High squint SAR imaging processing method under Doppler frequency distance time varying

Abstract

The invention relates to a high squint SAR imaging processing method under Doppler frequency distance time varying. The method comprises the following steps: firstly, carrying out linear walking correction and distance compression according to a squint angle; then distance partitioning and azimuth upsampling are carried out according to the pulse repetition frequency, the instantaneous Doppler bandwidth and the Doppler frequency time-varying quantity along with the distance, azimuth spectrum ambiguity caused by the change of the Doppler center frequency along with the slope distance is eliminated through azimuth spectrum extension, and sub-block splicing is carried out after upsampling of all sub-blocks is completed; carrying out azimuth space-variant resampling in a two-dimensional frequency domain, and eliminating azimuth space-variant in a range gate; then consistent compression and stot interpolation are carried out, and two-dimensional accurate focusing is completed; and finally, correcting image distance position time shift caused by linear walking correction, and intercepting an effective imaging area to obtain a focused SAR image. According to the method, the Doppler spectrum fuzzy problem caused by Doppler frequency distance time variation can be effectively eliminated, so that effective application of a traditional large squint algorithm is ensured, and a correctly focused SAR image is obtained.

Description

Technical Field

[0001] The present application relates to the field of SAR imaging processing, and in particular to a high squint SAR imaging processing method under time-varying Doppler frequency range. Background Art

[0002] In order to improve the efficiency of satellite applications, large-angle satellite attitude maneuvers are often used to observe targets at large squint angles. However, traditional large squint angle observations will reduce the effective width of the distance observation scene. Under the same viewing angle, the larger the squint angle, the smaller the effective observation width. Therefore, a new large squint imaging observation geometry is adopted in the prior art, which can ensure that the squint imaging has the same effective imaging width as the front side imaging. However, under this new large squint imaging observation geometry, the echo data has a more complex Doppler characteristic, that is, in addition to the change of the Doppler center frequency with the distance frequency under the traditional large squint, the change of the Doppler center frequency with the distance time domain is also introduced, which aggravates the two-dimensional spectrum blurring of the echo data, resulting in the traditional large squint imaging processing algorithm no longer being applicable. Summary of the invention

[0003] In order to overcome at least one deficiency in the prior art, the present application provides a high squint SAR imaging processing method under time-varying Doppler frequency range.

[0004] In a first aspect, a high squint SAR imaging processing method under time-varying Doppler frequency range is provided, comprising:

[0005] According to the central equivalent squint angle, equivalent speed and the frequency modulation slope of the transmitting signal, the echo data is corrected for linear movement and compressed for distance to obtain the corrected and compressed data;

[0006] According to the pulse repetition frequency, the instantaneous Doppler bandwidth and the rate of change of the Doppler frequency with the distance, the corrected and compressed data is divided into range sub-blocks to obtain a plurality of range sub-blocks;

[0007] Performing Doppler frequency shift, azimuth zero padding, and Doppler frequency shift compensation on each range sub-block to achieve upsampling, and obtaining upsampled sub-block data; splicing all upsampled sub-block data to obtain spliced ​​data;

[0008] Azimuthally resampling the spliced ​​data to obtain resampled data;

[0009] Perform consistent compression and Stolt interpolation on the resampled data to obtain focused data;

[0010] Calculate the image distance offset to the imaging position according to the equivalent speed, and perform distance offset correction on the focused data based on the image distance offset to obtain a corrected image;

[0011] The effective area of ​​the corrected image is intercepted to obtain the SAR image.

[0012] In one embodiment, linear motion correction and distance compression are performed on the echo data according to the central equivalent squint angle, the equivalent speed and the frequency modulation slope of the transmission signal. The linear motion correction and distance compression multiplication factors used are:

[0013]

[0014] Among them, H com is the linear motion correction and distance compression multiplication factor, f r is the distance frequency, t a is the azimuth time, K r is the frequency modulation slope of the transmitted signal, V r is the equivalent velocity, θ sq is the central equivalent oblique angle, c is the speed of light, f c For the carrier frequency.

