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

Abstract

The invention relates to a satellite-borne synthetic aperture radar in-orbit calibration method and system, and belongs to the technical field of satellite-borne SAR calibration, and the method comprises the steps: selecting a uniform scene target to execute a calibration task after a satellite enters an orbit and before a conventional observation task is executed, and obtaining an initial value of a distance direction beam pointing calibration parameter; in the conventional observation task, a pointing calibration area and a calibration observation period for the pointing calibration area are determined; based on business data of a conventional observation task, calculating a distance directional diagram for observing a pointing calibration area in a calibration observation period; and gradually comparing the observed distance directional diagrams, inverting the distance pointing deviation of the current observation relative to the previous observation, correcting the current distance directional diagram, updating the current distance beam pointing calibration parameters, and ensuring the accuracy of the conventional observation task business data in each calibration observation period. The on-orbit calibration method is efficient and convenient, and the task arrangement frequency is reduced.

Description

Technical Field

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

[0002] With the continuous development of satellite remote sensing technology, spaceborne synthetic aperture radar, as an important active microwave remote sensing sensor, is widely used in terrain mapping, ocean observation, environmental monitoring and many other fields. In order to ensure the accuracy and reliability of the data obtained by spaceborne SAR, it is very important to accurately calibrate its beam pointing.

[0003] The traditional method of calibrating the range beam pointing of satellite-borne SAR requires the arrangement of special calibration tasks and the calibration work must be carried out regularly to cope with the changes in beam pointing caused by various factors during the operation of the satellite. However, this traditional method has certain limitations. On the one hand, the arrangement of special calibration tasks is relatively frequent, which will occupy more satellite resources and mission planning time; on the other hand, the calibration process is relatively complex, and the calibration equipment and operation requirements are high, which increases the cost and difficulty of calibration.

[0004] Therefore, there is an urgent need for a more efficient, convenient on-orbit calibration method for spaceborne synthetic aperture radar that can reduce the number of task arrangements, so as to meet the requirements of current and future satellite remote sensing applications for SAR data accuracy and calibration efficiency. Summary of the invention

[0005] In view of the above analysis, the present invention aims to disclose an on-orbit calibration method and system for a spaceborne synthetic aperture radar; to solve the problem that traditional calibration methods require frequent arrangements for special calibration tasks, and to achieve regular estimation and calibration of radar range beam pointing in combination with operational mission data using a smaller number of calibration tasks.

[0006] The present invention discloses an on-orbit calibration method for a spaceborne synthetic aperture radar, which is used to calibrate the radar range beam pointing; the method comprises:

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

[0008] In routine observation missions, the on-orbit calibration steps are performed one by one; the pointing calibration area and the calibration observation period for the pointing calibration area in routine observation missions are determined; based on the business data of routine observation missions, the range direction pattern observed for the pointing calibration area in the calibration observation period is calculated; the observed range direction patterns are compared one by one to invert the range pointing deviation of the current observation relative to the previous observation, the current range direction pattern is corrected, and the current range beam pointing calibration parameters are updated to ensure the accuracy of each observation.

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

[0010] Step S201: After the satellite is in orbit and in working condition, a uniform scene target is selected and the SAR payload is arranged to perform differential beam observation, with the beam positions covering high, medium and low viewing angles;

[0011] Step S202: perform distance compression on the echo signal obtained by the difference beam observation and calculate the offset distance, accurately locate the sampling point position corresponding to the difference beam notch in the directional diagram, and calculate the offset distance d of the sampling point position. 0 ;

[0012] Step S203: Determine the satellite's line of sight vector and the satellite-to-ground slant distance R based on the satellite attitude data at the middle of the observation. 1 ;

[0013] Step S204: By calculating the satellite-to-ground slant distance R 1 Subtract the offset distance d 0 Get the actual slant distance R of the difference beam center 2 ; and calculate the slope distance R by iterative approximation 2 The corresponding lower viewing angle θ 2 ; The pointing deviation is calibrated to Δθ = θ 1 -θ 2 ;

[0014] Step S205: perform multiple measurements to obtain statistical results of SAR antenna beam pointing calibration parameter measurements; store the calibration parameters in an onboard memory as a reference for subsequent calibration.

