Fast time-domain imaging method for medium-high orbit SAR based on user-defined scene coordinate system

Abstract

The application provides a middle-high orbit SAR fast time domain imaging method based on a self-defined scene coordinate system, and relates to the technical field of radar imaging. The method comprises the following steps: firstly, according to the complex imaging geometric characteristics of the middle-high orbit SAR in the large squint mode, the analytical expression of the wave number support area in the squint mode is calculated. Secondly, aiming at the characteristics of the middle-high orbit SAR imaging, such as the super wide swath and the super long synthetic aperture time, the application provides a fast time domain imaging algorithm, which can effectively process the imaging data and meet the imaging mode of the middle-high orbit SAR. Finally, according to the arrangement mode of the imaging grid, combined with the characteristics that the radar beam footprint is not fixed and the ground sidelobe direction is not consistent, the imaging algorithm can meet the complex imaging geometric conditions of the middle-high orbit SAR in different orbit segments, downward angles and squint angles, and can self-define a two-dimensional direction flexible and non-orthogonal scene coordinate system, so as to meet the imaging processing requirements of the middle-high orbit SAR in different imaging modes.

Description

Technical Field

[0001] This invention relates to the field of radar imaging technology, and in particular to a fast time-domain imaging method for medium- and high-orbit SAR based on a custom scene coordinate system. Background Technology

[0002] Medium- and high-orbit SAR technology plays an irreplaceable role in large-scale, wide-area, and high-frequency observation of the Earth, enabling better target observation. The establishment of the scene coordinate system directly affects the imaging of medium- and high-orbit SAR data; selecting a suitable scene coordinate system ensures clear focusing and meets imaging mode requirements.

[0003] Traditional frequency-domain imaging algorithms perform a two-dimensional Fourier transform on the echo signal, applying range migration correction, range pulse compression, and azimuth compression in the frequency domain before converting the signal back to the time domain to obtain the focused SAR image. This algorithm primarily relies on the azimuth translation invariance of the signal. However, large-scene target signals in medium- and high-orbit SAR exhibit significant two-dimensional spatial variation, along with time-varying satellite velocities and pronounced trajectory curvature. This makes traditional frequency-domain algorithms unsuitable for the focusing imaging requirements of medium- and high-orbit SAR, resulting in poor image quality. When imaging echo signals, a scene coordinate system needs to be constructed. However, the construction of this system is based on a front-side-view imaging mode. In this mode, the sidelobe directions of the point response function on the ground are orthogonal. But in medium- and high-orbit SAR imaging, under complex imaging geometry conditions of different orbital segments, downward-facing angles, and oblique angles, the satellite beam footprint direction is not fixed, and the sidelobe directions of the point response function cannot always satisfy the orthogonality condition. This makes it unsuitable for imaging under complex geometries of different orbital segments, downward-facing angles, and oblique angles. Summary of the Invention

[0004] The purpose of this invention is to provide a fast temporal imaging method for medium- and high-orbit SAR based on a custom scene coordinate system, which solves the problems of poor imaging quality and the inapplicability of the constructed scene coordinate system to imaging complex geometries of different orbital segments, downward viewpoints, and oblique viewpoints.

[0005] To address the aforementioned technical problems, the embodiments of the present invention provide the following technical solutions:

[0006] The first aspect of this invention provides a fast temporal imaging method for medium- and high-orbit SAR based on a custom scene coordinate system, the method comprising:

[0007] Construct an echo signal model;

[0008] An adaptive scene coordinate system is established based on the imaging mode, and a grid is arranged under the scene coordinate system to obtain the scene coordinate system grid. The direction of the ground sidelobe is the direction of the sidelobe of the point response function of the echo signal on the imaging ground plane. The scene coordinate system includes two vectors, the u-axis and the v-axis, located on the surface tangent.

[0009] Project the spatial wavenumber center vector and the spatial wavenumber vector onto the u-axis and v-axis of the scene coordinate system grid, respectively, to determine the u-axis component and v-axis component of the corresponding wavenumber center vector, as well as the azimuth wavenumber domain and the range wavenumber domain.

[0010] The first-step compression function is determined based on the u-axis component and the v-axis component of the wavenumber center vector, and the echo signal is compressed using the first-step compression function to obtain the signal after the first-step compression.

[0011] The second-step compression function is determined based on the azimuth wavenumber domain and the range wavenumber domain, and the signal after the first-step compression is compressed using the second-step compression function to obtain the signal after two-step compression. The signal after two-step compression is then upsampled and subjected to inverse two-step spectral compression to obtain sub-aperture images at each level. The sub-aperture images at each level are then superimposed and stitched together to obtain a complete SAR image.

[0012] Compared to existing technologies, this invention provides a fast time-domain imaging method for medium- and high-orbit SAR based on a custom scene coordinate system. It constructs an echo signal model; adaptively establishes a scene coordinate system according to the imaging mode; and arranges a grid within the scene coordinate system to obtain a scene coordinate system grid. The ground sidelobe direction is the sidelobe direction of the point response function of the echo signal on the imaging ground plane. The scene coordinate system includes two vectors, the u-axis and v-axis, located on the surface tangent. The spatial wavenumber center vector and the spatial wavenumber vector of the point target are projected onto the u-axis and v-axis of the scene coordinate system grid, respectively, to determine the corresponding u-axis component and v-axis component of the wavenumber center vector, and the point target... The target's azimuth wavenumber domain and range wavenumber domain are used to compress the echo signal. A second compression function is determined based on the azimuth and range wavenumber domains, and this function is used to further compress the signal, resulting in a signal compressed in both steps. The compressed signal is then upsampled and subjected to inverse two-step spectral compression to obtain sub-aperture images. These sub-aperture images are then superimposed and stitched together to obtain a complete SAR image. In this way, a scene coordinate system can be flexibly established according to the imaging mode, so that the established scene coordinate system can meet both frontal and oblique imaging conditions, thus making it applicable to imaging with complex geometries of different orbital segments, downward and oblique angles. The echo signal can be compressed by the first-step compression function determined by the u-axis component and v-axis component of the wavenumber center vector, and the compressed signal can be compressed again by the second-step compression function determined by the azimuth wavenumber domain and the range wavenumber domain. This can achieve azimuth spectrum dealiasing of the echo signal, align the spectral center and the vertical spectrum of the echo signal, avoid the complex two-dimensional coupling spatial variation problem caused by orbit curvature and ultra-wide swath in medium and high orbit imaging, meet the requirements of medium and high orbit SAR focusing imaging, and result in better imaging quality. Attached Figure Description

[0013] The above and other objects, features, and advantages of exemplary embodiments of the present invention will become readily apparent upon reading the following detailed description with reference to the accompanying drawings. In the drawings, several embodiments of the invention are illustrated by way of example and not limitation, with the same or corresponding reference numerals denoteing the same or corresponding parts, wherein:

[0014] Figure 1 The flowchart of a fast temporal imaging method for medium-to-high orbit SAR based on a custom scene coordinate system is illustrated schematically. Figure 1 ;

[0015] Figure 2The flowchart of a fast temporal imaging method for medium-to-high orbit SAR based on a custom scene coordinate system is illustrated schematically. Figure 2 ;

[0016] Figure 3 The diagram illustrates the establishment of scene coordinate systems by the satellite in different imaging modes.

