A Spaceborne Along-Track-OffSAR Multi-Mode Integrated Frequency Domain Imaging Method

Abstract

This invention provides a multi-mode integrated frequency-domain imaging method for spaceborne non-tracking SAR. This method unifies the time-frequency relationship of multi-mode SAR signals and establishes a precise two-dimensional spatial variation model of the Doppler parameter range and azimuth. This method enables high-precision imaging in strip, sliding, and TOPS modes for spaceborne non-tracking SAR. This method addresses the issues of signal time-frequency inconsistency, Doppler parameter coupling, and severe spatial variation in spaceborne non-tracking SAR multi-mode imaging, addressing the shortcomings of existing technologies.

Description

Technical Field

[0001] The present invention belongs to the technical field of synthetic aperture radar, and in particular relates to a spaceborne non-tracking SAR multi-mode integrated frequency domain imaging method. Background Art

[0002] Synthetic Aperture Radar (SAR) is an active microwave remote sensing device with the characteristics of all-day, all-weather, two-dimensional high resolution, and strong penetration. It is an effective means of earth remote sensing and is of great significance to application fields such as disaster warning, environmental monitoring, and military reconnaissance.

[0003] Off-track imaging is a unique operating mode of spaceborne SAR. Compared to traditional spaceborne SAR, it generates a swath directly along the target terrain by continuously adjusting the beam pointing in both elevation and azimuth, rather than the traditional swath generation along the satellite's orbit. This fundamentally reduces echo data redundancy when imaging certain "off-track" scenarios, such as seismic zones and coastlines, significantly improving the observation efficiency of spaceborne SAR for narrow and long scenes, offering unique advantages. Conventional SAR modes such as swath, sliding focus, and Terrain Observation by Progressive Scans (TOPS) can all be applied to spaceborne SAR. Therefore, spaceborne SAR possesses multimodal characteristics. However, due to its more complex spatial configuration compared to traditional imaging modes, the coupling and spatial variation of imaging Doppler parameters are more severe, making traditional frequency-domain imaging algorithms inadequate for imaging. While time-domain imaging algorithms offer high accuracy and are suitable for spaceborne SAR imaging, they require the construction of filters for each target point, resulting in a significant computational load and algorithmic inefficiency.

[0004] Therefore, a multi-mode integrated frequency domain imaging method for spaceborne non-along-track SAR is needed. Summary of the Invention

[0005] To solve the above problems, the present invention provides a multi-mode integrated frequency domain imaging method for spaceborne non-tracking SAR, which can achieve high-precision imaging of spaceborne non-tracking SAR in strip mode, TOPS mode and sliding focusing mode.

[0006] A spaceborne non-along-track SAR multi-mode integrated frequency domain imaging method comprises the following steps:

[0007] S1: Acquire the multi-mode echo range-frequency domain signal S1 (f r ,t a );

[0008] S2: Based on the slant range history R(ta ) constructs a range migration azimuth space-varying model, and then corrects the multi-mode echo range frequency domain signal S1 (f r ,t a ) exists in the range migration azimuth space variation, and the multi-mode echo range frequency domain signal S2(f r ,t a );

[0009] S3:to S2(f r ,t a ) performs azimuth Fourier transform and obtains S2(f r ,t a )'s two-dimensional spectrum S3(f r ,f a );

[0010] S4: Based on the two-dimensional spectrum S3 (f r ,f a ) constructing a distance-azimuth two-dimensional space-varying model, and then obtaining a distance-direction nonlinear frequency modulation scaling factor and an azimuth-direction nonlinear frequency modulation scaling factor according to the distance-azimuth two-dimensional space-varying model;

[0011] S5: Use the distance-direction nonlinear frequency modulation factor to adjust the two-dimensional spectrum S3 (f r ,f a ) performs nonlinear frequency modulation scaling in the range direction to obtain the first scaling result, and then performs range migration correction and range compression on the first scaling result to obtain the signal S4 (f r ,f a );

[0012] S6: The azimuth nonlinear frequency modulation factor is used to adjust the phase-compensated S4 (f r ,f a ) performs nonlinear frequency modulation scaling in azimuth to obtain a second scaling result, and then performs azimuth compression on the second scaling result to obtain a two-dimensional image.

