High-resolution wide-swath spaceborne synthetic aperture radar imaging method

Abstract

This application relates to a high-resolution, wide-swath spaceborne synthetic aperture radar (SAR) imaging method. By preprocessing the two-dimensional echo signal acquired by the spaceborne SAR radar, a processed two-dimensional echo signal is obtained. This processed two-dimensional echo signal is data discretely sampled in polar coordinate format in the spatial frequency domain of the radar-illuminated target function. The processed two-dimensional echo signal is then converted from polar coordinate format sampling to rectangular format sampling to obtain a converted two-dimensional echo signal. Finally, an inverse fast Fourier transform is used to convert the converted two-dimensional echo signal from the spatial frequency domain to the image domain, thereby obtaining a high-resolution two-dimensional focused image of the target. This automatically compensates for orbital curvature and orbital non-coplanarity effects during the synthetic aperture time, thus overcoming the influence of orbital curvature, Earth's rotation, and spherical surface effects, achieving high-resolution, wide-swath spaceborne SAR imaging processing.

Description

Technical Field

[0001] This application relates to the field of radar imaging technology, and in particular to a high-resolution, wide-swath spaceborne synthetic aperture radar imaging method. Background Technology

[0002] With the development of radar imaging, numerous synthetic aperture radar (SAR) imaging processing methods have emerged. Based on the domain in which the main steps of the imaging processing methods are handled, these methods can be categorized into time-domain based and frequency-domain based methods. Typical examples of time-domain algorithms are the time-domain correlation method and the convolutional back-projection algorithm (or filtered back-projection algorithm). The time-domain correlation method essentially performs matched filtering directly in the two-dimensional time domain, thus achieving the best focusing accuracy. However, because the imaging process requires pixel-by-pixel calculations of the target in both dimensions, this algorithm has extremely low computational efficiency and almost no practical engineering application value. The convolutional back-projection algorithm utilizes the space-invariant property of range processing, achieving fast range filtering through FFT / IFFT, but the azimuth direction still relies on traditional time-domain correlation processing. Therefore, this algorithm significantly improves computational efficiency compared to the two-dimensional correlation method, but because azimuth processing still requires pixel-by-pixel calculations, its computational efficiency remains relatively low. Frequency domain algorithms, on the other hand, eliminate the spatial invariance of filters by performing special processing or approximation on the data, and finally achieve batch processing through FFT / IFFT operations, avoiding pixel-by-pixel operations and greatly improving the computational efficiency of the algorithms. Currently, typical frequency domain SAR algorithms include range-Doppler algorithms, scale-variable algorithms, range migration algorithms, polar coordinate format algorithms, and various improved algorithms derived from them.

[0003] While frequency domain algorithms offer engineering advantages, existing ones are based on two fundamental assumptions: the assumption of uniform linear motion of the radar track and the assumption of a flat terrain imaging scene. These assumptions generally hold true for airborne synthetic aperture radar (SAR) and, for spaceborne SAR, under traditional conditions of low resolution and narrow imaging swath. However, as resolution increases and synthetic aperture time lengthens, the curvilinear characteristics of satellite orbits become increasingly significant. Furthermore, for medium- and high-orbit SAR, the imaging swath increases dramatically, reaching hundreds or even thousands of kilometers. In such cases, traditional algorithms cannot overcome the effects of orbital curvature, Earth's rotation, and spherical surface effects, making them unsuitable for high-resolution, wide-swath spaceborne SAR imaging processing. Summary of the Invention

[0004] Therefore, it is necessary to provide a high-resolution, wide-swath spaceborne synthetic aperture radar imaging method that can overcome the effects of orbital curvature, Earth's rotation, and spherical surface effects, in order to address the aforementioned technical problems.

[0005] A high-resolution, wide-swath spaceborne synthetic aperture radar imaging method, the method comprising:

[0006] The two-dimensional echo signal acquired by the spaceborne synthetic aperture radar is preprocessed to obtain the processed two-dimensional echo signal. The processed two-dimensional echo signal is the data of the target function of the radar illumination in the polar coordinate format discrete sampling in the spatial frequency domain.