[0015] Furthermore, the on-orbit calibration step comprises:

[0016] Step S501, determining a pointing calibration area and a calibration observation period for the pointing calibration area in a conventional observation task; observing the area at the same incident angle in the conventional observation task according to the calibration observation period, and acquiring SAR echo data of each calibration observation;

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

[0018] Step S503: using the range direction pattern in the initial pointing calibration parameter as the initial value, comparing the range direction pattern of the current observation process with that of the previous observation process, determining the main lobe offset of the current observation relative to the previous observation, and inverting the range pointing deviation;

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

[0020] Further, step S501 includes:

[0021] Step S501-1, determining a pointing calibration area in a conventional observation task; the pointing calibration area is an area where the scattering characteristics are stable during the entire conventional observation task;

[0022] Step S501-2, determining the satellite's calibration observation period and coverage of the pointing calibration area according to the satellite orbit parameters;

[0023] Step S501-3: After the initial calibration of the Amazon rainforest is completed, a shooting of the pointing calibration area is arranged immediately as the first calibration observation of the on-orbit calibration, providing a reference initial value for the subsequent business data pointing calibration;

[0024] Step S501-4: In the subsequent business operation process, the same wave position is used to periodically observe the area, and auxiliary information including satellite attitude and orbit parameters are recorded each time.

[0025] Furthermore, the pointing calibration area is a flat area with stable backscattering characteristics including the Gobi Desert that is not affected by seasonal climate change;

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

[0027] Further, step S502 includes:

[0028] Step S502-1, performing channel merging and decompression on the observed SAR echo data to generate L0 level data of the area;

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

[0030] Step S502-3, accumulating the range-compressed data along the azimuth direction, and calculating the amplitude mean of each range gate to obtain a range direction pattern;

[0031] Step S502-4: normalize the range direction pattern and store it in the calibration database as a reference for the first service data calibration.

[0032] Further, step S503 includes:

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

[0034] Step S503-2, performing range-direction processing on the acquired data to obtain range-direction patterns of the calibration data of the second, third, ..., nth periods;

[0035] Step S503-3, using a cross-correlation algorithm to successively calculate the offset of the range direction pattern of the current observation processing and the previous observation processing;

[0036] Step S503-4: invert the slant distance corresponding to the offset, and then calculate the corresponding range-direction deviation angle according to the slant distance of the scene center during shooting.

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

[0038]

[0039] Among them, x(n) represents the directional pattern after the previous observation distance is compressed, y(n) represents the directional pattern after the current observation distance is compressed, and m is the offset; when the cross-correlation function R xy When (m) takes the 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 parameter is used as the initial value of x(n);

[0040] A positive offset indicates that the beam is offset away from the sub-satellite point, and a negative offset indicates that the beam is offset toward the sub-satellite point.

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

[0042]

[0043] Where c is the speed of light, F s is the sampling rate of the two shots before and after;

[0044] Obtain the center slant distance R from the satellite attitude data by converting the Euler angles c , the conversion formula is;

[0045]

[0046] Among them, R x(α), R y (β), R z (γ) are the rotation angles of the satellite’s x, y, and z axes, respectively. is the initial position of the sight vector [0,0,1] T ;

[0047] The satellite line of sight vector The straight line where the satellite is located intersects the earth's surface. The straight line distance between the satellite and the intersection is the slant distance R of the center of the scene. c ; And then calculate the deviation angle:

[0048]

[0049] A positive value indicates that the distance pointing is larger than the calibration value, and a negative value indicates that the distance pointing is smaller than the calibration value.

[0050] On the other hand, the present invention also discloses a system using the above-mentioned on-orbit calibration method of a spaceborne synthetic aperture radar, 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 the satellite performs routine observation tasks after entering orbit, and obtain the initial value of the range beam pointing calibration parameter;

[0052] The successive on-orbit calibration module is used in routine observation tasks to determine the pointing calibration area and the calibration observation period for the pointing calibration area; based on the business data of the routine observation task, the range direction pattern observed for the pointing calibration area in each calibration observation period is calculated; the observed range direction patterns are compared successively to invert the range pointing deviation of the current observation relative to the previous observation, correct the current range direction pattern, and update the current range beam pointing calibration parameters to ensure the accuracy of the business data of the routine observation task in each calibration observation period.

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

[0054] The on-orbit calibration method and system of a satellite-borne synthetic aperture radar disclosed in the present invention only need to arrange a special differential beam calibration task once after the satellite enters orbit, and subsequently use the satellite revisit cycle and operational mission data to achieve regular estimation and calibration of the radar range beam pointing, which greatly reduces the number of arrangements for special calibration tasks, saves satellite resources and mission planning time, and improves the efficiency and economy of the calibration work.

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

[0056] It can timely detect and correct the range beam pointing deviation caused by various factors (such as satellite attitude changes, component aging, etc.) during the radar's on-orbit operation, ensuring the accuracy and reliability of satellite-borne SAR data. It is of great significance to improve the quality of satellite remote sensing data and better serve application fields such as terrain surveying and environmental monitoring. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] The drawings are only for the purpose of illustrating particular embodiments and are not to be considered limiting of the present invention. Like reference symbols denote like components throughout the drawings.