[0013] Furthermore, the multi-mode echo range frequency domain signal S1 (f r ,t a ) is obtained as follows:

[0014] Build phase h1:

[0015]

[0016] Among them, f c is the carrier frequency, f r is the distance frequency, q1 and q2 are the first-order compensation coefficient and the second-order compensation coefficient respectively, c is the speed of light, t ais the imaging moment, j is the imaginary part;

[0017] After performing Fourier transform on the echo, it is multiplied by the phase h1 to obtain the multi-mode echo range frequency domain signal S1 (f r ,t a ).

[0018] Furthermore, the method for constructing the distance migration azimuth space-variant model is:

[0019] Solve using gradient descent method The corresponding azimuth time t p,new , and R(t a ) at t a =t p,new Taylor expansion at this point gives:

[0020] R(t a )=R 0,new +k 2,new (t a -t p,new ) 2 +k 3,new (t a -t p,new ) 3 +k 4,new (t a -t p,new ) 4

[0021] Among them, t a is the imaging moment, R 0,new 、k 2,new 、k 3,new and k 4,new are the zeroth, second, third and fourth order coefficients of Taylor expansion respectively, and the second order coefficient k 2,new There is a distance migration azimuth space variation as follows:

[0022] k 2,new =k 20 +k 21 t p,new

[0023] Among them, k 20 and k 21 are the azimuth zero-order space-varying coefficient and the first-order space-varying coefficient respectively.

[0024] Furthermore, the multi-mode echo range frequency domain signal S1 (f r ,t a The correction method for the range migration and azimuth space variation in ) is:

[0025] Construct the polynomial h2:

[0026]

[0027] Among them, f c is the carrier frequency, f r is the distance frequency, c is the speed of light, t a is the imaging moment, j is the imaginary part, and q3 is the third-order compensation coefficient;

[0028] The multi-mode echo distance frequency domain signal S1(f r ,t a ) is multiplied by the polynomial h2 to obtain the multi-mode echo range frequency domain signal S2(f r ,t a ).

[0029] Furthermore, the distance-azimuth two-dimensional space-variant model is constructed as follows:

[0030] The two-dimensional spectrum S3(f r ,f a ) at the distance frequency f r Performing Taylor expansion at , we get:

[0031]

[0032] Where, are the zeroth to fourth-order coefficients of Taylor expansion, and j is the imaginary part;

[0033] Among them, the zero-order coefficient There is a distance-azimuth two-dimensional space-time variable as follows:

[0034]

[0035] Where, f a is the azimuth frequency, λ is the wavelength, t p is the target synthetic aperture center moment, P 20 (R p ), P 21 (R p ) and P 22 (R p ) are slope distances R p The zero-order, first-order and second-order spatial variation coefficients of the azimuth secondary modulation phase within the range gate, P 30 (R p ), P 31 (R p ) are slope distances R p The zero-order and first-order spatially varying coefficients of the azimuth three-modulation phase within the range gate, P 40 (R p ) is the slope distance R p The azimuth fourth modulation phase coefficient at the range gate; coefficients A1 to A3 are expressed as:

[0036]

[0037] The coefficients K1 to K4 are expressed as:

[0038]

[0039] Among them, q1~q3 are the set first-order to third-order compensation coefficients, k1~k4 are the slant range history R(t a ) is expanded by Taylor series to obtain the first to fourth order coefficients, and:

[0040] R(t a )=R0+k1(t a -t p )+k2(t a -t p ) 2 +k3(t a -t p ) 3 +k4(t a -t p ) 4

[0041] Among them, t a is the imaging moment, R0 is t p The distance between the radar and the target to be measured at any moment;

[0042] First-order coefficient The distance space variation is as follows:

[0043]

[0044] Where, and They are The distance to zero-order, first-order and second-order spatial variation coefficients, R ref is the reference slope distance;

[0045] Second-order coefficients The distance space variation is as follows:

[0046]

[0047] Where, They are The distance to the zero-order and first-order spatial variable coefficients.

