A method for implementing a geosynchronous orbit sparse spotlight SAR imaging mode

Abstract

This invention discloses a method for implementing a geostationary orbit sparse spotting SAR imaging mode. The method involves constructing a schematic diagram of the wavefront selection for a geostationary orbit spotting SAR system; designing the wavefront of the geostationary orbit sparse spotting SAR imaging mode and calculating the mode parameters; calculating the yaw angle required for zero-Doppler attitude guidance of the geostationary orbit SAR satellite; constructing a radiation pattern model of the array antenna for the geostationary orbit sparse spotting SAR imaging mode; and conducting performance analysis of the geostationary orbit sparse spotting SAR imaging mode. This invention combines sparse microwave imaging with a geostationary orbit spotting SAR mode. Based on the existing SAR system, it uses a more accurate zebra pattern and appropriate wavefront design to obtain a larger mapping bandwidth, achieving the high-resolution, wide-swath observation objective of geostationary orbit spotting SAR. Simultaneously, it improves system performance by reducing range ambiguity and suppressing the influence of antenna rotation on orientation ambiguity.

Description

Technical Field

[0001] This invention belongs to the field of radar system mode design, specifically relating to a method for implementing a geosynchronous orbit sparse clustered SAR imaging mode. Background Technology

[0002] Synthetic Aperture Radar (SAR) is an active, high-resolution imaging radar that can be installed on aircraft, satellites, spacecraft, and other flight platforms to conduct all-day, all-weather observations of the Earth. It has broad application prospects in disaster prediction, environmental monitoring, and surface mapping.

[0003] Compared to traditional low-Earth orbit (LEO) SAR, geostationary orbit (GEO) SAR boasts advantages such as long operating range (over 36,000 kilometers), wide coverage, and strong staring capability. Therefore, the calculation formulas for system parameters and performance cannot be directly applied to conventional LEO SAR. Instead, it is necessary to derive parameter calculation and performance analysis methods suitable for GEO SAR based on the original physical meaning of the parameters and performance. The basic idea of ​​beamforming is to control the antenna beam direction during radar platform flight, allowing it to illuminate the imaged area for a longer period. This increases the coherent accumulation time of the echo, effectively increasing the synthetic aperture time and resulting in higher azimuth resolution, thus avoiding the problem of azimuth resolution being entirely limited by the actual antenna length.

[0004] Sparse SAR imaging is a new theory, system, and method in the field of SAR imaging. It combines the advantages of sparse signal processing and traditional SAR techniques, achieving performance comparable to traditional SAR by reducing the azimuth sampling rate and the amount of observation data. It also shows great potential in reducing hardware requirements and minimizing ambiguity. Combining a sparse SAR imaging system with geostationary orbit spotting mode allows for further optimization of radar system performance and the acquisition of a larger mapping bandwidth without altering the radar hardware. Summary of the Invention

[0005] Purpose of the invention: This invention provides a method for implementing a geosynchronous orbit sparse clustered SAR imaging mode. Without changing the existing radar hardware, it reduces the azimuth sampling frequency of geosynchronous orbit SAR to achieve the goal of high resolution and wide mapping band of geosynchronous orbit clustered SAR. At the same time, it improves system performance by reducing range ambiguity and suppressing the influence of antenna rotation on azimuth ambiguity.

[0006] Technical solution: The present invention provides a method for implementing a sparse-beam SAR imaging mode in geosynchronous orbit, comprising the following steps:

[0007] (1) Construct a zebra map of geosynchronous orbit spot SAR system;

[0008] (2) Design the wavefront of the geosynchronous orbit sparse spot SAR imaging mode and calculate the relevant parameters;

[0009] (3) Calculate the yaw angle required for zero-Doppler attitude guidance of a geosynchronous orbit SAR satellite;

[0010] (4) Construct a radiation pattern model of the array antenna for geosynchronous orbit sparse focused SAR imaging mode;

[0011] (5) Analyze the performance of the geosynchronous orbit sparse cluster SAR imaging mode based on the wave position and related parameters designed in step (2); if the designed wave position is interfered with or the range ambiguity is higher than -20dB, return to step (2); otherwise the design is completed.

