SAR image residual motion error compensation method based on two-dimensional frequency domain error model

Abstract

The application provides a SAR image residual motion error compensation method based on a two-dimensional frequency domain error model, which comprises the following steps: on the basis of a coarsely focused SAR image, taking a to-be-focused data as the center, selecting complex data near the to-be-focused data to form a data matrix; calculating residual motion error of the to-be-focused data in the azimuth aperture according to the position of the selected pixel and the track of the airplane and an ideal imaging track, calculating the mathematical form of the residual motion error in the two-dimensional frequency domain, and determining a two-dimensional frequency domain error compensation phase; performing two-dimensional Fourier transform on the selected data block to obtain two-dimensional frequency domain data, compensating the obtained two-dimensional frequency domain error phase into the two-dimensional frequency domain data; performing two-dimensional inverse Fourier transform on the compensated data block to obtain a two-dimensional time domain local image, and replacing the to-be-focused data with the center point data of the well-focused data block. The compensation precision of the application is greatly improved compared with the existing PTA algorithm, and the application can realize full-automatic engineering processing.

Description

TECHNICAL FIELD

[0001] The present application relates to synthetic aperture radar (SAR) signal processing technology, and in particular to a SAR image residual motion error compensation method based on an accurate two-dimensional frequency domain error model. BACKGROUND

[0002] Synthetic aperture radar (SAR) improves the azimuth resolution by the relative motion between the radar and the observed target, thereby obtaining a high-resolution radar image. Most of the existing SAR imaging processing algorithms are derived based on the ideal case that the radar is in uniform linear motion. However, when the carrier such as an airplane is in motion, due to the influence of atmospheric turbulence and other factors, the carrier is difficult to maintain the ideal uniform linear motion, and the actual trajectory of the carrier deviates from the ideal straight line trajectory, i.e., the carrier has motion errors, which will seriously affect the phase of the collected echo. If not compensated, the image after imaging processing will become very blurred. Therefore, how to accurately compensate for the phase error of the echo caused by the motion error is the key to airborne SAR imaging, especially when the SAR resolution is improved to the decimeter level or even the centimeter level, the requirement for the motion error compensation accuracy is further improved.

[0003] The existing SAR motion compensation usually divides the motion error into three parts: range-invariant motion error, range-dependent motion error and azimuth-dependent motion error, and processes them respectively. Literature [Moreira A, Huang Y. "Airborn SAR processing of highly squinted data using a chirp scaling approach with integrated motion compensation" IEEE Transactions on Geoscience and Remote Sensing, 1994, 32(5): 1029-1040.] proposes a two-step motion compensation algorithm, which compensates the motion error at the reference slant range, i.e. range-invariant motion error, before imaging, and compensates the range-dependent motion error after the range migration correction. Literature [Meng D, Hu D, Ding C. "A new approach airborne high Resolution SAR motion compensation for large trajectory deviations," Chinese Journal of Electronics, 2012(4): 764-769] proposes a method of using sinc interpolation to correct the range-dependent motion error. Compared with the two-step motion compensation, this method only needs one step operation to compensate the range-dependent motion error before the range migration correction, so it is called one-step motion compensation or direct motion compensation. This method considers the range-dependent motion error compensation before imaging, basically compensates the range-dependent phase error, but still leaves the azimuth-dependent error compensation.

[0004] To compensate the azimuth-varying phase error, a point-by-point compensation algorithm in image domain is proposed in [K.A.C.de Macedo and R.Scheiber,“Precise topography-and aperture-dependent motion compensation for airborne SAR,”IEEE Geoscience and Remote Sensing Letters,vol.2,no.2,pp.172-176,Apr.2005.] which is called PTA for short. However, there are two shortcomings in this algorithm. On the one hand, PTA ignores the influence of imaging process on the residual azimuth-varying phase error. PTA considers that the residual azimuth-varying phase error only varies along the azimuth direction before and after imaging, that is, the error only varies with the azimuth time in time domain and only varies with the azimuth frequency in frequency domain, and is independent of the range frequency. In fact, when the frequency domain imaging algorithm is used for processing, the residual error is affected by the imaging process after imaging, and not only varies with the azimuth frequency domain, but also varies with the range frequency domain. The component varying with the range frequency will cause the image envelope to be uneven, which is particularly evident in high-resolution SAR, and seriously reduces the quality of the SAR image. On the other hand, PTA calculates and compensates the residual error based on the ideal time-frequency relationship, ignoring the influence of the residual error on the time-frequency relationship, so the calculated and compensated residual error also has deviation, and the compensation effect is obviously poor in high-resolution SAR.