[0048] Furthermore, are all about the azimuth frequency f a function, then denote them as

[0049] The method for obtaining the distance nonlinear frequency modulation scaling factor and the azimuth nonlinear frequency modulation scaling factor is:

[0050] Calculate the distance nonlinear frequency modulation scaling factor coefficient C2(f a )、C3(f a )、Y3(f a ):

[0051]

[0052] Where f0 is the Doppler center frequency, for The distance at f0 is the first-order spatial variable coefficient, for The distance to the second-order spatial variation coefficient at f0;

[0053] Then the distance nonlinear frequency modulation scaling factor h4 is as follows:

[0054]

[0055] Where, t r is the distance time;

[0056] Calculate the azimuth nonlinear frequency modulation scaling factor coefficient c2(R p )、c3(R p )、c4(R p )、y3(R p )、y4(R p ):

[0057]

[0058] Where β is a constant greater than 1;

[0059] Then the azimuth nonlinear frequency modulation scaling factor h7 is as follows:

[0060]

[0061] Furthermore, the coefficient is the azimuth frequency f a and slope distance R p function of the reference slope distance R ref The coefficient at Marked as

[0062] The method for obtaining the first scaling result in step S5 is:

[0063] Build phase h3:

[0064]

[0065] The two-dimensional spectrum S3(f r ,f a ) is multiplied by the phase h3 and then a distance inverse Fourier transform is performed, and then the result of the distance inverse Fourier transform is multiplied by the distance nonlinear frequency modulation scaling factor h4 to obtain the first scaling result.

[0066] Furthermore, the signal S4 (f r ,f a ) is obtained as follows:

[0067] Construct phase h5:

[0068]

[0069] Where D2 and D3 are expressed as:

[0070]

[0071] The first scaling result is subjected to range Fourier transform and then multiplied by the phase h5. The product is then subjected to range inverse Fourier transform to obtain the signal S4 (f r ,f a ).

[0072] Furthermore, the method for obtaining the second scaling result in step S6 is:

[0073] Construct phase h6:

[0074] h6=exp[jy3(R p )f r 3 +jy4(R p )f r 3 ]

[0075] The signal S4(f r ,f a ) is multiplied by the phase h6 and then an azimuth inverse Fourier transform is performed, and then the azimuth inverse Fourier transform result is multiplied by the azimuth nonlinear frequency modulation scaling factor h7 to obtain the second scaling result.

[0076] Furthermore, the method for obtaining the two-dimensional image is:

[0077] Build phase h8:

[0078]

[0079] Where d2, d3 and d4 are expressed as:

[0080]

[0081] The second scaling result is subjected to azimuth Fourier transform and then multiplied by the phase h8. The product is then subjected to azimuth inverse Fourier transform to obtain a two-dimensional image.

[0082] Beneficial effects:

[0083] This invention provides a multi-mode integrated frequency-domain imaging method for spaceborne non-tracking SAR (SAR), achieving high-precision imaging in strip, sliding, and TOPS modes. By unifying the time-frequency relationship of multi-mode signals and establishing a precise two-dimensional range-azimuth spatial variation model for Doppler parameters, this method addresses the issues of signal time-frequency inconsistency, imaging Doppler parameter coupling, and severe spatial variation in multi-mode imaging for spaceborne non-tracking SAR, thus addressing the shortcomings of existing technologies. BRIEF DESCRIPTION OF THE DRAWINGS

[0084] Figure 1 A flowchart of a spaceborne non-along-track SAR multi-mode integrated frequency domain imaging method provided by the present invention;

[0085] Figure 2(a) is a schematic diagram of the spatial configuration of a spaceborne non-extended-track SAR operating in strip mode;

[0086] Figure 2(b) is a schematic diagram of the spatial configuration of a spaceborne non-extended-track SAR operating in the sliding-focusing mode;

[0087] Figure 2(c) is a schematic diagram of the spatial configuration of a spaceborne non-extended-track SAR operating in TOPS mode;

[0088] Figure 3 Schematic diagram of unified signal time-frequency relationship after preprocessing;

[0089] Figure 4 This is the imaging result of point targets in the strip mode of spaceborne non-along-track SAR;

[0090] Figure 5 This is the imaging result of point targets in the non-along-track SAR sliding mode;

[0091] Figure 6 This is the imaging result of point targets in the spaceborne non-along-track SAR TOPS mode. DETAILED DESCRIPTION

[0092] In order to enable those skilled in the art to better understand the solution of the present application, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application.

[0093] like Figure 1 As shown, a spaceborne non-along-track SAR multi-mode integrated frequency domain imaging method includes the following steps:

[0094] S1: Acquire the multi-mode echo range-frequency domain signal S1 (f r ,t a );

[0095] It should be noted that the spatial configuration of spaceborne non-extended-track SAR is as follows: Figure 2(a) to Figure 2(c) As shown in the figure, the satellite's mapping swath is no longer parallel to the sub-satellite point track. Instead, by continuously adjusting the pitch and azimuth beam pointing directions, the mapping swath is aligned with the target terrain direction, with an angle of θ with the sub-satellite point track.

