CN110020993B - MTF calculation method of spatial resampling GF-4 satellite based on target - Google Patents
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Abstract
The invention relates to the field of MTF sampling of GF-4 satellites, in particular to a method for calculating MTF of a real target image sequence shot by a panchromatic camera in a GF-4 satellite, wherein 100 frames of images of a target scene and a real target area image are captured by using the proposed MTF estimation method, the random change rhythm of the satellite attitude is checked through Kolmogorov-Smirnov test, Gaussian distribution is observed, a plurality of LSF curves are extracted according to a detailed program for estimating the on-orbit space characteristic of a GF-4 optical imaging system by using a novel target, and the difference of the MTF between a simulation image and a GF-4 data set comes from the setting of a PSF template and is not influenced by atmospheric scattering and the random error of satellite attitude vibration. The invention solves the problems that the influence of noise and accidental sampling errors on a single image is reduced, and a better LSF cannot be obtained.
Description
Technical Field
The invention relates to the field of MTF sampling of GF-4 satellites, in particular to a spatial resampling GF-4 satellite MTF calculation method based on a target.
Background
The Modulation Transfer Function (MTF) is an important parameter for evaluating the imaging quality of an optical system. Although the performance of the optical sensor is strictly tested in a laboratory before being transmitted, the MTF performance of the satellite camera is reduced due to the influence of satellite transmission vibration, the change of the space environment, atmospheric attenuation and other factors. Therefore, it is necessary to measure and monitor the change of the on-orbit MTF of the satellite-borne camera.
MTF measurement methods, such as the oblique-edge method, the star point method, and the impulse method, can be roughly classified into a method based on an artificial object and a method based on a natural object. Satellite targets are commonly used for calibration of low earth orbit satellites. For example, the oblique-blade method has been used to estimate the MTF for the satellite SPOT5 and the satellite Quickbird in orbit, KOMPSAT-3 and GOCI using the star point method to calculate the MTF. The on-track MTF of IKONOS is calculated by a pulse method. In the low-orbit (LEO) applications described above, it is necessary to lay a sufficiently large high-contrast target. The GF-4 satellite is a geosynchronous orbit (GEO) optical remote sensing satellite with ground resolution of 50 meters of a first panchromatic camera in China, and the arrangement of a large enough traditional artificial target is very difficult. Even if there is a star target, the image taken at one time is susceptible to strong noise interference. Another option is to use natural objects with strong contrast and curved edge shapes, such as roads and moons, as edge targets. However, this approach may suffer from considerable errors compared to artificial targets.
In view of the GF-4 gaze imaging mechanism, we propose an improved extended star target for MTF calibration. The sequential images taken by the GF-4 satellite camera in a short time have a relatively random distance to some extent. If such a spatially multi-sampled image sequence is used to compute the LSF, it will help to reduce the effect of noise and accidental sampling errors on the single image. And then fitting the optimal LSF by adopting a grid search and gradient descent parameter optimization method. To this end, we designed a method for MTF calculation based on spatially resampled GF-4 satellites from one target.
Disclosure of Invention
Aiming at the defects of the prior art, the invention provides a target-based MTF calculation method for a spatial resampling GF-4 satellite, which solves the problems that the influence of noise and accidental sampling errors on a single image is reduced, and a better LSF cannot be obtained.
The invention is realized by the following technical scheme:
a MTF calculation method for a spatial resampling GF-4 satellite based on a target comprises the following steps:
s1: for bulls-eye design, a uniform low-reflectivity region was chosen as the dark background, and a high-reflection radiance signature target (60%) with coverage 3 × 4GSD was set;
s2: performing LSF fitting, selecting a row containing the ARP in the target image as a single LSF to extract, and aligning all LSF curves according to the ARP as a center to obtain an n x 11 matrix;
s3: calculating MTF, fitting LSF, and performing Fourier change and normalization processing to obtain MTF of Nyquist point;
s4: obtaining a calculation result, firstly obtaining a simulation result, convolving a simulated target image sequence by using a known fuzzy kernel to check whether the calculated MTF is consistent with a preset value, then obtaining a GF-4 result, grabbing 100 frames of images of a target scene by using the MTF estimation method mentioned in the steps S1-S3 to obtain a real target area image, and using a detailed program of the target estimation GF-4 optical imaging system on-orbit space characteristics to extract a plurality of LSF curves, wherein the MTF is obtained by derivation according to the method provided in the steps S1-S3.