[0005] From the above status, it can be seen that PTA is a commonly used azimuth residual error compensation method, but its algorithm defects cannot accurately compensate the residual error in the SAR image, and it is difficult to effectively improve the focusing quality of the image in high-resolution SAR. SUMMARY

[0006] The purpose of the present application is to provide a SAR image residual motion error compensation method based on an accurate two-dimensional frequency domain error model. The method not only considers the two-dimensional spatial variation of the residual error in the two-dimensional frequency domain, but also considers the influence of the residual error on the time-frequency relationship. There is no approximation in the calculation process, and the compensation accuracy is greatly improved compared with the existing PTA algorithm.

[0007] The technical solution for achieving the purpose of the present application is a SAR image residual motion error compensation method based on a two-dimensional frequency domain error model, comprising the following steps:

[0008] Step 1, selecting local data: based on the coarsely focused SAR image, selecting N complex data in each of the nearby rows and columns as the center of a to-be-focused data to form an N*N data matrix;

[0009] Step 2, residual motion error calculation: calculate the residual motion error in the azimuth aperture corresponding to the point data according to the position of the selected pixel and the trajectory of the aircraft and the ideal imaging trajectory;

[0010] Step 3, two-dimensional frequency domain error model construction: calculate the mathematical form in the two-dimensional frequency domain according to the residual error in the azimuth aperture calculated in step 2, and determine the two-dimensional frequency domain error compensation phase;

[0011] Step 4, residual motion error compensation: perform two-dimensional Fourier transform on the data block selected in step 1 to obtain two-dimensional frequency domain data, and compensate the two-dimensional frequency domain error phase obtained in step 3 into the two-dimensional frequency domain data;

[0012] Step 5, update the compensated data: perform two-dimensional inverse Fourier transform on the data block compensated in step 4 to obtain a two-dimensional time domain local image. At this time, the center point data of the well-focused data block is used to replace the data to be focused in step 1.

[0013] Step 6, point-by-point processing to obtain the final precisely focused SAR image: repeat steps 1 to 5 until all data of the image are focused to obtain the final image result completely removing residual motion error.

[0014] Further, in step 1, based on the coarsely focused SAR image, 128 complex data in the vicinity of each row and column are selected around a certain data to be focused as the center to form a 128*128 data matrix.

[0015] Further, in step 2, the residual motion error in the azimuth aperture corresponding to the point data is calculated according to the position of the selected pixel and the trajectory of the aircraft and the ideal imaging trajectory. The specific method is as follows:

[0016] Suppose the carrier flies along the X direction and the Y direction is the illumination direction of the SAR. For a certain point T1 in the image, the trajectory range P1 to P2 corresponding to T1 is calculated according to the slant angle θ of the SAR data and the azimuth beam angle β.

[0017] According to the calculated trajectory range, the residual error corresponding to the center point of the selected data block in the trajectory range is calculated, that is, the residual motion error ΔR corresponding to T1 during the flight of the carrier from P1 to P2 is calculated.