[0096] The beam rotates in two dimensions, elevation and azimuth. Assume that at the imaging time t a , the angle between the beam center and the satellite velocity is α(t a ). When operating in strip mode, the angle between the beam center and the satellite velocity remains constant, that is:

[0097]

[0098] When working in the sliding focusing mode, the angle between the beam center and the satellite speed gradually increases, that is:

[0099]

[0100] When operating in TOPS mode, the angle between the beam center and the satellite speed gradually decreases, that is:

[0101]

[0102] At the same time, according to the non-extended SAR spatial configuration, the target's slant range history R(t a ) can be expressed using the Taylor series expansion slope distance model:

[0103] R(t a )=R0+k1(t a -t p )+k2(t a -t p ) 2 +k3(t a -t p ) 3 +k4(t a -t p ) 4

[0104] Where, t p is the target synthetic aperture center moment, R0 is t p The distance between the radar and the target at the moment, k1~k4 are the coefficients of each order of the Taylor series expansion slant range model.

[0105] Depend on Figure 2(a) to Figure 2(c)From the spatial configuration of the spaceborne non-extended-track SAR shown in the figure, it can be seen that at the imaging time t a , the Doppler center frequency f of the target illuminated by the center of the beam dc (t a )for:

[0106]

[0107] Where, v s is the satellite speed and λ is the wavelength.

[0108] By Taylor expanding the above equation, we can get the approximate solution of the Doppler center frequency:

[0109]

[0110] Where, f dc0 and f dc1 are the first-order and second-order coefficients of Taylor expansion respectively:

[0111]

[0112] Where α2 is the angle between the beam center and the satellite velocity at the imaging center moment.

[0113] To achieve spectrum orthogonalization and unify the time-frequency relationship of multi-mode SAR signals, we can first achieve spectrum orthogonalization by removing the motion, and then unify the time-frequency relationship of multi-mode spaceborne non-tracking SAR signals by removing the skew. Specifically, we construct the phase h1:

[0114]

[0115] Where, f c is the carrier frequency; f r is the distance frequency; q1 and q2 are the first-order and second-order compensation coefficients:

[0116]

[0117] The echo is Fourier transformed (FT) in the range direction and then multiplied by the phase h1 to achieve phase h1 compensation and obtain the multi-mode echo range frequency domain signal S1 (f r ,t a ), at this time, the unified signal time-frequency relationship is as follows Figure 3 As shown in the figure, the frequency domain signal expression of the non-tracking SAR multi-mode echo range S1(f r ,t a )for:

[0118]

[0119] Where kr is the frequency modulation slope.

[0120] S2: Based on the slant range history R(t a ) constructs a range migration azimuth space-varying model, and then corrects the multi-mode echo range frequency domain signal S1 (f r ,t a ) exists in the range migration azimuth space variation, and the multi-mode echo range frequency domain signal S2(f r ,t a );

[0121] According to the signal expression S1(f r ,t a ), the slant range history R(t a ) becomes:

[0122]

[0123] Using the gradient descent method, we can solve The corresponding azimuth time t p,new , and put the above formula at t a =t p,new Taylor expansion:

[0124] R(t a )=R 0,new +k 2,new (t a -t p,new ) 2 +k 3,new (t a -t p,new ) 3 +k 4,new (t a -t p,new ) 4

[0125] Where R 0,new 、k 2,new 、k 3,new and k 4,new are the coefficients of each order of Taylor expansion, and k 2,new There is space variation in azimuth:

[0126] k 2,new =k 20 +k 21 t p,new

[0127] Where k 20 and k 21 are the zero-order and first-order spatially varying coefficients in azimuth.

[0128] To this end, the polynomial h2 is introduced to correct the azimuth space variation of range migration:

[0129]

[0130] Where q3 is the third-order compensation coefficient:

[0131]

[0132] Finally, the multi-mode echo range frequency domain signal S1(f r ,t a ) is multiplied by the polynomial h2 to obtain the multi-mode echo range frequency domain signal S2(f r ,t a ).

[0133] S3:to S2(f r ,t a ) performs azimuth Fourier transform and obtains S2(f r ,t a )'s two-dimensional spectrum S3(f r ,f a );

[0134] It should be noted that the signal S1(f r ,t a ) After compensating the polynomial h2, the obtained signal expression S2(f r ,t a )for:

[0135]

[0136] make:

[0137]

[0138] Then the signal expression S2(f r ,t a ) can be rewritten as:

[0139]

[0140] In the formula, K1~K4 all have two-dimensional space variation of distance-azimuth.