[0012] Furthermore, the implementation process of step (1) is as follows:

[0013] To avoid transmitted pulse interference, the pulse repetition frequency (PRF) must meet the following conditions:

[0014]

[0015] Among them, R max and R min , representing the maximum and minimum slant ranges from the SAR satellite to the far and near ends of the mapping strip during the synthetic aperture time, respectively; n is the number of pulse cycles that the target echo travels to the radar within the observation area; c is the speed of light; Int(·) indicates taking its integer part; T p T is the pulse width. g For time intervals;

[0016] To avoid the strongest nadir echo affecting the received signal, the PRF should simultaneously meet the following requirements:

[0017]

[0018] Where H is the satellite altitude; to avoid interference from the transmitted pulse and the nadir echo, the relationship between the incident angle and the pulse repetition frequency is plotted as a waveform diagram.

[0019] Zebra maps were designed using the maximum and minimum slant ranges from SAR satellites to the far and near ends of the mapping strip during synthetic aperture time.

[0020]

[0021] A(xx midle )+B(yy middle )+C(zz midle ) = 0

[0022]

[0023] Among them, R x R y R z Let x be the radius of the Earth in each direction, (x) middle y middle , z middle (x, y, z) represents the coordinates of the satellite at the center time, and (x, y, z) represents the nearest and farthest endpoints (the farthest endpoint is indicated by the plus sign). θ α From a downward perspective, θ beam R is the beamwidth. middle R is the distance from the satellite to the Earth's center, and R is the distance from (x, y, z) to the Earth's center. r Let (x, y, z) be the distance from the satellite, and (A, B, C) be the plane normal vector of the zero Doppler plane at the center time. After obtaining the positions of the near and far endpoints of the observation area, the minimum slant range R is obtained by calculating the distance to the satellite. min and maximum slope distance R max .

[0024] Furthermore, the implementation process of step (2) is as follows:

[0025] The antenna is designed with a certain downsampling ratio, and the antenna height, pulse width, and peak transmit power remain unchanged. Based on the incident angle and pulse repetition frequency obtained in step (1), the center downward angle, near and far incident angles, and mapping strip width are calculated according to the satellite height and satellite velocity. The downsampling ratio is set to 50%.

[0026] Furthermore, the yaw angle mentioned in step (3) is achieved by the following formula:

[0027]

[0028] Where u is the latitudinal argument, i is the orbital inclination; ω s ω is the angular velocity of the satellite. g This is the Earth's rotational angular velocity.

[0029] Furthermore, step (4) is achieved through the following formula:

[0030]

[0031] Where λ is the wavelength, N is the number of antenna azimuth subarrays, and L ae Let θ be the azimuth dimension of a single-element antenna, θ be the observation angle of the target, θ0 be the beam scanning angle at the center time, and ω be the azimuth dimension of the antenna. r Let denot be the antenna turning angular rate, and Δt be the time delay from pulse transmission to reception.

[0032] Furthermore, step (5) involves analyzing the performance of the geosynchronous orbit sparse cluster SAR imaging mode, including azimuth ambiguity analysis and range ambiguity analysis.

[0033] Furthermore, the azimuth ambiguity analysis process for the geosynchronous orbit sparse clustered SAR imaging mode is as follows:

[0034] The azimuth ambiguity of synthetic aperture radar (SAR) is caused by the finite sampling in the azimuth direction and the non-band-limited nature of the Doppler spectrum of SAR. This results in the ambiguity signal being superimposed on the desired signal, affecting the imaging results. Different radar operating modes have different antenna control methods, leading to differences in the azimuth ambiguity calculation methods for each mode. The azimuth ambiguity ratio (AASR) of the geostationary orbit sparse spotting mode is expressed as:

[0035]

[0036] Among them, f d For echo Doppler frequency, B p Let f be the Doppler bandwidth, G(·) be the array antenna pattern, and m be the azimuth ambiguity region; for f d The calculation, under geosynchronous orbit SAR conditions, should start from the physical definition and be calculated according to the following formula:

[0037]

[0038] Among them, R r V is the vector pointing from the target point to the satellite's position. r For the satellite's velocity relative to the target point, in spotlight mode, the echo Doppler frequency is further changed to:

[0039]

[0040] Among them, V s The satellite velocity is given in the Earth-fixed coordinate system. Substituting this Doppler frequency into the AASR calculation formula yields a more accurate AASR.