[0017] Figure 4 A schematic diagram illustrating the scene coordinate system grid settings is provided.

[0018] Figure 5 A schematic diagram of the point response function profile of a point target is shown.

[0019] Figure 6 A schematic diagram of fast temporal imaging results of mid-to-high orbit SAR based on a custom scene coordinate system is shown. Detailed Implementation

[0020] Exemplary embodiments of the invention will now be described in more detail with reference to the accompanying drawings. While exemplary embodiments of the invention are shown in the drawings, it should be understood that the invention can be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided to enable a more thorough understanding of the invention and to fully convey the scope of the invention to those skilled in the art.

[0021] It should be noted that, unless otherwise stated, the technical or scientific terms used in this invention should have the ordinary meaning as understood by one of ordinary skill in the art.

[0022] For fast time-domain imaging of mid-to-high orbit SAR in scene coordinate systems, existing technologies require the application of traditional frequency-domain algorithms. These algorithms are mainly based on the azimuth translation invariance of signals. However, mid-to-high orbit SAR targets in large scenes exhibit severe two-dimensional spatial variation, along with time-varying satellite velocities and significant trajectory curvature. This makes traditional frequency-domain algorithms unsuitable for the focusing imaging requirements of mid-to-high orbit SAR, resulting in poor image quality. Scene coordinate systems are constructed based on a front-side-view imaging mode. In this mode, the sidelobe directions of the point response function on the ground are orthogonal. However, in mid-to-high orbit SAR imaging, under complex imaging geometries of different orbital segments, downward-facing angles, and oblique angles, the satellite beam footprint direction is not fixed, and the sidelobe directions of the point response function cannot always satisfy the orthogonality condition. This makes it unsuitable for imaging complex geometries of different orbital segments, downward-facing angles, and oblique angles. Therefore, this invention, considering the poor imaging quality of existing technologies and the inapplicability of the constructed scene coordinate systems to complex geometries of different orbital segments, downward-facing angles, and oblique angles, requires a processing method with better imaging quality and applicability to complex geometries of different orbital segments, downward-facing angles, and oblique angles.

[0023] Therefore, this invention chooses to construct an echo signal model; adaptively establish a scene coordinate system according to the imaging mode, and arrange a grid in the scene coordinate system to obtain the scene coordinate system grid. The ground sidelobe direction is the sidelobe direction of the point response function of the echo signal on the imaging ground plane. The scene coordinate system includes two vectors, the u-axis and the v-axis, located on the surface tangent. The spatial wavenumber center vector and the spatial wavenumber vector of the point target are projected onto the u-axis and v-axis of the scene coordinate system grid, respectively, to determine the corresponding u-axis component and v-axis component of the wavenumber center vector, the azimuth wavenumber domain of the point target, and the range wavenumber domain of the point target. In the wavenumber domain, the first-step compression function is determined based on the u-axis and v-axis components of the wavenumber center vector, and the echo signal is compressed using this function to obtain the signal after the first-step compression. In the azimuth and range wavenumber domains, the second-step compression function is determined, and the signal after the first-step compression is compressed using this function to obtain the signal after two steps of compression. The signal after two steps of compression is then upsampled and subjected to inverse two-step spectral compression to obtain sub-aperture images at each level. These sub-aperture images are then superimposed and stitched together to obtain a complete SAR image. In this way, a scene coordinate system can be flexibly established according to the imaging mode, so that the established scene coordinate system can meet both frontal and oblique imaging conditions, thus making it applicable to imaging with complex geometries of different orbital segments, downward and oblique angles. The echo signal can be compressed by the first-step compression function determined by the u-axis component and v-axis component of the wavenumber center vector, and the compressed signal can be compressed again by the second-step compression function determined by the azimuth wavenumber domain and the range wavenumber domain. This can achieve azimuth spectrum dealiasing of the echo signal, align the spectral center and the vertical spectrum of the echo signal, avoid the complex two-dimensional coupling spatial variation problem caused by orbit curvature and ultra-wide swath in medium and high orbit imaging, meet the requirements of medium and high orbit SAR focusing imaging, and result in better imaging quality.

[0024] The main idea of ​​this invention is to establish a scene coordinate system for imaging with complex geometry that is suitable for different orbital segments, downward angles and oblique angles, based on the imaging mode. The echo signal is compressed by the first step compression function and the second step compression function to achieve azimuth spectrum dealiasing of the echo signal. The echo signal is then compressed twice to obtain multi-level sub-aperture images. The multi-level sub-aperture images are superimposed and stitched together to obtain a complete SAR image.

[0025] The methods described in the embodiments of the present invention will be explained in detail below.

[0026] Figure 1 A flowchart illustrating a fast temporal imaging method for mid-to-high orbit SAR based on a custom scene coordinate system, as shown in this embodiment of the invention, is provided. Figure 1As shown, the method may include:

[0027] S101. Construct the echo signal model.

[0028] The full aperture includes multiple levels of sub-apertures, and each level of sub-aperture corresponds to the echo signals of multiple point targets.

[0029] Constructing the echo signal model includes:

[0030] First, determine the instantaneous slant range from point target P to satellite S using the following formula:

[0031]

[0032] Among them, t a For slow time, [X(t) a ),Y(t a ),Z(t a )] is for satellite S at t a The position of time, [x p ,y p ,z p [ ] represents the position of point target P.