[0141] For S2(f r ,t a ) performs azimuth Fourier transform to obtain the signal two-dimensional spectrum S3(f r ,f a ):

[0142]

[0143] Where, f ais the azimuth frequency, A1 to A3 are expressed as:

[0144]

[0145] S4: Based on the two-dimensional spectrum S3 (f r ,f a ) constructing a distance-azimuth two-dimensional space-varying model, and then obtaining a distance-direction nonlinear frequency modulation scaling factor and an azimuth-direction nonlinear frequency modulation scaling factor according to the distance-azimuth two-dimensional space-varying model;

[0146] Furthermore, based on the two-dimensional spectrum S3(f r ,f a ) The specific construction of the distance-azimuth two-dimensional space-variant model is as follows:

[0147] The two-dimensional spectrum S3(f r ,f a ) for f r Taylor expansion gives:

[0148]

[0149] Where, are the coefficients of various orders of Taylor expansion:

[0150]

[0151]

[0152]

[0153]

[0154] in, Related to the azimuth modulation term, a two-dimensional distance-azimuth space-varying model is established:

[0155]

[0156] Where, f a is the azimuth frequency, λ is the wavelength, t p is the target synthetic aperture center moment, P 20 (R p ), P 21 (R p ) and P 22 (R p ) are slope distances R p The zero-order, first-order and second-order spatial variation coefficients of the azimuth secondary modulation phase within the range gate, P 30 (R p ), P 31 (R p ) are slope distances Rp The zero-order and first-order spatially varying coefficients of the azimuth three-modulation phase within the range gate, P 40 (R p ) is the slope distance R p The azimuth quartic modulation phase coefficient at the range gate;

[0157] It is related to the distance migration. Since step S2 corrects the azimuth spatial variation of the migration, its azimuth spatial variation can be ignored. The distance spatial variation model is established:

[0158]

[0159] Where, and They are The distance to zero-order, first-order and second-order spatial variation coefficients, R ref is the reference slope distance.

[0160] It is related to the distance quadratic phase, and its azimuth space variation can be ignored. The distance space variation model is established:

[0161]

[0162] Where, They are The distance to the zero-order and first-order spatial variable coefficients.

[0163] It is related to the cubic phase of distance, and its distance and azimuth spatial variations can be ignored.

[0164] The following describes the calculation method of the nonlinear frequency modulation scaling factor h4 in the range and the nonlinear frequency modulation scaling factor h7 in the azimuth. It should be noted that: are all about the azimuth frequency f a function, so they can be written as

[0165] First, calculate the distance nonlinear frequency modulation coefficient C2(f a )、C3(f a )、Y3(f a ):

[0166]

[0167] Where f0 is the Doppler center frequency, which is usually 0.

[0168] Then the range-direction nonlinear frequency modulation scaling factor h4 is as follows:

[0169]

[0170] Where c is the speed of light, t r is the distance time;

[0171] Then, calculate the azimuth nonlinear frequency modulation scaling factor coefficient c2(R p )、c3(R p )、c4(R p )、y3(R p )、y4(R p ):

[0172]

[0173] Where β is a constant greater than 1;

[0174] Then the azimuth nonlinear frequency modulation scaling factor h7 is as follows:

[0175]

[0176] Where, t a The time of orientation.

[0177] S5: Use the distance-direction nonlinear frequency modulation factor to adjust the two-dimensional spectrum S3 (f r ,f a ) performs nonlinear frequency modulation scaling in the range direction to obtain the first scaling result, and then performs range migration correction and range compression on the first scaling result to obtain the signal S4 (f r ,f a );

[0178] It should be noted that step S5 mainly performs range-direction NCS (Nonlinear Chirp Scaling) to complete range migration correction and range compression. First, the range cubic phase is compensated in the two-dimensional spectrum and pre-filtered. The compensated phase h3 is:

[0179]

[0180] It should be noted that the coefficient is the azimuth frequency f a and slope distance R p function, Indicates the reference slope distance R ref The coefficient at

[0181] The two-dimensional spectrum S3(f r ,f a) is multiplied by the phase h3 and then a distance inverse Fourier transform is performed, and then the result of the distance inverse Fourier transform is multiplied by the distance nonlinear frequency modulation scaling factor h4 to obtain the first scaling result.

