Satellite-borne synthetic aperture radar in-orbit calibration method and system

Abstract

The application relates to an on-orbit calibration method and system of a space-borne synthetic aperture radar, and belongs to the technical field of space-borne SAR calibration. The method comprises the following steps: after a satellite is put into orbit, a uniform scene target is selected to perform a calibration task before a routine observation task is performed, and initial values of distance direction beam pointing calibration parameters are obtained; in the routine observation task, a pointing calibration area and a calibration observation period of the pointing calibration area are determined; based on business data of the routine observation task, a distance direction pattern of the pointing calibration area observed in the calibration observation period is calculated; the observed distance direction patterns are compared one by one, distance pointing deviations of current observation relative to previous observation are inversely calculated, the distance direction pattern of the current observation is corrected, the distance direction beam pointing calibration parameters of the current observation are updated, and the accuracy of the business data of the routine observation task in each calibration observation period is ensured. The on-orbit calibration method is efficient and convenient, and the number of task arrangements is reduced.

Description

Technical Field

[0001] This invention relates to the field of spaceborne synthetic aperture radar (SAR) calibration technology, and in particular to a spaceborne synthetic aperture radar on-orbit calibration method and system. Background Technology

[0002] With the continuous development of satellite remote sensing technology, spaceborne synthetic aperture radar (SAR), as an important active microwave remote sensing sensor, is widely used in many fields such as topographic mapping, ocean observation, and environmental monitoring. To ensure the accuracy and reliability of the data acquired by spaceborne SAR, precise beam pointing calibration is crucial.

[0003] Traditional spaceborne SAR range beam pointing calibration methods require dedicated calibration tasks and regular calibration to address beam pointing changes caused by various factors during satellite operation. However, this traditional method has certain limitations. On the one hand, scheduling dedicated calibration tasks is relatively frequent, consuming significant satellite resources and mission planning time. On the other hand, the calibration process is relatively complex, requiring sophisticated calibration equipment and operations, thus increasing calibration costs and difficulty.

[0004] Therefore, there is an urgent need for a more efficient, convenient, and mission-reducing on-orbit calibration method for spaceborne synthetic aperture radar to meet the requirements of current and future satellite remote sensing applications for SAR data accuracy and calibration efficiency. Summary of the Invention

[0005] Based on the above analysis, this invention aims to disclose an on-orbit calibration method and system for spaceborne synthetic aperture radar; to solve the problem that traditional calibration methods require frequent scheduling of dedicated calibration tasks, and to achieve periodic estimation and calibration of radar range beam pointing by combining operational task data with fewer calibration tasks.

[0006] This invention discloses an on-orbit calibration method for a spaceborne synthetic aperture radar, used for calibrating the range-direction beam pointing of the radar; comprising:

[0007] Before a routine observation mission, a single on-orbit calibration step is performed; after the satellite enters orbit and before performing a routine observation mission, a calibration mission is performed on a uniform scene target, including the Amazon rainforest, to obtain the initial values ​​of the range beam pointing calibration parameters.

[0008] In routine observation missions, the following steps are performed for on-orbit calibration: the pointing calibration area and the calibration observation period for the pointing calibration area are determined; based on the operational data of the routine observation mission, the range-direction beam pointing calibration parameters for the pointing calibration area during the calibration observation period are calculated; the range-direction beam pointing calibration parameters for the current observation are compared with the range-direction beam pointing calibration parameters for each observation, and the range-direction beam pointing calibration parameters for the current observation are updated to ensure the accuracy of each observation.

[0009] Furthermore, a single on-orbit calibration step includes:

[0010] Step S201: After the satellite is in orbit and ready for operation, select a uniform scene target and arrange the SAR payload to perform differential beam observation, with the beam positions covering high, medium and low angles;

[0011] Step S202: Compress the range of the echo signal obtained by the differential beam observation and calculate the offset distance to accurately locate the sampling point position corresponding to the differential beam notch in the radiation pattern, and calculate the offset distance d0 of the sampling point position.