[0015] In one embodiment, the length of each distance sub-block is:

[0016]

[0017] Among them, T block is the length of each range sub-block, PRF is the pulse repetition frequency, B a,ins is the instantaneous Doppler bandwidth, and α is the rate of change of Doppler frequency with distance.

[0018] In one embodiment, performing Doppler frequency shift, azimuth zero padding, and Doppler frequency shift compensation on each range sub-block to achieve upsampling, and obtaining up-sampled sub-block data includes:

[0019] According to the Doppler center frequency of each distance sub-block, the distance sub-block is Doppler-shifted to obtain the sub-block data after Doppler frequency shift;

[0020] Performing azimuth zero padding operation on the sub-block data after Doppler frequency shift to obtain sub-block data after Doppler spectrum expansion;

[0021] Doppler frequency shift compensation is performed on the sub-block data after Doppler spectrum expansion to obtain up-sampled sub-block data.

[0022] In one embodiment, in the azimuth zero-filling operation, the number of points that need to be expanded in the azimuth direction is:

[0023]

[0024] Among them, dN is the number of points that need to be expanded in azimuth, PRF′ is the sampling rate after spectrum expansion, PRF is the pulse repetition frequency, N ris the number of sampling points in the original distance direction, B a,ins is the instantaneous Doppler bandwidth, Δf dc,r is the magnitude of Doppler variation with slant range.

[0025] In one embodiment, the frequency coordinate of the azimuth resampling changes to:

[0026]

[0027] Among them, f a is the original azimuth frequency, f a ′ is the new azimuth frequency, f dc is the Doppler frequency corresponding to the central oblique angle, θ sq is the central equivalent oblique angle, V r is the equivalent speed, f c is the carrier frequency, f r is the distance frequency and c is the speed of light.

[0028] In one embodiment, the image distance to imaging position offset is calculated according to the equivalent speed using the following formula:

[0029]

[0030] Among them, Δt shift V is the displacement of the image distance to the imaging position, r is the equivalent velocity, θ sq is the central equivalent oblique angle, t a is the azimuth time, and c is the speed of light.

[0031] In a second aspect, a high squint SAR imaging processing device under time-varying Doppler frequency range is provided, comprising:

[0032] A correction and compression module is used to perform linear movement correction and distance compression on the echo data according to the central equivalent squint angle, equivalent speed and frequency modulation slope of the transmitting signal to obtain corrected and compressed data;

[0033] A distance sub-block division module is used to divide the corrected and compressed data into distance sub-blocks according to the pulse repetition frequency, the instantaneous Doppler bandwidth and the rate of change of the Doppler frequency with the distance to obtain a plurality of distance sub-blocks;

[0034] An upsampling module is used to perform Doppler frequency shift, azimuth zero padding, and Doppler frequency shift compensation on each range sub-block to achieve upsampling, and obtain upsampled sub-block data; all upsampled sub-block data are spliced ​​to obtain spliced ​​data;

[0035] An azimuth resampling module is used to perform azimuth resampling on the spliced ​​data to obtain resampled data;

[0036] A focusing module is used to perform consistent compression and Stolt interpolation on the resampled data to obtain focused data;

[0037] A distance offset correction module is used to calculate the image distance to imaging position offset according to the equivalent speed, and perform distance offset correction on the focused data based on the image distance to imaging position offset to obtain a corrected image;

[0038] The effective area interception module is used to intercept the effective area of ​​the corrected image to obtain a SAR image.

