JP2594908B2 - A method for measuring spatial resolution of synthetic aperture radar images. - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to human-readable and human-readable data obtained from imaging data of the ground surface by a synthetic aperture radar (hereinafter abbreviated as SAR) mounted on an artificial satellite or an aircraft. The present invention relates to a digital processing system for reproducing an image that can be reproduced, and more particularly to a spatial resolution measurement method suitable for evaluating the image quality of a reproduced image with high accuracy.

[Conventional technology]

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

Fig. 2 shows the entire SAR system. The SAR including the radar sensor 21 and the antenna 22 is mounted on an artificial satellite or the like, and performs imaging of the ground surface while moving in the direction of the arrow 24 on the flight path 23. The imaging data from the SAR is received by the ground station 25,
Video film 27 processed by data processor 26
, The data storage magnetic tape 28, and the like. In addition, 29 is a resolution cell, 30 is the range direction on the surface of the data collected by SAR, 31 is the azimuth direction,
32 indicates the antenna beam and 33 indicates the cutting width.

The outline of the processing of the data collected by SAR is described above. For details, refer to Proceedings of 13th International Symposium
rnational Symposium on Remote Sensing of Environme
nt) pp. 337-360 (April 1977).
ette) and Cumming (digital)
S.A.R. image formation airborne and satellite results ”(“ Digita
l SAR Image Formation Airborne and Satellite Resul
ts ").

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. 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,
Next, it is compressed in the azimuth direction. This is shown in FIG. FIG. 3A schematically shows a received image when there are only two microwave reflection points on the ground surface.
If the compression processing in the direction is performed, the original ground pattern as shown in B can be obtained. The range and azimuth compression processing are 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, an enormous processing time is required. Therefore, the speed is increased by using a fast Fourier transform (hereinafter, referred to as “FFT”), complex multiplication, and a fast inverse Fourier transform (hereinafter, referred to as “IFFT”). Is achieved. 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 simply a multiplication in the frequency domain after the FFT is performed, the result of the FFT calculation of each of the above two data is multiplied by each other, and the result of the IFFT is applied to the correlation processing result for one line. can get.

Although the SAR reproduced image is obtained by the above processing, it is necessary to measure the image quality of the reproduced image, particularly the spatial resolution, in order to check whether or not the reproduction processing has been performed normally.

Conventionally, methods for measuring the spatial resolution of SAR images include, for example, the Proceedings of the 1984 International Symposium on Noise and Crazy Radiation in Radars and Imaging Sensors.
rnational Symposium on Noise and Clutter Rejection
in Raders and Imaging Sensors) in 1984 by Ito et al., “Improvement on SA R Data Processing Technics Fork.
High Quality Image (Improvement on SAR Dat
a Processing Technique for High Quality Image) ”
In general, a bright point is selected from a reproduced image, and the degree of spatial expansion of the point is examined to determine the spatial resolution of the reproduced image.

[Problems to be solved by the invention]

The above prior art assumes that a point selected from a reproduced image corresponds to a point target on the ground surface in a vacuum, and if this assumption is not satisfied, the measurement accuracy of the spatial resolution decreases. There was a problem that no consideration was given to them.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems and to provide a method capable of measuring a spatial resolution with high accuracy even in a scene such as a sea without assuming the existence of a point target in a reproduced image. Is to do.

[Means for solving the problem]

The above object is achieved by calculating a self-interphase function of a SAR reproduced complex image and measuring a spatial resolution in the SAR image using the autocorrelation function.

[Action]

SAR is a sensor that measures the microwave complex reflectance of the earth's surface. G (t, r) represents the microwave complex reflectance distribution on the ground
far. Here, t is the azimuth direction coordinate, and r is the range direction coordinate. Since the complex reflectance at each point (t, r) is a vector sum of the reflectances of a number of minute microwave reflectors, it shows a large fluctuation statistically, and in particular, it is known that the phase changes completely randomly. Have been. Therefore, E is a probability averaging operator and E [g (t, r) g * (t ′, r ′)] = δ (tt ′) δ
(R−r ′) σ ² (t, r) (1) Here, * is a complex conjugate, δ is a delta function, and σ ² (t, r) is the average microwave reflection intensity at point (t, r).

On the other hand, if the point image response function of the SAR imaging system including the SAR image reproduction processing system is h (t, r), the SAR reproduced complex image f
(T, r) is f (t, r) = {g (t ', r') h (tt-, rr-) d
t′dr ′ (2) _{Let the} autocorrelation function of the reconstructed complex image be R _f (Δt, Δ
r), R _f (Δt, Δr) = ∫f (t, r) f * (t + Δt, r + Δr)
dtdr (3) f and R _f are quantities that change statistically. In particular, the expected value of the auto-correlation function R _f is calculated from the equations (1), (2), and (3). Here, if tt ′ and rr ′ are newly set as t and r, respectively, Becomes The equation (4), the expected value of the autocorrelation function of reproducing complex images f is found to be proportional to the autocorrelation function R _h of the point spread response function h.

By knowing the autocorrelation function of the point image response function, the spatial spread of the point image can be measured. Therefore, the spatial resolution can be measured without assuming the presence of the point image in the reproduced image.

〔Example〕

Hereinafter, an embodiment of the present invention will be described with reference to FIG. FIG. 1 is a processing flow for measuring the spatial resolution of a SAR image according to the present invention.

