JPS6273182A - Regeneration processing system of synthetic aperture radar image - Google Patents

Abstract

PURPOSE:To obtain a synthetic aperture radar regenerated image having high image quality, by calculating the power spectrum of the receiving data after the correction processing of a range curvature in a frequency region and estimating Doppler center frequency from the shape of said power spectrum. CONSTITUTION:The data collected by a synthetic aperture radar SAR is compressed in a range direction by a range compression processing part 101 and the initial value of a regeneration parameter is calculated by a regeneration parameter calculation processing part 102 to be stored in a regeneration parameter file 103. Next, the data is altered to a frequency region by an azimuth direction FFT processing part 104 and corrected by a range curvature correction processing part 105 and the estimation of Doppler center frequency is estimated by a Doppler center frequency estimation processing part 106 to correct the content of the regeneration parameter file 103. An azimuth point image pattern is formed using said parameter by an azimuth point image pattern forming processing part 107 and processed by processing parts 108-111 to obtain the final SAR regenerated image.

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to a synthetic aperture radar (Synthetic Aperture Radar) mounted on an artificial satellite or an aircraft.
The present invention relates to a digital processing method for reproducing a human-understandable image from imaging data obtained by dar (hereinafter referred to as rSARJ), and particularly to an image reproducing processing method suitable for rapidly reproducing images of island quality.

[Background of the invention]

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

Figure 1 shows the entire SAR system. Radar sensor 1. A SAR having an antenna 2 is mounted on an artificial satellite or the like and images the ground surface while moving in the direction of an arrow 4 on a flight path 3. Imaging data from the SAR is received by a ground station 5 and processed by a data processor 6 to create a video film 7.

A data storage magnetic table 8 is created, etc.

In addition, 9 indicates the decomposition cell, 10 indicates the range direction on the ground surface of the data collected by SAR, and 11 indicates the azimuth direction.
12 indicates the antenna beam, and 13 indicates the cutting width.

An overview of the processing of data collected by SAR will be described below. For details, please refer to the Proceedings of the 13th International Symposium on Remote Sensing of the Environment.

p337-360. April 1977 (Proceeded
ingsof 1 3th International
ional Symposium onRemot
e Sensing of Environmart
, p. 337-360° April, 1977).
``Digital composite amount Rorader image creation, airborne and satellite results'' by Cunning)
(” Digiatl S A RI mageFor
ation, Airborne and 5ate
lliteResults”).

In the third image of s A R, one point on the original image is distributed with the spread of a point image pattern h (t, r), and cannot be used as is. Here, r indicates the range direction, and t 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 situation is shown in FIG. A in Figure 2 shows two microwave reflection points on the ground surface.
This is a schematic diagram of a received image when only points exist, but if compression processing is performed in two directions, the original ground pattern as shown in B can be obtained. The range compression processing is performed by correlation processing with point image pattern data for each line of image data.

However, if you run the correlation process as is.

Since the processing takes a huge amount of time, speeding up is attempted by using fast Fourier transform (hereinafter referred to as rFFTJ), complex multiplication, and fast inverse Fourier transform (hereinafter referred to as rIFFTJ). To perform correlation processing using FFT, first, a point image pattern is generated by digital processing using a computer, and the FWT of both the point image pattern tone and one line of image data is calculated.

Correlation of two pieces of data is a simple multiplication in the frequency domain after calculating FFT, so the above 2
Take the product of the FFT calculation results of the data and apply it to the IFF
By performing T, correlation results for one line can be obtained.

Similar to the above range compression, azimuth compression also uses FFT to obtain correlation results at high speed.

However, even if the speed is increased using FFT as described above, the amount of calculation required for SAR image reproduction processing is still enormous, and an ultra-high-speed calculation device is required to perform the processing within a practical time. It is known that

On the other hand, in order to obtain a high-quality SAR image, it is necessary to accurately estimate the point image pattern. In particular, the point image pattern in the azimuth direction depends on the moving speed of the SAR sensor and the distance between the SAR sensor and the ground surface, so it differs from scene to scene. It is possible to calculate the point image pattern in the azimuth direction from data on the position and velocity of the SAR sensor obtained by other means, but these data contain errors, and usually automatic estimation of the point image pattern from the imaging data itself is not possible. SAR images of ZR image quality are often obtained by the focusing method.

