A spaceborne high squint multi-mode SAR integrated imaging method and device

Abstract

The present invention discloses a spaceborne high-squint multi-mode SAR integrated imaging method and device, belonging to the field of radar technology. The method comprises: performing Fourier transform on the multi-mode SAR data along the range direction, and performing azimuth de-squinting and range compression processing; eliminating Doppler spectrum aliasing based on azimuth sub-bands; performing reference point matched filtering on the data after eliminating Doppler spectrum aliasing, and performing range-wise blocking and differential matched filtering on the data after reference point matched filtering; performing two-dimensional frequency domain resampling on the data after range-wise blocking and differential matched filtering, and performing two-dimensional time domain blocking and differential matched filtering again to obtain a finely focused imaging result. The present invention realizes Doppler spectrum de-aliasing and full-scene fine focusing of the high-squint multi-mode spaceborne SAR while only increasing the amount of data.

Description

Technical Field

[0001] The present invention belongs to the field of radar technology, and in particular relates to a spaceborne high-squint multi-mode SAR integrated imaging method and device. Background Art

[0002] Spaceborne Synthetic Aperture Radar (SAR) offers all-day, all-weather, high-resolution Earth observation capabilities and is widely used in remote sensing applications such as global climate monitoring, marine and land resource surveying, and disaster monitoring. Staring / sliding spotlight SAR uses antenna beam rotation to extend the synthetic aperture time and improve azimuth resolution, resulting in higher spatial resolution than traditional strip-based SAR. Furthermore, its high-squint imaging capability significantly enhances spaceborne SAR's Earth observation capabilities, making it invaluable in urban mapping and disaster assessment.

[0003] Currently, frequency-domain imaging algorithms cannot be directly used for high-squint spaceborne staring / sliding spotlight SAR imaging due to inherent azimuth frequency-domain aliasing. Methods for addressing azimuth frequency-domain aliasing primarily include a two-step subaperture algorithm and a full-aperture algorithm. However, neither approach is suitable for high-resolution high-squint scenarios. The subaperture algorithm introduces phase discontinuities between subapertures, leading to paired echoes in the imaging results. The first step of the two-step spectral analysis (SPECAN) processing, deskewing, can only remove secondary phases from the signal. However, the tertiary phase in high-resolution high-squint SAR signals is non-negligible, and removing only the secondary phase will result in aliasing in the SAR signal after the azimuth Fourier transform. Appropriate upsampling of the SAR signal before SPECAN processing can address this issue, but the upsampling factor must be sufficiently large to address aliasing caused by higher-order signals. Summary of the Invention

[0004] To solve the above technical problems, the present invention provides a spaceborne high squint multi-mode SAR integrated imaging method and device, and designs a corresponding precise imaging processing solution, which can significantly improve the efficiency of subsequent precise imaging processing.

[0005] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0006] A spaceborne high squint multi-mode SAR integrated imaging method, the method comprising:

[0007] Step 1: Perform Fourier transform on the multi-mode SAR data along the range direction, and perform azimuth de-skewing and range compression processing;

[0008] Step 2: Doppler spectrum aliasing elimination based on azimuth sub-band;

[0009] Step 3: performing reference point matched filtering on the data after the Doppler spectrum aliasing elimination process, and performing range-direction blocking and differential matched filtering on the data after the reference point matched filtering process;

[0010] Step 4: Perform two-dimensional frequency domain resampling on the data after range-wise segmentation and differential matched filtering, and perform two-dimensional time domain segmentation and differential matched filtering again to obtain a finely focused imaging result.

[0011] In another aspect, the present invention provides a spaceborne high squint multi-mode SAR integrated imaging device, comprising:

[0012] The pre-processing module is used to perform Fourier transform on the multi-mode SAR data along the range direction, and perform azimuth de-skewing and range compression processing;

[0013] Aliasing elimination module, used for eliminating Doppler spectrum aliasing based on azimuth sub-band;

[0014] A filtering module is used to perform reference point matched filtering on the data after the Doppler spectrum aliasing elimination process, and to perform range-direction blocking and differential matched filtering on the data after the reference point matched filtering;

[0015] The imaging module is used to perform two-dimensional frequency domain resampling on the data after range-division blocking and differential matched filtering, and then perform two-dimensional time domain blocking and differential matched filtering again to obtain a finely focused imaging result.

