A high squint SAR imaging processing method with time-varying Doppler frequency and range

Abstract

The present application relates to a high squint SAR imaging processing method under Doppler frequency and distance time-varying conditions. First, linear motion correction and range compression are performed according to the squint angle; then, distance segmentation and azimuth upsampling are performed according to the pulse repetition frequency, instantaneous Doppler bandwidth, and the Doppler frequency with distance time-varying conditions. Azimuth spectrum expansion is used to eliminate azimuth spectrum ambiguity caused by Doppler center frequency with slant range variation, and sub-block splicing is performed after all sub-blocks have completed upsampling; then, azimuth space-varying resampling is performed in the two-dimensional frequency domain to eliminate azimuth space-varying conditions within the range gate; then, uniform compression and Stolt interpolation are performed to achieve 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. The present application can effectively eliminate the Doppler spectrum ambiguity problem caused by Doppler frequency and distance time-varying conditions, thereby ensuring the effective application of traditional high squint algorithms and obtaining correctly focused SAR images.

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 and range. Background Art

[0002] 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 reduce the effective width of the distance observation scene. Under the same downward viewing angle, the larger the squint angle, the smaller the effective observation width. Therefore, the existing technology adopts a new large squint imaging observation geometry, which can ensure that the squint imaging has the same effective imaging width as the front-view imaging. However, under this new large squint imaging observation geometry, the echo data has more complex Doppler characteristics. In other words, in addition to the change of the Doppler center frequency with the distance frequency under traditional large squint, the change of the Doppler center frequency with the distance time domain is also introduced. This aggravates the two-dimensional spectrum blurring of the echo data, making the traditional large squint imaging processing algorithm no longer 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 and 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 velocity 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 multiple range sub-blocks;

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

[0008] Performing azimuth resampling on the spliced ​​data to obtain resampled data;

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

[0010] 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 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 range compression are performed on the echo data according to the central equivalent squint angle, the equivalent velocity, and the frequency modulation slope of the transmitted signal. The linear motion correction and range compression multiplication factors used are:

[0013]

[0014] Among them, H com is the linear movement 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 transmitting signal, V r is the equivalent velocity, θ sq is the central equivalent oblique angle, c is the speed of light, f c is 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 upsampled sub-block data includes:

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

[0020] Performing azimuth zero padding 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 the Doppler change with slant range.

[0025] In one embodiment, the frequency coordinate of the azimuth resampling is changed 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 velocity, 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 based on the equivalent speed using the following formula:

[0029]

[0030] Where Δt shift is the offset 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.

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

[0032] The 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 velocity and frequency modulation slope of the transmission signal to obtain the 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 multiple 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, thereby obtaining upsampled sub-block data; and all upsampled sub-block data are spliced ​​to obtain spliced ​​data.

[0035] Azimuth resampling module, used for performing azimuth resampling on the spliced ​​data to obtain resampled data;

[0036] The 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 existing technology, the present application has the following beneficial effects: the high squint SAR imaging processing method of the present application under the time-varying Doppler frequency and distance first performs linear motion correction and range compression according to the squint angle; then performs distance blocking 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 by 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 uniform 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 in conjunction with the accompanying drawings, which together with the following detailed description are incorporated into 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 and 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] Exemplary embodiments of the present application are described below with reference to the accompanying drawings. For the sake of clarity and conciseness, not all features of actual embodiments are described in this specification. However, it should be understood that in the process of developing any such actual embodiment, many implementation-specific decisions may be made to achieve the developer's specific goals, and these decisions may vary from one implementation to another.

[0044] It is also necessary to explain here that, in order to avoid obscuring the present application due to unnecessary details, the accompanying drawings only show the device structure closely related to the solution according to the present application, 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 embodiments due to the following description with reference to the accompanying drawings. In this document, where feasible, the embodiments may be combined with each other, features between different embodiments may be replaced or borrowed, and one or more features may be omitted in one embodiment.

[0046] The present invention provides a method for processing high squint SAR imaging under time-varying Doppler frequency and distance. Figure 1 The flowchart of the high squint SAR imaging processing method under the time-varying Doppler frequency range is shown in FIG. Figure 1 , methods include:

[0047] Step S1 , performing linear motion 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.

[0048] Here, first, a range FFT (fast Fourier transform) is performed on the echo data, and then a range IFFT (inverse Fast Fourier Transform) is performed on 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 movement 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 transmitting signal, V r is the equivalent velocity, θ sq is the central equivalent oblique angle, c is the speed of light, f c is the carrier frequency.

[0052] Step S2 : 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 distance to obtain a plurality of range 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, and Doppler frequency shift compensation on each range sub-block to achieve upsampling, thereby obtaining upsampled sub-block data; and splicing all upsampled sub-block data to obtain spliced ​​data.

[0058] Specifically, first, 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 mean of all the Doppler center frequencies that vary with distance corresponding 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 with no ambiguity or overlap in the azimuth spectrum. After the Doppler frequency shift is performed, the azimuth FFT is performed.