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

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

[0060] Figure 3 A schematic diagram showing the comparison between a differential beam and a conventional beam in an embodiment of the present invention;

[0061] Figure 4 Schematic diagram of the geometric relationship between satellite and ground in an embodiment of the present invention;

[0062] Figure 5 This is a flow chart of successive on-orbit calibration in an embodiment of the present invention;

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

[0064] Figure 7 The figure is a schematic diagram of the connection components of the on-orbit calibration system of a space-borne synthetic aperture radar in an embodiment of the present invention. DETAILED DESCRIPTION

[0065] The preferred embodiments of the present invention will be described in detail below in conjunction with the accompanying drawings, wherein the accompanying drawings constitute a part of this application and are used to illustrate the principles of the present invention together with the embodiments of the present invention.

[0066] Embodiment 1

[0067] An embodiment of the present invention discloses an on-orbit calibration method for a spaceborne synthetic aperture radar, which is used to calibrate the radar range beam pointing; Figure 1 As shown, including:

[0068] Step S101, a single on-orbit calibration step before a routine observation mission: after the satellite enters orbit and before performing a routine observation mission, a uniform scene target including the Amazon rainforest is selected to perform a calibration task to obtain the initial value of the range beam pointing calibration parameter;

[0069] Step S102, in the routine observation task, the on-orbit calibration steps are performed successively; the pointing calibration area and the calibration observation period for the pointing calibration area in the routine observation task are determined; based on the business data of the routine observation task, the range direction pattern observed for the pointing calibration area in each calibration observation period is calculated; the observed range direction pattern is compared successively, the range pointing deviation of the current observation relative to the previous observation is inverted, the current range direction pattern is corrected, and the current range beam pointing calibration parameters are updated to ensure the accuracy of the business data of the routine observation task in each calibration observation period.

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

[0071] Step S201: After the satellite is in orbit and in working condition, a uniform scene target is selected and the SAR payload is arranged to perform differential beam observation, with the beam positions covering high, medium and low viewing angles;

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

[0073] Step S202, performing distance compression on the echo signal obtained by the difference beam observation and calculating the offset distance, accurately locating the sampling point position corresponding to the difference beam notch in the radiation pattern, and calculating the offset distance of the sampling point position;

[0074] include:

[0075] S202-1. Accumulate the range-compressed signals in the azimuth direction to obtain the range-direction difference beam antenna pattern. Since the radiation gain at the difference beam notch is low and the width is extremely narrow, such as Figure 3 As shown, the distance positioning is more accurate. The sampling point position corresponding to the notch can be accurately located in the direction diagram after the distance compression.

[0076] S202-2, subtract the above sampling points from half of the total sampling points in the range direction, and obtain the number of sampling points that deviate from the center. The offset distance d in the slant range direction can be further calculated. 0 .

[0077]

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

[0079] Step S203, determining the satellite's sight vector and the satellite-to-ground slant range on the ground based on the satellite attitude data at the intermediate observation time;

[0080] In such Figure 4 In the satellite-ground geometry shown, the line of sight vector The angle of view θ is converted from the attitude data. The surface of the earth can be represented by an ellipsoid equation, that is:

[0081]

[0082] R e is the Earth's equatorial radius, R p is the polar radius of the Earth. Find the intersection point with the above equation to reach the ground position (x 0 ,y 0 ,z 0 ). Since the satellite position (x, y, z) is known, the satellite-to-ground slant distance R can be calculated 1 , expressed as:

[0083]

[0084] Step S204: By calculating the satellite-to-ground slant distance R 1 Subtract the offset distance d 0 Get the actual slant distance R of the difference beam center 2 ; and calculate the slope distance R by iterative approximation 2 The corresponding lower viewing angle θ 2 ; The pointing deviation is calibrated to Δθ = θ 1 -θ 2 ;

[0085] During the iterative approximation process, the initial angle of iteration is the line of sight direction θ 1 , calculate the initial satellite-to-ground slant moment according to the initial angle, calculate the distance difference between the initial slant moment and the actual slant moment, and judge whether it is less than the set threshold; if not, adjust the initial angle for iterative approximation until the distance difference is less than the set threshold, and use the iterative approximation angle as the slant distance R 2 The lower viewing angle θ 2 .

[0086] Step S205: perform multiple measurements to obtain statistical results of SAR antenna beam pointing calibration parameter measurements; store the calibration parameters in an onboard memory as a reference for subsequent calibration.

