JPH0750502B2 - Synthetic aperture radar image reproduction processing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Use of the Invention] The present invention relates to a synthetic aperture radar (hereinafter referred to as SAR) mounted on an artificial satellite or an aircraft.
The present invention relates to a digital image processing method for reproducing an image that can be understood by humans from the imaged data according to the present invention, and particularly to an image reproduction processing method suitable for reproducing an image of high quality.

[Background of the Invention] In the field of remote sensing using artificial satellites or aircraft, SAR, which can obtain a high-resolution image in a microwave band that penetrates clouds, is attracting attention as a sensor for imaging the ground surface. .

Figure 1 shows the overall SAR system. The SAR including the radar sensor 1 and the antenna 2 is mounted on an artificial satellite or the like and moves on the flight path 3 in the direction of arrow 4 to image the ground surface. The image data from the SAR is received by the surface station 5 and processed by the data processor 6 to create a video film 7 and a data storage magnetic tape 8.
In addition, 9 is a resolution cell, 10 is the range direction on the ground surface of the data sampled by SAR, 11 is the same azimuth direction, 12 is the antenna beam, and 13 is the cutting width.

The outline of the processing of the data collected by SAR is described above.
For details, see the proceedings of 13th Interna.
tional Symposium on Remote Sensing of Environment,
P337-360, April, 1977 Bennette and Cumming
By “Digital SAR Image Fomation. Aiborne and sat
ellite Results "are described in the literature.

In the SAR received image, one point on the original image is distributed with the spread of the point image pattern h (x, y) and cannot be used as it is. Where x is the range direction,
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. This is shown in FIG.
FIG. 2A schematically shows a received image when only two microwave reflection points are present on the ground surface. However, if compression processing is performed in two directions, the original ground surface pattern as in B is obtained. Can be obtained. The range and azimuth compression processing is performed by correlation processing with point image pattern data for each line of image data. However, if the correlation processing is executed as it is, it will take an enormous amount of processing time. Therefore, the speed is increased by using the fast Fourier transform (hereinafter referred to as “FFT”) complex multiplication and the fast inverse Fourier transform (hereinafter referred to as “IFFT”). Is planned. In order to perform correlation processing using FFT, first, a point image pattern is generated by digital processing by a computer, and FFTs of both the point image pattern and one line of image data are calculated. Since the correlation calculation is a mere multiplication in the frequency domain after the FFT is applied, the product of the FFT calculation results of the above two data is taken and IFFT is performed to obtain the correlation processing result for one line.

As a result of the azimuth compression process, a SAR reproduction image is obtained,
The value at each point of the image data is a complex value corresponding to the complex reflectance of microwaves on the ground surface. However, in general, the phase of the complex number changes randomly and there is no information. Therefore, the SAR image finally generated is often given as a real number or an integer value obtained by discretizing the real number. However, if the absolute value of the complex image data after azimuth compression is taken as it is and converted into a real value, the spatial resolution will be deteriorated by Bennete et al. "Digital SAR Image Formation Aiborne and Sate".
llite Results "(Roceedings of th 13 th Internatio
nal Symposium on Remote Sensing of Environment, Apr
il 1977). On the other hand, the above-mentioned document describes a method of preventing the deterioration of spatial resolution by re-sampling (resampling) data as it is as complex-valued data prior to digitization.

However, in the conventional SAR reproduction processing method, since the relative phase between pixels of the image after azimuth compression is not sufficiently taken into consideration, a phase shift occurs between the lines, and as a result, the data that is out of phase in the resampling is interpolated. However, there is a drawback that image quality is deteriorated.

[Object of the Invention] The object of the present invention is to solve the above-mentioned drawbacks and to achieve high resolution.
Another object of the present invention is to provide a SAR image reproduction processing method capable of reproducing a high-quality SAR image.

[Outline of Invention]

First, the cause of the occurrence of phase shift in the image after azimuth compression will be described.

The azimuth direction point image pattern is represented as follows.

hn = exp [2πj (αn ² + βn)] (1) Here, n represents an integer sample number. j is an imaginary unit. α and β are quantities called reproduction parameters, α is a Doppler change rate, and β is a Doppler center frequency. α, β
Changes slowly in the range direction because it depends on the distance between the satellite and the target point on the ground and the relative speed. Therefore, the azimuth direction point image pattern is not constant in one scene.

The phase shift occurs in the following two cases (1) When the generation of the azimuth direction point image pattern is omitted.

Since the reproduction parameters α and β differ from line to line, it is necessary to generate an azimuth direction point image pattern for each line in the azimuth compression process. However, in an actual process, in order to reduce the amount of calculation, generation of a point image pattern may be omitted within the range of accuracy required for a reproduction parameter, and the same point image pattern may be used for a plurality of lines. However, in this method, the reproduction parameter changes discontinuously at the position of the line that updates the point image pattern, and a phase shift occurs there. For details of this phenomenon, see Japanese Patent Application No. 59-186317.
See (filed September 7, 1984).