[0033] Secondly, based on the instantaneous slant distance R s (t a The following formula is used to construct the echo signals of multiple point targets P corresponding to each sub-aperture level. These echo signals are the echo signals of point targets P in the scene after demodulation and range compression:

[0034]

[0035] Wherein, S(t) r ,t a ) represents the echo signal, t r Where c is the speed of light, λ is the wavelength of the signal, and W is the wavelength of light. P (t a ) represents the azimuth window, B represents the bandwidth, and R represents the... s The slope distance of the center of the synthetic aperture is j, which is an imaginary number, and t is... a This is a slow time.

[0036] Based on the above formula, performing a range-direction FFT on the echo signal yields the following expression for the echo signal of point target P in the range frequency domain:

[0037]

[0038]

[0039]

[0040] Among them, W P (ta ) represents the orientation window, W r Let K be the distance width of the wavenumber support region on the inclined plane, j be an imaginary number, and K be the distance width. r For wave number, K rc f is the center vector of the space beam. c For the signal carrier frequency, f r R(t) is the frequency in the distance direction, c is the speed of light, and R(t) is the frequency in the distance direction. a ;P) represents point target P at time t a The slant distance at time.

[0041] S102. Adaptively establish a scene coordinate system according to the imaging mode, and arrange a grid under the scene coordinate system to obtain the scene coordinate system grid.

[0042] The imaging modes include frontal and lateral view imaging mode, large strabismus imaging mode, and small strabismus imaging mode.

[0043] Specifically, a scene coordinate system is flexibly established according to the imaging mode to meet the requirement of maximizing the scene in a single imaging session and reduce redundant data.

[0044] S103. Project the spatial wavenumber center vector and the spatial wavenumber vector onto the u-axis and v-axis of the scene coordinate system grid, respectively, to determine the u-axis component and v-axis component of the corresponding wavenumber center vector, as well as the azimuth wavenumber domain and the range wavenumber domain.

[0045] Specifically, the spatial wavenumber center vector and the spatial wavenumber vector are projected onto the u-axis and v-axis of the scene coordinate system grid obtained in step S102, respectively, to determine the corresponding projection vector of the spatial wavenumber center vector on the u-axis of the scene coordinate system and the projection vector of the spatial wavenumber center vector on the v-axis of the scene coordinate system, in the azimuth wavenumber domain and the range wavenumber domain.

[0046] S104. Determine the first-step compression function based on the u-axis component and v-axis component of the wavenumber center vector, and use the first-step compression function to compress the echo signal to obtain the signal after the first-step compression.

[0047] Specifically, the first-step compression function is determined based on the u-axis component and v-axis component of the wavenumber center vector determined in step S103, and the first-step compression function is used to compress each echo signal corresponding to the echo signal model constructed in step S101 to obtain the signal after the first-step compression.

[0048] The first-step compression function, determined by the u-axis component and the v-axis component of the wavenumber center vector, can be used to align the spectral center of the echo signal.

[0049] S105. Determine the second-step compression function based on the azimuth wavenumber domain and the range wavenumber domain, and use the second-step compression function to compress the signal after the first-step compression to obtain the signal after two-step compression.

[0050] The second-step compression function, determined by the azimuth and range wavenumber domains, can be used to vertically align the spectrum of the signal obtained after the first-step compression. The resulting spectrum of the second-step compression function is vertically aligned, which improves the imaging quality of the image processed by the second-step compression function.

[0051] S106. The signal after two-step compression is upsampled and subjected to inverse two-step spectral compression to obtain sub-aperture images at each level. The sub-aperture images at each level are then superimposed and stitched together to obtain a complete SAR image.

[0052] Based on the above Figure 1 As can be seen from the implementation method, the embodiments of the present invention can flexibly establish a scene coordinate system according to the imaging mode, so that the established scene coordinate system can meet both frontal and oblique imaging conditions, thus making it applicable to imaging of complex geometries with different orbital segments, downward angles, and oblique angles. The echo signal can be compressed by the first-step compression function determined by the u-axis component and v-axis component of the wavenumber center vector, and the compressed signal can be compressed again by the second-step compression function determined by the azimuth wavenumber domain and the range wavenumber domain. This can achieve azimuth spectrum dealiasing of the echo signal, align the spectral center and the upper and lower spectra of the echo signal, avoid the complex two-dimensional coupling spatial variation problem caused by orbit curvature and ultra-wide swath in medium and high orbit imaging, meet the requirements of medium and high orbit SAR focusing imaging, and make the imaging quality better.

[0053] As a refinement and extension of the above embodiments, it can be achieved through... Figure 2 This document illustrates the specific operations for rapid time-domain imaging using mid-to-high orbit SAR based on a custom scene coordinate system. Figure 2 The flowchart below illustrates the fast temporal imaging method for medium-to-high orbit SAR based on a custom scene coordinate system in this embodiment of the invention. Figure 2 See Figure 2 As shown, the fast temporal imaging method for medium- and high-orbit SAR based on a custom scene coordinate system provided in this embodiment of the invention may include:

[0054] S201. Construct an echo signal model.

[0055] Step S201 is the same as step S101, so step S201 will not be described again here.

[0056] The specific operations for determining the direction of the ground azimuth sidelobe and the direction of the ground distance sidelobe based on the satellite's parameter orientation and the surface tangent plane normal vector include the following steps S202-S204.

[0057] S202. Based on the direction of the satellite's beam center and the direction of the satellite's velocity, determine the direction of the orthogonal component of the satellite's velocity direction relative to the direction of the satellite's beam center.

[0058] The point response function of the echo signal is the energy expression of the echo signal corresponding to the point target received by the radar, and the wavenumber support region is the transformation of the point target into the wavenumber domain.

[0059] Specifically, let V s Let ρ be the direction of the satellite's velocity, and ρ be the direction of the satellite's beam center. Then, the orthogonal component V of the satellite's velocity direction relative to the satellite's beam center direction is... ρr For: V ρr =(I-ρ·ρ T V s , where I is a 3×3 identity matrix.

[0060] S203. Set the azimuth sidelobe direction of the oblique plane to be parallel to the direction of the orthogonal component, and set the distance sidelobe direction of the oblique plane to be parallel to the direction of the satellite beam center.

[0061] Specifically, the point response function of the echo signal is oriented towards the sidelobe direction on the imaging oblique plane. sa Set the direction parallel to the orthogonal component determined in step S202, and direct the distance of the point response function of the echo signal on the imaging oblique plane towards the sidelobe direction e. sr Set the beam direction to be parallel to the satellite's center.