[0039] Compared with the prior art, the present application has the following beneficial effects: the high squint SAR imaging processing method under the time-varying Doppler frequency and distance of the present application first performs linear motion correction and distance compression according to the squint angle; then performs distance block division and azimuth upsampling according to the pulse repetition frequency, instantaneous Doppler bandwidth and Doppler frequency with distance time-varying variable, eliminates the azimuth spectrum ambiguity caused by the change of Doppler center frequency with slant range through azimuth spectrum expansion, and performs sub-block splicing after all sub-blocks have completed upsampling; then performs azimuth space-varying resampling in the two-dimensional frequency domain to eliminate the azimuth space-varying within the range gate; then performs consistent compression and Stolt interpolation to complete two-dimensional precise focusing; finally corrects the image distance position time shift caused by the linear motion correction, and intercepts the effective imaging area to obtain a well-focused SAR image. The present application can effectively eliminate the Doppler spectrum ambiguity problem caused by the time-varying Doppler frequency and distance, thereby ensuring the effective application of the traditional high squint algorithm and obtaining a correctly focused SAR image. The method has the advantages of high precision, robustness and strong practicality. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] The present application may be better understood by referring to the following description given in conjunction with the accompanying drawings, which together with the following detailed description are included in this specification and form a part of this specification. In the drawings:

[0041] Figure 1 A flowchart of a high squint SAR imaging processing method under time-varying Doppler frequency range is shown;

[0042] Figure 2 The structural block diagram of the high squint SAR imaging processing device under the time-varying Doppler frequency range is shown. DETAILED DESCRIPTION

[0043] The exemplary embodiments of the present application will be described below in conjunction with the accompanying drawings. For the sake of clarity and conciseness, not all features of the actual embodiments are described in the specification. However, it should be understood that many implementation-specific decisions can be made in the process of developing any such actual embodiments in order to achieve the specific goals of the developer, and these decisions may vary from embodiment to embodiment.

[0044] It is also necessary to explain here that, in order to avoid obscuring the present application due to unnecessary details, only the device structure closely related to the scheme according to the present application is shown in the drawings, while other details that are not closely related to the present application are omitted.

[0045] It should be understood that the present application is not limited to the described implementation forms due to the following description with reference to the accompanying drawings. In this article, where feasible, the embodiments can be combined with each other, features between different embodiments can be replaced or borrowed, and one or more features can be omitted in one embodiment.

[0046] The present invention provides a high squint SAR imaging processing method under time-varying Doppler frequency range. Figure 1 FIG. 1 shows a flowchart of a high squint SAR imaging processing method under time-varying Doppler frequency range, see FIG. Figure 1 , methods include:

[0047] Step S1, performing linear movement correction and distance compression on the echo data according to the central equivalent squint angle, the equivalent speed and the frequency modulation slope of the transmitting signal to obtain corrected and compressed data.

[0048] Here, firstly, a range FFT (fast Fourier transform) is performed on the echo data, and then a range IFFT (Inverse Fast Fourier Transform) is performed after obtaining the corrected and compressed data.

[0049] Specifically, the linear movement correction and distance compression multiplication factors used are:

[0050]

[0051] Among them, H com is the linear motion correction and distance compression multiplication factor, f r is the distance frequency, t a is the azimuth time, K r is the frequency modulation slope of the transmitted signal, V r is the equivalent velocity, θ sq is the central equivalent oblique angle, c is the speed of light, f c For the carrier frequency.

[0052] Step S2, dividing the corrected and compressed data into distance sub-blocks according to the pulse repetition frequency, the instantaneous Doppler bandwidth and the rate of change of the Doppler frequency with the distance to obtain a plurality of distance sub-blocks.

[0053] Specifically, the length of each distance sub-block in seconds is:

[0054]

[0055] Among them, T block is the length of each range sub-block, PRF is the pulse repetition frequency, B a,ins is the instantaneous Doppler bandwidth, and α is the rate of change of Doppler frequency with distance.

[0056] In order to ensure effective splicing of distance sub-blocks, the sub-block divisions must have a certain overlap (about 5%).

[0057] Step S3, performing Doppler frequency shift, azimuth zero padding operation, and Doppler frequency shift compensation on each range sub-block to achieve upsampling, and obtaining upsampled sub-block data; and splicing all upsampled sub-block data to obtain spliced ​​data.