[0061] Then, the azimuth zero padding operation is performed on the sub-block data after the Doppler frequency shift to obtain the sub-block data after the 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 the Doppler change 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 field and will not be described in detail.

[0067] After all range sub-blocks are up-sampled, sub-blocks are spliced ​​to eliminate the azimuth Doppler ambiguity caused by the time-varying Doppler frequency range.

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

[0069] Here, first a two-dimensional FFT is performed on the spliced ​​data, 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 velocity, f c is the carrier frequency, f r is the distance frequency, and c is the speed of light.

[0073] Step S5: performing uniform 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 velocity using the following formula:

[0077]

[0078] Where, Δt shift V is the offset 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 range compression are first performed based on the squint angle. Then, range segmentation and azimuth upsampling are performed based on the pulse repetition frequency, instantaneous Doppler bandwidth, and the time-varying variation of Doppler frequency with range. Azimuth spectrum expansion is used to eliminate azimuth spectrum ambiguity caused by the variation of Doppler center frequency with slant range. Sub-block splicing is performed after all sub-blocks have completed upsampling. Azimuth space-variant resampling is then performed in the two-dimensional frequency domain to eliminate azimuth space-variation within the range gate. Consistent compression and Stolt interpolation are then performed to achieve two-dimensional precise focusing. Finally, image range position time shifts caused by linear motion correction are corrected, and the effective imaging area is intercepted to obtain a well-focused SAR image. This embodiment can effectively eliminate the Doppler spectrum ambiguity caused by the time-varying Doppler frequency and range, thereby ensuring the effective application of traditional high squint algorithms and obtaining correctly focused SAR images. 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 corresponding high squint SAR imaging processing device under time-varying Doppler frequency and distance. Figure 2 The block diagram of the structure 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 motion correction and distance compression on the echo data according to the central equivalent squint angle, equivalent velocity 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] An upsampling module 23 is configured to perform Doppler frequency shift, azimuth zero padding, and Doppler frequency shift compensation on each range sub-block to achieve upsampling, thereby obtaining upsampled sub-block data; and to splice all the upsampled sub-block data 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 for calculating an image distance offset to an imaging position according to an equivalent velocity, and performing distance offset correction on the focused data based on the image distance 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 time-varying Doppler frequency and distance of this embodiment has the same inventive concept as the high-squint SAR imaging processing method under time-varying Doppler frequency and distance described above. Therefore, the specific implementation of the device can be found in the embodiment section of the high-squint SAR imaging processing method under time-varying Doppler frequency and distance described above, and its technical effects correspond to those of the above method, which will not be repeated here.

[0091] The above descriptions are merely examples of various embodiments of the present application, but the scope of protection of the present application is not limited thereto. Any modifications or substitutions that can be readily conceived by a person skilled in the art within the technical scope disclosed in the present application should be included within the scope of protection of the present application. Therefore, the scope of protection of the present application should be based on the scope of protection of the claims.

Claims

1. A high squint SAR imaging processing method under time-varying Doppler frequency and range, characterized in that: include: 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 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 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, thereby obtaining upsampled sub-block data; and 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 an image distance offset to an 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, wherein in, The echo data is corrected for linear motion and compressed based on the central equivalent squint angle, equivalent velocity, and frequency modulation slope of the transmitted signal. The multiplication factors used for linear motion correction and range compression are: Among them, H com is the linear movement 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 transmitting signal, V r is the equivalent velocity, θ sq is the central equivalent oblique angle, c is the speed of light, f c is the carrier frequency.

3. The method according to claim 1, wherein 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, wherein in, 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, including: According to the Doppler center frequency of each range sub-block, the range sub-block is Doppler shifted to obtain the sub-block data after Doppler shift; Performing an azimuth zero-padding operation on the Doppler-shifted sub-block data to obtain Doppler spectrum-expanded sub-block data; 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, wherein 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 the Doppler change with slant range.

6. The method according to claim 1, wherein 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 velocity, 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, wherein in, The image distance to imaging position offset is calculated based on the equivalent velocity using the following formula: Where, Δt shift V is the offset 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.

8. A high squint SAR imaging processing device under time-varying Doppler frequency and distance, characterized in that: include: The 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 velocity and frequency modulation slope of the transmission signal to obtain the corrected and compressed data; a distance sub-block division module, configured 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 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, thereby obtaining upsampled sub-block data; and all upsampled sub-block data are spliced ​​to obtain spliced ​​data. An azimuth resampling module, configured to perform azimuth resampling on the spliced ​​data to obtain resampled data; a focusing module, configured to perform consistent compression and Stolt interpolation on the resampled data to obtain focused data; a distance offset correction module, configured to calculate an image distance-to-imaging position offset according to the equivalent velocity, and perform 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 perform effective area interception on the corrected image to obtain a SAR image.

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