Mirror synthetic aperture radiometer calibration method based on an external source with known orientation

Abstract

An error correction method for a mirror synthetic aperture radiometer based on an externally sourced with known orientation, comprising the following steps: The mirror synthetic aperture system acquires target scene signals; the mirror synthetic aperture system acquires externally sourced signals (i.e., calibration data), and uses the acquired externally sourced signals to obtain phase error data for each antenna and channel; uses the obtained phase error data to correct the target scene signals to obtain a corrected correlation output function; obtains a cosine visibility function by solving the transfer equation formed by the correlation output function; reconstructs the brightness temperature image of the target scene by performing an inverse cosine transform on the cosine visibility function. In the present invention, the external source can be placed at any position, can be a noise source or a signal source, the calibration signal is easily obtained, channel phase errors can be corrected, and the imaging quality can be improved.

Description

Technical Field

[0001] The present invention relates to a calibration method for a mirror synthetic aperture radiometer based on an externally sourced with known azimuth, belonging to the technical field of microwave remote sensing and detection. Background Art

[0002] Without expanding the array size, the spatial resolution of a mirror synthetic aperture is better than twice that of a conventional synthetic aperture, achieving a high spatial resolution with fewer antenna elements.

[0003] System errors can lead to a decline in the quality of microwave radiation imaging of a mirror synthetic aperture. Existing error calibration methods require the calibration point to be at the intersection of the two reflectors. When collecting calibration data, the reflectors must be removed, and when collecting observation data, the reflectors need to be reinstalled. This makes the calibration operation complex, and repeatedly installing the reflectors will introduce additional errors and reduce the imaging quality. Summary of the Invention

[0004] The technical problem to be solved by the present invention is: overcoming the deficiencies of the prior art, providing a calibration method for a mirror synthetic aperture radiometer based on an externally sourced with known azimuth. Traditional error calibration methods require the calibration point to be at the intersection of the two reflectors. When collecting calibration data, the reflectors must be removed, and when collecting observation data, the reflectors need to be reinstalled. This makes the calibration operation complex, and repeatedly installing the reflectors will introduce additional errors and reduce the imaging quality. The calibration method disclosed by the present invention includes the following steps: the mirror synthetic aperture system collects the target scene signal; the mirror synthetic aperture system collects the externally sourced signal (i.e., calibration data), and uses the collected externally sourced signal to calculate the phase error data of each antenna and channel; uses the calculated phase error data to calibrate the target scene signal to obtain the calibrated correlation output function; obtains the cosine visibility function by solving the transfer equation formed by the correlation output function; reconstructs the brightness temperature image of the target scene by performing an inverse cosine transform on the cosine visibility function. The externally sourced of the present invention can be placed at any position, can be a noise source or a signal source, the calibration signal is easily obtained, can calibrate the channel phase error, and improve the imaging quality.

[0005] The object of the present invention is achieved by the following technical solutions:

[0006] A calibration method for a mirror synthetic aperture radiometer based on an externally sourced with known azimuth, comprising the following steps:

[0007] S1: The mirror synthetic aperture system collects the target scene signal;

[0008] S2: The mirror synthetic aperture system collects the externally sourced signal, and uses the collected externally sourced signal to determine the phase error data of each antenna and channel;

[0009] S3: Correct the target scene signal using the phase error data to obtain a corrected correlation output function;

[0010] S4: Obtain the cosine visibility function by solving the transfer equation formed by the correlation output function;

[0011] S5: Reconstruct the brightness temperature image of the target scene by performing an inverse cosine transform on the cosine visibility function.

[0012] Preferably, the external source signal includes the external source direct radiation signal and the radiation signal reflected by the double reflector.

[0013] Preferably, the azimuth of the external source signal is at any position.

[0014] Preferably, calculate the ideal correlation output function using the azimuth information of the external source, and obtain the positive and negative information of the ideal correlation output function; compensate the positive and negative information into the correlation output function of the collected external source signal to obtain the phase error data.