[0062] S204. Set the direction of the ground azimuth sidelobe to be parallel to the cross product of the direction of the oblique plane distance sidelobe and the normal vector of the surface tangent plane, and set the direction of the ground distance sidelobe to be parallel to the cross product of the direction of the oblique plane azimuth sidelobe and the normal vector of the surface tangent plane.

[0063] Based on the projection relationship between the wavenumber support region on the oblique plane and the ground plane, it can be known that the point response function of the echo signal is related to the distance e in the sidelobe direction on the ground plane. gr The point response function parallel to the point echo signal in the azimuth sidelobe direction e on the oblique plane sa The cross product of the cross product with the surface tangent plane normal vector n, the point response function of the echo signal in the azimuth sidelobe direction e on the ground plane. ga The point response function parallel to the echo signal is located at a distance e in the oblique plane towards the sidelobe. srThe cross product of the normal vector n of the tangent plane to the ground surface. After obtaining the azimuth direction of the point response function of the echo signal in the imaging ground plane and the distance direction of the point response function of the echo signal in the imaging ground plane, the orientation of the scene coordinate system can be set according to the sidelobe direction. The focusing effect of the image after imaging can reflect whether the scene coordinate system is properly established.

[0064] S205. When the imaging mode is frontal side view imaging mode, set the u-axis direction of the scene coordinate system to be consistent with the direction of the ground azimuth sidelobe, and set the v-axis direction of the scene coordinate system to be consistent with the direction of the ground distance sidelobe, so as to establish the scene coordinate system under frontal side view.

[0065] Specifically, when the imaging mode is the front-side view imaging mode, the u-axis direction of the scene coordinate system is set to be consistent with the ground azimuth sidelobe direction set in step S204, i.e., the azimuth sidelobe direction of the point response function of the echo signal on the imaging ground plane. The v-axis direction of the scene coordinate system is set to be consistent with the ground distance sidelobe direction set in step S204, i.e., the distance sidelobe direction of the point response function of the echo signal on the imaging ground plane, so as to establish the scene coordinate system under the front-side view.

[0066] S206. When the imaging mode is large squint imaging mode and small squint imaging mode, the u-axis direction of the scene coordinate system is set to be consistent with the direction of the ground azimuth sidelobe. The orthogonal direction of the u-axis is obtained according to the right-hand rule. The orthogonal direction of the u-axis direction of the scene coordinate system is determined as the v-axis direction of the scene coordinate system to establish the scene coordinate system under squint.

[0067] Based on the direction of the sidelobe at ground azimuth, the direction of the sidelobe at ground distance, and the imaging mode (i.e., frontal side view imaging mode, large squint imaging mode, and small squint imaging mode), a scene coordinate system can be flexibly established.

[0068] Since the direction of the SAR sensor scanning the scene is consistent with the beam footprint, the direction of the scene coordinate system u-axis can also be set to the direction of beam movement according to the imaging mode.

[0069] Figure 3This diagram illustrates the establishment of scene coordinate systems for a satellite in different imaging modes according to an embodiment of the present invention. In the frontal side-view imaging mode, the u-axis direction of the scene coordinate system is aligned with the azimuth-to-sidelobe direction of the echo signal's point response function on the imaging ground plane, and the v-axis direction of the scene coordinate system is aligned with the distance-to-sidelobe direction of the echo signal's point response function on the imaging ground plane, thus establishing a scene coordinate system under frontal side-view conditions. In the large and small squint imaging modes, the u-axis direction of the scene coordinate system is aligned with the azimuth-to-sidelobe direction of the echo signal's point response function on the imaging ground plane, and the direction orthogonal to the u-axis direction of the scene coordinate system is defined as the v-axis direction of the scene coordinate system, thus establishing a scene coordinate system under squint conditions. Different scene coordinate system settings are adopted for different imaging modes to meet the focusing performance requirements of the imaging.

[0070] S207. Determine the width of the wavenumber support zone in the ground range direction based on the oblique plane distance to the sidelobe direction, the ground distance to the sidelobe direction, and the oblique plane distance resolution; and determine the width of the wavenumber support zone in the ground azimuth direction based on the oblique plane azimuth sidelobe direction, the ground azimuth sidelobe direction, and the oblique plane azimuth resolution.

[0071] Specifically, the width of the wavenumber support region in the ground range direction is determined based on the oblique plane distance to the sidelobe direction, the ground distance to the sidelobe direction, and the oblique plane distance resolution. Similarly, the width of the wavenumber support region in the ground azimuth direction is determined based on the oblique plane azimuth sidelobe direction, the ground azimuth sidelobe direction, and the oblique plane azimuth resolution. This includes:

[0072] Step A1: Determine the ground tilt angle based on the inverse cosine of the product of the oblique plane distance from the sidelobe direction and the ground distance from the sidelobe direction. The ground tilt angle is the angle between the oblique plane distance from the sidelobe direction and the ground distance from the sidelobe direction.

[0073] Specifically, the inverse cosine of the product of the distance from the oblique plane to the sidelobe direction (i.e., the distance of the wavenumber support region of the echo signal on the ground plane) and the distance from the ground to the sidelobe direction (i.e., the distance of the point response function of the echo signal on the imaging ground plane) is used to determine the angle γ between the distance of the wavenumber support region of the echo signal on the ground plane to the sidelobe direction and the distance of the point response function of the echo signal on the imaging ground plane to the sidelobe direction. r .

[0074] Step A2: Determine the azimuth geoclimate based on the inverse cosine of the product of the azimuth sidelobe direction in the oblique plane and the azimuth sidelobe direction on the ground. The azimuth geoclimate is the angle between the azimuth sidelobe direction in the oblique plane and the azimuth sidelobe direction on the ground.

[0075] Specifically, the angle γ between the azimuth of the wavenumber support region of the echo signal on the ground plane and the azimuth of the point response function of the echo signal on the imaging ground plane is determined based on the inverse cosine of the product of the azimuth of the wavenumber support region of the echo signal on the ground plane and the azimuth of the point response function of the echo signal on the imaging ground plane. a .

[0076] Step A3: Determine the width of the wavenumber support region in the oblique plane distance direction based on the oblique plane distance resolution, and determine the width of the wavenumber support region in the oblique plane azimuth direction based on the oblique plane azimuth resolution.