[0058] Specifically, firstly, according to the Doppler center frequency of each range sub-block, a Doppler frequency shift is performed on the range sub-block to obtain sub-block data after the Doppler frequency shift.

[0059] Here, according to the sub-block division result and the Doppler center frequency f that varies with distance dc (t r ), determine the Doppler center frequency of each range sub-block i is the serial number of the range sub-block. Here, the average of all Doppler center frequencies that vary with distance and correspond to each range sub-block can be calculated as the Doppler center frequency of the range sub-block.

[0060] The purpose of Doppler frequency shift is to obtain sub-block data without ambiguity and folding of the azimuth spectrum. After Doppler frequency shift, azimuth FFT is performed.

[0061] Then, the sub-block data after Doppler frequency shift is subjected to azimuth zero padding operation to obtain the sub-block data after Doppler spectrum expansion;

[0062] Specifically, in the azimuth zero-filling operation, the number of points that need to be expanded in the azimuth direction is:

[0063]

[0064] Among them, dN is the number of points that need to be expanded in azimuth, PRF′ is the sampling rate after spectrum expansion, PRF is the pulse repetition frequency, N r is the number of sampling points in the original distance direction, B a,ins is the instantaneous Doppler bandwidth, Δf dc,r is the magnitude of Doppler variation with slant range.

[0065] Here, after the azimuth zero-padding operation is performed, the azimuth IFFT is performed.

[0066] Then, Doppler frequency shift compensation is performed on the sub-block data after Doppler spectrum expansion to obtain up-sampled sub-block data. Here, Doppler frequency shift compensation refers to phase compensation, which is a conventional technology in the art and will not be described in detail.

[0067] After all range sub-blocks have been upsampled, sub-block splicing is performed to eliminate the azimuth Doppler ambiguity problem caused by the time-varying Doppler frequency range.

[0068] Step S4, azimuth resampling is performed on the spliced ​​data to obtain resampled data, the purpose of which is to eliminate azimuth spatial variation within the range gate.

[0069] Here, the spliced ​​data is first subjected to a two-dimensional FFT, and then azimuth resampling is performed.

[0070] The frequency coordinate changes of azimuth resampling are:

[0071]

[0072] Among them, f a is the original azimuth frequency, f a ′ is the new azimuth frequency, f dc is the Doppler frequency corresponding to the central oblique angle, θ sq is the central equivalent oblique angle, V r is the equivalent speed, f c is the carrier frequency, f r is the distance frequency and c is the speed of light.

[0073] Step S5, performing consistent compression and Stolt interpolation on the resampled data to obtain focused data.

[0074] Here, RMA (Range Migration Algorithm) is used for consistent compression and Stolt interpolation to achieve two-dimensional precise focusing.

[0075] Step S6, calculating the image distance to imaging position offset according to the equivalent speed, and performing distance offset correction on the focused data based on the image distance to imaging position offset to obtain a corrected image.

[0076] Specifically, the image distance to imaging position offset is calculated according to the equivalent speed, using the following formula:

[0077]

[0078] Among them, Δt shift V is the displacement of the image distance to the imaging position, r is the equivalent velocity, θ sq is the central equivalent oblique angle, t a is the azimuth time, and c is the speed of light.

[0079] Step S7, performing effective area interception on the corrected image to obtain a SAR image.

[0080] Here, the effective area refers to the fully focused area in the corrected image, that is, the area composed of fully focused pixels.