[0015] Preferably, correct the target scene signal using the phase error data solved by the external source with a known azimuth to obtain a corrected correlation output function:

[0016]

[0017] where R i ′ j is the correlation output function containing the brightness temperature image information of the target scene with phase error, β ij is the phase error data solved by using the external source with a known azimuth, is the corrected correlation output function containing the brightness temperature image information of the target scene.

[0018] Preferably, the mirror synthetic aperture system includes an antenna array, a reflector, a receiving channel array, an A / D array, a correlator, etc.;

[0019] The antenna array is used to receive the radiation signal from the observation scene and the radiation signal reflected by the reflector;

[0020] The receiving channel array includes multiple receiving channels, one receiving channel corresponding to one antenna element, and the receiving channel down-converts, filters, and amplifies the signal received by the antenna element;

[0021] The A / D array corresponds one-to-one with the receiving channel array, and converts the analog signal in the receiving channel into a digital signal;

[0022] The correlator correlates the signals converted by the A / D array in pairs;

[0023] The output after correlation is the correlation function.

[0024] An image synthesis aperture radiometer calibration device based on an external source with known azimuth, comprising:

[0025] A processing module, which determines the phase error data of each antenna and channel by using the external source signal;

[0026] A calibration module, which calibrates the target scene signal by using the phase error data to obtain a calibrated correlation output function;

[0027] A solving module, which is used to solve the transfer equation formed by the correlation output function to obtain the cosine visibility function;

[0028] A reconstruction module, which reconstructs the brightness temperature image of the target scene by performing an inverse cosine transform on the cosine visibility function.

[0029] Preferably, the external source signal includes the directly radiated signal of the external source and the radiated signal reflected by the double reflector.

[0030] Preferably, the azimuth of the external source signal is at any position.

[0031] Preferably, the ideal correlation output function is calculated by using the azimuth information of the external source, and the positive and negative information of the ideal correlation output function is obtained; the positive and negative information is compensated into the correlation output function of the collected external source signal to obtain the phase error data.

[0032] Preferably, the calibration module calibrates the target scene signal by using the phase error data solved by the external source with known azimuth to obtain a calibrated correlation output function:

[0033]

[0034] wherein, R i ′ j is the correlation output function containing the brightness temperature image information of the target scene with phase error, β ij is the phase error data solved by using the external source with known azimuth, is the calibrated correlation output function containing the brightness temperature image information of the target scene.

[0035] The present invention has the following beneficial effects compared with the prior art:

[0036] (1) Compared with the prior art, the present invention corrects the phase error of the system, simplifies the calibration steps, avoids introducing additional errors, and improves the imaging accuracy after calibration;

[0037] (2) Compared with the prior art, in addition to collecting external source data, the present invention does not need to collect additional point source data to find calibration points, which simplifies the calibration steps;

[0038] (3) Compared with the prior art, the position of the external source is not limited to the intersection line of the double reflectors and can be at any position, which is flexible and easy to use;

[0039] (4) Compared with the prior art, when collecting the calibration data of the external source, there is no need to remove the reflector, and when collecting the data of the target scene, there is no need to install the reflector again, which simplifies the calibration steps;

[0040] (5) Compared with the prior art, since there is no need to remove the reflector and install it again, additional errors are avoided. Description of the Drawings

[0041] Figure 1 is a schematic diagram of a mirror synthetic aperture radiometer provided by an embodiment of the present invention;

[0042] Figure 2 is a schematic diagram of an equivalent antenna array provided by an embodiment of the present invention;

[0043] Figure 3 is a flowchart of a method for calibrating the error of a mirror synthetic aperture radiometer based on an external source with known azimuth provided by an embodiment of the present invention;

[0044] Figure 4 is a 24-element rectangular array for testing provided by an embodiment of the present invention;

[0045] Figure 5 is an uncalibrated noise source image provided by an embodiment of the present invention;

[0046] Figure 6 is a calibrated noise source image provided by an embodiment of the present invention;

[0047] Figure 7 is an uncalibrated stepped extended source image provided by an embodiment of the present invention;

[0048] Figure 8 is a calibrated stepped extended source image provided by an embodiment of the present invention. Detailed Embodiments

[0049] To make the objectives, technical solutions, and advantages of the present invention clearer, the embodiments of the present invention will be described in further detail below in conjunction with the accompanying drawings.

