Time sequence cross calibration method and system of synthetic aperture radar

Abstract

The invention relates to a time sequence cross calibration method and system of a synthetic aperture radar. The method comprises the following steps: screening out a target area for time sequence cross calibration; sAR image data pairs are generated one by one in the time range; adopting a window median method to extract a median matrix of a data slice at a single moment, obtaining a median matrix array, carrying out robust regression fitting of abnormal value elimination, and generating a radiometric calibration coefficient array; and processing the radiometric calibration coefficient array according to the final radiometric calibration coefficient reference value to obtain a final radiometric calibration coefficient. Interference of abnormal values is avoided by performing robust regression fitting of abnormal value elimination on a median matrix array; the radiation calibration coefficient corresponding to each single moment is calculated, the correlation of the radiation calibration coefficients at different moments is taken as a reference, the radiation calibration coefficient value measured at the to-be-calibrated moment and the radiation calibration coefficient value at the previous moment are combined, the final reference value of the radiation calibration coefficient is determined, and the calibration accuracy is improved.

Description

Technical Field

[0001] The invention relates to the technical field of synthetic aperture radar (SAR) radiation calibration, and in particular to a time series cross calibration method and system for a synthetic aperture radar. Background Art

[0002] As an active microwave imaging system, Synthetic Aperture Radar (SAR) can obtain high-resolution surface image information all day and all weather, and has become an important technical means in application fields such as environmental monitoring, disaster assessment and marine protection. With the continuous expansion of application fields, traditional qualitative remote sensing can no longer meet conventional needs. It is necessary to obtain accurate ground object scattering physical information from remote sensing images to realize the quantitative application of remote sensing data. The key to realizing SAR quantitative remote sensing application lies in SAR radiation calibration. Through this process, the relationship between the digital quantization value (DN value) of SAR image and the backscattering coefficient value of ground objects is established to realize the quantitative acquisition of surface physical parameters (such as soil moisture and dielectric constant). Radiation calibration is of great significance for the quantitative extraction and accurate interpretation of electromagnetic information of ground object target characteristics in SAR images. However, affected by factors such as component aging and changes in the external environment, the radiation performance of SAR satellite sensors will inevitably change during space operation, especially after the design life cycle. If the radiation calibration coefficient is not updated in time, the characteristic information of ground objects will be distorted, and the subtle changes of ground objects introduced by changes in external environmental factors may be covered up, thus affecting the accuracy of SAR quantitative remote sensing applications.

[0003] Existing SAR absolute radiometric calibration technologies can be divided into site calibration technology and radiation cross calibration technology. Site calibration technology uses an artificial calibrator deployed at the calibration site. The satellite to be calibrated passes through the calibration site and images the artificial calibrator to obtain the response energy of the artificial calibrator, and then jointly calculates the absolute radiometric calibration coefficient with the known theoretical value of the radar cross section (RCS) of the calibrator. However, the number of deployed artificial calibrators is limited, and the revisit period of SAR satellites is long, making it difficult to achieve high-frequency calibration. Radiation cross calibration technology is a new technology for achieving SAR radiometric calibration. This technology uses a satellite that has been calibrated with high precision as a reference star. The reference star and the satellite to be calibrated observe the same stable ground reference target. The satellite to be calibrated obtains the response energy of the reference target, and then jointly calculates the absolute radiometric calibration coefficient with the backscatter coefficient of the reference target obtained from the reference star. The reference target used in site calibration technology is the artificial calibrator, and the reference target used in radiation cross calibration technology is a stable ground target, which has a larger number, so it can effectively increase the calibration frequency.

[0004] However, in existing radiation cross-calibration technical solutions, interference is rarely considered, resulting in poor calibration accuracy. Summary of the invention

[0005] Based on this, in order to solve the above technical problems, a time series cross-calibration method and system for synthetic aperture radar are provided, which can improve the accuracy and efficiency of calibration.

[0006] A time series cross calibration method for synthetic aperture radar, the method comprising:

[0007] Determine a reference satellite and a satellite to be calibrated, determine a distribution target based on the reference satellite and the satellite to be calibrated, obtain position information of the distribution target, and perform region screening within the spatial range of the distribution target according to the position information to obtain a target region for time series cross calibration;

[0008] Acquire imaging parameters of the reference satellite and the satellite to be calibrated, determine a time range for time series cross calibration according to the imaging parameters and position information, cut data slices for time series cross calibration based on the position information, and generate SAR image data pairs corresponding to the distributed targets moment by moment within the time range;

[0009] The median matrix of the data slices at a single moment is extracted respectively by using the window median method to obtain a median matrix array, and the median matrix array is subjected to robust regression fitting with outliers removed to output the radiation calibration coefficient at the single moment;

[0010] Determine a reference radiation calibration coefficient, and obtain a final reference value of the radiation calibration coefficient according to the reference radiation calibration coefficient and the radiation calibration coefficient;

[0011] Within the time range, the radiation calibration coefficients corresponding to each single moment are calculated respectively to generate a radiation calibration coefficient array; the radiation calibration coefficient array is error processed according to the final reference value of the radiation calibration coefficient to obtain the final radiation calibration coefficient, thereby completing the time series cross calibration.

[0012] In one embodiment, obtaining the location information of the distribution target, and performing area screening within the spatial range of the distribution target according to the location information to obtain the target area for time series cross calibration includes:

[0013] Acquire the time-series SAR data of the reference satellite according to the position information, and pre-process the time-series SAR data to obtain a time-series RCS value image corresponding to the distribution target;

[0014] Based on the time series RCS value image, regional screening is performed within the spatial range of the distribution target to obtain a target area for time series cross calibration.

[0015] In one embodiment, performing regional screening within the spatial range of the distribution target based on the time-series RCS value image includes:

[0016] Performing a primary screening of regions based on the time-series RCS value image to obtain each primary screening region;

[0017] According to the position information of each of the initial screening areas, respectively cutting the time series RCS value slices;

[0018] Using the window median method, each time series backscatter coefficient corresponding to each time series RCS value slice is extracted respectively, and the average backscatter coefficient corresponding to the time series backscatter coefficient is calculated;

[0019] The target area is screened out from each of the initial screening areas according to the time series backscatter coefficient and the average backscatter coefficient through the root mean square error.