[0041] Furthermore, the range ambiguity analysis process for the geosynchronous orbit sparse clustered SAR imaging mode is as follows:

[0042] The range ambiguity ratio (RASR) for sparse-spotted geosynchronous orbit modes is expressed as:

[0043]

[0044] Where N is the number of echo signal samples within the data recording window, and S i and S aiThese represent the useful signal power and the ambiguous signal power at the receiver output at time i, respectively, where j is the pulse number, j = 0, ±1, ±2, ..., j = 0 indicates a useful pulse, and θ ij σ is the incident angle of the radar beam. ij 0 Given θ ij Normalized backscattering coefficient, G ij The range-direction antenna pattern, R ij Let i be the slant distance at time i;

[0045] Under geostationary orbit focused SAR conditions, considering the Earth's ellipsoidality, R is calculated. ij and θ ij The following formulas need to be combined:

[0046]

[0047] A(xx middle )+B(yy middle )+C(zz midle ) = 0

[0048]

[0049] The result obtained by solving the above equations simultaneously is the RASR.

[0050] Beneficial Effects: Compared with the prior art, the beneficial effects of this invention are as follows: 1. The zebra pattern design method proposed in this invention takes into account the slant range variation of geostationary orbit SAR satellites operating in spotting mode throughout the entire observation time, resulting in more accurate zebra patterns; 2. The geostationary orbit sparse spotting mode inherits the high-resolution imaging capability and wide observation band advantage of traditional spotting mode under geostationary orbit conditions, and further improves the mapping bandwidth on this basis; 3. This invention suppresses the influence of antenna azimuth rotation on azimuth ambiguity, resulting in more stable image quality; 4. This invention reduces the system's limitation on pulse repetition frequency, suppressing range ambiguity. Attached Figure Description

[0051] Figure 1 This is a flowchart of the present invention;

[0052] Figure 2 A schematic diagram of wavefront selection for geosynchronous orbit sparse clustered SAR imaging mode;

[0053] Figure 3 A schematic diagram showing the yaw angle that needs to be compensated under satellite attitude guidance;

[0054] Figure 4The antenna patterns of the array antenna in the sparse spotting SAR imaging mode in geosynchronous orbit are shown; where (a) is the antenna pattern when the turning angle is 0°; and (b) is the antenna pattern when the turning angle is 0.2°.

[0055] Figure 5 This is a schematic diagram of the azimuth ambiguity signal ratio; where (a) is a schematic diagram of the turning angle and azimuth ambiguity at different observation times within the synthetic aperture time, and (b) is the relationship between the azimuth ambiguity deterioration and the turning angle.

[0056] Figure 6 This is a schematic diagram of the distance ambiguity signal ratio. Detailed Implementation

[0057] The present invention will now be described in further detail with reference to the accompanying drawings.

[0058] like Figure 1 As shown, this invention provides a method for implementing a sparse-beam SAR imaging mode in geosynchronous orbit, specifically including the following steps:

[0059] Step 1: Construct a zebra map of the geosynchronous orbit spotting SAR system.

[0060] To avoid transmitted pulse interference, the pulse repetition frequency (PRF) must meet the following conditions:

[0061]

[0062] Among them, R max and R min Here, represents the maximum and minimum slant ranges from the SAR satellite to the far and near ends of the mapping strip, respectively, during the entire observation period; n is the number of pulse cycles that the target echo travels from the radar within the observation area; c is the speed of light; Int(·) indicates taking its integer part; T p T is the pulse width. g This is the time interval to ensure that data is recorded effectively; it is also known as the guard window width.

