Satellite-borne synthetic aperture radiometer antenna mutual coupling error correction method

Abstract

The invention discloses a method for correcting mutual coupling errors of antennas of a satellite-borne synthetic aperture radiometer. The method comprises the following steps of: 1, observing a target scene by the satellite-borne synthetic aperture radiometer to obtain a measured target scene visibility function; step 2, observing the flat cold air by the satellite-borne synthetic aperture radiometer to obtain a measured cold air visibility function; step 3, calculating system 1K response by using the measured cold air visibility function and the brightness temperature of the back lobe model when the cold air is observed; 4, correcting a target scene visibility function by using the 1K response calculated by the cold air visibility and combining the physical temperature of the receiver during scene observation; and 5, performing brightness temperature reconstruction on the corrected target scene visibility function to obtain a corrected reconstructed brightness temperature. According to the method, the error influence of antenna mutual coupling on the non-zero baseline of the synthetic aperture radiometer can be effectively corrected, the imaging quality of the satellite-borne synthetic aperture radiometer is improved, and the method has a wide market application prospect.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of space microwave remote sensing, and particularly relates to a method for correcting antenna mutual coupling error of a spaceborne synthetic aperture radiometer. BACKGROUND

[0002] The synthetic aperture radiometer uses small-aperture antennas to form an antenna array to observe a target scene. After filtering, amplification, mixing and A / D conversion of received signals, cross-correlation processing is performed. Any two units in the array can form a baseline pair, corresponding to a group of cross-correlation output values. The output of a single antenna unit corresponds to an autocorrelation output value. After absolute calibration and redundant averaging of all autocorrelation outputs and cross-correlation outputs, the visibility function corresponding to different baseline pairs can be obtained. The visibility function has a Fourier transform relationship with the scene brightness temperature. The reconstructed brightness temperature image in the field of view can be obtained by performing Fourier inverse transform on the visibility function.

[0003] Due to the small antenna spacing of the synthetic aperture radiometer, there is a coupling effect between the antenna units. When there is no antenna mutual coupling error, the measured cross-correlation output is positively correlated with the scene brightness temperature. The antenna mutual coupling makes the measured cross-correlation output positively correlated with the difference between the scene brightness temperature and the receiver physical temperature. If the antenna mutual coupling error is ignored for brightness temperature reconstruction, an error will be brought to the reconstructed image, resulting in image distortion. SUMMARY

[0004] The technical problem solved by the application is that, in view of the problem of image distortion caused by the antenna mutual coupling effect due to the small antenna spacing of the spaceborne synthetic aperture radiometer, a method for correcting antenna mutual coupling error of a spaceborne synthetic aperture radiometer is provided.

[0005] The technical solution of the application is: A method for correcting antenna mutual coupling error of a spaceborne synthetic aperture radiometer, characterized by comprising the following steps: step 1, observing a target scene by the spaceborne synthetic aperture radiometer, collecting correlation output matrix data of the target scene, and performing two-point calibration to obtain a measured visibility function of the target scene; Step 2, observing a flat cold space by the spaceborne synthetic aperture radiometer, collecting correlation output matrix data of the flat cold space, and performing two-point calibration to obtain a measured cold space visibility function; Step 3, calculating the 1K response by using the cold space visibility function and the backlobe model brightness temperature when the cold space is observed; Step 4, correcting the target scene visibility function by using the 1K response calculated by the cold space visibility and combining the receiver physical temperature when the scene is observed; Step 5, performing brightness temperature reconstruction on the corrected target scene visibility function to obtain the corrected reconstructed brightness temperature.

[0006] Further, in step 2, the measured target scene visibility function, for any two-element non-zero baseline pair (k, j), when the antenna mutual coupling is not negligible, its visibility function expression is:

[0007] wherein, is the visibility function of the target scene. is the target scene brightness temperature. and is the baseline normalized to wavelength . and are the position coordinates of the corresponding antenna j and antenna k . and are the normalized antenna voltage patterns, and and are the corresponding antenna solid angles, wherein, and are the directional cosines in the x-axis and y-axis, φ and θ are the azimuth and elevation angles, respectively. is the receiver physical temperature when observing the target scene, which is measured by the temperature sensor in real time.