[0012] Step S203: Based on the satellite attitude data at the intermediate observation time, determine the satellite's line-of-sight vector and the ground slant distance R1.

[0013] Step S204: Obtain the actual slant range R2 of the difference beam center by subtracting the offset distance d0 from the satellite-to-ground slant range R1; calculate the downward angle θ2 corresponding to the slant range R2 by iterative approximation; and calibrate the pointing deviation as Δθ=θ1-θ2;

[0014] Step S205: Perform multiple measurements and statistically analyze the measurement results of the SAR antenna beam pointing calibration parameters; store the calibration parameters in the on-board memory as a reference for subsequent calibration.

[0015] Furthermore, the successive on-orbit calibration steps include:

[0016] Step S501: Determine the pointing calibration area and the calibration observation period for the pointing calibration area in the routine observation task; according to the calibration observation period, observe the area at the same incident angle in the routine observation task to obtain SAR echo data for each calibration observation.

[0017] Step S502: Perform range compression processing on the SAR echo data of each calibration observation to obtain the range direction pattern;

[0018] Step S503: Using the range pattern in the initial pointing calibration parameters as the initial value, compare the range pattern of the current observation with that of the previous observation one by one to determine the main lobe offset of the current observation relative to the previous observation and to deduce the range pointing deviation.

[0019] Step S504: Correct the current range-direction pattern based on the inverted range-direction deviation, update the pointing calibration parameters, and ensure the accuracy of each observation.

[0020] Further, step S501 includes:

[0021] Step S501-1: Determine the pointing calibration area in the routine observation mission; the pointing calibration area is the region with stable scattering characteristics throughout the entire routine observation mission.

[0022] Step S501-2: Based on the satellite orbit parameters, determine the satellite's calibration observation cycle and coverage of the pointing calibration area;

[0023] Step S501-3: After completing the initial calibration of the Amazon rainforest, an image of the directional calibration area is immediately taken as the first calibration observation for successive on-orbit calibration, providing an initial reference value for subsequent operational data directional calibration.

[0024] Step S501-4: In the subsequent operational process, the same wavefront is used to periodically observe the area and record auxiliary information, including satellite attitude and orbital parameters, each time.

[0025] Furthermore, the calibration area is a flat region with stable backscattering characteristics, including the Gobi Desert, which is unaffected by seasonal climate change.

[0026] The calibration observation period for the calibration area is an integer multiple of the satellite revisit period determined by the satellite orbit parameters, and the incident angle for the calibration area remains the same each time a calibration observation is performed.

[0027] Further, step S502 includes:

[0028] Step S502-1: Perform channel merging and decompression on the observed SAR echo data to produce L0 level data for the region;

[0029] Step S502-2: Based on L0 level auxiliary data, construct a matched filter to perform range pulse compression on the echo data;

[0030] Step S502-3: Accumulate the range-compressed data along the azimuth direction and calculate the mean amplitude of each range gate to obtain the range direction pattern;

[0031] Step S502-4: Normalize the distance direction map and store it in the calibration database as a reference for the initial business data calibration.

[0032] Further, step S503 includes:

[0033] Step S503-1: Based on the pointing calibration area and the calibration observation cycle of the pointing calibration area, periodically obtain the corresponding L0 level data from the business data;

[0034] Step S503-2: Perform distance direction graph processing on the acquired data to obtain the distance direction graphs of the calibration data for the 2nd, 3rd, ..., nth periods;

[0035] Step S503-3: Calculate the offset of the distance direction pattern between the current observation and the previous observation using the cross-correlation algorithm.

[0036] Step S503-4: Invert the slant distance corresponding to the offset, and then calculate the corresponding distance pointing deviation angle based on the slant distance of the scene center at the time of shooting.