[0063] In addition, to avoid the strongest nadir echo affecting the received signal, the PRF should simultaneously meet the following requirements:

[0064]

[0065] Where H is the satellite altitude, and the relationship between the incident angle and the pulse repetition frequency is plotted as a waveform diagram to avoid interference from the transmitted pulse and the nadir echo.

[0066] The calculation methods for the maximum and minimum slant ranges of the SAR satellite to the far and near ends of the mapping zone during the entire observation period are as follows. First, since the antenna illumination range is an ellipse on the Earth's surface, it is necessary to determine the farthest and nearest points of the illumination area by simultaneously applying the following formulas:

[0067]

[0068] A(xx midle )+B(yy middle )+C(z—z middle ) = 0

[0069]

[0070] Among them, R x R y R z Let x be the radius of the Earth in each direction, (x) middle y middle , z middle (x, y, z) represents the coordinates of the satellite at the center time, and (x, y, z) represents the nearest and farthest endpoints (the farthest endpoint is indicated by the plus sign). θ α From a downward perspective, θ beam R is the beamwidth. middle R is the distance from the satellite to the Earth's center, and R is the distance from (x, y, z) to the Earth's center. r Let (x, y, z) be the distance from the satellite to (x, y, z), and (A, B, C) be the plane normal vector of the zero Doppler surface at the center time.

[0071] After obtaining the near and far endpoints of the observation area, the minimum slant range R is obtained by calculating the distance to the satellite. min and maximum slope distance R max Because geostationary orbit SAR has a long slant range, the slant range variation caused by antenna rotation will also increase. If the slant range calculation method based on a single satellite position used when calculating the low-orbit zebra pattern is applied, it may only meet the observation conditions at the center time and affect the observation results at other times. Using the maximum and minimum slant range zebra pattern design method can design the zebra pattern more accurately and avoid the above problems.

[0072] Step 2: Design the wavefront of the geosynchronous orbit sparse clustered SAR imaging mode and calculate the relevant parameters.

[0073] The design is carried out with a sampling ratio of 50%, the antenna height, pulse width and peak transmit power remain unchanged; the incident angle, pulse repetition frequency and satellite parameters such as satellite height and satellite speed are obtained by step (1) to calculate the center downward angle, near and far incident angles and mapping strip width.

[0074] Step 3: Calculate the yaw angle required for zero-Doppler attitude guidance of a geosynchronous orbit SAR satellite.

[0075] Due to the Earth's rotation, the actual direction of a satellite's orbit deviates from its inertial velocity direction. This characteristic is further amplified under geosynchronous orbit conditions, causing the central Doppler frequency and oblique angle to be non-zero, exacerbating distance migration effects and affecting image processing. Therefore, attitude guidance is needed to compensate for this. The method for calculating the yaw angle is as follows:

[0076]

[0077] Where u is the latitudinal argument, i is the orbital inclination; ω s ω is the angular velocity of the satellite. g Let be the Earth's rotational angular velocity. It is evident that this formula is independent of factors such as the satellite's left and right side views and its ascent and descent orbits. Introducing the result from this formula into the satellite orbit calculation formula allows the beam to be controlled at the zero Doppler plane at the center moment, thus mimicking the Doppler frequency variation under low-Earth orbit SAR spotting conditions, facilitating subsequent imaging processing.

[0078] Step 4: Construct an antenna pattern model for geosynchronous orbit sparse clustered SAR imaging mode.

[0079] Constructing the array antenna pattern model: The array antenna pattern model for geosynchronous orbit sparse spot-beam SAR imaging mode can be represented as:

[0080]

[0081] Where λ is the wavelength, N is the number of antenna azimuth subarrays, and L ae Let θ be the azimuth dimension of a single-element antenna, θ be the observation angle of the target, θ0 be the beam scanning angle at the center time, and ω be the azimuth dimension of the antenna. r Let denot be the antenna turning angular rate, and Δt be the time delay from pulse transmission to reception.