[0077] Specifically, the width of the wavenumber support region of the echo signal on the inclined plane is related to the bandwidth and the synthesis aperture time. Let B be the bandwidth, and R... s It is the slant distance of the center of the synthetic aperture, T a It is the synthesis aperture time, ρ r and ρ a These are the range resolution and azimuth resolution of the wavenumber support region of the echo signal in the slant range plane, respectively. The range width of the wavenumber support region of the echo signal on the inclined plane is W. r It is 2π and ρ r The ratio of the wavenumber support region to the azimuth width of the echo signal on the oblique plane, i.e., the azimuth width W of the wavenumber support region on the oblique plane. a It is 2π and ρ a The ratio of .

[0078] Step A4: Determine the ground distance width of the wavenumber support area based on the ratio of the distance width of the inclined plane to the cosine of the distance to the ground tilt angle, and determine the ground azimuth width of the wavenumber support area based on the ratio of the azimuth width of the inclined plane to the cosine of the azimuth tilt angle.

[0079] Specifically, since the point response function is the result of the two-dimensional fast Fourier transform of the wavenumber support region, according to the slicing theorem, the sidelobe direction is orthogonal to the edge of the wavenumber support region, and the azimuth oblique angle γ... a and the distance from the ground tilt angle γ r It can be obtained that the width W of the wavenumber support area in the direction of ground distance. gr The width W of the inclined plane of the wavenumber support region is... r With respect to the ground tilt angle γ r The ratio of cosine values, the azimuth width W of the wavenumber support area. ga The azimuth width W of the wavenumber support region in the oblique plane. a With azimuth angle γ a The ratio of cosine values.

[0080] S208. Determine the ground distance resolution based on the ground distance width of the wavenumber support area, and determine the ground azimuth resolution based on the ground azimuth width of the wavenumber support area.

[0081] Specifically, according to The ground range resolution, i.e., the range resolution of the wavenumber support area of ​​the echo signal on the ground plane, can be calculated, and based on... The ground azimuth resolution, i.e. the azimuth resolution of the wavenumber support area of ​​the echo signal on the ground plane, can be calculated.

[0082] S209. Based on the scene coordinate system under frontal or oblique view, the grid setting parameters, ground distance resolution, and ground azimuth resolution, arrange the grid in the scene coordinate system to obtain the scene coordinate system grid.

[0083] Specifically, based on the scene coordinate system established in step S205 under frontal or side view, or the scene coordinate system established in step S206 under oblique view, the grid setting parameters, and the ground distance resolution and ground azimuth resolution determined in step S208, a grid is arranged in the scene coordinate system to obtain the scene coordinate system grid.

[0084] The grid is arranged in the scene coordinate system. Using Digital Elevation Model (DEM) information, the grid center point and its corresponding tangent plane are obtained from GPS information and beam pointing. Based on the scene center point and the scene coordinate system, the imaging grid direction and grid spacing are set, with the grid spacing set to be smaller than the ground distance resolution ρ. gr and ground azimuth resolution ρ ga The latitude and longitude values ​​corresponding to each grid point can be calculated based on the distance interval, azimuth interval, and scene center point of the imaging grid, thereby completing the grid layout.

[0085] A flexible, non-orthogonal two-dimensional scene coordinate system grid can be custom-built based on the imaging grid settings. This method of building the scene coordinate system grid is applicable to various observation modes, making it more widely applicable. The imaging area can be flexibly selected, allowing for customization of the imaging process. The scene coordinate system grid is based on the surface imaging grid, making the coordinate system more closely fit the surface and more adaptable to terrain, suitable for various complex imaging geometric conditions.

[0086] S210. Project the spatial wavenumber center vector onto the u-axis and v-axis of the scene coordinate system grid, respectively, to obtain the u-axis component and v-axis component of the wavenumber center vector.

[0087] Specifically, the spatial wavenumber center vector is projected onto the u-axis and v-axis of the scene coordinate system grid obtained in step S209, respectively, to obtain the u-axis component of the wavenumber center vector, which is the projection vector of the spatial wavenumber center vector of the point target onto the u-axis of the scene coordinate system, and the v-axis component of the wavenumber center vector, which is the projection vector of the spatial wavenumber center vector of the point target onto the v-axis of the scene coordinate system.

[0088] Let e u and e v These are the unit vectors of the u-axis and v-axis of the scene coordinate system, respectively, with an included angle of θ. (In frontal and side view mode) In cases of strabismus If the spatial wavenumber center vector of the point target is projected onto the u-axis and v-axis of the scene coordinate system grid, then:

[0089] Project the spatial wavenumber center vector onto the u-axis and v-axis of the scene coordinate system grid according to the following formula:

[0090]

[0091]

[0092] Among them, K uc The u-axis component of the wavenumber center vector. Let e ​​be the spatial wavenumber center vector at point target P. u K is the unit vector of the u-axis of the scene coordinate system mesh. vc e is the v-axis component of the wavenumber center vector. v f is the unit vector of the v-axis of the scene coordinate system mesh. c denoted as the signal carrier frequency, c as the speed of light, S as the satellite position, and P as the point target position.

[0093] S211. Project the spatial wavenumber vector onto the u-axis and v-axis of the scene coordinate system grid to obtain the u-axis component and v-axis component of the spatial wavenumber vector.

[0094] Specifically, the spatial wavenumber vector is projected onto the u-axis and v-axis of the scene coordinate system grid according to the following formula:

[0095]

[0096]

[0097] Among them, K u This is the projection vector of the spatial wavenumber vector onto the u-axis of the scene coordinate system grid. Let e ​​be the space wavenumber vector. u K is the unit vector of the u-axis of the scene coordinate system mesh.v This is the projection vector of the spatial wavenumber vector onto the v-axis of the scene coordinate system grid. For, e v f is the unit vector of the v-axis of the scene coordinate system mesh. c For the signal carrier frequency, f r Here, is the range frequency, c is the speed of light, S is the satellite position, and P is the point target position.

[0098] Figure 4 This is a schematic diagram of the scene coordinate system mesh setting according to an embodiment of the present invention. Figure (a) shows the scene coordinate system mesh in three-dimensional space, and Figure (b) shows the scene coordinate system mesh in a two-dimensional image. u and v are the two coordinate axes of the scene coordinate system mesh, e u and e v These are its basis vectors, namely e u and e v These are the unit vectors of the u-axis and v-axis in the scene coordinate system, respectively, and their included angle when viewed from the side. It is 90°, in the case of strabismus.