[0018] Further, in step 3, the mathematical form in the two-dimensional frequency domain is calculated according to the residual error in the azimuth aperture calculated in step 2, and the two-dimensional frequency domain error compensation phase is determined. The specific method is as follows:

[0019] According to the residual error in the azimuth aperture calculated in step 2, the mathematical form in the two-dimensional frequency domain is calculated, and the azimuth time-frequency relationship when considering the error is:

[0020]

[0021]

[0022]

[0023] wherein c is the speed of light, f r is the range frequency, f c is the carrier frequency, R res (η) is the residual error of extraction, η is the azimuth time, f η is the azimuth frequency, v is the flight velocity, R0 is the shortest slant range corresponding to the center point of the selected data block;

[0024] The residual error of the two-dimensional frequency spectrum is calculated by numerical interpolation, and for each frequency point f η of the azimuth frequency domain, the is obtained by interpolation. According to the interpolated res , the corresponding R η (η) is obtained, that is, the residual error R(f η ,f r ) at the azimuth frequency f η and the range frequency f r , and has

[0025] For the obtained two-dimensional frequency domain residual error, a residual phase error model is further constructed:

[0026]

[0027] wherein,

[0028]

[0029] A SAR image residual motion error compensation system based on a two-dimensional frequency domain error model, based on the SAR image residual motion error compensation method, realizes SAR image residual motion error compensation based on a two-dimensional frequency domain error model, so as to realize accurate focusing of the SAR image.

[0030] A computer device, comprising a memory, a processor and a computer program stored on the memory and executable on the processor, when the processor executes the computer program, based on the SAR image residual motion error compensation method, realizes SAR image residual motion error compensation based on a two-dimensional frequency domain error model, so as to realize accurate focusing of the SAR image.

[0031] The application discloses a computer readable storage medium, which stores a computer program.

[0032] Compared with the prior art, the application has the following advantages: 1) the influence of residual azimuth variable error on imaging is considered, that is, the residual error after imaging is two-dimensional frequency related, while the traditional method considers that the residual error after imaging is only related to azimuth frequency; 2) the influence of residual error on time-frequency relationship is considered, and the residual error in the two-dimensional frequency domain is accurately calculated, while the traditional method ignores the influence of residual error on time-frequency relationship, and error calculation is based on ideal time-frequency relationship and only azimuth frequency compensation is performed; 3) the algorithm is easy to implement, can be fully automated, and is convenient for engineering implementation and does not need to modify the imaging program. BRIEF DESCRIPTION OF DRAWINGS

[0033] Figure 1 is a SAR image after conventional imaging processing.

[0034] Figure 2 is a SAR image compensated by a classical PTA method.

[0035] Figure 3 is a SAR image compensated by the method of the application.

[0036] Figure 4 is a flowchart of the application. DETAILED DESCRIPTION

[0037] In order to make the purpose, technical scheme and advantages of the application clearer, the application is further described in detail below with reference to the drawings and examples. It should be understood that the specific examples described herein are only used to explain the application and do not limit the application.

[0038] The application provides a SAR image residual motion error compensation method based on an accurate two-dimensional frequency domain error model, and the basic idea is to construct a residual error phase model in a SAR image through an accurate two-dimensional frequency domain error model, and to perform residual error compensation on a coarse-focused SAR image point by point. The specific processing flow is as follows:

[0039] Step 1, SAR image local data selection

[0040] On the basis of the coarse focusing SAR image, a N*N data matrix is formed by selecting N complex data in the nearby row and column of a certain data pixel point to be focused. Because the original SAR image is out of focus, the extracted region should contain the total energy of the center point. Too few points will reduce the frequency resolution, and too many points will increase the calculation. According to experience, N=128 can be taken.

[0041] Step 2, calculation of the signal acquisition range corresponding to the selected region

[0042] According to the center point of the data block selected in step 1 and the azimuth bandwidth of the SAR image, the azimuth time range corresponding to the center point of the data block is calculated, and the corresponding track range is extracted;

[0043] Step 3, aperture residual error calculation

[0044] According to the azimuth time range calculated in step 2, the residual error corresponding to the center point of the selected data block in the azimuth time is calculated;

[0045] Step 4, two-dimensional frequency domain residual error calculation

[0046] The azimuth time-frequency relationship considering error is

[0047]

[0048]

[0049]

[0050] Where c is the speed of light, f r is the range frequency, f c is the carrier frequency, R res (η) is the residual error extracted in step 3, η is the azimuth time, f η is the azimuth frequency, v is the flight speed, and R0 is the shortest slant range corresponding to the center point of the selected data block.