[0082] Step 5: Analyze the system performance of the geostationary orbit sparse cluster SAR imaging mode based on the designed wavefront and related parameters.

[0083] 1) Azimuth ambiguity analysis of geosynchronous orbit sparse clustered SAR imaging mode:

[0084] Azimuth ambiguity in SAR is caused by the limited sampling in the azimuth direction and the non-bandwidth characteristic of the SAR Doppler spectrum, resulting in the superposition of ambiguous signals onto the desired signal, thus affecting the imaging results. Different radar operating modes employ different antenna control methods, leading to variations in the azimuth ambiguity calculation methods for each mode. The azimuth ambiguity ratio in the geostationary orbit sparse-spot mode can be expressed as:

[0085]

[0086] Among them, f d For echo Doppler frequency, B p Let f be the Doppler bandwidth, G(·) be the array antenna pattern, and m be the azimuth ambiguity region. d The calculation, under geosynchronous orbit SAR conditions, should start from the physical definition and be calculated according to the following formula:

[0087]

[0088] Among them, R r V is the vector pointing from the target point to the satellite's position. r Let $\frac{ ...

[0089]

[0090] Among them, V s This represents the satellite velocity in the Earth-fixed coordinate system. Substituting this Doppler frequency into the AASR calculation formula yields a more accurate Azimuth Ambiguity Signal Ratio (AASR).

[0091] 2) Range ambiguity analysis of geosynchronous orbit sparse clustered SAR imaging mode:

[0092] Range ambiguity in SAR is mainly caused by excessively high PRF (Pressure Reflection Factor), leading to aliasing of the desired echo signal with echo signals from other areas, interfering with imaging of the mapping zone. The range ambiguity ratio in geostationary orbit sparse-spot mode can be expressed as:

[0093]

[0094]

[0095] Where N is the number of echo signal samples within the data recording window, and S i and S ai These represent the useful signal power and the ambiguous signal power at the receiver output at time i, respectively, where j is the pulse number, j = 0, ±1, ±2, ..., j = 0 indicates a useful pulse, and θ ij σ is the incident angle of the radar beam. ij 0 Given θ ij Normalized backscattering coefficient, G ij The range-direction antenna pattern, R ijLet R be the slant range at time i. Under geostationary orbit focused SAR conditions, the Earth's ellipsoidality needs to be considered, similar to the method used when calculating the zebra diagram, to determine R. ij and θ ij The following formulas should be combined:

[0096]

[0097] A(xx middle )+B(yy middle )+C(zz middle ) = 0

[0098]

[0099] The result obtained by solving the above equations can be substituted into the formula to calculate RASR. If the designed wavefront is disturbed or the distance ambiguity is higher than -20dB, return to step 2 to redesign; otherwise, the design is complete.

[0100] The following uses a set of typical geostationary orbit satellite parameters as an example to verify the design of the geostationary orbit sparse clustered SAR imaging mode of this invention. According to... Figure 2 The wave position selection diagram shown is used to calculate relevant parameters. The calculated parameters of the geosynchronous orbit sparse spot SAR imaging mode and the parameters of the conventional geosynchronous orbit spot SAR imaging mode are shown in Table 1.

[0101] Table 1. Comparison of parameters between the sparse beam pattern and the traditional beam pattern in geosynchronous orbit.

[0102]

[0103]

[0104] Compared with the traditional geostationary orbit spotting mode, the geostationary orbit sparse spotting mode reduces the pulse repetition frequency to 50% of the original, while increasing the range mapping bandwidth from 120km to 240km, while the azimuth resolution, beamwidth, and satellite velocity remain unchanged.