[0016] In a third aspect, the present invention provides an electronic device comprising: one or more processors; a memory for storing one or more programs; wherein, when the one or more programs are executed by the one or more processors, the one or more processors implement the aforementioned spaceborne high-squint multi-mode SAR integrated imaging method.

[0017] In a fourth aspect, the present invention provides a computer-readable storage medium having executable instructions stored thereon, which, when executed by a processor, enables the processor to implement the aforementioned spaceborne high-squint multi-mode SAR integrated imaging method.

[0018] The beneficial effects of the present invention are:

[0019] The present invention provides an effective and precise imaging processing method for a spaceborne SAR system operating in a high-squint multi-mode. Through azimuth sub-band splicing processing, Doppler spectrum dealiasing of a high-squint multi-mode (spotlight / sliding spotlight) spaceborne SAR is achieved with only a small increase in data volume. The method also features low data processing difficulty and good focusing effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1This is a schematic diagram of a spaceborne high squint multi-mode SAR integrated imaging method according to the present invention;

[0021] Figure 2 Schematic diagram of Doppler aliasing elimination based on azimuth sub-band;

[0022] Figure 3 Schematic diagram of azimuth single sub-band spectrum and sub-band composite spectrum;

[0023] Figure 4 Schematic diagram of the impulse response function and two-dimensional spectrum of a point target after focusing;

[0024] Figure 5 Schematic diagram of the time-frequency relationship of the echo and the time-frequency relationship after focusing. DETAILED DESCRIPTION

[0025] The description of the embodiments in this specification should be combined with the corresponding drawings, which should be considered a complete part of this specification. In the drawings, the shapes and thicknesses of the embodiments may be exaggerated and indicated for simplicity or convenience. Furthermore, the various structural components in the drawings will be described separately. It is worth noting that components not shown in the drawings or not described in words are known to those of ordinary skill in the art.

[0026] The description of the embodiments herein and any references to directions and orientations are for ease of description only and are not to be construed as limiting the scope of the present invention. The following description of the preferred embodiments may involve combinations of features, which may exist independently or in combination. The present invention is not specifically limited to the preferred embodiments. The scope of the present invention is defined by the claims.

[0027] The present invention provides a spaceborne high-squint multi-mode SAR integrated imaging method and device. The method comprises the following steps: performing Fourier transform along the range direction on the initial echo data of the multi-mode (spotlight / sliding spotlight mode) SAR, and performing azimuth de-skewing and range compression on the transformed data; performing azimuth sub-band splicing to eliminate Doppler spectrum aliasing; firstly, the azimuth time domain is divided into blocks, and Fourier transform is performed along the azimuth direction to obtain azimuth frequency domain data, which is padded with zeros on both sides to achieve upsampling; then, performing inverse Fourier transform along the azimuth direction, performing azimuth time domain skewing, and finally performing Fourier transform along the azimuth direction and performing two-dimensional frequency domain splicing to obtain a signal eliminating azimuth spectrum aliasing; and designing a corresponding precise imaging processing scheme for the spaceborne SAR data after the azimuth sub-band splicing processing to obtain a finely focused imaging result.

[0028] The embodiments of the present invention are described in detail below with reference to the accompanying drawings.

[0029] like Figure 1FIG. 1 is a flow chart of a spaceborne high squint multi-mode SAR imaging processing method and a spaceborne high squint multi-mode SAR integrated imaging method proposed by the present invention, wherein the method comprises the following steps:

[0030] Step 1: Perform Fourier transform on the multi-mode (staring spotlight / sliding spotlight) SAR data along the range direction, and perform azimuth deskewing and range compression.