The azimuth direction point image pattern h(t) is h(t)=A(t)exp(-k t2+β
t) (1). Here, t is the coordinate in the azimuth direction, k and β are quantities called reproduction parameters, k is the Doppler frequency change rate, and β is the Doppler center frequency. A is called an antenna pattern and is related to the intensity distribution in the azimuth direction of microwaves emitted from the SAR sensor. Although the ratio and β are estimated using completely different methods, especially regarding the estimation of the Doppler center frequency β,
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This method is described in a document titled ``).The method first reproduces an image using roughly estimated reproduction parameters, then Fourier transforms the image data again in the azimuth direction to obtain the power spectrum. This method estimates the Doppler center frequency from its shape.According to this document, the center frequency can be estimated more accurately than the method of simply Fourier transforming the data after range compression in the azimuth direction and calculating the Doppler center frequency from the power spectrum. It is said that it is possible to estimate the

However, according to the method in the above document, it is necessary to generate a point image pattern in the azimuth direction, perform FFT of the point spread pattern, multiply it with received data, and IFFT the result, and then perform FFT to obtain the power spectrum. However, there is a drawback that estimation of the Doppler center frequency requires a large amount of calculation and processing time, which accounts for about 10% or more of the entire SAR reproduction process.

[Purpose of the invention]

It is an object of the present invention to provide a SAR image reproduction processing method that overcomes the drawbacks of the above-mentioned conventional techniques, estimates Doppler center frequency accurately and at high speed, and enables reproduction of high-quality SAR images. It is in.

[Summary of the invention]

In the literature by Lee (F, LL) et al., the reason why the Doppler center frequency cannot be accurately estimated from the azimuth direction power spectrum of the received data after range compression is explained by the effect of so-called "wrapping", that is, only partially microwaves. This is assumed to be due to the reflection effects from each point on the ground surface that received the irradiation. However, the true cause is range curvature distortion caused by the distance between the SAR sensor and each point on the ground changing due to movement of the SAR sensor, as will be shown below.

If the received data after range compression is f(t, r) (r: range direction coordinate, t: azimuth direction coordinate), its azimuth direction Fourier transform F (w + r) is F(ω,
r) = but (f(t, r)) = fdr oG (ω
, r O) A((J) hr(r-r O-aω2)e
It can be written as xp[-ja (1+21), where B and is the Fourier transform with respect to t.

ω is the frequency in the azimuth direction, A(ω) is the microwave intensity distribution in the azimuth direction (antenna pattern), 'hr is the spot pattern in the range direction after range compression, a is a constant representing range curvature distortion, and α is the reproduction parameter This is the amount determined by A(ω) is when ω is equal to the Doppler center frequency,
Take the largest value.

hr(r) is a function with a peak at r100, and (2) shows that the information of one point on the earth's surface is distributed on a quadratic curve of r=r(1+aω2), and G(ωtro) is the function of the earth's surface. When the microwave reflectance is g(tlro), it is the Fourier transform in the azimuth direction. That is, G(ω, r,))=but(
g(t, r o)) (3).