[0008] For the visibility of the zero baseline, there is no antenna mutual coupling error influence, and its visibility function expression is:

[0009] Further, in step 2, the measured cold space visibility function, when the antenna mutual coupling is not negligible, its visibility function expression is:

[0010] wherein, is the target scene brightness temperature. is the receiver physical temperature when observing the cold space, which is measured by the temperature sensor in real time. , respectively represent the measured cold space visibility function corresponding to the non-zero baseline pair and the cold space visibility function corresponding to the zero baseline.

[0011] Further, in step 2, the timing selection criteria for cold space observation are: 1) the satellite-borne synthetic aperture radiometer beam points to a uniform cold space region (near the polar region), avoiding the influence of strong point sources such as the galaxy, the sun, and the moon; 2) the backlobe is directed to a scene where the main region is the ocean, avoiding land areas.

[0012] The timing of cold air observation needs to be planned in advance, and simulation is carried out in combination with a global brightness temperature distribution model, a radiation transmission model and an orbit numerical model to select a cold air region observed by a main lobe after the cold air as a relatively stable region and a position of an ocean region with minimum land pollution observed by a back lobe.

[0013] Further, in step 3, the earth longitude and latitude of the back lobe field of view can be obtained according to satellite position, orientation and other information. The corresponding back lobe brightness temperature distribution model can be obtained from the earth longitude and latitude and the global brightness temperature distribution model. The contribution of the back lobe can be calculated in combination with the average directional diagram of the antenna back lobe.

[0014] In the formula, is the average directional diagram of the back lobe, is the antenna directional diagram solid angle corresponding to the average antenna directional diagram.The 1K response of the system can be obtained from the cold air visibility, in combination with the cold air brightness temperature and the receiver physical temperature during cold air observation :

[0015] Further, in step 4, the 1K response calculated by the cold air visibility is combined with the receiver physical temperature during scene observation to correct the target scene visibility function, and the calculation formula is as follows:

[0016] Further, in step 4, the system impulse response matrix G matrix is used for brightness temperature reconstruction:

[0017] In the formula, PG is the pseudo-inverse matrix of the G matrix, which can be solved by using the MP generalized inverse and other regularization methods. is the reconstructed brightness temperature vector, is the corrected scene visibility vector.

[0018] Compared with the prior art, the present application has the following advantages: (1) The present application can effectively correct the error term caused by antenna mutual coupling and improve the quality of the reconstructed brightness temperature image by observing the cold air in orbit, processing the cold air visibility by using the back lobe model brightness temperature, obtaining the system 1K response, and correcting the target scene visibility in combination with the real-time measured receiver physical temperature.

[0019] (2) The antenna mutual coupling error can be effectively corrected without changing the hardware conditions of the spaceborne synthetic aperture radiometer system. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 A flow chart of the present application; Figure 2 An array arrangement schematic diagram; Figure 3 An original scene brightness temperature distribution diagram; Figure 4 A main-back lobe brightness temperature distribution diagram during cold space observation. Wherein, (a) is a main lobe cold space brightness temperature distribution, and (b) is a back lobe brightness temperature distribution; Figure 5 Simulation results before correction. Wherein, (a) is a reconstructed brightness temperature diagram, and (b) is an error diagram; Figure 6 Simulation results after correction. Wherein, (a) is a reconstructed brightness temperature diagram, and (b) is an error diagram. DETAILED DESCRIPTION

[0021] In order to make the purpose, technical scheme and advantages of the present application more clear and understandable, the present application is further described in detail below in combination with the drawings and examples. It should be understood that the specific examples described herein are only used to explain the present application, and are not used to limit the present application.