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

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

[0089] Specifically, they include:

[0090] Step S501-1, determining a pointing calibration area in a conventional observation task; the pointing calibration area is an area where the scattering characteristics are stable during the entire conventional observation task;

[0091] like Figure 6 As shown, a flat area with stable backscattering characteristics including the Gobi Desert which is not affected by seasonal climate changes is preferred.

[0092] Step S501-2, determining the satellite's calibration observation period and coverage of the pointing calibration area according to the satellite orbit parameters;

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

[0094] For example, assuming that the satellite revisit period is 10 days and a calibration observation is performed using three revisit periods, the calibration observation period is 30 days.

[0095] Step S501-3: After the initial calibration of the Amazon rainforest is completed, a shooting of the pointing calibration area is arranged immediately as the first calibration observation of the on-orbit calibration, providing a reference initial value for the subsequent business data pointing calibration;

[0096] Step S501-4: In the subsequent business operation process, the same wave position is used to periodically observe the area, and auxiliary information including satellite attitude and orbit parameters are recorded each time.

[0097] Step S502, performing range compression processing on the SAR echo data of each calibration observation to obtain a 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 of the area;

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

[0101] The radar transmission signal is s(t), and the decompressed echo complex signal is s(t,τ), where t is the fast time in the range direction and τ is the slow time in the azimuth direction; the matched filter signal is s * (t), expressed as the conjugate of the transmitted signal s(t), the signal after range compression is:

[0102]

[0103] Step S502-3, accumulating the range-compressed data along the azimuth direction, and calculating the amplitude mean of each range gate to obtain a range direction pattern;

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

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

[0106] The mean value of the signal amplitude is calculated along the first dimension, which can be expressed as:

[0107] s c (t) = [abs(s 1 ) … abs(s m )]

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

[0109] Step S502-4: normalize the range direction pattern and store it in the calibration database as a reference for the first service data calibration.

[0110] The main lobe pattern in the range direction is normalized using the minimum and maximum value normalization to obtain the normalized range pattern s n (t);

[0111]

[0112] Get the normalized distance direction map s n (t).

[0113] Step S503: using the range direction pattern in the initial pointing calibration parameter as the initial value, comparing the range direction pattern of the current observation process with the previous observation process one by one, determining the main lobe offset of the current observation process relative to the previous observation process, and inverting the range pointing deviation;

[0114] Step S503 includes:

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

[0116] Step S503-2, performing range-direction processing on the acquired data to obtain range-direction patterns of the calibration data of the second, third, ..., nth periods;

[0117] Step S503-3, using a cross-correlation algorithm to successively calculate the offset of the range direction pattern of the current observation processing and the previous observation processing;

[0118] The cross-correlation function expression is:

[0119]

[0120] Among them, x(n) represents the directional diagram after the previous observation distance is compressed, y(n) represents the directional diagram after the current observation distance is compressed, and m is the offset. xy When (m) reaches the maximum value, the corresponding m is the offset; the distance direction pattern in the initial pointing calibration parameter is taken as the initial value of x(n);

[0121] A positive offset indicates that the beam is offset away from the sub-satellite point, and a negative offset indicates that the beam is offset toward the sub-satellite point.

[0122] Step S503-4, invert the slant distance corresponding to the offset, and then calculate the corresponding distance pointing deviation angle according to the slant distance of the scene center during shooting;

[0123] Based on the premise that the wave position parameters of the two shots are the same, the sampling rate F of the two shots is s , according to the inversion of the corresponding slope distance of the offset m, the expression is:

[0124]

[0125] Where c is the speed of light,

[0126] Obtain the center slant distance R from the satellite attitude data by converting the Euler angles c , the conversion formula is;

[0127]

[0128] Among them, R x (α), R y (β), R z (γ) are the rotation angles of the satellite’s x, y, and z axes, respectively. is the initial position of the sight vector [0,0,1] T .

[0129] The satellite line of sight vector The straight line where the satellite is located intersects the earth's surface. The straight line distance between the satellite and the intersection is the slant distance R of the center of the scene. c ; And then calculate the deviation angle:

[0130]

[0131] A positive value indicates that the distance pointing is larger than the calibration value, and a negative value indicates that the distance pointing is smaller than the calibration value.

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

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

[0134] In summary, the on-orbit calibration method of a satellite-borne synthetic aperture radar disclosed in an embodiment of the present invention only needs to arrange a special differential beam calibration task once after the satellite enters orbit, and then the satellite revisit period and operational mission data can be used to achieve regular estimation and calibration of the radar range beam pointing, which greatly reduces the number of arrangements for special calibration tasks, saves satellite resources and mission planning time, and improves the efficiency and economy of the calibration work.