[0007] The processed two-dimensional echo signal is converted from polar coordinate sampling in the spatial frequency domain to rectangular sampling to obtain the converted two-dimensional echo signal.

[0008] The inverse fast Fourier transform is used to convert the transformed two-dimensional echo signal from the spatial frequency domain to the image domain in order to obtain a high-resolution two-dimensional focused image of the target.

[0009] In one embodiment, the preprocessing of the two-dimensional echo signal acquired by the spaceborne synthetic aperture radar to obtain the processed two-dimensional echo signal includes:

[0010] The two-dimensional echo signal acquired by the spaceborne synthetic aperture radar is sequentially subjected to pulse compression, range resampling, phase compensation, and Fourier transform to obtain the processed two-dimensional echo signal.

[0011] In one embodiment, the expression for pulse compression is:

[0012] ;

[0013] in, For location and time, For distance and time, For two-dimensional echo signals acquired by spaceborne synthetic aperture radar, This is a two-dimensional echo signal after pulse compression processing. For the fast Fourier transform along the distance-time, This is the inverse fast Fourier transform along the distance-time axis. This is the reference function for pulse compression. , For range frequency, It represents the conjugate of the radar transmitted signal spectrum.

[0014] In one embodiment, the expression for the distance resampling is:

[0015] ;

[0016] in, The distance and time before resampling. This represents the distance-time after resampling. The speed of electromagnetic wave propagation. The distance from the center of the Earth to the radar. This is the radius of the Earth.

[0017] In one embodiment, the phase compensation function of the phase compensation is:

[0018] ;

[0019] in, The speed of electromagnetic wave propagation. The carrier frequency of the signal before the previous resampling step. The carrier frequency of the signal after the previous distance resampling step. This is the distance from the previous step before resampling. The distance is the distance after resampling from the previous step, where j is the imaginary unit. This is the phase compensation function.

[0020] In one embodiment, the step of converting the processed two-dimensional echo signal from polar coordinate sampling in the spatial frequency domain to rectangular sampling to obtain the converted two-dimensional echo signal includes:

[0021] One-dimensional resampling is performed on the range and azimuth directions of the processed two-dimensional echo signal to obtain the converted two-dimensional echo signal.

[0022] In one embodiment, the expressions for the two one-dimensional resampling directions, range and azimuth, are as follows:

[0023] ,

[0024] ;

[0025] in, The distance frequency before resampling. This represents the distance frequency after resampling. This represents the azimuth time before azimuth resampling. This represents the azimuth time after azimuth resampling. This is the instantaneous azimuth angle of the radar. The instantaneous elevation angle of the radar. and It is a function of azimuth and time. for The derivative value at the time of the aperture center. For radar azimuth time The instantaneous azimuth angle, This is the carrier frequency of the signal after the previous distance resampling.

[0026] In one embodiment, the step of employing inverse fast Fourier transform to convert the transformed two-dimensional echo signal from the spatial frequency domain to the image domain to obtain a target two-dimensional high-resolution focused image includes:

[0027] After performing space-invariant non-coplanar phase error compensation on the transformed two-dimensional echo signal, a range-direction inverse Fourier transform is performed to obtain the transformed two-dimensional echo signal.

[0028] After performing range-space-variable non-coplanar phase error compensation on the transformed two-dimensional echo signal, an inverse Fourier transform in the azimuth direction is performed to obtain a high-resolution two-dimensional focused image of the target.

[0029] In one embodiment, the expression for the compensation function of the space-invariant non-coplanar phase error compensation is:

[0030] ;

[0031] in, The compensation function for non-coplanar phase error compensation that is empty and invariant. The speed of electromagnetic wave propagation. The carrier frequency of the signal after the previous distance resampling step. This represents the distance frequency after resampling. The Z-axis coordinate value of the 3D coordinates of the scene center point. For function inverse function, This is the instantaneous azimuth angle of the radar. The instantaneous elevation angle of the radar. for The derivative value at the time of the aperture center. For radar azimuth time The instantaneous azimuth angle.