[0037] Furthermore, the expression for the cross-correlation function is:

[0038]

[0039] Where x(n) represents the radiation pattern after range compression of the previous observation, y(n) represents the radiation pattern after range compression of the current observation, and m is the offset; when the cross-correlation function R xy When (m) reaches its maximum value, the corresponding m is the offset of the range direction pattern; when calculating the cross-correlation function, the range direction pattern in the initial pointing calibration parameters is used as the initial value of x(n);

[0040] A positive offset indicates that the beam is shifted away from the nadir point, while a negative offset indicates that the beam is shifted closer to the nadir point.

[0041] Furthermore, based on the offset m, the corresponding slant distance d is inverted, and the expression is:

[0042]

[0043] Where c is the speed of light, F s The sampling rate for the two consecutive shots;

[0044] The slant distance R at the center of the scene is calculated from satellite attitude data using Euler angle transformation. c The conversion formula is:

[0045]

[0046] Among them, R x (α), R y (β), R z (γ) represents the rotation angles of the satellite along the x, y, and z axes, respectively. The initial position of the line-of-sight vector is [0,0,1]. T ;

[0047] satellite line-of-sight vector Find the intersection point between the line containing the satellite and the Earth's surface. The straight-line distance between the satellite and the intersection point is the slant distance R from the center of the scene. c Then calculate the deviation angle:

[0048]

[0049] A positive value indicates that the distance is greater than the calibrated value, while a negative value indicates that the distance is less than the calibrated value.

[0050] Another aspect of the present invention discloses a system employing the on-orbit calibration method for spaceborne synthetic aperture radar as described above, comprising: a single on-orbit calibration module and a successive on-orbit calibration module; wherein,

[0051] The single on-orbit calibration module is used to select a uniform scene target, including the Amazon rainforest, to perform a calibration task before performing routine observation tasks after the satellite enters orbit, and obtain the initial values ​​of the range beam pointing calibration parameters.

[0052] The successive on-orbit calibration module is used in routine observation missions to determine the pointing calibration area and the calibration observation period for the pointing calibration area; based on the operational data of the routine observation mission, it calculates the range directional pattern of the pointing calibration area observed in each calibration observation period; it compares the observed range directional patterns one by one, retrieves the range pointing deviation of the current observation relative to the previous observation, corrects the current range directional pattern, and updates the current range beam pointing calibration parameters to ensure the accuracy of the operational data of the routine observation mission within each calibration observation period.

[0053] This invention can achieve one of the following beneficial effects:

[0054] The on-orbit calibration method and system for spaceborne synthetic aperture radar disclosed in this invention only requires one dedicated differential beam calibration task after the satellite enters orbit. Subsequently, the range beam pointing of the radar can be periodically estimated and calibrated using satellite revisit period and operational task data. This greatly reduces the number of dedicated calibration tasks, saves satellite resources and mission planning time, and improves the efficiency and economy of calibration work.

[0055] The pointing deviation is inverted by comparing the main lobe shift of the range pattern of the same region at different times. This method is simple and effective, has relatively low requirements for equipment and operation, and is easy to implement in actual satellite operation. It has strong engineering operability and practicality.

[0056] The ability to promptly detect and correct range beam pointing deviations caused by various factors (such as satellite attitude changes and component aging) during radar operation in orbit ensures the accuracy and reliability of spaceborne SAR data. This is of great significance for improving the quality of satellite remote sensing data and better serving applications such as topographic mapping and environmental monitoring. Attached Figure Description

[0057] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts.

[0058] Figure 1 This is a flowchart of the on-orbit calibration method for spaceborne synthetic aperture radar in an embodiment of the present invention;

[0059] Figure 2 This is a flowchart of a single on-orbit calibration process in an embodiment of the present invention;

[0060] Figure 3 This is a schematic diagram comparing differential beams and conventional beams in an embodiment of the present invention;

[0061] Figure 4 This is a schematic diagram of the satellite-to-ground geometric relationship in an embodiment of the present invention;

[0062] Figure 5 This is a flowchart illustrating the successive on-orbit calibration process in an embodiment of the present invention.