[0050] Figure 1 shows a schematic diagram of a mirror synthetic aperture radiometer, Figure 2 shows a schematic diagram of an equivalent antenna array. For the convenience of description, only the parts related to the present invention are shown and are described in detail as follows:

[0051] The mirror aperture radiometer includes an antenna array and a reflector, a receiving channel array, an A / D array, a correlator, etc. The antenna array receives the radiation signal from the observed scene and the radiation signal reflected by the reflector. i ,y i ,0) of antenna a i Four signals will be received, namely the direct incident signal, the signal reflected once by the reflector, and the signal reflected twice by the double reflector. i and a j Indicates that antenna a i Three mirror antennas will be formed relative to the double reflector, and their coordinates are (-x i ,y i )、(x i ,-y i ) and (-x i ,-y i ). The receiving channel array includes multiple receiving channels, one receiving channel corresponds to one antenna unit, and the receiving channel down-converts, filters and amplifies the signal received by the antenna unit. The A / D array corresponds to the receiving channel array one by one, and converts the analog signal in the receiving channel into a digital signal. The correlator correlates the signals converted by the A / D array in pairs. The output after correlation is the correlation function.

[0052] like Figure 3 As shown, the mirror synthetic aperture radiometer correction method based on an external source with a known position provided by the present invention specifically includes the following steps:

[0053] 1. The mirror-image synthetic aperture system collects target scene signals. The output of the antenna array is a dual-antenna cross-correlation output function. Antenna a i Antenna a receives the radiation signal from the observation scene and the radiation signal reflected by the reflector. j Receiving the radiation signal from the observation scene and the radiation signal reflected by the reflector, the cross-correlation output function of the dual antennas is:

[0054]

[0055] in is the direction cosine, θ is the pitch angle of the observed target scene, is the azimuth of the observed target scene, and T(ξ,η) is the brightness temperature of the target scene.

[0056] Define a two-dimensional cosine visibility function:

[0057]

[0058] The output of the dual-antenna received signal in the two-dimensional mirror comprehensive aperture system is:

[0059] R ij = CV(x j - x i , y j - y i ) - CV(x j - x i , y j + y i ) + CV(x j + x i , y j - y i ) - CV(x j + x i , y j + y i )

[0060] For any two antennas, an equation similar to the above equation can be obtained, and these equations can be combined into a system of linear equations:

[0061]

[0062] where L is the number of antennas, M and N are the maximum sampling frequencies in two directions, R is the correlation output vector, P is the transfer matrix, and CV is the cosine visibility vector.

[0063] Add the error of the mirror synthetic aperture to the correlation function:

[0064]

[0065] 2. Collect external source calibration data at any position. The correlation output of the external source is:

[0066]

[0067] Calculate the correlation output of the ideal external source according to the azimuth information (ξ1, η1) of the external source:

[0068]

[0069] Obtain the positive and negative sign information of the correlation output of the ideal external source for all antenna pairs, compensate it into the correlation output of the collected external source, and calculate the error of the correlation output of the collected external source

[0070]

[0071] 3. Use the solved error to correct the target scene signal to obtain the corrected correlation output:

[0072]

[0073] 4. The cosine visibility function is obtained by solving a linear equation system containing the scene brightness temperature image information;

[0074] CV = A·R

[0075] Where A is the pseudo-inverse of P, and algorithms such as the MP generalized inverse method, Tikhonov regularization method, and truncated singular value method can be used. The relevant output of the mirror synthetic aperture belongs to the first kind of Fredholm integral equation. The first kind of integral equation with a completely continuous operator is unstable even if it has a solution. In addition, the measured data actually obtained inevitably contains systematic errors and noise, which will lead to a serious deviation of the approximate solution from the true solution. Due to the underdetermination of the matrix, the matrix inversion based on the MP generalized inverse is very ill-posed. In order to obtain a stable approximate solution to the ill-posed problem, a regularization method is needed. The basic idea of the regularization method is to redefine the concept of the solution to the ill-posed problem using some additional information of the specific problem, and then introduce a stabilization functional to give a stable method for approximating the solution to the original problem, that is, to find a suitable inverse operator to obtain the stable approximate solution.