[0031] Select the reference point coordinates in the slope distance plane , the Earth-centered rectangular coordinates of the geolocation reference point, the slant range history and deslant function of the reference point can be expressed as:

[0032] (1)

[0033] in, is the Earth-centered rectangular coordinate of the reference point, is the coordinate of the reference point of the slope distance plane, Represents the distance from the radar to the target at the moment of the aperture center. represents the coordinates of the satellite in the Earth-centered Earth-fixed frame, represents the radar carrier frequency, represents the distance frequency, represents the linear modulation frequency in the range direction, represents the speed of light, represents the imaginary unit, and represents the polynomial fitting coefficients, represents the exponential function, It represents the azimuth time, and its formula is:

[0034] (2)

[0035] in, Indicates the number of sampling points in the azimuth direction, represents the pulse repetition frequency;

[0036] Step 2: Doppler spectrum aliasing elimination based on azimuth sub-band (schematic diagram as shown in Figure 2 The specific processing steps are as follows:

[0037] Step 2.1: Split the multi-mode SAR signal processed in step 1 into sub-blocks, and the position time of each block of data after segmentation is , which can be expressed as follows:

[0038] (3)

[0039] in Represents the center of orientation, Represents the number of sampling points of the current sub-block, Represents the sampling time.

[0040] Step 2.2: Perform azimuth Fourier transform on each azimuth sub-block data and fill zeros in the azimuth frequency domain to ensure that the Doppler spectrum of the SAR signal is not aliased after azimuth modulation recovery.

[0041] Step 2.3: Perform azimuth modulation recovery on each azimuth sub-block data based on the phase compensation function. The phase compensation function used to recover azimuth modulation is:

[0042] (4)

[0043] in, Represents the slant range history after the azimuth sub-block performs the target linear range walk correction (LRWC). Its expression is:

[0044] (5)

[0045] The azimuth frequency expression after azimuth modulation is restored as:

[0046] (6)

[0047] in, Represents the upsampling multiple, Represents the azimuth sampling frequency after processing, It represents the Doppler centroid of the azimuth sub-block data after phase compensation, and its formula is:

[0048] (7)

[0049] Step 2.4: Splice the processed azimuth sub-block data into a whole block of data and calculate the total azimuth frequency. The comprehensive formula is:

[0050] (8)

[0051] in, is the number of azimuth sub-blocks. Sub-block data splicing can be completed by adding the positions according to formulas (7) and (8). The new azimuth frequency and direction time The expression is:

[0052] (9)

[0053] Where, Indicates the number of azimuth points after the azimuth sub-bands are spliced.

[0054] Figure 3The schematic diagram of the azimuth sub-band spectrum and the sub-band composite spectrum in this embodiment shows the spectrum after the azimuth sub-blocks are spliced.

[0055] Step 3: Perform reference point matched filtering on the data processed in step 2, and perform range-direction blocking and differential matched filtering on the data after reference point matched filtering;

[0056] The expression of the reference point matched filter is:

[0057] (10)

[0058] in, represents the signal phase spectrum, ,in, Indicates the second, third, and fourth order Taylor expansion coefficients of the slant range history to the azimuth time. represents the Doppler frequency, which is expressed as:

[0059] (11)

[0060] After the reference point matched filtering, the image is divided into blocks along the range in the coarsely focused image domain. After the blocks are divided, fine focusing is performed on each block, which is the differential matched filtering. The filter formula is:

[0061] (12)

[0062] in, Indicates the distance to the focus position, Represents the two-dimensional spectrum corresponding to the radar beam scanning point at the moment of the azimuth aperture center.

[0063] Step 4: Perform two-dimensional frequency domain resampling on the data after range-wise segmentation and differential matched filtering, and perform two-dimensional time domain segmentation and differential matched filtering again to obtain a finely focused imaging result.

[0064] Step 4.1, perform two-dimensional frequency domain range resampling based on the improved Stolt interpolation, and perform two-dimensional frequency domain azimuth resampling by two-dimensional frequency domain azimuth interpolation to obtain a coarsely focused SAR image after full-aperture processing;

[0065] Two-dimensional frequency domain distance resampling can be regarded as an improved Stolt interpolation, and the function expression is:

[0066] (13)

[0067] Where, Indicated by distance frequency and azimuth frequency Get the resampled distance frequency Function mapping relationship;

[0068] Two-dimensional frequency domain azimuth resampling can be regarded as the generalization of the improved Stolt interpolation in the azimuth direction. It is implemented through two-dimensional frequency domain azimuth interpolation. Its function expression is:

[0069] (14)

[0070] Where, Indicated by distance frequency and azimuth frequency Get the resampled azimuth frequency Function mapping relationship.