[0081] In this embodiment, linear motion correction and distance compression are first performed according to the squint angle; then, distance segmentation and azimuth upsampling are performed according to the pulse repetition frequency, instantaneous Doppler bandwidth and Doppler frequency as a function of distance time variable, and azimuth spectrum ambiguity caused by Doppler center frequency changing with slant distance is eliminated by azimuth spectrum expansion, and sub-block splicing is performed after all sub-blocks have completed upsampling; then, azimuth space-variant resampling is performed in the two-dimensional frequency domain to eliminate azimuth space-variant within the range gate; then, uniform compression and Stolt interpolation are performed to complete two-dimensional precise focusing; finally, the image distance position time shift caused by the linear motion correction is corrected, and the effective imaging area is intercepted to obtain a well-focused SAR image. This embodiment can effectively eliminate the Doppler spectrum ambiguity problem caused by the time-varying Doppler frequency distance, thereby ensuring the effective application of the traditional large squint algorithm and obtaining a correctly focused SAR image. This method has the advantages of high accuracy, robustness and strong practicality.

[0082] Based on the same inventive concept as the high squint SAR imaging processing method under time-varying Doppler frequency and distance, this embodiment also provides a high squint SAR imaging processing device under time-varying Doppler frequency and distance corresponding thereto. Figure 2 The structural block diagram of the high squint SAR imaging processing device under the time-varying Doppler frequency range is shown, including:

[0083] The correction and compression module 21 is used to perform linear movement correction and distance compression on the echo data according to the central equivalent squint angle, the equivalent speed and the frequency modulation slope of the transmission signal to obtain corrected and compressed data;

[0084] The distance sub-block division module 22 is used to divide the corrected and compressed data into distance sub-blocks according to the pulse repetition frequency, the instantaneous Doppler bandwidth and the rate of change of the Doppler frequency with the distance to obtain a plurality of distance sub-blocks;

[0085] The upsampling module 23 is used to perform Doppler frequency shift, azimuth zero padding operation, and Doppler frequency shift compensation on each range sub-block to achieve upsampling, and obtain upsampled sub-block data; all upsampled sub-block data are spliced ​​to obtain spliced ​​data;

[0086] An azimuth resampling module 24 is used to perform azimuth resampling on the spliced ​​data to obtain resampled data;

[0087] A focusing module 25 is used to perform consistent compression and Stolt interpolation on the resampled data to obtain focused data;

[0088] A distance offset correction module 26 is used to calculate the image distance to imaging position offset according to the equivalent speed, and perform distance offset correction on the focused data based on the image distance to imaging position offset to obtain a corrected image;

[0089] The effective area interception module 27 is used to intercept the effective area of ​​the corrected image to obtain a SAR image.

[0090] The high squint SAR imaging processing device under the time-varying Doppler frequency distance of this embodiment has the same inventive concept as the high squint SAR imaging processing method under the time-varying Doppler frequency distance of the above text. Therefore, the specific implementation method of the device can be seen in the embodiment part of the high squint SAR imaging processing method under the time-varying Doppler frequency distance of the above text, and its technical effect corresponds to the technical effect of the above method, which will not be repeated here.

[0091] The above are only various implementations of the present application, but the protection scope of the present application is not limited thereto. Any person skilled in the art who is familiar with the present technical field can easily think of changes or substitutions within the technical scope disclosed in the present application, which should be included in the protection scope of the present application. Therefore, the protection scope of the present application should be based on the protection scope of the claims.

Claims

1. A high squint SAR imaging processing method under time-varying Doppler frequency range, characterized in that: include: According to the central equivalent squint angle, equivalent speed and the frequency modulation slope of the transmitting signal, the echo data is corrected for linear movement and compressed for distance to obtain the corrected and compressed data; Dividing the corrected and compressed data into range sub-blocks according to the pulse repetition frequency, the instantaneous Doppler bandwidth and the rate of change of the Doppler frequency with the distance to obtain a plurality of range sub-blocks; Performing Doppler frequency shift, azimuth zero padding, and Doppler frequency shift compensation on each range sub-block to achieve upsampling, and obtaining upsampled sub-block data; splicing all upsampled sub-block data to obtain spliced ​​data; Performing azimuth resampling on the spliced ​​data to obtain resampled data; Performing consistent compression and Stolt interpolation on the resampled data to obtain focused data; Calculating the image distance offset to the imaging position according to the equivalent speed, and performing distance offset correction on the focused data based on the image distance offset to the imaging position to obtain a corrected image; The effective area of ​​the corrected image is intercepted to obtain a SAR image.