[0051] Because R res (η) is a function of η, and η is a function of f η , it can be written as η(f η ), so R res (η) is also a function of f η , the key is to solve R η (η) corresponding to f res . However, the time-frequency relationship is coupled with the derivative of the residual error, and the error often has no fixed form, so R res (η) and f ηSince there is no precise analytical equation between them, the above formula is difficult to solve analytically. To obtain the residual error in the two-dimensional frequency domain, this invention uses numerical interpolation to calculate the residual error of the two-dimensional spectrum. For each frequency point f in the azimuth frequency domain... η The corresponding value can be obtained through interpolation. Then based on interpolation Get the corresponding R res (η), which is at the azimuth frequency f η The distance frequency is f r The residual error R(f) at the location η ,f r ), and have

[0052] Step 5: Error Phase Construction and Compensation

[0053] For the two-dimensional frequency domain residual error obtained in step 4, a residual phase error model is further constructed. The data block obtained in step 1 is subjected to a two-dimensional Fourier transform to obtain two-dimensional frequency domain data, and the above-mentioned error phase is compensated. Then, a two-dimensional inverse Fourier transform is performed to obtain the data block after the two-dimensional error is compensated.

[0054] The constructed phase error model is as follows:

[0055]

[0056] in,

[0057]

[0058] Step 6: Data Replacement

[0059] For the data block that has been compensated for phase error obtained in step 5, replace the data at the corresponding position of the original data with the data corresponding to the center pixel;

[0060] Step 7: Complete Data Processing

[0061] Repeat steps 1 to 6 until all pixels in the region of interest of the SAR image have been processed, and the compensated SAR image will be obtained.

[0062] This invention also proposes a SAR image residual motion error compensation system based on a two-dimensional frequency domain error model. Based on the SAR image residual motion error compensation method, residual motion error compensation of SAR images based on a two-dimensional frequency domain error model is realized, thereby achieving accurate focusing of SAR images.

[0063] The computer device comprises a memory, a processor and a computer program stored on the memory and executable on the processor, and when the processor executes the computer program, residual motion error compensation of a SAR image based on a two-dimensional frequency domain error model is realized based on the residual motion error compensation method of the SAR image, so that accurate focusing of the SAR image is realized.

[0064] A computer readable storage medium has a computer program stored thereon, and when the computer program is executed by a processor, residual motion error compensation of a SAR image based on a two-dimensional frequency domain error model is realized based on the residual motion error compensation method of the SAR image, so that accurate focusing of the SAR image is realized.

[0065] In summary, the present application constructs a residual error phase model in the SAR image by an accurate two-dimensional frequency domain error model, selects a local data block point by point on the coarsely focused SAR image, and compensates the residual phase in the two-dimensional frequency domain.

[0066] Embodiment

[0067] In order to illustrate the effectiveness of the scheme of the present application, certain airborne SAR data is selected for experiment.

[0068] The experimental results are compared as shown in the table. Figures 1-2 As shown in the table, the flight height of the carrier is 400 meters, and the building height in the imaging area is 70 meters. After imaging, the residual motion error caused by the height difference is large, and the floor defocusing is serious. On the basis of the original image, the residual error of the defocused building area is compensated by using the traditional method and the present application. It can be seen that the image focusing is better after the error compensation by the present application, which shows that the error compensation method of the present application is effective.

[0069] The technical features of the above embodiments can be combined in any way. In order to make the description simple, all possible combinations of the technical features in the above embodiments are not described, but as long as the combinations of the technical features do not exist, they should be considered as the scope of the description.

[0070] The above embodiments only express several embodiments of the present application, and the description is more specific and detailed, but it should not be understood as a limitation on the scope of the patent. It should be pointed out that for ordinary skilled in the art, without departing from the concept of the present application, a number of modifications and improvements can be made, which are within the scope of the present application. Therefore, the scope of the patent of the present application should be subject to the appended claims.