Preferably, the LSF in S2 is calculated as,
the PSF (point spread function) is used to characterize the impulse response of the imaging system, and the LSF represents the integrated result of the PSF along the line source direction.
Preferably, the LSF fitting is performed using a sequence of images of satellite attitude angle changes, thereby causing random micro-displacements.
Preferably, the traditional fitting function of the central part and the tail part of the LSF is a simple addition of a Gaussian function and an exponential function, and a deeper relational formula between the Gaussian function and the exponential function is,
f (x) represents the LSF fitting value, θ 1 represents the center of the fitted LSF, θ 2 θ 4 and θ 6 are weight coefficients, θ 3 is the standard deviation of the gaussian function, θ 5 represents the slope of the exponential function, and x represents the LSF abscissa.
Preferably, to obtain the optimum θ
Wherein y is the LSF true value, f is the fitting value, and λ is the regular term coefficient.
Preferably, the grid search and the iteration of the gradient descent method can obtain
Alpha is the step size of the iteration,
for each derived gradient, j is the number of iterations.
The invention has the beneficial effects that: the invention has reasonable structural design, and the method calculates the MTF in the real target image sequence shot by the panchromatic camera in the GF-4 satellite. The time interval for full color imaging is 5 seconds, so the target scene can be considered unchanged during multiple imaging of several minutes. Using the mentioned MTF estimation method, 100 frames of images of the target scene, the real target area image, are captured. The target was laid out in the east of the province of the Heilongjiang province (46.04N, 125.59E) in 2016, 7, 26, and the random variation rhythm of satellite attitude was examined by Kolmogorov-Smirnov test, observing Gaussian distribution, and according to a detailed procedure for estimating the in-orbit spatial characteristics of a GF-4 optical imaging system using a target, the extracted plurality of LSF curves, the difference in MTF between the simulated image and the GF-4 dataset came from the PSF template setup, and were not affected by atmospheric scattering and random errors in satellite attitude vibration. Since the estimated MTF is very close to the known MTF derived from the PSF in this section, the method proposed herein is verified to be reliable and convincing, and the problem that better LSF cannot be obtained while reducing the influence of noise and accidental sampling errors on a single image is solved.
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In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, it is obvious that the drawings in the following description are only some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to the drawings without creative efforts.
FIG. 1 is a flow chart of the present invention;
FIG. 2 is a schematic representation of a target of the present invention;
FIG. 3 is a graph of raw LSF extracted from the target of FIG. 2;
FIG. 4 is a schematic diagram of a matrix formed by a plurality of LSFs
Detailed Description
In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are some, but not all, embodiments of the present invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
Referring to FIGS. 1-4: a MTF calculation method for a spatial resampling GF-4 satellite based on a target comprises the following steps:
s1: for bulls-eye design, a uniform low-reflectivity region was chosen as the dark background, and a high-reflection radiance signature target (60%) with coverage 3 × 4GSD was set;
s2: performing LSF fitting, selecting a row containing the ARP in the target image as a single LSF to extract, and aligning all LSF curves according to the ARP as a center to obtain an n x 11 matrix;
s3: calculating MTF, fitting LSF, and performing Fourier change and normalization processing to obtain MTF of Nyquist point;
s4: obtaining a calculation result, firstly obtaining a simulation result, convolving a simulated target image sequence by using a known fuzzy kernel to check whether the calculated MTF is consistent with a preset value, then obtaining a GF-4 result, grabbing 100 frames of images of a target scene by using the MTF estimation method mentioned in the steps S1-S3 to obtain a real target area image, and using a detailed program of the target estimation GF-4 optical imaging system on-orbit space characteristics to extract a plurality of LSF curves, wherein the MTF is obtained by derivation according to the method provided in the steps S1-S3.
Specifically, the LSF in S2 is calculated as,
the PSF (point spread function) is used to characterize the impulse response of the imaging system, and the LSF represents the integrated result of the PSF along the line source direction.
Specifically, the LSF fitting is performed using an image sequence in which the attitude angle of the satellite changes, thereby causing random micro-displacements.