[0020] In one of the embodiments, cutting data slices for time series cross calibration based on the position information and generating SAR image data pairs corresponding to the distribution target moment by moment within the time range include:

[0021] Based on the location information, cut out reference satellite RCS value data slices and to-be-calibrated satellite DN value data slices of the target area;

[0022] Determining a radiation calibration requirement, and filtering out SAR image data from the data slices within the time range according to the radiation calibration requirement;

[0023] The SAR image data are respectively cropped to obtain SAR image data pairs corresponding to the distribution targets.

[0024] In one embodiment, the median matrix of the data slices at a single moment is extracted respectively by using the window median method to obtain a median matrix array, including:

[0025] Get the set window size;

[0026] According to the window size, a window median method is used to extract a median matrix from the RCS value data slice and the DN value data slice respectively to obtain an RCS value median matrix and a DN value median matrix;

[0027] The RCS value median matrix and the DN value median matrix at each single moment in the time range are collected to obtain an RCS value median matrix array and a DN value median matrix array.

[0028] In one embodiment, performing robust regression fitting for outlier removal on the median matrix array and outputting the radiation calibration coefficient at the single moment includes:

[0029] Perform robust regression fitting using all data points in the DN value median matrix as independent variables and all data points in the RCS value median matrix array as dependent variables to obtain fitting results;

[0030] Calculating a residual array of data points of the fitting result, and calculating a residual control interval based on the residual array;

[0031] Data points outside the residual control interval are treated as outliers and removed, and the median matrix array after the outliers are removed is subjected to robust regression fitting again until all data points are within the residual control interval;

[0032] The fitting result corresponding to all data points being within the residual control interval is used as the radiation calibration coefficient at the single moment.

[0033] In one of the embodiments, obtaining a final reference value of the radiation calibration coefficient according to the reference radiation calibration coefficient and the radiation calibration coefficient includes:

[0034] The reference radiation calibration coefficient and the average value of the radiation calibration coefficient are calculated, and the average value is used as the final reference value of the radiation calibration coefficient.

[0035] In one of the embodiments, error processing is performed on the radiation calibration coefficient array according to the final reference value of the radiation calibration coefficient to obtain the final radiation calibration coefficient, including:

[0036] Respectively calculating the difference between each radiation calibration coefficient in the radiation calibration coefficient array and the reference radiation calibration coefficient;

[0037] Based on the gap value, several target radiation calibration coefficients are selected for weighted sum calculation to obtain a final radiation calibration coefficient.

[0038] In one embodiment, the method further comprises:

[0039] Obtaining the radiation calibration coefficient at each moment, and generating a SAR radiation cross calibration coefficient time series curve according to the radiation calibration coefficient;

[0040] A satellite sensor attenuation model is constructed according to the timing curve.

[0041] A time series cross-calibration system for synthetic aperture radar, the system comprising:

[0042] A region screening module is used to determine a reference satellite and a satellite to be calibrated, determine a distribution target based on the reference satellite and the satellite to be calibrated, obtain position information of the distribution target, and perform region screening within the spatial range of the distribution target according to the position information to obtain a target region for time series cross calibration;

[0043] A data pair acquisition module, used to acquire imaging parameters of the reference satellite and the satellite to be calibrated, determine a time range for time series cross calibration according to the imaging parameters and position information, cut data slices for time series cross calibration based on the position information, and generate SAR image data pairs corresponding to the distributed targets moment by moment within the time range;

[0044] A single moment radiation calibration coefficient acquisition module is used to respectively extract the median matrix of the data slice at a single moment by using the window median method to obtain a median matrix array, and perform robust regression fitting for outlier removal on the median matrix array to output the radiation calibration coefficient at the single moment;

[0045] A calibration coefficient adjustment module, used to determine a reference radiation calibration coefficient, and obtain a final reference value of the radiation calibration coefficient according to the reference radiation calibration coefficient and the radiation calibration coefficient;

[0046] The cross calibration module is used to calculate the radiation calibration coefficients corresponding to each single moment within the time range, generate a radiation calibration coefficient array; perform error processing on the radiation calibration coefficient array according to the final reference value of the radiation calibration coefficient, obtain the final radiation calibration coefficient, and complete the time series cross calibration.

[0047] The above-mentioned synthetic aperture radar time series cross-calibration method and system avoid the interference of outliers and improve the accuracy of radiation calibration coefficients by performing robust regression fitting on the median matrix array to eliminate outliers; by generating SAR image data pairs at each moment within the time range, and then calculating the radiation calibration coefficients corresponding to each single moment respectively, taking the correlation of the radiation calibration coefficients between different moments as a reference, combining the radiation calibration coefficient value measured at the time to be calibrated and the radiation calibration coefficient value at the previous moment, the final reference value of the radiation calibration coefficient is determined, thereby improving the accuracy of calibration and effectively reducing the uncertainty of a single calibration. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 A diagram of an application environment of a time series cross-calibration method for a synthetic aperture radar in one embodiment;

[0049] Figure 2 A schematic diagram of a flow chart of a time series cross-calibration method for a synthetic aperture radar in one embodiment;

[0050] FIG3( a ) is a schematic diagram showing the position distribution of 9 target areas in one embodiment;

[0051] FIG3( b ) is a schematic diagram of the histogram distribution of 9 target areas in one embodiment;

[0052] Figure 4 A schematic diagram of a time series cross-reference target data pair composition for time series cross-calibration in one embodiment;

[0053] Figure 5 A schematic diagram of a process of obtaining a target area median matrix array using a window median method in an embodiment;

[0054] Figure 6 A result diagram of a SAR radiation cross-calibration coefficient timing curve in one embodiment;

[0055] Figure 7 It is a schematic flow chart of a time series cross calibration method of a synthetic aperture radar in another embodiment;

[0056] Figure 8 It is a structural block diagram of a time series cross calibration system of a synthetic aperture radar in one embodiment;

[0057] Fig. 9 FIG. 4 is a diagram showing the internal structure of a computer device in one embodiment. DETAILED DESCRIPTION