Claims

1. A method for residual motion error compensation of SAR images based on a two-dimensional frequency domain error model, characterized in that, The method comprises the following steps: Step 1, selecting local data: on the basis of a coarsely focused SAR image, selecting N complex data in each of the nearby rows and columns of a certain to-be-focused data as the center to form an N*N data matrix; Step 2, residual motion error calculation: calculating the residual motion error in the azimuth aperture corresponding to the data point according to the position of the selected pixel and the trajectory of the aircraft and the ideal imaging trajectory; Step 3, two-dimensional frequency domain error model construction: calculating the mathematical form of the residual error in the azimuth aperture calculated in step 2 in the two-dimensional frequency domain, and determining the two-dimensional frequency domain error compensation phase, the specific method being: According to the residual error in the azimuth aperture calculated in step 2, the mathematical form of the residual error in the two-dimensional frequency domain is calculated, and the azimuth time-frequency relationship when the error is considered is: where c is the speed of light, f r is the range rate, f c is the carrier frequency, R res (η) is the extracted residual error, η is the azimuth time, f η is the azimuth frequency, v is the flight speed, and R0 is the shortest slant range corresponding to the center point of the selected data block; The residual error of the two-dimensional spectrum is calculated by numerical interpolation, and for each frequency point f η of the azimuth frequency domain The corresponding is obtained by interpolation res (η), that is, the residual error R(f η ,f r ) at the azimuth frequency f η and the distance frequency f r , and has For the two-dimensional frequency domain residual error obtained, a residual phase error model is further constructed: Wherein, Step 4, residual motion error compensation: performing two-dimensional Fourier transform on the data block selected in step 1 to obtain two-dimensional frequency domain data, and compensating the two-dimensional frequency domain error phase obtained in step 3 into the two-dimensional frequency domain data; Step 5, updating the data after compensation: performing two-dimensional inverse Fourier transform on the data block compensated in step 4 to obtain a local image in the two-dimensional time domain, at this time the local data block has been well focused, and the center point data of the well-focused data block is used to replace the to-be-focused data in step 1; Step 6, point-by-point processing to obtain the final precisely focused SAR image: repeating steps 1 to 5 until all the data of the image are focused to obtain the final image result completely removing the residual motion error.

2. The SAR image residual motion error compensation method based on a two-dimensional frequency domain error model according to claim 1, characterized in that, In step 1, on the basis of a coarsely focused SAR image, 128 complex data in each of the nearby rows and columns of a certain to-be-focused data are selected as the center to form a 128*128 data matrix.

3. The method of claim 1, wherein the SAR image residual motion error compensation based on a two-dimensional frequency domain error model is characterized by, In step 2, the residual motion error in the azimuth aperture corresponding to the data point is calculated according to the position of the selected pixel and the trajectory of the aircraft and the ideal imaging trajectory, and the specific method is: Suppose the carrier aircraft flies along the X direction, and the Y direction is the illumination direction of the SAR, for a certain point T1 in the image, the trajectory range P1 to P2 corresponding to T1 is calculated according to the squint angle θ of the SAR data and the azimuth beam angle β; According to the calculated trajectory range, the residual error corresponding to the center point of the selected data block in the trajectory range is calculated, that is, the residual motion error ΔR corresponding to T1 during the flight of the carrier aircraft from P1 to P2 is calculated.

4. A SAR image residual motion error compensation system based on a two-dimensional frequency domain error model, characterized in that, The SAR image residual motion error compensation method according to any one of claims 1-3 realizes SAR image residual motion error compensation based on a two-dimensional frequency domain error model, thereby realizing precise focusing of the SAR image.

5. A computer device comprising a memory, a processor, and a computer program stored on the memory and executable on the processor, wherein when the processor executes the computer program, the SAR image residual motion error compensation method according to any one of claims 1-3 is realized to realize SAR image residual motion error compensation based on a two-dimensional frequency domain error model, thereby realizing precise focusing of the SAR image.

6. A computer readable storage medium, having stored thereon a computer program, which, when executed by a processor, implements the SAR image residual motion error compensation based on a two-dimensional frequency domain error model to realize accurate focusing of a SAR image based on the SAR image residual motion error compensation method according to any one of claims 1-3.

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