[0063] Figure 6 This is a schematic diagram of the Gobi Desert region in an embodiment of the present invention;

[0064] Figure 7 This is a schematic diagram showing the components and connections of the on-orbit calibration system for a spaceborne synthetic aperture radar in an embodiment of the present invention. Detailed Implementation

[0065] Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which form part of this application and, together with the embodiments of the present invention, serve to illustrate the principles of the present invention.

[0066] Example 1

[0067] One embodiment of the present invention discloses an on-orbit calibration method for a spaceborne synthetic aperture radar, used for calibrating the range-direction beam pointing of the radar; such as Figure 1 As shown, it includes:

[0068] Step S101: Before the routine observation mission, a single on-orbit calibration step; after the satellite enters orbit and before performing the routine observation mission, select a uniform scene target including the Amazon rainforest to perform a calibration mission once to obtain the initial values ​​of the range beam pointing calibration parameters.

[0069] Step S102: In routine observation missions, perform successive on-orbit calibration steps; determine the pointing calibration area and the calibration observation period for the pointing calibration area in routine observation missions; based on the operational data of routine observation missions, calculate the range directional pattern of the pointing calibration area observed in each calibration observation period; compare the observed range directional patterns one by one, deduce the range pointing deviation of the current observation relative to the previous observation, correct the current range directional pattern, update the current range beam pointing calibration parameters, and ensure the accuracy of the operational data of routine observation missions in each calibration observation period.

[0070] Specifically, in a single on-orbit calibration step before a routine observation mission, such as Figure 2 As shown, it includes:

[0071] Step S201: After the satellite is in orbit and ready for operation, select a uniform scene target and arrange the SAR payload to perform differential beam observation, with the beam positions covering high, medium and low angles;

[0072] Uniform scene targets are generally selected from the Amazon rainforest, because they have large and stable backscattering characteristics, no strong scattering targets to interfere, and cover high, medium and low viewpoints to be representative; thus, they can provide reliable reference data.

[0073] Step S202: Compress the range of the echo signal obtained by the differential beam observation and calculate the offset distance to accurately locate the sampling point position corresponding to the differential beam notch in the radiation pattern and calculate the offset distance of the sampling point position.

[0074] include:

[0075] S202-1. The range-compressed signal is accumulated in the azimuth direction to obtain the range-direction difference beam pattern. Because the radiation gain is low and the beamwidth is extremely narrow at the difference beam notch, ... Figure 3 As shown, this method provides more accurate range-based positioning. The sampling point corresponding to the notch can be accurately located by etching the range-compressed direction map.

[0076] S202-2. Subtract half of the total number of sampling points in the distance direction from the above sampling points to obtain the number of sampling points that deviate from the center. The offset distance d0 in the slant range direction can then be calculated.

[0077]

[0078] Where N is the number of sampling points off-center, f s Where is the sampling rate, and c is the speed of light.

[0079] Step S203: Determine the satellite's line-of-sight vector and the ground slant distance based on the satellite attitude data at the intermediate observation time.

[0080] In such Figure 4 In the shown star-ground geometry, the line-of-sight vector The downward angle θ is obtained by converting attitude data. The Earth's surface can be represented by an ellipsoidal equation, namely:

[0081]

[0082] R e R is the radius of the Earth's equator. p The Earth's polar radius. Finding the intersection point with the above equation yields the ground position (x0, y0, z0). Since the satellite position (x, y, z) is known, the satellite-to-ground slant distance R1 can be calculated, expressed as:

[0083]

[0084] Step S204: Obtain the actual slant range R2 of the difference beam center by subtracting the offset distance d0 from the satellite-to-ground slant range R1; calculate the downward angle θ2 corresponding to the slant range R2 by iterative approximation; and calibrate the pointing deviation as Δθ=θ1-θ2;

[0085] During the iterative approximation process, the initial angle of the iteration is the line-of-sight direction θ1. The initial star-ground slant moment is calculated based on the initial angle. The distance difference between the initial slant moment and the actual slant moment is calculated, and it is determined whether it is less than the set threshold. If it is not less than the threshold, the initial angle is adjusted and iterative approximation is carried out until the distance difference is less than the set threshold. The angle of iterative approximation is taken as the downward angle θ2 of the slant distance R2.