[0182] Finally, the first scaling result is subjected to range Fourier transform, the phase h5 is compensated to achieve range migration correction and range compression, and the inverse Fourier transform is performed to transform the signal into the range-Doppler domain, thus obtaining the signal S4 (f r ,f a ). Here, the phase h5 is:

[0183]

[0184] Where D2 and D3 are expressed as:

[0185]

[0186] S6: The azimuth nonlinear frequency modulation factor is used to adjust the phase-compensated S4 (f r ,f a ) performs nonlinear frequency modulation scaling in azimuth to obtain a second scaling result, and then performs azimuth compression on the second scaling result to obtain a two-dimensional image.

[0187] It should be noted that step S6 is mainly to perform azimuth NCS and complete azimuth compression. First, pre-filtering is performed in the range Doppler domain, and the compensated phase h6 is:

[0188] h6=exp[jy3(R p )f r 3 +jy4(R p )f r 3 ]

[0189] Then, the signal S4(f r ,f a ) is multiplied by the phase h6 and then an azimuth inverse Fourier transform is performed, and then the azimuth inverse Fourier transform result is multiplied by the azimuth nonlinear frequency modulation scaling factor h7 to obtain the second scaling result.

[0190] Finally, the second scaling result is subjected to azimuth Fourier transform and then multiplied by the phase h8, and the product is then subjected to azimuth inverse Fourier transform to obtain a two-dimensional image. Here, the phase h8 is:

[0191]

[0192] Where d2, d3 and d4 are expressed as:

[0193]

[0194] The effects of the present invention are further illustrated below through point target simulation tests using three modes of spaceborne non-along-track SAR: strip, sliding focus, and TOPS.

[0195] Experiment 1: Spaceborne non-tracking SAR strip mode point target simulation test

[0196] The simulation parameters of the spaceborne non-along-track SAR strip mode are shown in Table 1.

[0197] Table 1 Simulation parameters of spaceborne non-tracking SAR strip mode

[0198]

[0199]

[0200] The strip pattern is imaged using the multi-mode integrated frequency domain imaging method of the spaceborne non-tracking SAR proposed in this invention. The imaging results are as follows: Figure 4 The upper left, center, and lower right point targets were selected for resolution, peak sidelobe ratio, and integrated sidelobe ratio evaluation. The evaluation results are shown in Table 2.

[0201] Table 1. Evaluation results of point targets in the strip mode of spaceborne non-along-track SAR

[0202]

[0203] Experiment 2: Spaceborne non-along-track SAR point target simulation test in sliding focusing mode

[0204] The simulation parameters of the spaceborne non-along-track SAR sliding mode are shown in Table 3.

[0205] Table 3 Simulation parameters of the spaceborne non-along-track SAR sliding focusing mode

[0206]

[0207]

[0208] The sliding mode is imaged using the multi-mode integrated frequency domain imaging method of the spaceborne non-tracking SAR proposed in this invention. The imaging results are as follows: Figure 5 The upper left, center, and lower right point targets were selected for resolution, peak sidelobe ratio, and integrated sidelobe ratio evaluation. The evaluation results are shown in Table 4.

[0209] Table 4. Evaluation results of point targets in the non-along-track SAR sliding focus mode

[0210]

[0211] Experiment 3: Spaceborne non-tracking SAR TOPS mode point target simulation test

[0212] The simulation parameters of spaceborne non-along-track SAR TOPS mode are shown in Table 5.

[0213] Table 5 Spaceborne non-tracking SAR TOPS mode simulation parameters

[0214]

[0215]

[0216] The TOPS mode is imaged using the spaceborne non-tracking SAR multi-mode integrated frequency domain imaging method proposed in this invention. The imaging results are as follows: Figure 6 The upper left, center, and lower right point targets were selected for resolution, peak sidelobe ratio, and integrated sidelobe ratio evaluation. The evaluation results are shown in Table 6.

[0217] Table 6. Point target evaluation results of spaceborne non-along-track SAR TOPS mode

[0218]

[0219] As can be seen, the present invention provides a method for integrated frequency-domain imaging of multi-mode spaceborne non-tracking SAR (SAR), unifying the time-frequency relationship of multi-mode SAR signals and establishing a precise two-dimensional spatial variation model of the Doppler parameter range-azimuth. This method enables high-precision imaging in strip, sliding, and TOPS modes for spaceborne non-tracking SAR. This method addresses the issues of signal time-frequency inconsistency, imaging Doppler parameter coupling, and severe spatial variation in multi-mode spaceborne non-tracking SAR imaging, thus addressing the shortcomings of existing technologies.