[0032] In one embodiment, the expression for the compensation function of the non-coplanar phase error compensation for the distance spatial variation is:

[0033] ;

[0034] in, For the non-coplanar phase error compensation of the range spatial variation, The Z-axis coordinates of the target at the m-th distance gate.

[0035] The aforementioned high-resolution, wide-swath spaceborne synthetic aperture radar (SAR) imaging method preprocesses the two-dimensional echo signals acquired by the spaceborne SAR radar to obtain processed two-dimensional echo signals. These processed two-dimensional echo signals are discretely sampled data of the radar-illuminated target function in the spatial frequency domain in polar coordinate format. The processed two-dimensional echo signals are then transformed from polar coordinate format sampling to rectangular format sampling to obtain transformed two-dimensional echo signals. Finally, an inverse fast Fourier transform is used to transform the transformed two-dimensional echo signals from the spatial frequency domain to the image domain, thereby obtaining a high-resolution two-dimensional focused image of the target. This automatically compensates for orbital curvature and orbital non-coplanarity effects during the synthetic aperture time, thus overcoming the influence of orbital curvature, Earth's rotation, and spherical surface effects, achieving high-resolution, wide-swath spaceborne SAR imaging processing. Attached Figure Description

[0036] Figure 1 This is a flowchart illustrating a high-resolution wide-swath spaceborne synthetic aperture radar imaging method in one embodiment;

[0037] Figure 2 This is a schematic diagram of the geometric relationship for data acquisition by a spaceborne synthetic aperture radar in one embodiment. Detailed Implementation

[0038] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0039] In one embodiment, such as Figure 1 As shown, a high-resolution, wide-swath spaceborne synthetic aperture radar imaging method is provided. Taking the application of this method to a terminal as an example, the method includes the following steps:

[0040] Step S220: Preprocess the two-dimensional echo signal acquired by the spaceborne synthetic aperture radar to obtain the processed two-dimensional echo signal. The processed two-dimensional echo signal is the data of the target function of the radar illumination in the polar coordinate format discrete sampling in the spatial frequency domain.

[0041] The preprocessing can involve sequentially performing pulse compression, range resampling, phase compensation, and one-dimensional Fourier transform on the two-dimensional echo signal acquired by the spaceborne synthetic aperture radar. After this preprocessing, the resulting two-dimensional echo signal is data of discrete sampling in polar coordinate format of the radar illumination target function in the spatial frequency domain.

[0042] Among them, such as Figure 2 The diagram shows the geometric relationship of data acquisition by a spaceborne synthetic aperture radar. In the Earth-fixed coordinate system, the instantaneous position of the radar is the distance from the radar to the center of the Earth. Azimuth and pitch angle Given a target point located on Earth (with radius r), On the sphere, its position is The instantaneous distance of the radar reaching the target point is The distance to the radar is An equidistant sphere intersects the Earth's surface at a circle. Assume the distance from the center of the sphere to the plane containing this circle is... The quantities mentioned above that are related to the instantaneous position of the radar are all functions of azimuth and time. For the sake of simplification, their azimuth and time dependence has been omitted here.

[0043] Assuming the radar transmits a linear frequency modulated signal, after demodulation and ignoring the echo amplitude, the acquired two-dimensional echo signal can be expressed as:

[0044] ;

[0045] in, For the acquired two-dimensional echo signal, For location and time, The distance is time, and j is the imaginary unit. The frequency modulation slope of the transmitted signal. The speed of electromagnetic wave propagation. The carrier frequency for the transmitted signal. This represents the instantaneous distance from the target to the radar.

[0046] In one embodiment, preprocessing the two-dimensional echo signal acquired by the spaceborne synthetic aperture radar to obtain the processed two-dimensional echo signal includes: sequentially performing pulse compression, range resampling, phase compensation, and Fourier transform on the two-dimensional echo signal acquired by the spaceborne synthetic aperture radar to obtain the processed two-dimensional echo signal.

[0047] Among them, pulse compression can be range-directed pulse compression processing, which can be achieved by matched filtering or other methods, such as Decirp processing, etc.