[0086] Step S205: Perform multiple measurements and statistically analyze the measurement results of the SAR antenna beam pointing calibration parameters; store the calibration parameters in the on-board memory as a reference for subsequent calibration.

[0087] Specifically, in routine observation missions, during the successive on-orbit calibration steps, such as Figure 5 As shown, it includes:

[0088] Step S501: Determine the pointing calibration area and the calibration observation period for the pointing calibration area in the routine observation task; according to the calibration observation period, observe the area at the same incident angle in the routine observation task to obtain SAR echo data of each calibration observation.

[0089] Specifically, including:

[0090] Step S501-1: Determine the pointing calibration area in the routine observation mission; the pointing calibration area is the region with stable scattering characteristics throughout the entire routine observation mission.

[0091] like Figure 6As shown, flat areas with stable backscattering characteristics, including the Gobi Desert, that are not affected by seasonal climate change are preferred.

[0092] Step S501-2: Based on the satellite orbit parameters, determine the satellite's calibration observation cycle and coverage of the pointing calibration area;

[0093] The calibration observation period for the calibration area is an integer multiple of the satellite revisit period determined by the satellite orbit parameters, and the incident angle for the calibration area remains the same each time a calibration observation is performed.

[0094] For example, assuming the satellite revisit period is 10 days, and a calibration observation is performed every 3 revisit periods, then the calibration observation period is 30 days.

[0095] Step S501-3: After completing the initial calibration of the Amazon rainforest, an image of the directional calibration area is immediately taken as the first calibration observation for successive on-orbit calibration, providing an initial reference value for subsequent operational data directional calibration.

[0096] Step S501-4: In the subsequent operational process, the same wavefront is used to periodically observe the area and record auxiliary information, including satellite attitude and orbital parameters, each time.

[0097] Step S502: Perform range compression processing on the SAR echo data of each calibration observation to obtain the range direction pattern;

[0098] Step S502 includes:

[0099] Step S502-1: The ground production system performs channel merging and decompression on the observed SAR echo data to produce L0 level data for the area;

[0100] Step S502-2: Based on L0 level auxiliary data, construct a matched filter to perform range pulse compression on the echo data;

[0101] The radar transmitted signal is s(t), and the decompressed echo signal is s(t,τ), where t is the range-direction fast time and τ is the azimuth-direction slow time; the matched filter signal is s * Let (t) be the conjugate of the transmitted signal s(t). Then the distance-compressed signal is:

[0102]

[0103] Step S502-3: Accumulate the range-compressed data along the azimuth direction and calculate the mean amplitude of each range gate to obtain the range direction pattern;

[0104] Distance after compression s o(t) The two-dimensional signal form is:

[0105] The first dimension (column direction) is the orientation direction, with a length of n; the second dimension (row direction) is the distance direction, with a length of m.

[0106] The mean of the signal amplitude along the first azimuth direction can be expressed as:

[0107] s c (t)=[abs(s1) … abs(s m )]

[0108] Where abs(s) i ), where i = 1…m represents the mean amplitude of each range gate signal. At this point, the main lobe pattern in the range direction has been obtained;

[0109] Step S502-4: Normalize the distance direction map and store it in the calibration database as a reference for the initial business data calibration.

[0110] The range pattern s is obtained by normalizing the main lobe pattern in the range direction using minimum-maximum normalization. n (t);

[0111]

[0112] The normalized distance-direction pattern s is obtained n (t).

[0113] Step S503: Using the range pattern in the initial pointing calibration parameters as the initial value, compare the range pattern of the current observation with the previous observation one by one to determine the main lobe offset of the current observation relative to the previous observation and to deduce the range pointing deviation.