[0105] Figure 3 This is a schematic diagram of the yaw angle that needs to be compensated under satellite attitude guidance. Figure 4 Antenna pattern for sparse spotlight SAR imaging mode in geosynchronous orbit. Figure 4 (a) shows the case when the steering angle is 0°. Figure 4 In the middle (b), the case is when the turning angle is 0.2°. It can be seen that when beam scanning is performed, grating lobes will be generated in the antenna pattern. Figure 5 Azimuth ambiguity analysis for geosynchronous orbit sparse clustered SAR imaging mode. Figure 5In the middle (a), the relationship between the turning angle and azimuth ambiguity at different observation times during the synthetic aperture time is shown. Compared with the traditional geostationary orbit spot SAR imaging mode, the geostationary orbit sparse spot SAR imaging mode has an increased azimuth ambiguity ratio of about 18dB, and the overall ambiguity is greater than -20dB. Figure 5 (b) is a schematic diagram showing the worsening of azimuth ambiguity as the steering angle changes. It can be seen that compared with traditional geostationary orbit spot SAR, geostationary orbit sparse spot SAR suppresses the influence of antenna rotation on azimuth ambiguity. Figure 6 Range ambiguity analysis of the sparse-spot SAR imaging mode in geostationary orbit was performed. Compared with the traditional geostationary orbit spot SAR imaging mode, the range ambiguity ratio of the sparse-spot SAR imaging mode in geostationary orbit was reduced by 19 dB.

[0106] In summary, the geosynchronous orbit sparse clustered SAR imaging mode reduces the system's limitation on pulse repetition frequency without changing the SAR platform hardware, and designs reasonable wave positions based on more accurate wave position maps, resulting in a larger mapping bandwidth. At the same time, it suppresses the influence of antenna rotation on orientation ambiguity and reduces range ambiguity.

[0107] The embodiments described above are merely examples of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention.

Claims

1. A method for implementing a sparse-beam SAR imaging mode in geosynchronous orbit, characterized in that, Includes the following steps: (1) Construct a zebra map of geosynchronous orbit spot SAR system; (2) Design the wavefront of the geosynchronous orbit sparse spot SAR imaging mode and calculate the relevant parameters; (3) Calculate the yaw angle required for zero-Doppler attitude guidance of a geosynchronous orbit SAR satellite; (4) Construct a radiation pattern model of the array antenna for geosynchronous orbit sparse focused SAR imaging mode; (5) Analyze the performance of the geosynchronous orbit sparse cluster SAR imaging mode based on the wave position and related parameters designed in step (2); if the designed wave position is interfered with or the range ambiguity is higher than the preset value, return to step (2); otherwise the design is completed. The implementation process of step (1) is as follows: To avoid transmitted pulse interference, the pulse repetition frequency (PRF) must meet the following conditions: Among them, R max and R min These represent the maximum and minimum slant ranges from the SAR satellite to the far end and near end of the mapping strip, respectively, within the synthetic aperture time. n is the number of pulse cycles the target echo takes to reach the radar within the observation area, c is the speed of light, and Int(·) represents taking its integer part. p T is the pulse width. g For time intervals; To avoid the strongest nadir echo affecting the received signal, the PRF should simultaneously meet the following requirements: Where H is the satellite altitude; to avoid interference from transmitted pulses and nadir echoes, the relationship between the incident angle and the pulse repetition frequency is plotted as a waveform diagram. Zebra maps were designed using the maximum and minimum slant ranges from SAR satellites to the far and near ends of the mapping strip during synthetic aperture time. A(x-x middle )+B(y-y middle )+C(z-z middle )=0 Among them, R x R y R z Let x be the radius of the Earth in each direction. middle y middle , z middle Let (x, y, z) be the coordinates of the satellite at the center time, and let θ be the coordinates of the nearest and farthest endpoints. α From a downward perspective, θ beam R is the beamwidth. middle R is the distance from the satellite to the Earth's center, and R is the distance from (x, y, z) to the Earth's center. r Let (x, y, z) be the distance from the satellite, and (A, B, C) be the plane normal vector of the zero Doppler surface at the center time. After obtaining the positions of the near and far endpoints of the observation area, the minimum slant range R is obtained by calculating the distance to the satellite. min and maximum slope distance R max ; The yaw angle mentioned in step (3) is achieved by the following formula: Where u is the latitudinal argument, i is the orbital inclination; ω s ω is the angular velocity of the satellite. g This is the Earth's rotational angular velocity.