[0048] In one embodiment, pulse compression employs matched filtering. Considering that the acquired two-dimensional echo signal is discrete sampled data, matched filtering is typically implemented in the frequency domain. Its processing steps include range-directed Fast Fourier Transform (FFT), reference function multiplication, and inverse Fourier Transform (IFFT). Therefore, the expression for pulse compression can be:

[0049] ;

[0050] in, For location and time, For distance and time, For two-dimensional echo signals acquired by spaceborne synthetic aperture radar, This is a two-dimensional echo signal after pulse compression processing. For the fast Fourier transform along the distance-time, This is the inverse fast Fourier transform along the distance-time axis. This is the reference function for pulse compression. , For range frequency, It represents the conjugate of the radar transmitted signal spectrum.

[0051] The two-dimensional echo signal after pulse compression processing is as follows:

[0052] ;

[0053] in, This is a two-dimensional echo signal after pulse compression processing. For the transmission signal bandwidth, .

[0054] The two-dimensional echo signal after pulse compression is further resampled in the range direction. Mathematically, this is equivalent to replacing the range variable in the pulse-compressed two-dimensional echo signal as follows:

[0055] ;

[0056] in, Let the function mapping relationship between the independent variables before and after the variable substitution satisfy the following condition: Therefore, the resampled two-dimensional echo signal can be expressed as:

[0057] ;

[0058] in, This is the resampled two-dimensional echo signal.

[0059] In one embodiment, the expression for distance-oriented resampling is:

[0060] ;

[0061] in, The distance and time before resampling. This represents the distance-time after resampling. The speed of electromagnetic wave propagation. The distance from the center of the Earth to the radar. This is the radius of the Earth.

[0062] Phase compensation can be used to correct the phase of the resampled two-dimensional echo signal. The phase compensation function is as follows:

[0063] ;

[0064] in, The speed of electromagnetic wave propagation. The carrier frequency of the signal before the previous resampling step. The carrier frequency of the signal after the previous distance resampling step. This is the distance from the previous step before resampling. The distance is the distance after resampling from the previous step, where j is the imaginary unit. This is the phase compensation function.

[0065] The phase-compensated two-dimensional echo signal is as follows:

[0066] ;

[0067] in, This is the two-dimensional echo signal after phase compensation.

[0068] Furthermore, a range-to-Fourier transform (RTF) is performed on the phase-compensated two-dimensional echo signal. This RTF can be a range-to-fast Fourier transform (FFT). After performing the RTF on the phase-compensated two-dimensional echo signal, a processed two-dimensional echo signal is obtained. The expression for the processed two-dimensional echo signal can be expressed as follows:

[0069] ;

[0070] in, This is the processed two-dimensional echo signal.

[0071] Furthermore, according to Figure 1 The imaging geometry is shown, where:

[0072] ;

[0073] Substituting the processed two-dimensional echo signal into the expression, we obtain...

[0074] .

[0075] Step S240: Convert the processed two-dimensional echo signal from polar coordinate sampling in the spatial frequency domain to rectangular sampling to obtain the converted two-dimensional echo signal.

[0076] The conversion from polar coordinate sampling in the spatial frequency domain to rectangular sampling can be achieved through one-dimensional resampling in the range and azimuth directions.

[0077] It should be understood that after preprocessing, the acquired two-dimensional echo signal becomes a two-dimensional sample in the three-dimensional spectral space of the objective function, with the sampling position changing from the polar radius. Azimuth and pitch angle The decision, among which, The frequency is radial. To facilitate the transformation of data from the spatial frequency domain to the spatial image domain using IFFT, the sampling format needs to be adjusted, changing the polar coordinate format sampled data to a rectangular format sampled data. This is essentially a two-dimensional resampling process. In practice, to improve algorithm efficiency, the above process can be decomposed into two one-dimensional resampling operations: a range resampling and an azimuth resampling.

[0078] In one embodiment, converting the processed two-dimensional echo signal from polar coordinate sampling in the spatial frequency domain to rectangular sampling to obtain the converted two-dimensional echo signal includes: performing one-dimensional resampling on the range and azimuth directions of the processed two-dimensional echo signal to obtain the converted two-dimensional echo signal.

