the other hand, in the azimuth compression processing in the frequency space that is usually performed in the image reproduction processing of the synthetic aperture radar, the blur in the range direction due to the so-called range walk, which is the spread of the signal in the diagonal direction, is pointed out in the following document. .

"War Jin and C. WU: SAR Correlation Algorithm Adapted to Large Area" by M. W. Jin and C. Wu (IAE Transaction on Geoscience and Remote Sensing GE-22 Vol. 6, November 1984) [MYJin and C.WU:
"A SAR Correlation Algorithm which Accommod
ates Large-Range Migration "(IE ³ Trans.on Geosc
ience and Remote Sensing, volGE-22, no.6, Novemb
er1984)] In this document, it is shown that the range-direction blur caused by the range walk is theoretically obtained, and the range can be corrected and removed by slightly changing the range chirp rate from the true value.

As described above, conventionally, in order to correct the blur in the range direction of the synthetic aperture radar, the cause of the blur is caused by the error of the pulse chirp rate (about 0.2% in the case of SEASAT-I) and the azimuth in the frequency space. The range walk blurring caused by the compression process is divided into two, and the correct pulse chirp rate correction amount is calculated for each by the autofocus method for the former and theoretical calculation from the Doppler center frequency for the latter, and added. However, there is a problem that it is unobtrusive because a method such as finding a new range chirp rate is used.

[Object of the Invention]

The present invention has been made to eliminate the above-mentioned drawbacks, and its purpose is to estimate the sum of the error of the initial pulse chirp rate and the fine adjustment of the pulse chirp rate for correction of range walk blur accompanying frequency space processing. However, it is another object of the present invention to provide a method for focusing a synthetic aperture radar image that removes blur in the range direction.

[Outline of Invention]

In the present invention for achieving the above object, a means for obtaining multi-look image data in the range (orbital vertical direction) is provided, and a shift in the range direction of the image among a plurality of range-look image data obtained by the means is provided. It is characterized in that the amount of measurement is measured, the range chirp rate correction amount necessary for simultaneously removing the blur in the range direction due to the above two different factors is obtained, and the synthetic aperture radar image reproduction processing is performed by the corrected range chirp rate. .

Example of Invention

Embodiments of the present invention will be described below with reference to the drawings. Second
FIG. 1 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 the MTI is Fourier transformed in the range direction by the FFT means 2. The point spread function creating means 4 creates a linear chirp signal from the initial pulse chirp rate storage device 3 and performs a Fourier transform. The multiplication means 5
Image 1 of the Fourier-transformed point spread function and FFT received image
Each line is multiplied in the frequency space. The frequency band cutout unit 6 divides the range-direction frequency image after the multiplication into two for each line, and first sends the high frequency image to the IFFT unit 7. The transposing means 8 vertically transposes the range compressed image (for one look) from the IFFT means 7 and sends it to the azimuth direction compression system 9. The azimuth direction compression system 9 comprises an FFT means 10, a range curvature correction means 11, a Fourier transform point image multiplication means 12, and an IFFT means 13. The first range look image that has also been subjected to azimuth compression is temporarily stored in the image storage device 14. Next, the frequency band cutout means 6 sends the remaining low frequency image divided into two to the IFFT means 7. A second range look image is obtained by the transposing means 8 and the azimuth direction compression system 9, and an image storage device is provided.
The positional deviation in the range from the first look image in 14 is obtained by the positional deviation detection device 15. The range direction point image correction means 16 obtains a corrected pulse chirp rate from the positional deviation amount and the initial pulse chirp rate stored in the storage device 3. The point spread function creating means 17 creates a modified point spread function and sends it to the multiplication means 5. The received image data is again subjected to range compression, transposition, and azimuth compression processing by the modified point spread function, and reproduced image data 18 having no blur in the range direction is output to MT2.

EFFECTS OF THE INVENTION According to the present invention, the error of the initial pulse chirp rate is estimated from the amount of deviation between the range look images of only the range compression created specially, and used for the correction of range walk blurring accompanying frequency space processing. The required amount of pulse chirp rate correction is theoretically obtained from the Doppler center frequency, and there is no need for a procedure to obtain a new pulse chirp rate considering both factors. It has the effect of focusing.

[Brief description of drawings]

FIG. 1 is a block diagram showing the overall system configuration of a synthetic aperture radar, and FIG. 2 is a block configuration diagram of a synthetic aperture radar image reproduction processing system showing an embodiment of the present invention. Explanation of symbols 1 ... Received image data, 2 ... FFT means, 3 ... Initial pulse chirp rate storage device, 4 ... Point spread function creating means, 5
...... Multiplication means, 6 …… Frequency band cutout means, 7 …… IF
FT means, 8 ... Transposition means, 9 ... Azimuth direction compression system, 10 ... FFT means, 11 ... Range coverage correction means, 12 ... Multiplication means, 13 ... IFFT means, 14 ... Image storage device , 15 …… Position shift detection device, 16 …… Range direction point image correction means, 17 …… Point image function creation means, 18 …… Reproduced image data.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Fuminobu Furumura 1099, Ozenji, Aso-ku, Kawasaki-shi, Kanagawa Inside the Hitachi, Ltd. system development laboratory (56) Reference JP-A-58-186068 (JP, A)

Claims (1)

[Claims] 1. An image processing method for reproducing an image from image data obtained by a synthetic aperture radar, wherein the image data is first divided into frequency bands in the range direction and compressed, and the image data thus divided and compressed are taken in the azimuth direction. To create multi-look image data in the range direction, measure the amount of shift in the range of the images between the created multi-look image data, and measure the error in the initial pulse chirp rate from the measured shift amount. The value obtained by adding the pulse chirp rate deviation for the range direction image blur removal due to the range walk blur in the frequency space processing is calculated, and the initial pulse chirp rate is corrected based on the added value, and the corrected initial A method for focusing a synthetic aperture radar image, which is characterized by reproducing an image based on a pulse chirp rate.