[0220] Of course, the present invention may have many other embodiments. Without departing from the spirit and essence of the present invention, those skilled in the art may of course make various corresponding changes and modifications based on the present invention, but these corresponding changes and modifications should all fall within the scope of protection of the claims attached to the present invention.

Claims

1. A spaceborne non-along-track SAR multi-mode integrated frequency domain imaging method, characterized in that: The following steps are involved: S1: Acquire the multi-mode echo range-frequency domain signal S1 (f r ,t a ); S2: Based on the slant range history R(t a ) constructs a range migration azimuth space-varying model, and then corrects the multi-mode echo range frequency domain signal S1 (f r ,t a ) exists in the range migration azimuth space variation, and the multi-mode echo range frequency domain signal S2(f r ,t a ); S3:to S2(f r ,t a ) performs azimuth Fourier transform and obtains S2(f r ,t a )'s two-dimensional spectrum S3(f r ,f a ); S4: Based on the two-dimensional spectrum S3 (f r ,f a ) constructing a distance-azimuth two-dimensional space-varying model, and then obtaining a distance-direction nonlinear frequency modulation scaling factor and an azimuth-direction nonlinear frequency modulation scaling factor according to the distance-azimuth two-dimensional space-varying model; S5: Use the distance-direction nonlinear frequency modulation factor to adjust the two-dimensional spectrum S3 (f r ,f a ) performs nonlinear frequency modulation scaling in the range direction to obtain the first scaling result, and then performs range migration correction and range compression on the first scaling result to obtain the signal S4 (f r ,f a ); S6: The azimuth nonlinear frequency modulation factor is used to adjust the phase-compensated S4 (f r ,f a ) performs nonlinear frequency modulation scaling in azimuth to obtain a second scaling result, and then performs azimuth compression on the second scaling result to obtain a two-dimensional image.

2. The spaceborne non-along-track SAR multi-mode integrated frequency domain imaging method according to claim 1, characterized in that: The multi-mode echo range frequency domain signal S1 (f r ,t a ) is obtained as follows: Build phase h1: Among them, f c is the carrier frequency, f r is the distance frequency, q1 and q2 are the first-order compensation coefficient and the second-order compensation coefficient respectively, c is the speed of light, t a is the imaging moment, j is the imaginary part; After performing Fourier transform on the echo, it is multiplied by the phase h1 to obtain the multi-mode echo range frequency domain signal S1 (f r ,t a ).

3. The method for spaceborne non-along-track SAR multi-mode integrated frequency domain imaging according to claim 1, wherein: The method for constructing the distance migration azimuth space-variant model is as follows: Solve using gradient descent method The corresponding azimuth time t p,new , and R(t a ) at t a =t p,new Taylor expansion at this point gives: R(t a )=R 0,new +k 2,new (t a -t p,new ) 2 +k 3,new (t a -t p,new ) 3 +k 4,new (t a -t p,new ) 4 Among them, t a is the imaging moment, R 0,new 、k 2,new 、k 3,new and k 4,new are the zeroth, second, third and fourth order coefficients of Taylor expansion respectively, and the second order coefficient k 2,new There is a distance migration azimuth space variation as follows: k 2,new =k 20 +k 21 t p,new Among them, k 20 and k 21 are the azimuth zero-order space-varying coefficient and the first-order space-varying coefficient respectively.

4. The method for spaceborne non-along-track SAR multi-mode integrated frequency domain imaging according to claim 1, wherein: The multi-mode echo range frequency domain signal S1 (f r ,t a The correction method for the range migration and azimuth space variation in ) is: Construct the polynomial h2: Among them, f c is the carrier frequency, f r is the distance frequency, c is the speed of light, t a is the imaging moment, j is the imaginary part, and q3 is the third-order compensation coefficient; The multi-mode echo distance frequency domain signal S1(f r ,t a ) is multiplied by the polynomial h2 to obtain the multi-mode echo range frequency domain signal S2(f r ,t a ).