[0079] In one embodiment, the expressions for the two one-dimensional resampling directions, range and azimuth, are as follows:

[0080] ,

[0081] ;

[0082] in, The distance frequency before resampling. This represents the distance frequency after resampling. This represents the azimuth time before azimuth resampling. This represents the azimuth time after azimuth resampling. This is the instantaneous azimuth angle of the radar. The instantaneous elevation angle of the radar. and It is a function of azimuth and time. for The derivative value at the time of the aperture center. For radar azimuth time The instantaneous azimuth angle, This is the carrier frequency of the signal after the previous distance resampling.

[0083] In this context, one-dimensional resampling in the range direction mathematically involves replacing the range frequency with a variable, i.e.:

[0084] ;

[0085] The echo signal after one-dimensional resampling at the above distance is represented as follows:

[0086] ;

[0087] in, This is the echo signal after one-dimensional resampling in the range direction.

[0088] Furthermore, the echo signal after one-dimensional resampling in the range direction is resampled in the azimuth direction to eliminate... The coupling of the two independent variables in the coefficient term is mathematically equivalent to the following variable substitution:

[0089] ;

[0090] or its inverse function ;

[0091] in, This represents the functional mapping relationship between the independent variables before and after azimuth resampling.

[0092] Therefore, the expression for the signal after azimuth resampling is:

[0093] ;

[0094] in, This is the signal after azimuth resampling, i.e., the converted two-dimensional echo signal.

[0095] Step S260: Inverse Fast Fourier Transform is used to convert the converted two-dimensional echo signal from the spatial frequency domain to the image domain in order to obtain a high-resolution two-dimensional focused image of the target.

[0096] In one embodiment, an inverse fast Fourier transform is used to convert the transformed two-dimensional echo signal from the spatial frequency domain to the image domain to obtain a high-resolution two-dimensional focused image of the target, including:

[0097] After performing space-invariant non-coplanar phase error compensation on the transformed two-dimensional echo signal, a range-direction inverse Fourier transform is performed to obtain the transformed two-dimensional echo signal. After performing range-space-variant non-coplanar phase error compensation on the transformed two-dimensional echo signal, an azimuth-direction inverse Fourier transform is performed to obtain a high-resolution two-dimensional focused image of the target.

[0098] In one embodiment, the compensation function expression for space-invariant non-coplanar phase error compensation is:

[0099] ;

[0100] in, The compensation function for non-coplanar phase error compensation that is empty and invariant. The speed of electromagnetic wave propagation. The carrier frequency of the signal after the previous distance resampling step. This represents the distance frequency after resampling. The Z-axis coordinate value of the 3D coordinates of the scene center point. For function inverse function, This is the instantaneous azimuth angle of the radar. The instantaneous elevation angle of the radar. for The derivative value at the time of the aperture center. For radar azimuth time The instantaneous azimuth angle.

[0101] In one embodiment, the compensation function expression for the non-coplanar phase error compensation of the distance spatial variation is:

[0102] ;

[0103] in, For the non-coplanar phase error compensation of the range spatial variation, The Z-axis coordinates of the target at the m-th distance gate.

[0104] It should be understood that when the Earth's rotation effect is negligible and the non-coplanar characteristics of the radar track are not considered, the converted two-dimensional echo signal... Since the last term in the expression for the converted two-dimensional echo signal is always equal to zero, it can be ignored. However, considering the non-negligible effect of Earth's rotation, compensation can be applied to the last term in the expression for the converted two-dimensional echo signal. This compensation involves two steps: the first step is space-invariant non-coplanar phase error compensation, using the scene center as a reference. This compensation involves multiplying the expression for the converted two-dimensional echo signal by the following space-invariant non-coplanar phase error compensation function:

[0105] ;

[0106] in, The compensation function for non-coplanar phase error compensation that is empty and invariant. The speed of electromagnetic wave propagation. The carrier frequency of the signal after the previous distance resampling step. This represents the distance frequency after resampling. The Z-axis coordinate value of the 3D coordinates of the scene center point. For function inverse function, This is the instantaneous azimuth angle of the radar. The instantaneous elevation angle of the radar. for The derivative value at the time of the aperture center. For radar azimuth time The instantaneous azimuth angle.