[0022] The error correction process is shown in Figure 1 . The specific method steps and simulation examples are as follows: 1. Target scene data acquisition The spaceborne synthetic aperture radiometer observes the target scene, and the collected correlation output matrix data is two-point calibrated to obtain the measured visibility function. For any two elements of a non-zero baseline pair (k, j), when the antenna mutual coupling cannot be ignored, the expression of the visibility function is: (1) In the formula, is the visibility function of the target scene. is the brightness temperature of the target scene. and is the baseline normalized to the wavelength . and are the position coordinates of the corresponding antennas j and k . and are the normalized antenna voltage patterns, and and are the corresponding antenna solid angles. In the formula, and are the direction cosines of the x and y axes, respectively, φ and θ are the azimuth and elevation angles, respectively. is the receiver physical temperature when observing the target scene, which is measured by the temperature sensor in real time.

[0023] For the visibility of the zero baseline, its expression is: (2) 2. Flat cold space data acquisition Since the cold space brightness temperature is relatively stable and can provide a larger uniform scene, the 1K response of the synthetic aperture radiometer (the visibility corresponding to the observation scene of 1K) can be obtained by observing the cold space, which is used for the subsequent correction of the visibility of the target scene.

[0024] The selection criteria for the timing of cold space observation are: 1) The satellite-borne synthetic aperture radiometer beam points to a uniform cold space area (near the position of the silver pole), avoiding the influence of strong point sources such as the galaxy, the sun, and the moon; 2) The main area of the back lobe is the ocean scene, avoiding land areas. The ocean area is relatively uniform, and the land area has large brightness temperature differences and is contaminated by RFI. The timing of cold space observation needs to be simulated in advance by combining global brightness temperature distribution models, radiation transfer models, and orbit numerical models to select positions where the cold space observed by the main lobe after the cold space is relatively stable, and the back lobe observation area is the ocean area with the least land pollution. The visibility of the cold space can be expressed as: (3) In the formula, is the target scene brightness temperature. is the receiver physical temperature when observing the cold space, which is measured by the temperature sensor in real time. Generally, the receiver physical temperatures when observing the earth and observing the cold space are different. 、 and represent the measured non-zero baseline corresponding to the cold space visibility function, and the zero baseline corresponding to the cold space visibility function.

[0025] 3. 1K response calculation For a flat cold space, the brightness temperature in the field of view is basically the same in different directions. Then the cold space brightness temperature can be equal to a constant, The cold space data collected in addition to the contribution of the cold space brightness temperature is also important. When the satellite-borne synthetic aperture radiometer observes the cold space, the earth is located in the antenna back lobe, and its brightness temperature contribution is relatively reduced a lot, but since the earth brightness temperature value is higher than the cold space brightness temperature value, the contribution of the earth cannot be ignored. According to the satellite position, azimuth, and other information, the earth longitude and latitude of the back lobe field of view can be obtained. The corresponding back lobe brightness temperature distribution model can be obtained from the earth longitude and latitude and the global brightness temperature distribution model. Let the back lobe corresponding model brightness temperature be , combined with the antenna back lobe pattern and other information, the contribution of the back lobe can be calculated, and the contribution of the back lobe is subtracted from the zero baseline, so that the zero baseline can be calculated

[0026] (4) In the formula, is the average back lobe pattern, is the average antenna pattern corresponding to the antenna pattern solid angle. From the cold space visibility and combined with the cold space brightness temperature and the receiver physical temperature during the cold space observation, the 1K response of the system can be obtained (5).

[0027] 4. Scene visibility correction The 1K response calculated by the cold space visibility is combined with the receiver physical temperature during the scene observation to correct the scene visibility.

[0028] (6).

[0029] Since the antenna mutual coupling error only affects the visibility function corresponding to the non-zero baseline, the correction only modifies the visibility corresponding to the non-zero baseline.