[0058] In order to make the purpose, technical solution and advantages of the present application more clearly understood, the present application is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0059] The time series cross calibration method of synthetic aperture radar provided in the embodiment of the present application can be applied to Figure 1 In the application environment shown. Figure 1As shown, the application environment includes a computer device 110. The computer device 110 can determine the reference satellite and the satellite to be calibrated, determine the distribution target based on the reference satellite and the satellite to be calibrated, obtain the location information of the distribution target, and perform regional screening within the spatial range of the distribution target according to the location information to obtain the target area for time series cross calibration; the computer device 110 can obtain the imaging parameters of the reference satellite and the satellite to be calibrated, determine the time range for time series cross calibration according to the imaging parameters and the location information, cut the data slices for time series cross calibration based on the location information, and generate SAR image data pairs corresponding to the distribution target one by one within the time range; the computer device 110 can use The window median method extracts the median matrix of the data slices at a single moment respectively, obtains the median matrix array, and performs robust regression fitting to remove outliers on the median matrix array, and outputs the radiation calibration coefficient at a single moment; the computer device 110 can determine the reference radiation calibration coefficient, and obtain the final reference value of the radiation calibration coefficient according to the reference radiation calibration coefficient and the radiation calibration coefficient; the computer device 110 can calculate the radiation calibration coefficient corresponding to each single moment within the time range, and generate a radiation calibration coefficient array; the radiation calibration coefficient array is error processed according to the final reference value of the radiation calibration coefficient, and the final radiation calibration coefficient is obtained to complete the time series cross calibration. Among them, the computer device 110 can be, but not limited to, various personal computers, laptops, smart phones, robots, unmanned aerial vehicles and other devices.

[0060] In one embodiment, Figure 2 As shown, a time series cross-calibration method for synthetic aperture radar is provided, comprising the following steps:

[0061] Step 202, determine the reference satellite and the satellite to be calibrated, determine the distribution target based on the reference satellite and the satellite to be calibrated, obtain the position information of the distribution target, and perform area screening within the spatial range of the distribution target according to the position information to obtain the target area for time series cross calibration.

[0062] First, the computer device can screen out the target area for time series cross calibration, and prepare for selecting the location information of multiple target areas for subsequent calibration. When screening the target area, the SAR image histogram distribution statistical characteristics and time series scattering stability of the target area are mainly used as the measurement criteria to screen the target area for subsequent time series cross calibration. The operator can select the reference satellite and the satellite to be calibrated through the computer device. Specifically, the operator can select a calibrated high-radiometric precision SAR satellite as a reference satellite to perform radiometric calibration for the satellite to be calibrated.

[0063] In this embodiment, the image data used can be Sentinel-1 series satellite images. As part of the European Space Agency's Copernicus Earth Observation Program, the Sentinel-1 series satellites can ensure higher revisit efficiency and coverage, and provide SAR satellite image data for a long time, which is convenient for subsequent time series cross-calibration experiments. For example, the user can select the Sentinel-1A satellite as the reference satellite and the Sentinel-1B satellite as the satellite to be calibrated through a computer device, and select the distribution targets that are repeatedly irradiated by both satellites and have a short imaging time interval on the ESA official website, and finally select the Los Angeles city area for subsequent experiments.

[0064] In one embodiment, a time series cross-calibration method for a synthetic aperture radar is provided, which may also include a process of performing area screening to obtain a target area, and the specific process includes: acquiring time series SAR data of a reference satellite according to position information, and preprocessing the time series SAR data to obtain a time series RCS value image corresponding to a distributed target; performing area screening within the spatial range of the distributed target based on the time series RCS value image to obtain a target area for time series cross-calibration.

[0065] The computer device can select a distributed target that is repeatedly illuminated by both satellites and has a short imaging time interval to increase the frequency of time series cross-calibration. In this embodiment, the computer device can obtain the time series SAR data of different months within a one-year time range of the reference satellite based on the location information of the distributed target, wherein the group of data is required to have the same ascending and descending orbits, polarization modes and irradiation areas and similar incident angles, and the imaging interval does not exceed 60 days. Then, after orbit correction, radiation calibration, coherent speckle filtering, geocoding and other preprocessing, the time series RCS value image of the distributed target is obtained. Specifically, taking the selected target area as the Los Angeles urban area as an example, according to the location information of the Los Angeles urban area, the time series SAR data of different months within a one-year time range of the Sentinel-1A satellite is obtained, the group of data has similar incident angles, the same ascending and descending orbits, polarization modes and irradiation areas, and the imaging interval does not exceed 60 days. The specific information is shown in the following table:

[0066]

[0067]

[0068] Next, the computer equipment can perform orbit correction, radiation calibration, coherent speckle filtering, geocoding and other pre-processing on the data to obtain a time-series RCS value image of the Los Angeles city area.

[0069] In one embodiment, a time series cross-calibration method for a synthetic aperture radar is provided, which may also include a process of area screening, and the specific process includes: performing initial screening of the area based on a time series RCS value image to obtain each preliminary screening area; cropping the time series RCS value slices according to the position information of each preliminary screening area; using the window median method to extract each time series backscatter coefficient corresponding to each time series RCS value slice, and calculating the average backscatter coefficient corresponding to the time series backscatter coefficient; and screening out the target area from each preliminary screening area according to the time series backscatter coefficient and the average backscatter coefficient through the root mean square error.

[0070] The computer equipment can perform the initial screening of the target area within the spatial range of the acquired distribution target. In order to reduce the generation of abnormal values ​​in the subsequent calibration process, the area with uniform RCS value image brightness, no abnormal bright spots, single-peak distribution of image histogram, approximately symmetrical left and right, and short-tail distribution at the tail is selected as the initial screening area. Then, according to the position information of each initial screening area, the time series RCS value slices are cut and produced respectively, and the time series backscattering coefficient is extracted by the window median method, and the scattering stability of the initial screening area is judged by the root mean square error (RMSE). Among them, the calculation formula of the root mean square error can be expressed as: Among them, σ i and are the backscatter coefficient value of the target area time series RCS value slice and the average backscatter coefficient value of all slices, respectively, and N is the number of time series slices.

[0071] In this embodiment, the time root mean square error (RMSE) less than 0.5dB can be used as the screening standard. If the RMSE of the initial screening area is less than 0.5dB, it is determined to be a scattering stable area and used as the target area for subsequent time series cross calibration. In this embodiment, 9 target areas are finally screened, and their specific locations are shown in Figure 3(a), and the distribution histogram is shown in Figure 3(b).