[0107] The residual distance migration after compensation is generally negligible; therefore, the expression for the signal after compensation for the space-invariant non-coplanar phase error is:

[0108] ;

[0109] in, The signal after compensation for non-coplanar phase error, which is empty and invariant. Let X be the X-axis coordinate of the target on the m-th distance gate. The value is the Y-axis coordinate of the target at the m-th distance gate.

[0110] Furthermore, performing an IFFT in the range direction allows for range-oriented focused imaging, yielding...

[0111] ;

[0112] in, For the range direction, the Sink function, This is the transformed two-dimensional echo signal.

[0113] With the distance information available, we can further perform distance-space-variable non-coplanar phase error compensation on the last term, which is the transformed two-dimensional echo signal multiplied by the distance-space-variable non-coplanar phase error compensation function:

[0114] ;

[0115] in, For the non-coplanar phase error compensation of the range spatial variation, The Z-axis coordinates of the target at the m-th distance gate.

[0116] The signal after compensation for the non-coplanar phase error of the distance spatial variable is:

[0117] ;

[0118] in, This is the signal after compensation for the non-coplanar phase error of the distance spatial variation. These are the coordinates of the image's distance position.

[0119] At this point, performing an inverse azimuth Fourier transform (IFFT) allows for focused imaging in the azimuth direction, yielding the desired result.

[0120] ;

[0121] in, For the azimuth direction, the Sink function is... The target is a two-dimensional high-resolution focused image, where x is the image's orientation coordinate.

[0122] It should be understood that the inverse fast Fourier transform (IFFT) is used to convert the transformed two-dimensional echo signal from the spatial frequency domain to the image domain to obtain a high-resolution two-dimensional focused image of the target. The two-dimensional IFFT transforms the data from the two-dimensional spatial frequency domain to the two-dimensional image domain. Simultaneously, spatial invariance and spatial variation compensations for radar platform non-coplanar effects are added before the range and azimuth IFFTs, respectively. This can further automatically overcome the effects of orbital curvature, Earth's rotation, and spherical surface effects.

[0123] The aforementioned high-resolution, wide-swath spaceborne synthetic aperture radar (SAR) imaging method preprocesses the two-dimensional echo signals acquired by the spaceborne SAR radar to obtain processed two-dimensional echo signals. These processed two-dimensional echo signals are discretely sampled data of the radar-illuminated target function in the spatial frequency domain in polar coordinate format. The processed two-dimensional echo signals are then transformed from polar coordinate format sampling to rectangular format sampling to obtain transformed two-dimensional echo signals. Finally, an inverse fast Fourier transform is used to transform the transformed two-dimensional echo signals from the spatial frequency domain to the image domain, thereby obtaining a high-resolution two-dimensional focused image of the target. This automatically compensates for orbital curvature and orbital non-coplanarity effects during the synthetic aperture time, thus overcoming the influence of orbital curvature, Earth's rotation, and spherical surface effects, achieving high-resolution, wide-swath spaceborne SAR imaging processing.

[0124] Furthermore, the high-resolution wide-swath spaceborne synthetic aperture radar imaging method of this application is directly based on spherical surface modeling, and can also accurately compensate and correct the influence of the spherical surface on imaging processing.

[0125] It should be understood that, although Figure 1 The steps in the flowchart are shown sequentially as indicated by the arrows, but these steps are not necessarily executed in the order indicated by the arrows. Unless otherwise specified herein, there is no strict order in which these steps are executed, and they can be performed in other orders. Figure 1 At least some of the steps in the process may include multiple sub-steps or multiple stages. These sub-steps or stages are not necessarily completed at the same time, but can be executed at different times. The execution order of these sub-steps or stages is not necessarily sequential, but can be executed in turn or alternately with other steps or at least some of the sub-steps or stages of other steps.

[0126] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0127] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this patent application should be determined by the appended claims.