[0030] 5. Reconstructing scene brightness temperature The corrected visibility and the scene brightness temperature have a Fourier transform relationship. Under the ideal condition that all unit antenna patterns are the same, the scene brightness temperature can be obtained from the visibility by using Fourier inverse transform. However, for the actual system, the unit antenna patterns have differences, so the system impulse response matrix G matrix is used to reconstruct the brightness temperature (7).

[0031] In the formula, PG is the pseudo-inverse matrix of the G matrix, which can be obtained by using the MP generalized inverse and other regularization methods. is the reconstructed brightness temperature vector, is the corrected scene visibility vector.

[0032] In order to further illustrate the satellite-borne synthetic aperture radiometer antenna mutual coupling error correction method provided by the embodiment of the application, the following will be described in detail in combination with specific examples: In this embodiment, a two-dimensional Y-shaped array with 18 units of a single arm is simulated, Figure 2 is the array arrangement diagram of the Y-shaped array used in simulation, and the minimum spacing of the antenna units is 0.8 times the wavelength. The simulation target scene setting includes two regions: earth + cold space, and the earth region is set as a typical H polarized ocean scene, and the cold space region brightness temperature is set as 2.7K, as shown in Figure 3 ​The main lobe cold air brightness temperature is 2.7K, the back lobe brightness temperature of the earth region is 150K, and the cold air region is 2.7K when the cold air is observed in the simulation, as shown in the figure. Figure 4 The receiver physical temperature is set to 310K when the target scene is observed, and the receiver physical temperature is set to 300K when the flat cold air is observed. The brightness temperature error is calculated as shown in the following formula: (8) In the formula, n is the number of brightness temperature pixels. is the reconstructed brightness temperature, is the original scene brightness temperature.

[0033] From the simulation results, it can be seen that Figure 5 is the result before error correction, compared with the original scene brightness temperature, the reconstructed brightness temperature has obvious error, and the brightness temperature distribution is inconsistent with the original scene brightness temperature distribution, and the error is 40.43K. Figure 6 is the result after correction by the method, the error is obviously improved, the reconstructed brightness temperature is close to the original scene brightness temperature, and the error is 0.54K. It is proved that the method can effectively reduce the influence of the antenna mutual coupling error of the synthetic aperture radiometer.

[0034] Therefore, the antenna mutual coupling error correction method of the synthetic aperture radiometer on the satellite proposed in the application can effectively correct the error caused by the antenna mutual coupling, and improve the quality of the reconstructed brightness temperature image.

[0035] The application can effectively correct the error caused by the antenna mutual coupling on the non-zero baseline of the synthetic aperture radiometer, improve the quality of the reconstructed brightness temperature image of the synthetic aperture radiometer, and make the reconstructed brightness temperature distribution more consistent with the original scene brightness temperature distribution.

[0036] The application can effectively improve the detection accuracy of the synthetic aperture radiometer on the satellite, and is an antenna mutual coupling error correction method of the synthetic aperture radiometer on the satellite.

[0037] The above-mentioned embodiments are only the preferred specific embodiments of the application, and the usual changes and replacements made by those skilled in the art within the technical scheme range of the application should be included in the protection range of the application.

Claims

1. A method for correcting the mutual coupling error of a spaceborne synthetic aperture radiometer antenna, characterized in that, Includes the following steps: Step 1: The spaceborne integrated aperture radiometer observes the target scene, collects relevant output matrix data of the target scene, and performs two-point calibration to obtain the measured target scene visibility function. Step 2: The spaceborne integrated aperture radiometer observes the flat cold sky, collects relevant output matrix data of the flat cold sky, and performs two-point calibration to obtain the measured cold sky visibility function. Step 3: Calculate the system's 1K response using the measured cold air visibility function and the back lobe model brightness temperature when observing the cold air; Step 4: Correct the target scene visibility function by using the 1K response calculated from the cold air visibility and combining it with the receiver's physical temperature during scene observation; Step 5: Reconstruct the brightness temperature of the corrected target scene visibility function to obtain the corrected reconstructed brightness temperature.