Specifically, the traditional fitting functions of the central part and the tail part of the LSF are simple addition of Gaussian and exponential functions, and the deeper relational formula between the Gaussian function and the exponential function is as follows,
f (X) represents the LSF fitting value, θ 1 represents the center of the fitted LSF, θ 2 θ 4 and θ 6 are weight coefficients, θ 3 is the standard deviation of the gaussian function, θ 5 represents the slope of the exponential function, and X represents the LSF abscissa.
Specifically, to obtain the optimum θ, there are
Wherein y is the LSF true value, f is the fitting value, and λ is the regular term coefficient.
Specifically, the iteration through the grid search and the gradient descent method can obtain
Alpha is the step size of the iteration,
for each derived gradient, j is the number of iterations.
The invention has reasonable structural design, and when the invention is used, the method calculates the MTF in the real target image sequence shot by the panchromatic camera in the GF-4 satellite. The time interval for full color imaging is 5 seconds, so the target scene can be considered unchanged during multiple imaging of several minutes. Using the mentioned MTF estimation method, 100 frames of images of the target scene, the real target area image, are captured. The target was laid out in the east of the province of the Heilongjiang province (46.04N, 125.59E) in 2016, 7, 26, and the random variation rhythm of satellite attitude was examined by Kolmogorov-Smirnov test, observing Gaussian distribution, and according to a detailed procedure for estimating the in-orbit spatial characteristics of a GF-4 optical imaging system using a target, the extracted plurality of LSF curves, the difference in MTF between the simulated image and the GF-4 dataset came from the PSF template setup, and were not affected by atmospheric scattering and random errors in satellite attitude vibration. Since the estimated MTF is very close to the known MTF derived from the PSF in this section, the method proposed herein is verified to be reliable and convincing, and the problem that better LSF cannot be obtained while reducing the influence of noise and accidental sampling errors on a single image is solved.
The above examples are only intended to illustrate the technical solution of the present invention, but not to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some technical features may be equivalently replaced; and such modifications or substitutions do not depart from the spirit and scope of the corresponding technical solutions of the embodiments of the present invention.
Claims (3)
1. A MTF calculation method for a spatial resampling GF-4 satellite based on a target comprises the following steps:
s1: for the bullseye design, selecting a uniform low-reflectivity area as a dark background, and setting a high-reflection radiation characteristic target covering 3 multiplied by 4 GSD;
s2: performing LSF fitting, selecting a row containing the ARP in the target image as a single LSF to extract, and aligning all LSF curves according to the ARP as a center to obtain an n x 11 matrix;
s3: calculating MTF, fitting LSF, and performing Fourier change and normalization processing to obtain MTF of Nyquist point;
s4: obtaining a calculation result, namely firstly, convolving the simulated target image sequence by using a known fuzzy kernel to obtain a simulation result so as to check whether the calculated MTF is consistent with a preset value;
obtaining a GF-4 result, grabbing 100 frames of images of the target scene by the MTF estimation method mentioned in the steps S1-S3 to obtain a real target area image, and using a detailed program of target estimation of the on-orbit spatial characteristics of the GF-4 optical imaging system to extract a plurality of LSF curves, wherein the MTF is derived according to the method proposed in the steps S1-S3;
the LSF in S2 is calculated by the formula PSF (Point spread function)
The LSF represents the integral result of the PSF along the line source direction;
the LSF fitting is carried out by utilizing an image sequence of which the satellite attitude angle changes so as to cause random micro displacement;
the center and tail of the LSF are conventional fitting functions, which are simple additions of gaussian and exponential functions, with a more deep relational formulation between gaussian and exponential functions,
fθ(x) Denote the LSF fit value, θ 1 denotes the center of the fitted LSF, θ 2, θ 4 and θ 6 are weighting coefficients, θ 3 is the standard deviation of the gaussian function, θ 5 denotes the slope of the exponential function, and x denotes the LSF abscissa.
2. The method of claim 1, wherein the MTF is calculated based on spatially resampled GF-4 satellites from a target, wherein: to obtain the optimum theta
Wherein y is the LSF true value, f is the fitting value, and λ is the regular term coefficient.
3. The method of claim 2, wherein the MTF is calculated based on spatially resampled GF-4 satellites from a target, wherein: through iteration, can obtain
Alpha is the step size of the iteration,
for each derived gradient, j is the number of iterations.
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