[0072] Step 204, obtaining the imaging parameters of the reference satellite and the satellite to be calibrated, determining the time range for time series cross calibration according to the imaging parameters and the position information, cropping the data slices for time series cross calibration based on the position information, and generating SAR image data pairs corresponding to the distributed targets at each moment within the time range.

[0073] Among them, obtaining SAR image data pairs is used to obtain data for time series cross-calibration, which is a set of cross-reference target data pairs in the time dimension, where the data pairs at each moment include reference star RCS value data slices and DN value data slices of the star to be calibrated in multiple target areas, which can be used to obtain the radiation calibration coefficient at a single moment.

[0074] Specifically, the computer device can determine the time range for implementing the time series calibration according to the imaging time of the reference satellite and the satellite to be calibrated, the obtained target area location and imaging parameters and other information. In this embodiment, taking the selected target area as the Los Angeles urban area as an example, the computer device can screen the Los Angeles urban area data of the Sentinel-1A satellite and the Sentinel-1B satellite provided on the official website of the European Space Agency, and select the Sentinel-1A satellite VV polarization image from August 21, 2019 to December 8, 2021 as the Sentinel-1B satellite VV polarization image from August 15, 2019 to December 2, 2021 for time series cross-calibration according to the imaging time, imaging parameters and target area location information and other conditions.

[0075] In one embodiment, a time series cross-calibration method for a synthetic aperture radar is provided, which may also include a process of generating data pairs, and the specific process includes: cutting out reference satellite RCS value data slices and to-be-calibrated satellite DN value data slices of the target area based on location information; determining radiation calibration requirements, and filtering out SAR image data from the data slices within a time range according to the radiation calibration requirements; and performing data cutting on the SAR image data respectively to obtain SAR image data pairs corresponding to the distributed targets.

[0076] Within a certain time range, the computer equipment can obtain the SAR image data pairs of the reference satellite and the satellite to be calibrated about the distributed targets one by one, requiring that the orbit, polarization mode and illumination area of ​​each pair of data satellites are the same and the incident angle is similar, and the imaging time interval does not exceed 10 days. Specifically, the operations for the reference satellite and the satellite to be calibrated data pairs at each moment include: preprocessing the reference satellite and the satellite to be calibrated data; cutting the data slices for time series cross calibration according to the position information of multiple selected target areas; and finally obtaining the time series cross reference target data pairs for time series cross calibration. Among them, each data pair belongs to a single moment, and contains reference satellite RCS value slices and satellite DN value slices for multiple target areas.

[0077] Specifically, in this embodiment, according to the determined time range, the computer device can screen the SAR image data pairs that meet the radiation cross calibration requirements one by one, and the specific requirements are: the incident angle of each pair of data is similar, the satellite orbit, polarization mode and irradiation area are the same, and the imaging time interval does not exceed 10 days. Finally, a total of 69 pairs of images within the time range are screened, and some image data information is shown in the following table:

[0078]

[0079]

[0080] Then, the computer equipment can operate on these 69 pairs of images one by one, including: preprocessing the reference satellite and the satellite data to be calibrated; cutting the data pairs according to the location information of the 9 target areas to obtain data slices for time series cross calibration; finally, 69 pairs of time series cross reference target data pairs are obtained, and the composition diagram is shown in the figure. Figure 4 Each data pair is for a single moment and contains 9 reference satellite and to-be-calibrated satellite data slices in the target area.

[0081] Step 206, using the window median method to extract the median matrix of the data slices at a single moment, obtain a median matrix array, and perform robust regression fitting on the median matrix array to remove outliers, and output the radiation calibration coefficient at a single moment.

[0082] The computer device operates on the RCS value slice and DN value slice data pairs at a single moment, and can obtain the radiation calibration coefficient at a single moment to achieve SAR radiation calibration at a single moment.

[0083] In one embodiment, a time series cross-calibration method for a synthetic aperture radar is provided, which may further include a process of obtaining a median matrix, and the specific process includes: obtaining a set window size; extracting a median matrix from RCS value data slices and DN value data slices respectively using a window median method according to the window size to obtain an RCS value median matrix and a DN value median matrix; and collecting the RCS value median matrix and the DN value median matrix at each single moment within a time range to obtain an RCS value median matrix array and a DN value median matrix array.

[0084] The computer equipment can use the data slices of multiple target areas at a single moment to perform radiation calibration. First, the window size is set to 50*50, and the window median method is used to extract the median matrix of the 9 target area images of the reference satellite and the satellite to be calibrated in the data pair, a total of 18 data slices, including RCS value slices and DN value slices, using the equal interval median method. The schematic diagram is as follows: Figure 5 Finally, the median matrix array R of the RCS values ​​of multiple target areas at the time to be calibrated is obtained. m and DN value median matrix array D m Among them, the same index positions of the two median matrix arrays are the RCS value median matrix and the DN value median matrix of the same target area.

[0085] In one embodiment, a time series cross-calibration method for a synthetic aperture radar is provided, which may further include a robust regression fitting process, wherein the specific process includes: performing robust regression fitting using all data points in a DN value median matrix as independent variables and all data points in an RCS value median matrix array as dependent variables to obtain a fitting result; calculating a residual array of data points of the fitting result, and calculating a residual control interval based on the residual array; treating data points outside the residual control interval as outliers and removing them, and performing robust regression fitting again on the median matrix array after removing the outliers until all data points are within the residual control interval; using the fitting result corresponding to all data points being within the residual control interval as the radiation calibration coefficient at a single moment.

[0086] The computer device can output the result R m and D m Perform robust regression fitting based on outlier removal. The specific steps include: m All data points are independent variables, R m All data points are used as dependent variables for robust regression fitting, and then the residual array of the data points, the upper control limit (UpperControlLimit, UCL) and the lower control limit (LowerControlLimit, LCL) of the fitting results are calculated. The calculation formula can be expressed as: UCL=μ R +2σ R ; LCL = μ R -2σ R ; Where R is the residual array of data points, y i is the fitted value of the ith data point, is the actual value of the ith data point, UCL and LCL are the upper and lower limits of residual control, μ R is the residual mean, σ R In this embodiment, [UCL, LCL] is used as the residual control interval, and the data points outside the interval are removed and then robust regression fitting is performed again, and it is iterated repeatedly until all data points are within the residual control interval. The output fitting radiation calibration coefficient is K t .