2. The method for correcting the mutual coupling error of a spaceborne synthetic aperture radiometer antenna according to claim 1, characterized in that: In step 1, the visibility function of the measured target scene, for any non-zero baseline pair (k,j) composed of two units, when the antenna mutual coupling is not negligible, has the following expression: （1） In the formula, It is the visibility function of the target scene; Brightness temperature for the target scene; and Normalization to wavelength The baseline; and The corresponding antenna j and antenna k Position coordinates; and This is the normalized antenna voltage pattern. and It is the corresponding antenna solid angle; where, and The direction cosines on the x and y axes are... φ and θ These are the azimuth and elevation angles, respectively. This is the physical temperature of the receiver when observing the target scene; this temperature is measured in real time by a temperature sensor.

3. For visibility at zero baseline, the visibility function expression is: （2）。 4. The method for correcting the mutual coupling error of a spaceborne synthetic aperture radiometer antenna according to claim 1, characterized in that: In step 2, the measured cold-air visibility function, when antenna mutual coupling is not negligible, has the following expression: （3） In the formula, Brightness temperature for the target scene; This is the physical temperature of the receiver when observing cold air; this temperature is measured in real time by a temperature sensor. , These represent the cold air visibility function corresponding to the non-zero baseline and the cold air visibility function corresponding to the zero baseline, respectively.

5. The method for correcting the mutual coupling error of a spaceborne synthetic aperture radiometer antenna according to claim 1, characterized in that: In step 2, the spaceborne synthetic aperture radiometer observes the flat cold sky. Specifically, the timing criteria for cold sky observation are as follows: 1) The beam of the spaceborne synthetic aperture radiometer is pointed to a uniform cold sky region (near the galactic pole) to avoid the influence of strong point sources such as the Milky Way, the Sun, and the Moon; 2) The back lobe is aligned with a scene where the main area is the ocean to avoid land areas. The timing of cold air observations needs to be planned in advance. Simulations should be carried out using global brightness temperature distribution models, radiative transfer models, and orbital numerical models to select locations where the cold air region observed by the main lobe after the cold air overturns is relatively stable, and the observation region by the back lobe is an ocean region with minimal land pollution.

6. The method for correcting the mutual coupling error of a spaceborne synthetic aperture radiometer antenna according to claim 1, characterized in that: In step 3, the calculation of the system's 1K response using the cold-sky visibility function and the back lobe model brightness temperature during cold-sky observation is specifically as follows: The Earth's latitude and longitude of the back lobe field of view are obtained based on the satellite's position and azimuth; the corresponding back lobe brightness temperature distribution model is obtained from the Earth's latitude and longitude and the global brightness temperature distribution model; let the model brightness temperature corresponding to the back lobe be... By combining the average radiation pattern of the antenna back lobe, the contribution of the back lobe is calculated from zero baseline. Subtracting the contribution of the dorsal lobe from the mid-wave, the cold air brightness temperature can be calculated. : （4） In the formula, The average orientation pattern of the dorsal lobe; The antenna pattern solid angle corresponding to the average antenna pattern; The brightness temperature of the model corresponding to the back lobe; Based on cold air visibility and combined with cold air brightness temperature and the physical temperature of the receiver during cold space observation The system's 1K response was obtained. : （5）。 7. The method for correcting the mutual coupling error of a spaceborne synthetic aperture radiometer antenna according to claim 1, characterized in that: In step 4, the target scene visibility function is corrected by using the 1K response calculated from cold air visibility and combining it with the receiver's physical temperature during scene observation. The calculation formula is as follows: （6）。 8. The method for correcting the mutual coupling error of a spaceborne synthetic aperture radiometer antenna according to claim 1, characterized in that: In step 4, the brightness temperature is reconstructed from the corrected target scene visibility function. Specifically, this is done using the system impulse response matrix G; the formula is: （7） In the formula, PG is the pseudo-inverse of matrix G, which can be obtained by using the MP generalized inverse and other regularization methods; To reconstruct the brightness temperature vector, This is the corrected scene visibility vector.

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