[0114] Step S503 includes:

[0115] Step S503-1: Based on the pointing calibration area and the calibration observation cycle of the pointing calibration area, periodically obtain the corresponding L0 level data from the business data;

[0116] Step S503-2: Perform distance direction graph processing on the acquired data to obtain the distance direction graphs of the calibration data for the 2nd, 3rd, ..., nth periods;

[0117] Step S503-3: Calculate the offset of the distance direction pattern between the current observation and the previous observation using the cross-correlation algorithm.

[0118] The expression for the cross-correlation function is:

[0119]

[0120] Where x(n) represents the radiation pattern after range compression of the previous observation, y(n) represents the radiation pattern after range compression of the current observation, and m is the offset. When R xy When (m) reaches its maximum value, the corresponding m is the offset; the initial value of x(n) is the distance direction pattern in the initial pointing calibration parameters;

[0121] A positive offset indicates that the beam is shifted away from the nadir point, while a negative offset indicates that the beam is shifted closer to the nadir point.

[0122] Step S503-4: Invert the slant distance corresponding to the offset, and then calculate the corresponding distance pointing deviation angle based on the slant distance of the scene center at the time of shooting;

[0123] Based on the premise that the wavefront parameters are the same in the two consecutive shots, the sampling rate F of the two shots is... s The slope distance corresponding to the offset m is inverted using the following expression:

[0124]

[0125] Where c is the speed of light.

[0126] The slant distance R at the center of the scene is calculated from satellite attitude data using Euler angle transformation. c The conversion formula is:

[0127]

[0128] Among them, R x (α), R y (β), R z (γ) represents the rotation angles of the satellite along the x, y, and z axes, respectively. The initial position of the line-of-sight vector is [0,0,1]. T .

[0129] satellite line-of-sight vector Find the intersection point between the line containing the satellite and the Earth's surface. The straight-line distance between the satellite and the intersection point is the slant distance R from the center of the scene. c Then calculate the deviation angle:

[0130]

[0131] A positive value indicates that the distance is greater than the calibrated value, while a negative value indicates that the distance is less than the calibrated value.

[0132] Step S504: Correct the current range-direction pattern based on the inverted range-direction deviation, update the pointing calibration parameters, and ensure the accuracy of each observation.

[0133] In this step, the beam control parameters are adjusted based on the deviation angle to correct the beam pointing. The new range pattern is then stored in the database as a reference pattern for the next pointing calibration.

[0134] In summary, the on-orbit calibration method for spaceborne synthetic aperture radar disclosed in this invention only requires one dedicated differential beam calibration task after the satellite enters orbit. Subsequently, the range beam pointing of the radar can be periodically estimated and calibrated using the satellite revisit cycle and operational task data. This greatly reduces the number of dedicated calibration tasks, saves satellite resources and mission planning time, and improves the efficiency and economy of calibration work.

[0135] The pointing deviation is inverted by comparing the main lobe shift of the range pattern of the same region at different times. This method is simple and effective, has relatively low requirements for equipment and operation, and is easy to implement in actual satellite operation. It has strong engineering operability and practicality.

[0136] The ability to promptly detect and correct range beam pointing deviations caused by various factors (such as satellite attitude changes and component aging) during radar operation in orbit ensures the accuracy and reliability of spaceborne SAR data. This is of great significance for improving the quality of satellite remote sensing data and better serving applications such as topographic mapping and environmental monitoring.

[0137] Example 2

[0138] One embodiment of the present invention discloses an on-orbit calibration system for a spaceborne synthetic aperture radar, used for calibrating the range-direction beam pointing of the radar; such as Figure 7 As shown, it includes: a single on-orbit calibration module and a successive on-orbit calibration module; wherein,

[0139] The single on-orbit calibration module is used to select a uniform scene target, including the Amazon rainforest, to perform a calibration task before performing routine observation tasks after the satellite enters orbit, and obtain the initial values ​​of the range beam pointing calibration parameters.