Since it can be assumed that the phase of the microwave reflectance g(Lpro) on the earth's surface changes completely randomly, E(g(t, ro)g (t' + r'O' ))= δ
(t-to)δ(II” Oar o')σ2(t,
ro) (4). Here δ is the delta function,
is the complex conjugate, and σ2 (shir.) is the average microwave irradiation intensity at the ground point corresponding to (tlro). Therefore, E<acω+ r O)G (<111 r o')
)=fdtdt'E(g(t,rO)g(t',
rO'))exp(-jωt+jωt')=fdt
dt'δ(g(t,t')δ(r(1-ro')f
f2(t, rO)6Xp(-j(alt+jωt')
Here 6 (rO)=fdtcr”(t, ra)
(6) Furthermore, E(IF(ω,rl)=fdrodro' E(G”
(c+>, r(+')) IA (October 2hr (r-rO
-aω2)hr"(r-r□' -a(i+2)=f
dro a2(ro)A(ω121hr(r-ro-
aω2) (7) Since hr(r) has a peak at rγ0, E(l F(ω, r)F ) ~ σ2(r
+aω2]A(ω)F (8). The Doppler center frequency is estimated by finding the peak position of IA(ω)12, but according to equation (8), the azimuth direction power spectrum of the received data after range compression has the average microwave reflection intensity σ2(r+aω2). Since there is ω-dependence throughout the range, the Doppler center frequency cannot be estimated with high accuracy.

Based on the above principle, estimation of the Doppler center frequency can be performed using σ2(r+
It can be seen that it is sufficient to remove the ω-dependence of aω2). Range curvature correction processing in the frequency domain is exactly r
Since it corresponds to the coordinate transformation of '=r+aω2, high estimation accuracy can be obtained by estimating the Doppler center frequency on the data after the range curvature correction process. This is the gist of the present invention.

[Embodiments of the invention]

Embodiments of the present invention will be described below with reference to FIGS. 3 and 4. First, SAR received data is compressed in the range direction by range compression processing 101. Next, initial values of the playback parameters are calculated by the playback parameter calculation process 102 and stored in the playback parameter file 103. Next, the range-compressed received data is subjected to azimuth direction FFT processing 104.
After changing to the frequency domain, range curvature distortion is corrected by range curvature correction processing 105.

Next, Doppler center frequency estimation processing 106 is performed on the range curvature corrected received data, and the contents of the reproduction parameter file 103 are corrected. Azimuth point image pattern generation processing 1 using the modified reproduction parameters
07. Same FFT processing] 08. Range curvature corrected received data and its complex multiplication processing 109. IFFT processing 11
A final SAR reconstructed image is obtained by 0 multi-look addition/quantization processing 111.

FIG. 4 shows the principle of the Doppler center frequency estimation process 106. The horizontal axis 120 is the frequency in the azimuth direction, the vertical axis 121 is the power spectrum intensity, and the curve 122 is the power change of the range curvature-corrected received data. Usually, the power of several lines is averaged to reduce the effects of noise. The initial value 123 of the Doppler center frequency is the value calculated in the reproduction parameter calculation process 102 in FIG.
is corrected to a value 124 where the areas of the left and right portions surrounded by the curve 122 and the horizontal axis 120 are equal, and this is used as the estimated value of the Doppler center frequency. Here, the estimation is made using a value that makes the left and right areas equal, but the effect of the present invention is of course still the same even if the peak of the curve 122 is directly determined.

〔Effect of the invention〕

As described above, according to the present invention, in order to estimate the Doppler center frequency using the shape of the power spectrum of the received data after range curvature correction, the influence of the microwave reflection intensity distribution on the ground surface is removed. Since the Doppler center frequency can be estimated at high speed and with high precision, it is possible to obtain a high-quality SAR reconstructed image.

[Brief explanation of drawings]

Fig. 1 is a diagram showing the entire SAR system, Fig. 2 is a diagram showing an example of an original image, a received image, and a compressed image, and Fig. 3 is a diagram showing an example of the processing procedure according to the invention method. l+
FIG. 4, a flowchart of the embodiment, is a diagram showing the principle of Doppler center frequency estimation processing in FIG. 3. Agent: Patent Attorney Katsuo Ogawa □ '42 Color Figure 3

Claims (1)

[Claims] In a synthetic aperture radar image reproduction processing method that includes means for estimating the Doppler center frequency from the azimuth direction power spectrum of received data, the power spectrum of the received data after range curvature correction processing in the frequency domain is determined, and the shape of the power spectrum is calculated. A synthetic aperture radar image reproduction processing method characterized by estimating a Doppler center frequency from.