[0099] S212. Determine the azimuth wavenumber domain and the range wavenumber domain based on the u-axis component of the wavenumber center vector, the v-axis component of the wavenumber center vector, the u-axis component of the spatial wavenumber vector, and the v-axis component of the spatial wavenumber vector.

[0100] Specifically, based on the u-axis component of the wavenumber center vector, the v-axis component of the wavenumber center vector, the u-axis component of the spatial wavenumber vector, the v-axis component of the spatial wavenumber vector, and the following first formula, the azimuth wavenumber domain and the range wavenumber domain are determined:

[0101]

[0102]

[0103] Among them, K u ′ represents the azimuth wavenumber domain, K u K represents the u-axis component of the space wavenumber vector. uc K is the u-axis component of the wavenumber center vector. rc R(t) is the center vector of the space beam. a ;u,v) is in t a The slant range K from the satellite to the point target at any given time. v ′ represents the range wavenumber domain of the point target, K v K represents the v-axis component of the space wavenumber vector. vc is the v-axis component of the wavenumber center vector.

[0104] S213. Determine the first-step compression function based on the u-axis component and v-axis component of the wavenumber center vector, and use the first-step compression function to compress the echo signal to obtain the signal after the first-step compression.

[0105] To uniformly move the wavenumber spectral support range of all points and targets in the imaging scene coordinate system grid to the grid center, a time-domain compression function, i.e., the first-step spectral compression function, is constructed based on the spatial variation of the spectral center. Specifically, the first-step compression function is determined based on the u-axis and v-axis components of the wavenumber center vector. This first-step compression function is then used to compress the echo signal, yielding the signal after the first-step compression, including:

[0106] The first step compression function is determined based on the u-axis component and v-axis component of the wavenumber center vector, as well as the following second formula:

[0107]

[0108] H sc-1 =exp{jΦ sc-1}

[0109] Φ sc-1 =∫K uc du+K vc dv=K rc R(t a ;u,v)+C

[0110] Wherein, S(t) r ,t a )·H sc-1 For the signal after the first compression step, S(t) r ,t a ) represents the echo signal, W P (t a ) represents the orientation window, t a Where t is slow time, c is the speed of light, B is bandwidth, and t is slow time. r To save time, R s The slant distance of the synthetic aperture center is denoted by j, where j is an imaginary number, λ is the signal wavelength, and K is the slant distance. rc R(t) is the center vector of the space beam. a ;u,v) is in t a The slant range Φ from the satellite to the point target at any given time. sc-1 For phase, K uc K is the u-axis component of the wavenumber center vector. vc Let v be the v-axis component of the wavenumber center vector, and C be an arbitrary constant;

[0111] The echo signal is substituted into the second formula for compression, that is, the echo signal is compressed in the time domain to obtain the signal after the first step of compression.

[0112] S214. Determine the second-step compression function based on the azimuth wavenumber domain and the range wavenumber domain, and use the second-step compression function to compress the signal after the first-step compression to obtain the signal after two-step compression.

[0113] To align the support regions of the wavenumber spectrum in different range frequency domains, a second-step compression function is determined based on the azimuth and range wavenumber domains. Specifically, the second-step compression function is determined based on the azimuth and range wavenumber domains, as well as the following third formula:

[0114] H sc-2 =exp{jΦ sc-2}

[0115]

[0116]

[0117] Among them, H sc-2 This is the compression function for the second step, where j is an imaginary number and Φ is... sc-2 For phase, K′ y K′ represents the range of the distance-wavenumber support region for the target point after the first step of compression. u For the azimuth wavenumber domain, v ref For the distance from the center unit, K′ v For the range wavenumber domain, R(t) is the spatially varying tilt angle of the spectrum. a ;u,v) is in t a The slant distance from the satellite to the target point at any given time.

[0118] The signal compressed in the first step is compressed using the second compression function. This involves using the range of the target point after the first compression, which is the range of the target point after time-domain compression. The range of the target point after the first compression, which is the range of the target point after time-domain compression, is substituted into the third formula above for compression. In other words, the echo signal is compressed in the range, frequency, azimuth, and time domains to obtain the signal after two compression steps.

[0119] S215. The signal after two-step compression is upsampled and subjected to inverse two-step spectral compression to obtain sub-aperture images at each level.

[0120] S216. Overlay two adjacent sub-aperture images from each level of sub-aperture to obtain multiple distance sub-scene images.

[0121] Specifically, the two adjacent sub-aperture images obtained in step S215 are superimposed to obtain the next level sub-image. The next level sub-images are then superimposed to each other until each level sub-aperture image is synthesized into a single sub-image, i.e., multiple distance sub-scene images.

[0122] Multiple distance sub-scene images are images with complete orientation, but incomplete distance orientation.

[0123] S217. Stitch together multiple range sub-scene images to obtain a complete SAR image.

[0124] Specifically, the multiple range sub-scene images obtained in step S216 are stitched together to obtain a complete SAR image, which is the final imaging result. This yields complete images in both the azimuth and range directions.

[0125] This invention first calculates the analytical expression for the wavenumber support region in the large oblique-view mode of medium- and high-orbit SAR, based on the complex imaging geometry. Secondly, considering the characteristics of ultra-wide swath and ultra-long synthetic aperture time in medium- and high-orbit SAR imaging, a fast temporal imaging algorithm is proposed, which can effectively process imaging data and meet the imaging requirements of medium- and high-orbit SAR. Finally, the proposed imaging algorithm, based on the arrangement of the imaging grid and considering the non-fixed radar beam footprint and its inconsistency with the direction of the ground sidelobes, can customize and establish a flexible, non-orthogonal two-dimensional scene coordinate system under the complex imaging geometry conditions of medium- and high-orbit SAR imaging in different orbital segments, downward viewing angles, and oblique viewing angles, thus meeting the imaging processing requirements of medium- and high-orbit SAR in different imaging modes.

[0126] The effectiveness of the complete SAR image obtained by this invention can be further illustrated by the following simulation experiments.

[0127] Experiment 1:

[0128] This simulation experiment utilizes the fast temporal imaging method for medium- and high-orbit SAR based on a custom scene coordinate system proposed in this invention. Simulation parameters are set according to the medium- and high-orbit SAR satellites, and point targets are set for imaging processing. Specific simulation parameters are shown in Table 1.