[0140] The successive on-orbit calibration module is used in routine observation missions to determine the pointing calibration area and the calibration observation period for the pointing calibration area; based on the operational data of the routine observation mission, it calculates the range directional pattern of the pointing calibration area observed in each calibration observation period; it compares the observed range directional patterns one by one, retrieves the range pointing deviation of the current observation relative to the previous observation, corrects the current range directional pattern, and updates the current range beam pointing calibration parameters to ensure the accuracy of the operational data of the routine observation mission within each calibration observation period.

[0141] The specific technical details and beneficial effects of this embodiment are the same as those disclosed in Embodiment 1. Please refer to them for details, and they will not be repeated here.

[0142] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for on-orbit calibration of a spaceborne synthetic aperture radar, characterized in that, Used for radar range-direction beam pointing calibration; including: Before a routine observation mission, a single on-orbit calibration step is performed; after the satellite enters orbit and before performing a routine observation mission, a calibration mission is performed on a uniform scene target, including the Amazon rainforest, to obtain the initial values ​​of the range beam pointing calibration parameters. In routine observation missions, the following steps are performed for on-orbit calibration: The pointing calibration area and the calibration observation cycle for that area are determined. Based on the operational data from the routine observation missions, the range-direction beam pointing calibration parameters for each calibration observation cycle are calculated. The range-direction beam pointing calibration parameters are compared sequentially to determine the range-direction deviation between the current and previous observations, correcting the current range-direction beam pointing calibration parameters and ensuring the accuracy of the operational data for each calibration observation cycle. The successive on-orbit calibration steps include: Step S501: Determine the pointing calibration area and the calibration observation period for the pointing calibration area in the routine observation task; according to the calibration observation period, observe the area at the same incident angle in the routine observation task to obtain SAR echo data for each calibration observation. Step S502: Perform range compression processing on the SAR echo data of each calibration observation to obtain the range direction pattern; Step S503: Using the range pattern in the initial pointing calibration parameters as the initial value, compare the range pattern of the current observation with the previous observation one by one to determine the main lobe offset of the current observation relative to the previous observation and to deduce the range pointing deviation. Step S504: Correct the current range-direction pattern based on the inverted range-direction deviation, update the pointing calibration parameters, and ensure the accuracy of each observation.

2. The on-orbit calibration method for spaceborne synthetic aperture radar according to claim 1, characterized in that, A single on-orbit calibration procedure includes: Step S201: After the satellite is in orbit and ready for operation, select a uniform scene target and arrange the SAR payload to perform differential beam observation, with the beam positions covering high, medium and low angles; Step S202: Perform range compression on the echo signal acquired by differential beam observation and calculate the offset distance to accurately locate the sampling point position corresponding to the differential beam notch in the radiation pattern, and calculate the offset distance of the sampling point position. ; Step S203: Based on the satellite attitude data at the intermediate observation time, determine the satellite's line-of-sight vector and the ground slant distance. ; Step S204: By measuring the satellite-to-ground slant distance Subtract offset distance Obtain the actual slant range of the difference beam center The slant distance was calculated through iterative approximation. Corresponding downward perspective The calibrated pointing deviation is... ; This refers to the initial angle during the iterative approximation process. Step S205: Perform multiple measurements and statistically analyze the measurement results of the SAR antenna beam pointing calibration parameters; store the calibration parameters in the on-board memory as a reference for subsequent calibration.

3. The on-orbit calibration method for spaceborne synthetic aperture radar according to claim 1, characterized in that, Step S501 includes: Step S501-1: Determine the pointing calibration area in the routine observation mission; the pointing calibration area is the region with stable scattering characteristics throughout the entire routine observation mission. Step S501-2: Based on the satellite orbit parameters, determine the satellite's calibration observation cycle and coverage of the pointing calibration area; Step S501-3: After completing the initial calibration of the Amazon rainforest, an image of the directional calibration area is immediately taken as the first calibration observation for successive on-orbit calibration, providing an initial reference value for subsequent operational data directional calibration. Step S501-4: In the subsequent operational process, the same wavefront is used to periodically observe the area and record auxiliary information, including satellite attitude and orbital parameters, each time.