(2) Look extraction in multi-look processing.

In the SAR image reproduction processing, multi-look processing is performed to reduce speckle noise called speckle noise peculiar to the SAR image. In the multi-look state, when azimuth compression is performed, one line of data is divided into L in the circumferential complement region, and each of them is independently Fourier-transformed to reproduce L images,
This is done by adding each image data. L is called the multi-look number and is usually 4 in satellite SAR. Each image reproduced at this time is called a look.

The signals are distributed around the Doppler center frequency in the circumferential complement region, and each look is also cut out using this center frequency as a reference.
Since the Doppler center frequency changes in the range direction, the look cutout position also differs from line to line. However, since the original data is a discrete sample series, the clipping position also takes an integer value and changes discontinuously. A phase shift occurs at this discontinuous change. That is, the number of looks L =
4. Original data Fk (k = 0.1 ... N-1, N is the number of data points)
And the cutout position of a certain look Then, the compressed data The cutting position changed by 1.

The compressed data _n for But between f _n and f _n The relationship is established. Expression (4) indicates that a phase shift occurs due to a discontinuous change in the cutout position.

In any of the two examples described above, a phase shift occurs between lines, and if resampling is performed with this being a complex number, the image quality deteriorates, specifically, stripes appear on the reproduced image. . In particular, the phase shift due to the discontinuous change in the look cutout position is an unavoidable problem.

The first feature of the present invention is that the phase shift is corrected by multiplication of the phase rotation factor.

First, let C _o be the cut-out center position of a certain look, which is determined from the Doppler center frequency. Generally, C _o changes continuously and takes a non-integer value. However, since the data is a discrete sample series, the integer value K _o K _o = int (C _o ) (5) that is the closest to C _o is actually used. It will be cut out in the center. Where int is a function that represents rounding to the nearest whole number. Let f _n be the azimuth-compressed data obtained by applying the inverse Fourier transform to the data cut out around K _o Becomes In order to prevent a phase shift at a place where K _o changes discontinuously, the clipping center position may be set to a non-integer value, but in the method of the present invention, this is a phase rotation in a real region equivalent to a shift in the frequency domain. It is realized by multiplication of factors. That is, if the required shift amount is δK _o , then δK _o = C _o −int (C _o ) (7), and using this, the phase shift corrected azimuth compression result _n is uncorrected according to the following equation. Predetermined phase rotation factor for each line for azimuth-compressed data f _n It is obtained by multiplying by.

From (6) and (8) Therefore, the azimuth compression result by the method of the present invention corresponds to the one obtained by cutting out a non-integer _Co, and the phase shift due to the discontinuous change can be eliminated. ΔK _o in equation (7) is called a look cutout position adjustment parameter. Where δK _o
Is not uniquely determined by equation (7), and there is a constant indefiniteness in correcting the phase shift between lines.

Next, a second feature of the present invention is to change the sample interval in the range direction in the frequency domain in order to prevent the image quality deterioration.
This is to be performed at the same time as the range curvature correction process for correcting the distortion caused by the distance change between the satellite and the ground target point. The principle of the method of improving the spatial resolution without re-sampling rows with complex numbers is that the frequency band of the signal is widened when the complex data is converted into real numbers, and the sampling theorem cannot be satisfied. In the method of the present invention, the sample interval in the range direction is changed with respect to the data before azimuth compression, so that the deterioration of the image quality due to the phase shift between lines is avoided. All the phases of the data before azimuth compression are uniform, and the above problem does not exist in the method of the present invention. The sample interval in the azimuth direction is changed by performing complex resampling only in the horizontal direction after azimuth compression.

The method of the present invention will be described with reference to FIG. On the image data 31 obtained by FFT the SAR image data compressed in the range direction 33 in the azimuth direction 34, the information of each point on the ground surface is distributed on the curved curve 32. The curve 32 depends on the reproduction parameter. In the range curvature correction, the point 35 on this curve is obtained from the neighboring point 36 of the image 31 by interpolation in the range direction of the complex data. With respect to the data after the range compression, the relative phase between the points of the image is correctly maintained, and therefore the phase shift does not occur in the interpolation here.

In the method of the present invention, the sample interval in the range direction is changed by calculating the curve to be corrected at an interval corresponding to the changed sample interval, and then performing the above interpolation. Therefore, the image data after range coverage correction 37
Is expanded in the range direction as compared with the data 31 before correction.

Example of Invention

An embodiment of the present invention will be described below with reference to FIGS. First, an embodiment for correcting the phase shift will be described with reference to FIGS.

FIG. 4 is an overall flow of SAR image reproduction processing.