The reproduced complex image data stored on the magnetic tape 1 or the magnetic disk 2 is input to a computer in a processing block 3.
Using the artificial satellite shown in FIG. 1, this reconstructed complex image data is converted into a received signal of the SRA system by a fast Fourier transform, a multiplication of the point image pattern by the FET, respectively, in both the range direction and the azimuth direction. It is obtained by performing a series of compression processing called fast inverse Fourier transform. In the above equation (2), the reproduced complex image data is represented as f (t, r). However, since these actual compression processes are also performed on discrete points,
The obtained reproduced complex image data is also expressed as a function of the sample numbers i and j. In other words, this reproduced complex image data is
f _ij . i and j are subscripts indicating the image position of the digital image, where 1 ≦ i ≦ M ₁ and 1 ≦ j ≦ M ₂ . Here, M ₁ and M ₂ represent the size of the reproduced complex image data. Next, in processing block 4, the self-interphase function R _ij , _-N≤i≤N , -N≤j≤N of the input reproduced complex image data is calculated by the following equation.

Here, Σ in equation (5) indicates that the sum of the subscripts k and l is obtained within the range of the region S on the reproduced complex image whose resolution is to be measured. For example, the region S is a region slightly narrowed inward with respect to the full size of the M ₁ × M ₂ reproduced complex image.
On the other hand, N represents the size of the autocorrelation function obtained for the resolution measurement. In a normal SAR image, since the resolution does not exceed several pixels, N may be a value of about 2 to 5.

From the above equation (4), R _ij obtained here is proportional to the autocorrelation function Q _ij of the point image response function h _ij of the SAR image reproduction system to be measured. This is because, when the sum of the entire image (within the range of the region S) is calculated by the equation (5), the phase of the complex reflectance g of the equation (4) changes at random, so that the original image, that is, the photographed image is obtained. It takes advantage of the fact that the effects of the terrain itself are averaged. The proportionality coefficient is obtained by summing the square of the absolute value of σ of the microwave complex reflectance on the ground over the entire image (within the range of the region S), as given by Expression (4). Since the object of the present invention is to determine the shape of the point spread response function, there is no need to consider this proportionality factor. By definition, Q _ij = Σh _kl h * _{k + i} , _{1 + j} . The meaning of Σ is the same as in equation (5).

In the next processing block 5, the self-interphase function _Rij is regarded as two-dimensional data and two-dimensional Fourier transform is performed. _Assuming that the result is F ｛R｝ _mn , F ｛R｝ _mn = F ｛Q｝ _mn = | F ｛h｝ _mn | ² (6) where F ｛Q｝ _mn and F ｛h｝ _mn are respectively , A two-dimensional Fourier transform of the autocorrelation function Qij of the point image response function and the point image response function hij. The Fourier transform of the autocorrelation function of the function h is the absolute value of the Fourier transform of the function h, which is 2
Raising to the power is a well-known formula in signal processing.

In the processing block 6, the square root (root) of the Fourier-transformed data {R} _mn is obtained. Therefore, from equation (6), F {h} _mn is obtained as a result of this processing. Further data processing block 7, after route conversion, i.e. by two-dimensional inverse Fourier transform of F {h} _mn, determine the point spread response function h _ij.

In the processing block 8, the spatial spread of the point image response function _hij obtained in the processing block 7 is obtained and output as a spatial resolution measurement result 9. That is, the data of the point image response function _hij obtained in the processing block 7 is (2N + 1) × (2N +
Since the two-dimensional discrete data of 1) is used, for example, a continuous function that best fits each value along the coordinate axis in one direction of the two-dimensional discrete data is estimated, and a half value of the peak value of the continuous function is given. The width between the coordinates, that is, the half-value width is obtained, and is set as the spatial resolution measurement result 9 in the coordinate axis direction.

In the processing of this embodiment, the point image response function is estimated from the autocorrelation function of the point image response function assuming the linear phase property of the point image response function h. However, the effect of the present invention does not change.

〔The invention's effect〕

According to the present invention, it is possible to measure a spatial resolution by deriving a point image response function from a reproduced complex image without deeming an image in the reproduced complex image whose spread of the imaging target itself is unknown as a point image. Therefore, there is an effect that measurement can be performed even in a scene such as the sea and measurement can be performed with high accuracy.

[Brief description of the drawings]

FIG. 1 is a processing flow of the invention method, FIG. 2 is a diagram showing an entire SAR system, and FIG. 3 is a diagram showing the principle of SAR image reproduction.

Claims (1)

(57) [Claims] 1. A synthesis method in which a Fourier transform is performed in each of a range direction and an azimuth direction of a received signal of a synthetic aperture radar, a Fourier transform result is multiplied by a Fourier transform of a point image pattern, and the multiplied result is obtained by performing an inverse Fourier transform. In the method of measuring the spatial resolution of a reproduced complex image of an aperture radar, an autocorrelation function of the reproduced complex image is calculated, the autocorrelation function is subjected to two-dimensional Fourier transform, and a result of the two-dimensional Fourier transform is subjected to a square root process. A combination comprising the steps of: performing a two-dimensional inverse Fourier transform on the processing result to create a point spread response function of the reproduced complex image; and obtaining a spatial resolution from the spread of the point spread response function. A method for measuring the spatial resolution of aperture radar images.