[0087] Step 208, determining a reference radiation calibration coefficient, and obtaining a final reference value of the radiation calibration coefficient according to the reference radiation calibration coefficient and the radiation calibration coefficient.

[0088] The existing radiation cross calibration method only uses a single moment image during calibration, and does not consider that due to changes in external environmental factors such as weather, the scattering characteristics of the ground objects in the image may be distorted, which leads to the uncertainty of the calibration result at a single moment. It is necessary to comprehensively refer to the radiation calibration coefficients of the previous moment and the calibration results of the time to be calibrated. Therefore, the radiation calibration coefficient with the smallest calibration error within a fixed time range (36 days) before the time to be calibrated is selected as the reference radiation calibration coefficient K. o , providing a reference for the time to be calibrated.

[0089] In one embodiment, a time series cross-calibration method for a synthetic aperture radar is provided, which may also include a process of determining a reference value, wherein the specific process includes: calculating a reference radiation calibration coefficient and a mean value between the radiation calibration coefficients, and using the mean value as a final reference value of the radiation calibration coefficient.

[0090] In this embodiment, if only the reference radiation calibration coefficient K is used o As a reference, the actual radiation performance changes at the current moment may be ignored, so the final reference value K of the radiation calibration coefficient is set r Set as reference radiation calibration factor K o and radiation calibration factor K t The mean value of is calculated as:

[0091] Step 210, within the time range, respectively calculate the radiation calibration coefficients corresponding to each single moment to generate a radiation calibration coefficient array; perform error processing on the radiation calibration coefficient array according to the final reference value of the radiation calibration coefficient to obtain the final radiation calibration coefficient, and complete the time series cross calibration.

[0092] Next, the 9 target areas can be calibrated separately to obtain the R m and D m Each corresponding index position is the RCS value slice and DN value slice of the same target area. Therefore, robust regression fitting based on outlier elimination is performed on each index position, and finally the radiation calibration coefficient array K of each area is obtained. v , which contains the radiation cross calibration results of 9 target areas. The formula can be expressed as: K v =[K 1 ,K 2 ,K 3 ···K n ]; among them, K i is the radiation calibration coefficient of the i-th area. Since the quality of the target area is different, the calibration results of each area are not completely accurate, so K o and K t As a benchmark, in K vThe same number of elements with the smallest errors between the two are selected as the radiation calibration coefficient array K for the final weighted summation.

[0093] In one embodiment, a time series cross-calibration method for a synthetic aperture radar is provided that may further include an error processing process, the specific process including: respectively calculating the difference values ​​between each radiation calibration coefficient in the radiation calibration coefficient array and the reference radiation calibration coefficient; based on the difference values, selecting several target radiation calibration coefficients for weighted summation calculation to obtain a final radiation calibration coefficient.

[0094] Among them, K v Although the array contains the radiation cross calibration results of 9 target areas, the quality of the target areas is different, so not every area has an accurate calibration result. Therefore, 3 areas with K are selected. o The elements with the smallest error, 3 and K t The element with the smallest error is used as the final radiation calibration coefficient array K for weighted summation. Calculate the difference between each element in K and K r The error is calculated by weighting the elements in K one by one according to the error size. If the error is large, a low weight is given, and if the error is small, a high weight is given. The final radiation calibration coefficient K at that moment is obtained by weighted summation. f .

[0095] Specifically, the computer device can calculate the correlation between each element in the radiation calibration coefficient array K and the radiation calibration coefficient reference value K r The error is calculated by weighting the elements in K one by one according to the error size. The elements with large errors are given low weights, and the elements with small errors are given high weights. The weighted sum is used to obtain the final radiation calibration coefficient K at that moment. f , the specific calculation formula can be expressed as: E i =|K i -K r |; Among them, K i is the element in the i-th radiation calibration coefficient array K, E i K i With K r The absolute value of the error, W i is the normalized weight, and K i The final result K is obtained after weighted summation f .

[0096] In one embodiment, a time series cross-calibration method for a synthetic aperture radar is provided, which may also include a process of constructing an attenuation model. The specific process includes: obtaining a radiation calibration coefficient at each moment, and generating a SAR radiation cross-calibration coefficient time series curve based on the radiation calibration coefficient; and constructing a satellite sensor attenuation model based on the time series curve.

[0097] In this embodiment, the computer device can obtain the radiation calibration coefficient at a single moment in the time dimension for the 69 pairs of time series calibration data obtained, and finally obtain the SAR radiation cross calibration coefficient time series curve and construct the satellite sensor attenuation model based on the curve. The schematic diagram of the SAR radiation cross calibration coefficient time series curve is shown in FIG. Figure 6 shown.

[0098] In one embodiment, Figure 7 As shown, a time series cross calibration method for synthetic aperture radar is provided, which mainly includes four parts: determining the target area range, obtaining the time series calibration data pair, obtaining the single moment radiation calibration coefficient, and SAR time series cross calibration, among which:

[0099] Determination of the target area range: The target area for subsequent time series cross calibration is screened mainly based on the statistical characteristics of the SAR image histogram distribution and the stability of the time series scattering in the target area;

[0100] Time series calibration data pair acquisition: mainly realizes the production of time series cross-reference target data pairs for time series cross-calibration, and performs SAR satellite time series data acquisition, preprocessing and slice cutting;

[0101] Acquisition of radiation calibration coefficients at a single moment: Mainly for the cross-reference target data at a single moment obtained during the acquisition process of time series calibration data, the slices are cross-calibrated for radiation. First, the median method is used to extract stable pixel points in all target areas, and then all stable pixel points are used to perform robust regression fitting based on outlier removal to obtain the initial radiation calibration coefficient at the current moment, and the radiation calibration coefficient values ​​at the current moment and the previous moment are combined to determine the reference value of the radiation calibration coefficient; then, robust regression fitting based on outlier removal is performed on each region to obtain the radiation calibration coefficient value of a single region, and the error between the radiation calibration coefficient value of each region and the reference value of the radiation calibration coefficient is calculated to assign weights to them one by one, and finally the weighted sum of the radiation calibration coefficient values ​​of each region is taken to obtain the final radiation calibration coefficient at that moment;

[0102] SAR time series cross-calibration: It mainly completes the iteration of step S3 in the time dimension, uses the time series cross-reference target data obtained in step S2 to slice, and obtains the radiation calibration coefficient of S3 in the time dimension. Finally, the SAR radiation calibration coefficient time series curve can be obtained and the satellite sensor attenuation model can be constructed based on this curve.