The observation data is first compressed in the range direction by the range compression processing 401. Next, the satellite orbit prior to azimuth compression
A reproduction parameter calculation process 402 for obtaining a reproduction parameter from the posture information is performed, and an azimuth compression process 403 is performed using the obtained reproduction parameter to perform compression in the azimuth direction. Next, distortion correction resampling 404 for correcting the geometric distortion of the compressed result and simultaneously changing the sampling period of the image data is performed. This resampling is performed as a complex number to prevent deterioration of spatial resolution. After that, the data is converted into a real number by performing a normization process 405, a multi-look addition process 406 that is a noise reduction process is performed, and then a quantization process 407 is performed to obtain a final image.

FIG. 5 shows in detail the processing for one line of the azimuth compression processing 403 according to the present invention in the flow of FIG.

First, the FFT 41 is applied to the observation data after the range compression and converted into the frequency domain, and then the range curvature correction 42 is performed to correct the distortion due to the change in the distance between the satellite and the ground surface target point. Next, the azimuth reference function generation processing 43 is performed according to the value of the reproduction parameter corresponding to the processing line, and the FFT 44
Give. The results of the processing 42 and 44 are multiplied by the complex multiplication processing 45. Next, look cutout processing for multi-look processing
Prior to 47, the look cutout position adjustment parameter calculation process 46 is performed to obtain the parameter Δk _o . The IFFT 48 is applied to the data after the look cut out to obtain the azimuth-compressed data, and the phase rotation factor multiplication processing 49 is further applied using the parameter δk _o, and the azimuth compression processing of one line is completed. 50 is an additional process according to the present invention.

As described above, according to this embodiment, since the error due to the integer conversion of the look cutout position is performed by the multiplication of the phase rotation factor, the phase shift between the lines of the complex image after the azimuth compression does not occur, and the complex number data of the complex number data is not generated. It is possible to prevent deterioration of image quality due to resampling and to reproduce a high quality SAR image.

Next, an embodiment for preventing the image quality deterioration will be described with reference to FIGS. 6 and 7.

FIG. 6 is a flowchart of the entire SAR image reproduction processing.

The observation data is first compressed in the range direction by the range compression processing 601. Next, prior to the azimuth compression, a reproduction parameter calculation process 602 for obtaining a reproduction parameter from the orbit / attitude information of the satellite is performed, and an azimuth compression process 603 is performed using the obtained reproduction parameter to perform compression in the azimuth direction. Then, in process 604, the complex number data is subjected to norm addition and multi-look addition, distortion correction resampling 605 is performed to correct the geometric distortion contained in the image, and the result is subjected to quantum processing 606 to obtain the final SAR image. .

FIG. 7 is a detailed flow of the azimuth compression processing 604 according to the present invention in the flow of FIG.

First, FFT operation processing 51 is applied to the data after range compression,
After the conversion into the frequency domain, in step 52, the satellite is corrected for distortion due to the change in the distance from the ground surface target point, and at the same time, the sampling interval is changed by resampling in the range direction. Next, azimuth reference function generation processing 53 is performed according to the value of the reproduction parameter, and the result is FF.
T arithmetic processing 54 is performed. The results of the processing 52 and the processing 54 are multiplied by the complex multiplication processing 55, and the IFFT calculation processing 56 is applied to the result, whereby an azimuth-compressed complex image is obtained. Further, the azimuth direction resampling process 57 is performed to change the sample interval in the azimuth direction for the image.

As described above, according to the present embodiment, since the complex resampling in the range direction that causes the image quality deterioration is performed at the same time as the range curvature correction, the above image quality deterioration does not occur, and the high image quality is a high resolution SAR image. Has the effect of being able to regenerate. In addition, since distortion correction resampling for correcting geometric distortion is performed on the image after normization and multi-look addition, the amount of calculation required for distortion correction is small, and good high-speed processing is possible. There is also.

〔The invention's effect〕

According to the present invention, it is possible to correct a relative phase shift between lines of a complex image after azimuth compression in the SAR image reproduction processing, and thus it is possible to reproduce a high-quality SAR image without image quality deterioration due to complex number resampling. The effect is that you can do it.

Furthermore, since complex resampling in the range direction, which causes image quality deterioration due to phase shift in the SAR image reproduction process, can be performed simultaneously with range curvature correction, it is possible to reproduce high-quality, high-resolution SAR images without the above image quality deterioration. There is an effect that can be.

[Brief description of drawings]

FIG. 1 is an overall configuration diagram of the SAR system, FIG. 2 is a diagram showing the principle of SAR image reproduction, and FIG. 3 is an explanatory diagram of a process for changing the range direction sample interval at the same time as the range curvature correction, and FIG. , FIG. 6 is a flowchart of the entire SAR image reproduction process, FIG. 5 is a detailed flowchart of the process 403 of FIG. 4, and FIG. 7 is a detailed flowchart of the process 603 of FIG.

Claims (1)

[Claims] 1. An image reproduction processing method for reproducing an image from image data obtained by a synthetic aperture radar, wherein the reproduced complex pixel is reproduced by multiplying a predetermined phase rotation factor for each line. A method of reproducing a synthetic aperture radar image, characterized by correcting a phase shift existing between lines of complex pixels.