4. The on-orbit calibration method for spaceborne synthetic aperture radar according to claim 3, characterized in that, The calibration area is a flat region with stable backscattering characteristics, including the Gobi Desert, which is unaffected by seasonal climate changes. The calibration observation period for the calibration area is an integer multiple of the satellite revisit period determined by the satellite orbit parameters, and the incident angle for the calibration area remains the same each time a calibration observation is performed.

5. The on-orbit calibration method for spaceborne synthetic aperture radar according to claim 1, characterized in that, Step S502 includes: Step S502-1: Perform channel merging and decompression on the observed SAR echo data to produce L0 level data for the region; Step S502-2: Based on L0 level auxiliary data, construct a matched filter to perform range pulse compression on the echo data; Step S502-3: Accumulate the range-compressed data along the azimuth direction and calculate the mean amplitude of each range gate to obtain the range direction pattern; Step S502-4: Normalize the distance direction map and store it in the calibration database as a reference for the initial business data calibration.

6. The on-orbit calibration method for spaceborne synthetic aperture radar according to claim 1, characterized in that, Step S503 includes: Step S503-1: Based on the pointing calibration area and the calibration observation cycle of the pointing calibration area, periodically obtain the corresponding L0 level data from the business data; Step S503-2: Perform distance direction graph processing on the acquired data to obtain the distance direction graphs of the calibration data for the 2nd, 3rd, ..., nth periods; Step S503-3: Calculate the offset of the distance direction pattern between the current observation and the previous observation using the cross-correlation algorithm. Step S503-4: Invert the slant distance corresponding to the offset, and then calculate the corresponding distance pointing deviation angle based on the slant distance of the scene center at the time of shooting.

7. The on-orbit calibration method for spaceborne synthetic aperture radar according to claim 6, characterized in that, The expression for the cross-correlation function is: in, This represents the radiation pattern after the previous observation distance compression. This represents the radiation pattern after the current observation distance is compressed. m It is the offset; when the cross-correlation function When the maximum value is reached, the corresponding m This represents the offset of the range-direction pattern; when calculating the cross-correlation function, the range-direction pattern in the initial pointing calibration parameters is used as... The initial value; A positive offset indicates that the beam is shifted away from the nadir point, while a negative offset indicates that the beam is shifted closer to the nadir point.

8. The on-orbit calibration method for spaceborne synthetic aperture radar according to claim 6, characterized in that, Based on offset m Inversion corresponding slant distance The expression is: Where c is the speed of light; The sampling rate for the two consecutive shots; The slant distance to the center of the scene is calculated from satellite attitude data using Euler angle transformation. The conversion formula is: in, These represent the rotation angles along the satellite's x, y, and z axes, respectively. The initial position of the line-of-sight vector ; satellite line-of-sight vector The slant distance between the satellite and the intersection point of the line containing the satellite and the Earth's surface is the center-of-field slant distance. Then calculate the deviation angle: A positive value indicates that the distance is greater than the calibrated value, while a negative value indicates that the distance is less than the calibrated value.

9. A system employing the on-orbit calibration method for spaceborne synthetic aperture radar as described in any one of claims 1-8, characterized in that, include: Single-time on-orbit calibration module and successive on-orbit calibration module; among which, The single on-orbit calibration module is used to select a uniform scene target, including the Amazon rainforest, to perform a calibration task before performing routine observation tasks after the satellite enters orbit, and obtain the initial values ​​of the range beam pointing calibration parameters. The successive on-orbit calibration module is used in routine observation missions to determine the pointer calibration area and the calibration observation cycle for the pointer calibration area; Based on the operational data from routine observation missions, the range-direction pattern of the pointing area observed in each calibration observation cycle is calculated. The range-direction pattern of each observation is compared sequentially to deduce the range-direction deviation of the current observation relative to the previous observation. The range-direction pattern of the current observation is corrected, and the range-direction beam pointing calibration parameters of the current observation are updated to ensure the accuracy of the operational data of routine observation missions in each calibration observation cycle.

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