5. The method for spaceborne non-along-track SAR multi-mode integrated frequency domain imaging according to claim 1, wherein: The method for constructing the distance-azimuth two-dimensional space-variant model is as follows: The two-dimensional spectrum S3(f r ,f a ) at the distance frequency f r Performing Taylor expansion at , we get: Where, are the zeroth to fourth-order coefficients of Taylor expansion, and j is the imaginary part; Among them, the zero-order coefficient There is a distance-azimuth two-dimensional space-time variable as follows: Where, f a is the azimuth frequency, λ is the wavelength, t p is the target synthetic aperture center moment, P 20 (R p ), P 21 (R p ) and P 22 (R p ) are slope distances R p The zero-order, first-order and second-order spatial variation coefficients of the azimuth secondary modulation phase within the range gate, P 30 (R p ), P 31 (R p ) are slope distances R p The zero-order and first-order spatially varying coefficients of the azimuth three-modulation phase within the range gate, P 40 (R p ) is the slope distance R p The azimuth fourth modulation phase coefficient at the range gate; coefficients A1 to A3 are expressed as: The coefficients K1 to K4 are expressed as: Among them, q1~q3 are the set first-order to third-order compensation coefficients, k1~k4 are the slant range history R(t a ) is expanded by Taylor series to obtain the first to fourth order coefficients, and: R(t a )=R0+k1(t a -t p )+k2(t a -t p ) 2 +k3(t a -t p ) 3 +k4(t a -t p ) 4 Among them, t a is the imaging moment, R0 is t p The distance between the radar and the target to be measured at any moment; First-order coefficient The distance space variation is as follows: Where, and They are The distance to zero-order, first-order and second-order spatial variation coefficients, R ref is the reference slope distance; Second-order coefficients The distance space exists as follows: Where, They are The distance to the zero-order and first-order spatial variable coefficients.

6. The method for spaceborne non-along-track SAR multi-mode integrated frequency domain imaging according to claim 5, characterized in that: are all about the azimuth frequency f a function, then denote them as The method for obtaining the distance nonlinear frequency modulation scaling factor and the azimuth nonlinear frequency modulation scaling factor is: Calculate the distance nonlinear frequency modulation scaling factor coefficient C2(f a )、C3(f a )、Y3(f a ): Where f0 is the Doppler center frequency, for The distance at f0 is the first-order spatial variable coefficient, for The distance to the second-order spatial variation coefficient at f0; Then the range-direction nonlinear frequency modulation scaling factor h4 is as follows: Where, t r is the distance time; Calculate the azimuth nonlinear frequency modulation scaling factor coefficient c2(R p )、c3(R p )、c4(R p )、y3(R p )、y4(R p ): Where β is a constant greater than 1; Then the azimuth nonlinear frequency modulation scaling factor h7 is as follows:

7. The method for spaceborne non-along-track SAR multi-mode integrated frequency domain imaging according to claim 6, wherein: coefficient is the azimuth frequency f a and slope distance R p function of the reference slope distance R ref The coefficient at Marked as The method for obtaining the first scaling result in step S5 is: Build phase h3: The two-dimensional spectrum S3(f r ,f a ) is multiplied by the phase h3 and then a distance inverse Fourier transform is performed, and then the result of the distance inverse Fourier transform is multiplied by the distance nonlinear frequency modulation scaling factor h4 to obtain the first scaling result.

8. The spaceborne non-along-track SAR multi-mode integrated frequency domain imaging method according to claim 7, characterized in that: The signal S4 (f r ,f a ) is obtained as follows: Construct phase h5: Where D2 and D3 are expressed as: The first scaling result is subjected to range Fourier transform and then multiplied by the phase h5. The product is then subjected to range inverse Fourier transform to obtain the signal S4 (f r ,f a ).

9. The method for spaceborne non-along-track SAR multi-mode integrated frequency domain imaging according to claim 6, wherein: The method for obtaining the second scaling result in step S6 is: Construct phase h6: h6=exp[jy3(R p )f r 3 +jy4(R p )f r 3 ] The signal S4(f r ,f a ) is multiplied by the phase h6 and then an azimuth inverse Fourier transform is performed, and then the azimuth inverse Fourier transform result is multiplied by the azimuth nonlinear frequency modulation scaling factor h7 to obtain the second scaling result.

10. The spaceborne non-along-track SAR multi-mode integrated frequency domain imaging method according to claim 9, characterized in that: The method for obtaining the two-dimensional image is: Construct phase h8: Where d2, d3 and d4 are expressed as: The second scaling result is subjected to azimuth Fourier transform and then multiplied by the phase h8. The product is then subjected to azimuth inverse Fourier transform to obtain a two-dimensional image.

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