[0076] Therefore, considering the systematic errors and noise, the stable approximate solution of the cosine visibility function is obtained by using the truncated singular value decomposition and Tikhonov regularization.

[0077] 5. The scene brightness temperature image is reconstructed by inverse cosine transform. The two-dimensional cosine visibility function can be obtained by solving a linear equation system, and then the scene brightness temperature image can be reconstructed by two-dimensional inverse cosine transform.

[0078]

[0079] Among them, u and v represent the spatial sampling frequencies in two dimensions.

[0080] To further illustrate the mirror synthetic aperture radiometer calibration method based on an external source with known azimuth provided by the embodiments of the present invention, it is described in detail below in combination with specific examples:

[0081] Example 1: Point source scene brightness temperature image

[0082] In this example, the calibration method for the mirror synthetic aperture radiometer based on an external source with known azimuth is simulated and verified. Figure 4 The array used for simulation is a 24-element rectangular array.

[0083] The specific steps are as follows:

[0084] (1) For the 24-element rectangular array, the mirror synthetic aperture system collects the radiation signals of the noise source scene;

[0085] (2) Collect the external source calibration data and calculate the error of the relevant output of the collected external source;

[0086] (3) The uncorrected noise source brightness temperature image, such as Figure 5 ;

[0087] (4) Use the error of the correlation output of the obtained external source to correct the noise source signal data to obtain the corrected noise source brightness temperature image, such as Figure 6 .

[0088] Judging from the simulation results, Figure 5 is the uncorrected noise source brightness temperature image, the result is distorted, and the noise source cannot be distinguished. Figure 6 is the corrected noise source brightness temperature image, the noise source contour is clear, the background is pure, and using the external source with known azimuth can correct the phase error of the antenna and the channel together to obtain an image with higher quality.

[0089] Embodiment 2: Step-expanded source scene brightness temperature image

[0090] In this embodiment, the calibration method of the mirror synthetic aperture radiometer for the external source with known azimuth is verified by simulation.

[0091] The specific steps are as follows:

[0092] (1) For a 24-element rectangular array, the mirror synthetic aperture system collects the radiation signals of the noise source scene;

[0093] (2) Collect the external source calibration data and calculate the phase error of the correlation output of the collected external source;

[0094] (3) The uncorrected step-expanded source brightness temperature image, such as Figure 7 ;

[0095] (4) Use the error of the correlation output of the obtained external source to correct the step-expanded source signal data to obtain the corrected step-expanded source brightness temperature image, such as Figure 8 .

[0096] Judging from the simulation results, Figure 7 is the uncorrected step-expanded source brightness temperature image, the result is distorted, and the step-expanded source contour cannot be distinguished. Figure 8 is the corrected step-expanded source brightness temperature image, the step-expanded source contour is clear, the background is pure, and using the external source with known azimuth can correct the phase error of the antenna and the channel together to obtain an image with higher quality.

[0097] The above embodiments demonstrate the effects of the present invention: the present invention can correct the phase error of the antenna and the channel and improve the imaging quality. Thus, the problem in the prior art that the reflector needs to be repeatedly installed and removed, thereby introducing additional errors, is solved, and the calibration position of the external source is flexible, convenient and easy to use.

[0098] The content not detailedly described in the specification of the present invention belongs to the well-known technology of those skilled in the art.