[0103] In this embodiment, the computer device first uses the statistical characteristics of the distributed target image histogram and the stability of the time series scattering as the standard to screen the target area for time series cross calibration, and obtains the time series cross calibration data pair after preprocessing and slicing. Then, the radiation calibration coefficient at a single moment is obtained. In order to solve the problem that the distributed target scattering stable pixel points extracted by the prior art solution have abnormal value interference and the calibration result at a single moment has uncertainty, the following scheme is adopted: all pixels are used to perform robust regression fitting based on abnormal value elimination to obtain the initial radiation calibration coefficient at the current moment, and the radiation calibration coefficient reference value is determined by combining this value with the radiation calibration coefficient value at the previous moment; robust regression fitting based on abnormal value elimination is performed on each region to obtain the radiation calibration coefficient value of a single region, and the error between the radiation calibration coefficient value of each region and the reference value of the radiation calibration coefficient is calculated and weighted one by one; weighted summation is performed to obtain the final radiation calibration coefficient at the moment. Then, according to the above scheme, the radiation calibration coefficient is obtained at each moment, and finally the SAR radiation calibration coefficient time series curve is obtained and the satellite sensor attenuation model is constructed based on the curve. The main implementation object of this application is the spaceborne synthetic aperture radar image to be calibrated, and the main work is to perform time series cross-calibration on it.

[0104] In this embodiment, after the existing radiation cross-calibration method uses the median method or the mean method to complete the extraction of scattering stable pixels, the ordinary least squares method is used to establish a calibration model to obtain the radiation calibration coefficient of the satellite to be calibrated. The problem of outlier interference in the scattering stable pixels is not considered, and a robust regression method based on outlier elimination is proposed for fitting the radiation calibration coefficient; in view of the problem that the calibration of each time point in the existing method only uses a single-time image, but there may be errors in the scattering characteristics of the ground objects in the single-time image, resulting in uncertainty in the calibration result at a single time, the correlation of the radiation calibration coefficients between different times is used as a reference, and the radiation calibration coefficient value measured at the time to be calibrated and the radiation calibration coefficient value at the previous time is combined to determine the final reference value of the radiation calibration coefficient, and then the final radiation calibration coefficient is obtained by weighting and weighted summation.

[0105] A time series cross-calibration method for a synthetic aperture radar is provided in the present application. To address the problem of outlier interference in the distributed target scattering stable pixel points extracted by the prior art solution, a robust regression method based on outlier elimination is used to fit the radiation calibration coefficient. To address the problem of uncertainty in the calibration results at a single moment in the prior art solution, the present application uses the correlation of the radiation calibration coefficients at different moments as a reference, combines the radiation calibration coefficient value measured at the moment to be calibrated and the radiation calibration coefficient value at the previous moment, determines the final reference value of the radiation calibration coefficient, and then obtains the final radiation calibration coefficient through weighting and weighted summation.

[0106] It should be understood that, although the steps in the above-mentioned flowcharts are shown in sequence according to the indication of the arrows, these steps are not necessarily executed in sequence according to the order indicated by the arrows. Unless there is a clear explanation in this article, the execution of these steps is not strictly limited in order, and these steps can be executed in other orders. Moreover, at least a part of the steps in the above-mentioned flowcharts may include a plurality of sub-steps or a plurality of stages, and these sub-steps or stages are not necessarily executed at the same time, but can be executed at different times, and the execution order of these sub-steps or stages is not necessarily to be carried out in sequence, but can be executed in turn or alternately with other steps or at least a part of the sub-steps or stages of other steps.

[0107] In one embodiment, Figure 8 As shown, a time series cross calibration system for synthetic aperture radar is provided, comprising: a region screening module 810, a data pair acquisition module 820, a single moment radiation calibration coefficient acquisition module 830, a calibration coefficient adjustment module 840 and a cross calibration module 850, wherein:

[0108] The region screening module 810 is used to determine the reference satellite and the satellite to be calibrated, determine the distribution target based on the reference satellite and the satellite to be calibrated, obtain the position information of the distribution target, and perform region screening within the spatial range of the distribution target according to the position information to obtain the target region for time series cross calibration;

[0109] The data pair acquisition module 820 is used to acquire the imaging parameters of the reference satellite and the satellite to be calibrated, determine the time range for time series cross calibration according to the imaging parameters and the position information, cut the data slices for time series cross calibration based on the position information, and generate SAR image data pairs corresponding to the distributed targets at each moment within the time range;

[0110] The single moment radiation calibration coefficient acquisition module 830 is used to respectively extract the median matrix of the single moment data slices by using the window median method to obtain the median matrix array, and perform robust regression fitting for outlier removal on the median matrix array to output the radiation calibration coefficient of the single moment;

[0111] The calibration coefficient adjustment module 840 is used to determine the reference radiation calibration coefficient, and obtain the final reference value of the radiation calibration coefficient according to the reference radiation calibration coefficient and the radiation calibration coefficient;

[0112] The cross calibration module 850 is used to calculate the radiation calibration coefficients corresponding to each single moment within the time range and generate a radiation calibration coefficient array; perform error processing on the radiation calibration coefficient array according to the final reference value of the radiation calibration coefficient to obtain the final radiation calibration coefficient and complete the time series cross calibration.

[0113] In one embodiment, the region screening module 810 is also used to obtain the time series SAR data of the reference satellite according to the position information, and pre-process the time series SAR data to obtain a time series RCS value image corresponding to the distribution target; based on the time series RCS value image, regional screening is performed within the spatial range of the distribution target to obtain a target area for time series cross-calibration.