[0129] Table 1 Simulation parameters of the fast temporal imaging method for medium- and high-orbit SAR based on a custom scene coordinate system

[0130]

[0131] The fast temporal imaging method for medium- and high-orbit SAR based on a custom scene coordinate system proposed in this invention was used to conduct simulation experiments on point targets in medium- and high-orbit SAR imaging mode, and the point response function profile of the selected point targets was obtained. Figure 5This is a cross-sectional view of the point response function of a point target according to an embodiment of the present invention, as shown below. Figure 5 As shown in the figure, Figure (a) is a two-dimensional contour map, Figure (b) is an azimuth sidelobe profile, and Figure (c) is a range sidelobe profile. From the azimuth and range sidelobe profiles corresponding to the point target, it can be seen that the sidelobe profiles of the selected point target are good in both the azimuth and range directions, and there is no phenomenon of excessively large sidelobes. This indicates that the time-domain imaging algorithm proposed in this invention can achieve accurate focusing of medium and high orbit SAR data in the mode of long synthetic aperture time and large scene.

[0132] Experiment 2:

[0133] This simulation experiment utilizes the fast temporal imaging method for medium-to-high orbit SAR based on a custom scene coordinate system proposed in this invention. It uses stitched LEO SAR images to simulate a medium-to-high orbit SAR imaging processing scene. Simulation parameters are set with a satellite data acquisition time of 450 seconds. The scene is established on the Earth's surface, with a size of 85km × 85km. Considering the data volume of raw and intermediate data during imaging processing, as well as the computational power of processing nodes, the entire image is divided into several or dozens of blocks, and each block is processed sequentially in parallel. In this simulation experiment, the SAR data is processed using the method of this invention to verify the imaging focusing performance of this method in a simulated large scene, such as... Figure 6 As shown.

[0134] Figure 6 This is a schematic diagram of the fast temporal imaging results of medium-to-high orbit SAR based on a custom scene coordinate system according to an embodiment of the present invention. The imaging processing results of an 85km × 85km scene are shown using simulation data. Figure 6 The distribution of rivers, mountains, bridges, and even cargo ships is clearly visible, indicating good image focus quality and no aliasing throughout the scene. This demonstrates that the generalized fast Cartesian decomposition back projection imaging algorithm proposed in this invention can achieve accurate focusing of medium- and high-orbit SAR data in oblique-view and large-scene modes. Furthermore, Experiment 2 also verified the processing speed of this imaging algorithm, achieving an imaging processing time of 35 seconds on a single Tesla A100 GPU, showcasing the algorithm's high efficiency in processing large amounts of medium- and high-orbit SAR data.

[0135] In summary, the entire simulation experiment verified the correctness, effectiveness, and efficiency of the complete SAR image obtained by this invention.

[0136] The above are merely specific embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A fast temporal imaging method for medium- and high-orbit SAR based on a custom scene coordinate system, characterized in that, The method includes: Construct an echo signal model; An adaptive scene coordinate system is established based on the imaging mode, and a grid is arranged within this scene coordinate system to obtain the scene coordinate system grid. The ground sidelobe direction is the sidelobe direction of the point response function of the echo signal on the imaging ground plane. The scene coordinate system includes two vectors located on the surface tangent. u shaft and v axis; The space wavenumber center vector and the space wavenumber vector are respectively located in the scene coordinate system grid. u shaft and v Projecting onto the axis determines the u-axis component and v-axis component of the corresponding wavenumber center vector, as well as the azimuth wavenumber domain and range wavenumber domain. The first-step compression function is determined based on the u-axis component and the v-axis component of the wavenumber center vector, and the echo signal is compressed using the first-step compression function to obtain the signal after the first-step compression. The second compression function is determined based on the azimuth wavenumber domain and the range wavenumber domain, and the signal after the first compression is compressed using the second compression function to obtain the signal after two compression steps. The signal after two-step compression is upsampled and then subjected to inverse two-step spectral compression to obtain sub-aperture images at each level. The sub-aperture images at each level are then superimposed and stitched together to obtain a complete SAR image.

2. The method according to claim 1, characterized in that, The ground sidelobe direction includes the ground distance sidelobe direction and the ground azimuth sidelobe direction. The imaging mode includes a frontal side-view imaging mode, a large oblique-view imaging mode, and a small oblique-view imaging mode. The step of adaptively establishing a scene coordinate system according to the imaging mode and arranging a mesh in the scene coordinate system to obtain a scene coordinate system mesh includes: Based on the satellite's parameter orientation and the surface tangent plane normal vector, determine the direction of the ground azimuth sidelobe and the direction of the ground distance sidelobe; When the imaging mode is the front-side view imaging mode, the scene coordinate system is... u The axis direction is set to be consistent with the direction of the ground azimuth sidelobe, and the scene coordinate system is... v The axis direction is set to be consistent with the direction of the ground distance from the side lobe to establish the scene coordinate system under the front side view; When the imaging mode is the large strabismus imaging mode and the small strabismus imaging mode, the scene coordinate system is... u The direction of the axial direction is set to be consistent with the direction of the ground azimuth sidelobe. The orthogonal direction of the u-axis is obtained according to the right-hand rule. The scene coordinate system is then... u The direction orthogonal to the axis is defined as the scene coordinate system. v The axial direction is used to establish the scene coordinate system under the oblique view; Based on the scene coordinate system under the frontal or oblique view, the grid setting parameters, the ground distance resolution, and the ground azimuth resolution, a grid is arranged in the scene coordinate system to obtain the scene coordinate system grid. The ground distance resolution is the distance resolution of the wavenumber support region of the echo signal on the ground plane, and the ground azimuth resolution is the azimuth resolution of the wavenumber support region of the echo signal on the ground plane.

3. The method according to claim 2, characterized in that, The satellite's parameter orientation includes the satellite's beam center direction and velocity direction. Determining the ground azimuth sidelobe direction and the ground distance sidelobe direction based on the satellite's parameter orientation and the surface tangent plane normal vector includes: Based on the beam center pointing of the satellite and the velocity direction of the satellite, determine the direction of the orthogonal component of the satellite's velocity direction relative to the beam center pointing of the satellite; The azimuth sidelobe direction of the oblique plane is set to be parallel to the direction of the orthogonal component, and the distance sidelobe direction of the oblique plane is set to be parallel to the beam center direction of the satellite. The azimuth sidelobe direction of the oblique plane is the azimuth sidelobe direction of the point response function of the echo signal on the imaging oblique plane, and the distance sidelobe direction of the oblique plane is the distance sidelobe direction of the point response function of the echo signal on the imaging oblique plane. The ground azimuth sidelobe direction is set to be parallel to the cross product of the oblique plane distance sidelobe direction and the surface tangent plane normal vector, and the ground distance sidelobe direction is set to be parallel to the cross product of the oblique plane azimuth sidelobe direction and the surface tangent plane normal vector.