[0099] Although the present invention has been disclosed above with the preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make possible changes and modifications to the technical solution of the present invention by using the methods and technical content disclosed above without departing from the spirit and scope of the present invention. Therefore, all simple modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention without departing from the technical solution of the present invention shall fall within the protection scope of the technical solution of the present invention.

Claims

1. A calibration method for a mirror synthetic aperture radiometer based on an external source with known azimuth, characterized in that It includes the following steps: S1: The mirror synthetic aperture system collects the target scene signals; S2: The mirror synthetic aperture system collects the external source signals, determines the phase error data of each antenna and channel by using the collected external source signals; calculates the ideal correlation output function by using the azimuth information of the external source, and obtains the positive and negative information of the ideal correlation output function; compensates the positive and negative information into the correlation output function of the collected external source signals to obtain the phase error data; Collect the external source calibration data at any position, and the correlation output of the external source is: Calculate the correlation output of the ideal external source according to the azimuth information (ξ1, η1) of the external source: Obtain the positive and negative sign information of the correlation output of the ideal external source for all antenna pairs, compensate it into the correlation output of the collected external source, and calculate the error of the correlation output of the collected external source S3: Use the phase error data to correct the target scene signals to obtain the corrected correlation output function; Among them, R' ij is the relevant output function of the target scene brightness temperature image information containing phase errors, and β ij is the phase error data solved using an external source with a known azimuth, is the relevant output function of the corrected target scene brightness temperature image information; S4: Obtain the cosine visibility function by solving the transfer equation formed by the correlation output function; S5: Reconstruct the brightness temperature image of the target scene by performing an inverse cosine transform on the cosine visibility function.

2. The calibration method according to claim 1, wherein The external source signals include the directly radiated signals of the external source and the radiated signals reflected by the double reflector.

3. The calibration method according to claim 1, wherein The azimuth of the external source signal is at any position.

4. The calibration method according to any one of claims 1 to 3, characterized in that, The mirror synthetic aperture system includes an antenna array, a reflector, a receiving channel array, an A / D array, and a correlator; The antenna array is used to receive the radiated signals from the observation scene and the radiated signals reflected by the reflector; The receiving channel array includes multiple receiving channels, one receiving channel corresponds to one antenna unit, and the receiving channel down-converts, filters, and amplifies the signals received by the antenna unit; The A / D array corresponds to the receiving channel array one by one, and converts the analog signals in the receiving channels into digital signals; The correlator correlates the signals after being converted by the A / D array pairwise; The output after correlation is the correlation function.

5. A calibration device for a mirror synthetic aperture radiometer based on an external source with known orientation, characterized in that, It includes: A processing module that determines the phase error data of each antenna and channel by using the external source signals; Calculates the ideal correlation output function by using the azimuth information of the external source, and obtains the positive and negative information of the ideal correlation output function; compensates the positive and negative information into the correlation output function of the collected external source signals to obtain the phase error data; Collect the external source calibration data at any position, and the correlation output of the external source is: Calculate the correlation output of the ideal external source according to the azimuth information (ξ1, η1) of the external source: Obtain the positive and negative sign information of the correlation output of the ideal external source for all antenna pairs, compensate it into the correlation output of the collected external source, and calculate the error of the correlation output of the collected external source A calibration module that uses the phase error data to correct the target scene signals to obtain the corrected correlation output function; uses the phase error data solved by the external source with known azimuth to correct the target scene signals to obtain the corrected correlation output function: wherein, R' ij is the relevant output function of the target scene brightness temperature image information containing phase error, and β ij is the phase error data solved by using an external source with known azimuth, is the relevant output function of the corrected target scene brightness temperature image information; A solving module for solving the transfer equation formed by the correlation output function to obtain the cosine visibility function; A reconstruction module that reconstructs the brightness temperature image of the target scene by performing an inverse cosine transform on the cosine visibility function.

6. The calibration device according to claim 5, wherein The external source signals include the directly radiated signals of the external source and the radiated signals reflected by the double reflector.

7. The calibration device according to claim 5, wherein The azimuth of the external source signal is at an arbitrary position.

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