2. The method for implementing a sparse-beam SAR imaging mode in geosynchronous orbit according to claim 1, characterized in that, The implementation process of step (2) is as follows: The antenna is designed with a certain downsampling ratio, and the antenna height, pulse width, and peak transmit power remain unchanged. Based on the incident angle and pulse repetition frequency obtained in step (1), the center downward angle, near and far incident angles, and mapping strip width are calculated according to the satellite height and satellite speed parameters.

3. The method for implementing a sparse-beam SAR imaging mode in geosynchronous orbit according to claim 2, characterized in that, Step (4) is achieved through the following formula: Where λ is the wavelength, N is the number of antenna azimuth subarrays, and L ae Let θ be the azimuth dimension of a single-element antenna, θ be the observation angle of the target, θ0 be the beam scanning angle at the center time, and ω be the azimuth dimension of the antenna. r Let denot be the antenna turning angular rate, and Δt be the time delay from pulse transmission to reception.

4. The method for implementing a sparse-beam SAR imaging mode in geosynchronous orbit according to claim 3, characterized in that, Step (5) describes the analysis of the performance of the geosynchronous orbit sparse cluster SAR imaging mode, including azimuth ambiguity analysis and range ambiguity analysis.

5. The method for implementing a sparse-beam SAR imaging mode in geosynchronous orbit according to claim 4, characterized in that, The azimuth ambiguity analysis process for the geosynchronous orbit sparse clustered SAR imaging mode is as follows: The azimuth ambiguity of synthetic aperture radar (SAR) is caused by the finite sampling in the azimuth direction and the non-band-limited nature of the Doppler spectrum of SAR. This results in the ambiguity signal being superimposed on the desired signal, affecting the imaging results. Different radar operating modes have different antenna control methods, leading to differences in the azimuth ambiguity calculation methods for each mode. The azimuth ambiguity ratio (AASR) of the geostationary orbit sparse spotting mode is expressed as: Among them, f d For echo Doppler frequency, B p Let f be the Doppler bandwidth, G(·) be the array antenna pattern, and m be the azimuth ambiguity region; for f d The calculation, under geosynchronous orbit SAR conditions, starts from the physical definition and is performed according to the following formula: Among them, R r V is the vector pointing from the target point to the satellite's position. r For the satellite's velocity relative to the target point, in spotlight mode, the echo Doppler frequency is further changed to: Among them, V s The satellite velocity is given in the Earth-fixed coordinate system. Substituting this Doppler frequency into the AASR calculation formula yields a more accurate AASR.

6. The method for implementing a sparse-beam SAR imaging mode in geosynchronous orbit according to claim 4, characterized in that, The process of range ambiguity analysis for the geosynchronous orbit sparse clustered SAR imaging mode is as follows: The range ambiguity ratio (RASR) for sparse-spotted geosynchronous orbit modes is expressed as: Where M is the number of echo signal samples within the data recording window, and S... p and S ap These represent the useful signal power and the ambiguous signal power at the receiver output at time p, respectively, where q is the pulse number, q = 0, ±1, ±2, ..., indicating the useful pulse, and θ... pq The incident angle of the radar beam. Given θ pq Normalized backscattering coefficient, G pq The range-direction antenna pattern, R pq Let p be the slant distance; Under geostationary orbit focused SAR conditions, considering the Earth's ellipsoidality, R is calculated. ij and θ ij The following formulas need to be combined: A(x-x middle )+B(y-y middle )+C(z-z middle )=0 The result obtained by solving the above equations simultaneously is the RASR.

7. The method for implementing a sparse-beam SAR imaging mode in geosynchronous orbit according to claim 6, characterized in that, The preset value is -20dB.

8. The method for implementing a sparse-beam SAR imaging mode in geosynchronous orbit according to claim 7, characterized in that, The downsampling ratio is set to 50%.

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