[0071] Step 4.2: Fine-focus the coarsely focused SAR image after full-aperture processing. First, divide the data after 2D frequency domain resampling into 2D blocks, and then perform phase multiplication and amplitude multiplication on the current sub-block to complete differential matched filtering. The signal phase expression that needs to be compensated is:

[0072] (15)

[0073] in, It represents the amplitude modulation term of the two-dimensional spectrum of the SAR original echo signal, and its expression is:

[0074] (16)

[0075] and It represents the azimuth frequency and range frequency of the SAR signal after resampling, and its expression is:

[0076] (17)

[0077] Where, and Respectively represent the number of sampling points in the azimuth and distance directions of the current sub-block, Represents the range sampling rate.

[0078] Figure 4 It shows the point target simulation fine focusing processing results and two-dimensional spectrum in this embodiment. Figure 5 The time-frequency relationship between the SAR echo and the focused SRA image under the azimuth sub-band splicing processing in this embodiment is shown in the figure. Represents the total azimuth time in the SAR signal domain, Represents the total Doppler bandwidth of the SAR signal, represents the azimuth time in the SAR image domain. It can be seen that the proposed method can well focus on point targets and, in principle, control the significant increase in data volume and improve imaging processing efficiency.

[0079] On the other hand, the present invention provides a spaceborne high squint multi-mode SAR integrated imaging device, the modules of which can implement the steps of the aforementioned method, specifically including:

[0080] The pre-processing module is used to perform Fourier transform on the multi-mode SAR data along the range direction, and perform azimuth de-skewing and range compression processing;

[0081] Aliasing elimination module, used for eliminating Doppler spectrum aliasing based on azimuth sub-band;

[0082] A filtering module is used to perform reference point matched filtering on the data after the Doppler spectrum aliasing elimination process, and to perform range-direction blocking and differential matched filtering on the data after the reference point matched filtering;

[0083] The imaging module is used to perform two-dimensional frequency domain resampling on the data after range-division blocking and differential matched filtering, and then perform two-dimensional time domain blocking and differential matched filtering again to obtain a finely focused imaging result.

[0084] In a third aspect, the present invention provides an electronic device comprising: one or more processors; a memory for storing one or more programs; wherein, when the one or more programs are executed by the one or more processors, the one or more processors implement the aforementioned spaceborne high-squint multi-mode SAR integrated imaging method.

[0085] In a fourth aspect, the present invention provides a computer-readable storage medium having executable instructions stored thereon, which, when executed by a processor, enables the processor to implement the aforementioned spaceborne high-squint multi-mode SAR integrated imaging method.

[0086] It should be noted that, in this document, the terms "comprises," "includes," or any other variations thereof are intended to encompass non-exclusive inclusion, such that a process, method, article, or terminal device that includes a series of elements includes not only those elements but also other elements not explicitly listed, or also includes elements inherent to such process, method, article, or terminal device. In the absence of further limitations, an element defined by the phrase "comprises a..." does not exclude the presence of other identical elements in the process, method, article, or terminal device that includes the element.

[0087] The foregoing description is of a preferred embodiment of the present invention. It should be noted that, although preferred embodiments of the present invention have been described, those skilled in the art, once understanding the basic inventive concepts of the present invention, may make various improvements and modifications without departing from the principles of the present invention, and such improvements and modifications should also be considered within the scope of protection of the present invention. Therefore, the appended claims are intended to be interpreted as covering the preferred embodiments and all variations and modifications that fall within the scope of the embodiments of the present invention.

Claims

1. A spaceborne high squint multi-mode SAR integrated imaging method, characterized in that: The method comprises: Step 1: Perform Fourier transform on the multi-mode SAR data along the range direction, and perform azimuth de-skewing and range compression processing; Step 2: Doppler spectrum aliasing elimination based on azimuth sub-band; including: Step 2.1: Split the pre-processed multi-mode SAR data into sub-blocks; Step 2.2, perform azimuth Fourier transform on each azimuth sub-block data, and fill zeros in the azimuth frequency domain; Step 2.3, performing azimuth modulation recovery on each azimuth sub-block data based on the phase compensation function; Step 2.4, stitching the processed azimuth sub-block data into a whole block of data, and synthesizing the total azimuth frequency; Step 3: performing reference point matched filtering on the data after the Doppler spectrum aliasing elimination process, and performing range-direction blocking and differential matched filtering on the data after the reference point matched filtering process; Step 4: Perform two-dimensional frequency domain resampling on the data after range-wise segmentation and differential matched filtering, and perform two-dimensional time domain segmentation and differential matched filtering again to obtain a finely focused imaging result.