[0114] In one embodiment, the region screening module 810 is also used to perform initial screening of regions based on the time series RCS value image to obtain each initial screening region; according to the position information of each initial screening region, the time series RCS value slices are respectively cropped; the time series backscatter coefficients corresponding to each time series RCS value slice are respectively extracted using the window median method, and the average backscatter coefficient corresponding to the time series backscatter coefficient is calculated; the target region is screened out from each initial screening region according to the time series backscatter coefficient and the average backscatter coefficient through the root mean square error.

[0115] In one embodiment, the data pair acquisition module 820 is also used to crop reference satellite RCS value data slices and to-be-calibrated satellite DN value data slices of the target area based on the location information; determine the radiation calibration requirements, and filter out SAR image data from the data slices within a time range according to the radiation calibration requirements; and perform data cropping on the SAR image data to obtain SAR image data pairs corresponding to the distributed targets.

[0116] In one embodiment, the single-moment radiation calibration coefficient acquisition module 830 is also used to obtain a set window size; according to the window size, the window median method is used to extract the median matrix of the RCS value data slices and the DN value data slices to obtain the RCS value median matrix and the DN value median matrix; the RCS value median matrix and the DN value median matrix of each single moment in the collection time range are obtained to obtain the RCS value median matrix array and the DN value median matrix array.

[0117] In one embodiment, the single-moment radiation calibration coefficient acquisition module 830 is also used to perform robust regression fitting with all data points in the DN value median matrix as independent variables and all data points in the RCS value median matrix array as dependent variables to obtain a fitting result; calculate the data point residual array of the fitting result, and calculate the residual control interval based on the residual array; treat the data points outside the residual control interval as outliers and eliminate them, and perform robust regression fitting again on the median matrix array after eliminating the outliers until all data points are within the residual control interval; and use the fitting result corresponding to all data points being within the residual control interval as the radiation calibration coefficient for a single moment.

[0118] In one embodiment, the calibration coefficient adjustment module 840 is further used to calculate the reference radiation calibration coefficient and the average value between the radiation calibration coefficients, and use the average value as the final reference value of the radiation calibration coefficient.

[0119] In one embodiment, the calibration coefficient adjustment module 840 is further used to respectively calculate the difference between each radiation calibration coefficient in the radiation calibration coefficient array and the reference radiation calibration coefficient; based on the difference value, several target radiation calibration coefficients are selected for weighted sum calculation to obtain the final radiation calibration coefficient.

[0120] In one embodiment, the cross calibration module 850 is further used to obtain the radiation calibration coefficient at each moment, and generate a SAR radiation cross calibration coefficient time series curve according to the radiation calibration coefficient; and construct a satellite sensor attenuation model according to the time series curve.

[0121] In one embodiment, a computer device is provided. The computer device may be a terminal, and its internal structure diagram may be as follows: Fig. 9 As shown. The computer device includes a processor, a memory, a network interface, a display screen and an input device connected through a system bus. Among them, the processor of the computer device is used to provide computing and control capabilities. The memory of the computer device includes a non-volatile storage medium and an internal memory. The non-volatile storage medium stores an operating system and a computer program. The internal memory provides an environment for the operation of the operating system and the computer program in the non-volatile storage medium. The network interface of the computer device is used to communicate with an external terminal through a network connection. When the computer program is executed by the processor, a time series cross-calibration method for a synthetic aperture radar is implemented. The display screen of the computer device can be a liquid crystal display screen or an electronic ink display screen, and the input device of the computer device can be a touch layer covered on the display screen, or a key, trackball or touchpad set on the computer device shell, or an external keyboard, touchpad or mouse, etc.

[0122] Those skilled in the art will understand that Fig. 9 The structure shown in the figure is only a block diagram of a part of the structure related to the solution of the present application, and does not constitute a limitation on the computer device to which the solution of the present application is applied. The specific computer device may include more or fewer components than those shown in the figure, or combine certain components, or have a different arrangement of components.

[0123] In one embodiment, a computer device is provided, including a memory and a processor. The memory stores a computer program, and the processor implements the steps of a time series cross-calibration method for a synthetic aperture radar when executing the computer program.

[0124] In one embodiment, a computer-readable storage medium is provided, on which a computer program is stored. When the computer program is executed by a processor, the steps of a time series cross-calibration method for a synthetic aperture radar are implemented.

[0125] Those skilled in the art can understand that all or part of the processes in the above-mentioned embodiment methods can be completed by instructing the relevant hardware through a computer program, and the computer program can be stored in a non-volatile computer-readable storage medium. When the computer program is executed, it can include the processes of the embodiments of the above-mentioned methods. Among them, any reference to memory, storage, database or other media used in the embodiments provided in this application can include non-volatile and / or volatile memory. Non-volatile memory can include read-only memory (ROM), programmable ROM (PROM), electrically programmable ROM (EPROM), electrically erasable programmable ROM (EEPROM) or flash memory. Volatile memory can include random access memory (RAM) or external cache memory. As an illustration and not limitation, RAM is available in many forms, such as static RAM (SRAM), dynamic RAM (DRAM), synchronous DRAM (SDRAM), double data rate SDRAM (DDRSDRAM), enhanced SDRAM (ESDRAM), synchronous link (Synchlink) DRAM (SLDRAM), memory bus (Rambus) direct RAM (RDRAM), direct memory bus dynamic RAM (DRDRAM), and memory bus dynamic RAM (RDRAM).

[0126] The technical features of the above embodiments may be combined arbitrarily. To make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0127] The above-mentioned embodiments only express several implementation methods of the present application, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for a person of ordinary skill in the art, several variations and improvements can be made without departing from the concept of the present application, and these all belong to the protection scope of the present application. Therefore, the protection scope of the patent of the present application shall be subject to the attached claims.