Claims

1. A high-resolution, wide-swath spaceborne synthetic aperture radar imaging method, characterized in that, The method includes: The two-dimensional echo signal acquired by the spaceborne synthetic aperture radar is preprocessed to obtain the processed two-dimensional echo signal. The processed two-dimensional echo signal is the data of the target function of the radar illumination in the polar coordinate format discrete sampling in the spatial frequency domain. The processed two-dimensional echo signal is converted from polar coordinate sampling in the spatial frequency domain to rectangular sampling to obtain the converted two-dimensional echo signal. The inverse fast Fourier transform is used to convert the transformed two-dimensional echo signal from the spatial frequency domain to the image domain in order to obtain a high-resolution two-dimensional focused image of the target. The step of converting the processed two-dimensional echo signal from polar coordinate sampling in the spatial frequency domain to rectangular sampling to obtain the converted two-dimensional echo signal includes: The range and azimuth directions of the processed two-dimensional echo signal are resampled in one dimension to obtain the converted two-dimensional echo signal. The expressions for the two one-dimensional resampling in the range and azimuth directions are as follows: ， ； in, The distance frequency before resampling. This represents the distance frequency after resampling. This represents the azimuth time before azimuth resampling. This represents the azimuth time after azimuth resampling. This is the instantaneous azimuth angle of the radar. The instantaneous elevation angle of the radar. and It is a function of azimuth and time. for The derivative value at the time of the aperture center. For radar azimuth time The instantaneous azimuth angle, The carrier frequency of the signal after the previous distance resampling; The method employs inverse fast Fourier transform to convert the transformed two-dimensional echo signal from the spatial frequency domain to the image domain to obtain a high-resolution two-dimensional focused image of the target, including: After performing space-invariant non-coplanar phase error compensation on the transformed two-dimensional echo signal, a range-direction inverse Fourier transform is performed to obtain the transformed two-dimensional echo signal. After performing range-space-variable non-coplanar phase error compensation on the transformed two-dimensional echo signal, an azimuth-direction inverse Fourier transform is performed to obtain a target two-dimensional high-resolution focused image. The expression for the compensation function of the space-invariant non-coplanar phase error compensation is: ； in, The compensation function for non-coplanar phase error compensation that is empty and invariant. The speed of electromagnetic wave propagation. The carrier frequency of the signal after the previous distance resampling step. This represents the distance frequency after resampling. The Z-axis coordinate value of the three-dimensional coordinates of the scene center point. For function inverse function, This is the instantaneous azimuth angle of the radar. The instantaneous elevation angle of the radar. for The derivative value at the time of the aperture center. For radar azimuth time The instantaneous azimuth angle; The compensation function expression for the non-coplanar phase error compensation of the distance spatial variation is: ； in, For the non-coplanar phase error compensation of the range spatial variation, The Z-axis coordinates of the target at the m-th distance gate.

2. The method according to claim 1, characterized in that, The preprocessing of the two-dimensional echo signal acquired by the spaceborne synthetic aperture radar to obtain the processed two-dimensional echo signal includes: The two-dimensional echo signal acquired by the spaceborne synthetic aperture radar is sequentially subjected to pulse compression, range resampling, phase compensation, and Fourier transform to obtain the processed two-dimensional echo signal.

3. The method according to claim 2, characterized in that, The expression for pulse compression is: ； in, For location and time, For distance and time, For two-dimensional echo signals acquired by spaceborne synthetic aperture radar, This is a two-dimensional echo signal after pulse compression processing. For the fast Fourier transform along the distance-time, This is the inverse fast Fourier transform along the distance-time axis. This is the reference function for pulse compression. , For range frequency, It represents the conjugate of the radar transmitted signal spectrum.

4. The method according to claim 3, characterized in that, The expression for the distance resampling is: ； in, This represents the distance-time after resampling. The speed of electromagnetic wave propagation. The distance from the Earth's center to the radar. This is the radius of the Earth.

5. The method according to claim 4, characterized in that, The phase compensation function is as follows: ； in, The speed of electromagnetic wave propagation. The carrier frequency of the signal before the previous resampling step. The carrier frequency of the signal after the previous distance resampling step. This is the distance from the previous step before resampling. The distance is the distance after resampling from the previous step, where j is the imaginary unit. This is the phase compensation function.

Images