2. The method for integrated high-squint multi-mode SAR imaging of spaceborne according to claim 1, characterized in that: In step 1, the multi-mode SAR data includes staring beamforming and sliding beamforming.

3. The method for integrated high squint multi-mode SAR imaging of spaceborne according to claim 1, characterized in that: The phase compensation function in step 2.3 is: （4） in, represents the distance frequency, Indicates the direction and time, represents the radar carrier frequency, represents the speed of light, represents the imaginary unit, Represents the slant range history after the azimuth sub-block performs the target linear range wandering correction. It is the azimuth time of each azimuth sub-block data after segmentation.

4. The method for integrated high-squint multi-mode SAR imaging of spaceborne according to claim 3, characterized in that: The total azimuth frequency in step 2.4 for: （8） in, is the number of azimuth sub-blocks, To recover the azimuth frequency after azimuth modulation: （6） in, Represents the upsampling multiple, Represents the azimuth sampling frequency after processing, represents the Doppler centroid of the azimuth sub-block data after phase compensation, Represents the number of sampling points of the current sub-block.

5. The method for integrated spaceborne high squint multi-mode SAR imaging according to claim 4, characterized in that: The expression of the reference point matched filter in step 3 is: （10） in, represents the signal phase spectrum; After the reference point matched filtering, the image is divided into blocks along the range in the coarsely focused image domain. After the blocks are divided, fine focusing is performed on the individual blocks, i.e., differential matched filtering. The filter formula of the differential matched filtering is: （12） in, Indicates the distance to the focus position, Represents the two-dimensional spectrum corresponding to the radar beam scanning point at the moment of the azimuth aperture center.

6. The spaceborne high squint multi-mode SAR integrated imaging method according to claim 1, characterized in that: The step 4 comprises: Step 4.1, perform two-dimensional frequency domain range resampling based on the improved Stolt interpolation, and perform two-dimensional frequency domain azimuth resampling by two-dimensional frequency domain azimuth interpolation to obtain a coarsely focused SAR image after full-aperture processing; Step 4.2: performing two-dimensional time domain blocking on the coarsely focused SAR image after full aperture processing, and then performing phase multiplication and amplitude multiplication on the sub-blocks after the two-dimensional time domain blocking to complete differential matched filtering.

7. A spaceborne high squint multi-mode SAR integrated imaging device, characterized in that: include: The pre-processing module is used to perform Fourier transform on the multi-mode SAR data along the range direction, and perform azimuth de-skewing and range compression processing; Aliasing elimination module, used for eliminating Doppler spectrum aliasing based on azimuth sub-band; including: The pre-processed multi-mode SAR data is divided into sub-blocks; Perform azimuth Fourier transform on each azimuth sub-block data and fill zeros in the azimuth frequency domain; Perform azimuth modulation recovery on each azimuth sub-block data based on a phase compensation function; The processed azimuth sub-block data are spliced into a whole block of data, and the total azimuth frequency is integrated; A filtering module is used to perform reference point matched filtering on the data after the Doppler spectrum aliasing elimination process, and to perform range-direction blocking and differential matched filtering on the data after the reference point matched filtering; The imaging module is used to perform two-dimensional frequency domain resampling on the data after range-wise blocking and differential matched filtering, and then perform two-dimensional time domain blocking and differential matched filtering again to obtain a finely focused imaging result.

8. An electronic device, characterized in that: include: one or more processors; a memory for storing one or more programs; When one or more programs are executed by the one or more processors, the one or more processors implement the spaceborne high squint multi-mode SAR integrated imaging method according to any one of claims 1 to 6.

9. A computer-readable storage medium, characterized in that Executable instructions are stored thereon, and when the instructions are executed by a processor, the processor can implement the space-borne high-squint multi-mode SAR integrated imaging method as described in any one of claims 1-6.

Images