Claims

1. A time series cross calibration method for synthetic aperture radar, characterized in that: The method comprises: Determine a reference satellite and a satellite to be calibrated, determine a distribution target based on the reference satellite and the satellite to be calibrated, obtain position information of the distribution target, and perform region screening within the spatial range of the distribution target according to the position information to obtain a target region for time series cross calibration; Acquire imaging parameters of the reference satellite and the satellite to be calibrated, determine a time range for time series cross calibration according to the imaging parameters and position information, cut data slices for time series cross calibration based on the position information, and generate SAR image data pairs corresponding to the distributed targets moment by moment within the time range; The median matrix of the data slices at a single moment is extracted respectively by using the window median method to obtain a median matrix array, and the median matrix array is subjected to robust regression fitting with outliers removed to output the radiation calibration coefficient at the single moment; Determine a reference radiation calibration coefficient, and obtain a final reference value of the radiation calibration coefficient according to the reference radiation calibration coefficient and the radiation calibration coefficient; Within the time range, the radiation calibration coefficients corresponding to each single moment are calculated respectively to generate a radiation calibration coefficient array; the radiation calibration coefficient array is error processed according to the final reference value of the radiation calibration coefficient to obtain the final radiation calibration coefficient, thereby completing the time series cross calibration.

2. The time series cross calibration method of synthetic aperture radar according to claim 1, characterized in that: Acquiring the location information of the distribution target, and performing area screening within the spatial range of the distribution target according to the location information to obtain a target area for time series cross calibration, including: Acquire the time-series SAR data of the reference satellite according to the position information, and pre-process the time-series SAR data to obtain a time-series RCS value image corresponding to the distribution target; Based on the time series RCS value image, regional screening is performed within the spatial range of the distribution target to obtain a target area for time series cross calibration.

3. The time series cross calibration method of synthetic aperture radar according to claim 2, characterized in that: Performing regional screening within the spatial range of the distribution target based on the time series RCS value image includes: Performing a primary screening of regions based on the time-series RCS value image to obtain each primary screening region; According to the position information of each of the initial screening areas, respectively cutting the time series RCS value slices; Using the window median method, each time series backscatter coefficient corresponding to each time series RCS value slice is extracted respectively, and the average backscatter coefficient corresponding to the time series backscatter coefficient is calculated; The target area is screened out from each of the initial screening areas according to the time series backscatter coefficient and the average backscatter coefficient through the root mean square error.

4. The time series cross calibration method of synthetic aperture radar according to claim 1, characterized in that: The method includes cutting data slices for time series cross calibration based on the position information and generating SAR image data pairs corresponding to the distribution target moment by moment within the time range, including: Based on the location information, cut out reference satellite RCS value data slices and to-be-calibrated satellite DN value data slices of the target area; Determining a radiation calibration requirement, and filtering out SAR image data from the data slices within the time range according to the radiation calibration requirement; The SAR image data are respectively cropped to obtain SAR image data pairs corresponding to the distribution targets.

5. The time series cross calibration method of synthetic aperture radar according to claim 4, characterized in that: The median matrix of the data slices at a single moment is extracted respectively by using the window median method to obtain a median matrix array, including: Get the set window size; According to the window size, a window median method is used to extract a median matrix from the RCS value data slice and the DN value data slice respectively to obtain an RCS value median matrix and a DN value median matrix; The RCS value median matrix and the DN value median matrix at each single moment in the time range are collected to obtain an RCS value median matrix array and a DN value median matrix array.

6. The time series cross calibration method of synthetic aperture radar according to claim 5, characterized in that: Performing a robust regression fitting to remove outliers on the median matrix array and outputting the radiation calibration coefficient at the single moment includes: Perform robust regression fitting using all data points in the DN value median matrix as independent variables and all data points in the RCS value median matrix array as dependent variables to obtain fitting results; Calculating a residual array of data points of the fitting result, and calculating a residual control interval based on the residual array; Data points outside the residual control interval are treated as outliers and removed, and the median matrix array after the outliers are removed is subjected to robust regression fitting again until all data points are within the residual control interval; The fitting result corresponding to all data points being within the residual control interval is used as the radiation calibration coefficient at the single moment.

7. The time series cross calibration method of synthetic aperture radar according to claim 1, characterized in that: Obtaining a final reference value of the radiation calibration coefficient according to the reference radiation calibration coefficient and the radiation calibration coefficient includes: The reference radiation calibration coefficient and the average value of the radiation calibration coefficient are calculated, and the average value is used as the final reference value of the radiation calibration coefficient.

8. The time series cross calibration method of synthetic aperture radar according to claim 1, characterized in that: The radiation calibration coefficient array is subjected to error processing according to the final reference value of the radiation calibration coefficient to obtain the final radiation calibration coefficient, including: Respectively calculating the difference between each radiation calibration coefficient in the radiation calibration coefficient array and the reference radiation calibration coefficient; Based on the gap value, several target radiation calibration coefficients are selected for weighted sum calculation to obtain a final radiation calibration coefficient.

9. The time series cross calibration method of synthetic aperture radar according to claim 1, characterized in that: The method further comprises: Obtaining the radiation calibration coefficient at each moment, and generating a SAR radiation cross calibration coefficient time series curve according to the radiation calibration coefficient; A satellite sensor attenuation model is constructed according to the timing curve.

10. A time series cross calibration system for synthetic aperture radar, characterized in that: The system comprises: A region screening module is used to determine a reference satellite and a satellite to be calibrated, determine a distribution target based on the reference satellite and the satellite to be calibrated, obtain position information of the distribution target, and perform region screening within the spatial range of the distribution target according to the position information to obtain a target region for time series cross calibration; A data pair acquisition module, used to acquire imaging parameters of the reference satellite and the satellite to be calibrated, determine a time range for time series cross calibration according to the imaging parameters and position information, cut data slices for time series cross calibration based on the position information, and generate SAR image data pairs corresponding to the distributed targets moment by moment within the time range; A single moment radiation calibration coefficient acquisition module is used to respectively extract the median matrix of the data slice at a single moment by using the window median method to obtain a median matrix array, and perform robust regression fitting for outlier removal on the median matrix array to output the radiation calibration coefficient at the single moment; A calibration coefficient adjustment module, used to determine a reference radiation calibration coefficient, and obtain a final reference value of the radiation calibration coefficient according to the reference radiation calibration coefficient and the radiation calibration coefficient; The cross calibration module is used to calculate the radiation calibration coefficients corresponding to each single moment within the time range, generate a radiation calibration coefficient array; perform error processing on the radiation calibration coefficient array according to the final reference value of the radiation calibration coefficient, obtain the final radiation calibration coefficient, and complete the time series cross calibration.

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