4. The method according to claim 3, characterized in that, Before arranging a mesh in the scene coordinate system according to the scene coordinate system, mesh setting parameters, ground distance resolution, and ground azimuth resolution to obtain the scene coordinate system mesh, the method further includes: The wavenumber support region's ground range width is determined based on the oblique plane distance to the sidelobe direction, the ground distance to the sidelobe direction, and the oblique plane distance resolution. Similarly, the wavenumber support region's ground azimuth width is determined based on the oblique plane azimuth sidelobe direction, the ground azimuth sidelobe direction, and the oblique plane azimuth resolution. The oblique plane distance resolution is the range resolution of the wavenumber support region of the echo signal on the imaging oblique plane. The oblique plane azimuth resolution is the azimuth resolution of the wavenumber support region of the echo signal on the imaging oblique plane. The wavenumber support region's ground range width is the range width of the wavenumber support region of the echo signal on the ground plane. The wavenumber support region's ground azimuth width is the azimuth width of the wavenumber support region of the echo signal on the ground plane. The ground distance resolution is determined based on the ground distance width of the wavenumber support area, and the ground azimuth resolution is determined based on the ground azimuth width of the wavenumber support area.

5. The method according to claim 4, characterized in that, The step of determining the ground range width of the wavenumber support region based on the oblique plane distance to the sidelobe direction, the ground distance to the sidelobe direction, and the oblique plane distance resolution, and determining the ground azimuth width of the wavenumber support region based on the oblique plane azimuth sidelobe direction, the ground azimuth sidelobe direction, and the oblique plane azimuth resolution, includes: The ground-distance inclination angle is determined based on the inverse cosine of the product of the product of the oblique plane distance to the sidelobe direction and the ground distance to the sidelobe direction. The ground-distance inclination angle is the angle between the oblique plane distance to the sidelobe direction and the ground distance to the sidelobe direction. The azimuth tilt angle is determined based on the inverse cosine of the product of the azimuth sidelobe direction of the oblique plane and the azimuth sidelobe direction of the ground. The azimuth tilt angle is the angle between the azimuth sidelobe direction of the oblique plane and the azimuth sidelobe direction of the ground. The wavenumber support region is determined by the oblique plane range resolution and the wavenumber support region is determined by the oblique plane azimuth resolution. The wavenumber support region oblique plane range width is the range width of the wavenumber support region of the echo signal on the imaging oblique plane, and the wavenumber support region oblique plane azimuth width is the azimuth width of the wavenumber support region of the echo signal on the imaging oblique plane. The distance width of the wavenumber support zone is determined by the ratio of the distance width of the inclined plane to the cosine of the distance inclination angle, and the azimuth width of the wavenumber support zone is determined by the ratio of the azimuth width of the inclined plane to the cosine of the azimuth inclination angle.

6. The method according to claim 1, characterized in that, The spatial wavenumber center vector and the spatial wavenumber vector of the point target are respectively located in the scene coordinate system grid. u shaft and v Projecting onto the axis determines the u-axis component and v-axis component of the corresponding wavenumber center vector, the azimuth wavenumber domain of the point target, and the range wavenumber domain of the point target, including: The spatial wavenumber center vectors are respectively placed on the scene coordinate system grid. u shaft and v Projecting onto the axis yields the u-axis component and the v-axis component of the wavenumber center vector; The spatial wavenumber vectors are respectively placed on the scene coordinate system grid. u shaft and v Projecting onto the axis yields the u-axis component and v-axis component of the spatial wavenumber vector. The u-axis component of the spatial wavenumber vector is the spatial wavenumber vector within the scene coordinate system grid. u The projection vector of the axis, wherein the v-axis component of the spatial wavenumber vector is the spatial wavenumber vector in the scene coordinate system grid. v The projection vector of the axis; Based on the u-axis component of the wavenumber center vector, the v-axis component of the wavenumber center vector, the u-axis component of the spatial wavenumber vector, the v-axis component of the spatial wavenumber vector, and the following first formula, the azimuth wavenumber domain and the range wavenumber domain are determined: in, For the azimuth wavenumber domain, The u-axis component of the space wavenumber vector is... The u-axis component of the wavenumber center vector is... The center vector of the space beam. In order to be in t a The slant distance from the satellite to the point target at any given time. For the range wavenumber domain, The v-axis component of the space wavenumber vector is... The v-axis component of the wavenumber center vector is given.

7. The method according to claim 6, characterized in that, The step of determining the first-step compression function based on the u-axis component and the v-axis component of the wavenumber center vector, and then using the first-step compression function to compress the echo signal to obtain the signal after one-step compression, includes: The first step compression function is determined based on the u-axis component of the wavenumber center vector, the v-axis component of the wavenumber center vector, and the following second formula: in, This refers to the signal compressed in the first step. The echo signal, For orientation windows, For slow time, At the speed of light, For bandwidth, To save time, The slope distance of the center of the synthetic aperture. It is an imaginary number. For the signal wavelength, The center vector of the space beam. In order to be in t a The slant distance from the satellite to the point target at any given time. For phase, The u-axis component of the wavenumber center vector is... The v-axis component of the wavenumber center vector is... It is an arbitrary constant; The echo signal is substituted into the second formula for compression to obtain the signal after the first step of compression.

8. The method according to claim 6, characterized in that, The step of determining the second compression function based on the azimuth wavenumber domain and the range wavenumber domain includes: The second-step compression function is determined based on the azimuth wavenumber domain, the range wavenumber domain, and the following third formula: in, This is the compression function for the second step. It is an imaginary number. For phase, The range of the distance wavenumber support region for the target point after the first step of compression. For the azimuth wavenumber domain, For distance from the center unit, For the range wavenumber domain, The spatial tilt angle of the spectrum. In order to be in t a The slant distance from the satellite to the target point at any given time.

9. The method according to claim 1, characterized in that, The process of superimposing and stitching the sub-aperture images at each level to obtain a complete SAR image includes: By overlaying two adjacent sub-aperture images from each level of sub-aperture, multiple distance sub-scene images are obtained; The multiple distance sub-scene images are stitched together to obtain the complete SAR image.

Images