2. The method according to claim 1, characterized in that in, According to the central equivalent squint angle, equivalent velocity and the frequency modulation slope of the transmitting signal, the echo data is corrected for linear motion and compressed for distance. The linear motion correction and distance compression multiplication factors used are: Among them, H com is the linear motion correction and distance compression multiplication factor, f r is the distance frequency, t a is the azimuth time, K r is the frequency modulation slope of the transmitted signal, V r is the equivalent velocity, θ sq is the central equivalent oblique angle, c is the speed of light, f c For the carrier frequency.

3. The method according to claim 1, characterized in that The length of each distance sub-block is: Among them, T block is the length of each range sub-block, PRF is the pulse repetition frequency, B a,ins is the instantaneous Doppler bandwidth, and α is the rate of change of Doppler frequency with distance.

4. The method according to claim 1, characterized in that in, Doppler frequency shift, azimuth zero padding, and Doppler frequency shift compensation are performed on each range sub-block to achieve upsampling, and upsampled sub-block data is obtained, including: According to the Doppler center frequency of each distance sub-block, the distance sub-block is Doppler-shifted to obtain the sub-block data after Doppler frequency shift; Performing an azimuth zero-filling operation on the sub-block data after the Doppler frequency shift to obtain sub-block data after Doppler spectrum expansion; Doppler frequency shift compensation is performed on the sub-block data after Doppler spectrum expansion to obtain up-sampled sub-block data.

5. The method according to claim 4, characterized in that In the azimuth zero-filling operation, the number of points that need to be expanded in the azimuth direction is: Among them, dN is the number of points that need to be expanded in azimuth, PRF′ is the sampling rate after spectrum expansion, PRF is the pulse repetition frequency, N r is the number of sampling points in the original distance direction, B a,ins is the instantaneous Doppler bandwidth, Δf dc,r is the magnitude of Doppler variation with slant range.

6. The method according to claim 1, characterized in that The frequency coordinate change of the azimuth resampling is: Among them, f a is the original azimuth frequency, f a ′ is the new azimuth frequency, f dc is the Doppler frequency corresponding to the central oblique angle, θ sq is the central equivalent oblique angle, V r is the equivalent speed, f c is the carrier frequency, f r is the distance frequency and c is the speed of light.

7. The method according to claim 1, characterized in that in, The image distance to imaging position offset is calculated according to the equivalent speed, using the following formula: Among them, Δt shift is the displacement of the image distance to the imaging position, V r is the equivalent velocity, θ sq is the central equivalent oblique angle, t a is the azimuth time, and c is the speed of light.

8. A high squint SAR imaging processing device under time-varying Doppler frequency range, characterized in that: include: A correction and compression module is used to perform linear movement correction and distance compression on the echo data according to the central equivalent squint angle, equivalent speed and the frequency modulation slope of the transmitting signal to obtain corrected and compressed data; A distance sub-block division module is used to divide the corrected and compressed data into distance sub-blocks according to the pulse repetition frequency, the instantaneous Doppler bandwidth and the rate of change of the Doppler frequency with the distance to obtain a plurality of distance sub-blocks; An upsampling module is used to perform Doppler frequency shift, azimuth zero padding, and Doppler frequency shift compensation on each range sub-block to achieve upsampling, and obtain upsampled sub-block data; all upsampled sub-block data are spliced ​​to obtain spliced ​​data; An azimuth resampling module, used for performing azimuth resampling on the spliced ​​data to obtain resampled data; A focusing module, used for performing consistent compression and Stolt interpolation on the resampled data to obtain focused data; A distance offset correction module, used for calculating the image distance to imaging position offset according to the equivalent speed, and performing distance offset correction on the focused data based on the image distance to imaging position offset to obtain a corrected image; The effective area interception module is used to intercept the effective area of ​​the corrected image to obtain a SAR image.

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