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

[0136] It can timely detect and correct the range beam pointing deviation caused by various factors (such as satellite attitude changes, component aging, etc.) during the radar's on-orbit operation, ensuring the accuracy and reliability of satellite-borne SAR data. It is of great significance to improve the quality of satellite remote sensing data and better serve application fields such as terrain surveying and environmental monitoring.

[0137] Embodiment 2

[0138] An embodiment of the present invention discloses an on-orbit calibration system for a spaceborne synthetic aperture radar, which is used to calibrate the radar range beam pointing; 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 the satellite performs routine observation tasks after entering orbit, and obtain the initial value of the range beam pointing calibration parameter;

[0140] The successive on-orbit calibration module is used in routine observation tasks to determine the pointing calibration area and the calibration observation period for the pointing calibration area; based on the business data of the routine observation task, the range direction pattern observed for the pointing calibration area in each calibration observation period is calculated; the observed range direction patterns are compared successively to invert the range pointing deviation of the current observation relative to the previous observation, correct the current range direction pattern, and update the current range beam pointing calibration parameters to ensure the accuracy of the business data of the routine observation task in 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 will not be described in detail here.

[0142] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by any technician familiar with the technical field within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. A method for on-orbit calibration of a spaceborne synthetic aperture radar, characterized in that: Used to calibrate the radar range beam pointing direction; including: Before the routine observation mission, a single on-orbit calibration step is performed; after the satellite enters orbit and before performing the routine observation mission, a uniform scene target including the Amazon rainforest is selected to perform a calibration mission to obtain the initial value of the range beam pointing calibration parameter; In routine observation missions, the on-orbit calibration steps are performed one by one; the pointing calibration area and the calibration observation period for the pointing calibration area in the routine observation mission are determined; based on the business data of the routine observation mission, the range direction pattern observed for the pointing calibration area in each calibration observation period is calculated; the observed range direction patterns are compared one by one to invert the range pointing deviation of the current observation relative to the previous observation, the current range direction pattern is corrected, and the current range beam pointing calibration parameters are updated to ensure the accuracy of the business data of the routine observation mission in each calibration observation period.

2. The on-orbit calibration method of a spaceborne synthetic aperture radar according to claim 1, characterized in that: The single on-orbit calibration steps include: Step S201: After the satellite is in orbit and in working condition, a uniform scene target is selected and the SAR payload is arranged to perform differential beam observation, with the beam positions covering high, medium and low viewing angles; Step S202, performing distance compression on the echo signal obtained by the difference beam observation and calculating the offset distance, accurately locating the sampling point position corresponding to the difference beam notch in the directional diagram, and calculating the offset distance d0 of the sampling point position; Step S203, determining the satellite's sight vector and the satellite-to-ground slant range R1 based on the satellite attitude data at the middle observation moment; Step S204, obtain the actual slant distance R2 of the difference beam center by subtracting the offset distance d0 from the satellite-to-ground slant distance R1; and calculate the lower viewing angle θ2 corresponding to the slant distance R2 by iterative approximation; calibrate the pointing deviation as Δθ=θ1-θ2; Step S205: perform multiple measurements to obtain statistical results of SAR antenna beam pointing calibration parameter measurements; store the calibration parameters in an onboard memory as a reference for subsequent calibration.

3. The on-orbit calibration method of a spaceborne synthetic aperture radar according to claim 1, characterized in that: The steps of on-orbit calibration include: Step S501, determining a pointing calibration area and a calibration observation period for the pointing calibration area in a conventional observation task; observing the area at the same incident angle in the conventional observation task according to the calibration observation period, and acquiring SAR echo data of each calibration observation; Step S502, performing range compression processing on the SAR echo data of each calibration observation to obtain a range direction pattern; Step S503: using the range direction pattern in the initial pointing calibration parameter as the initial value, comparing the range direction pattern of the current observation process with that of the previous observation process, determining the main lobe offset of the current observation relative to the previous observation, and inverting the range pointing deviation; Step S504: correct the current range-to-direction pattern according to the inverted range-to-direction deviation, update the pointing calibration parameters, and ensure the accuracy of each observation.

4. The on-orbit calibration method of a spaceborne synthetic aperture radar according to claim 3, characterized in that: Step S501 includes: Step S501-1, determining a pointing calibration area in a conventional observation task; the pointing calibration area is an area where the scattering characteristics are stable during the entire conventional observation task; Step S501-2, determining the satellite's calibration observation period and coverage of the pointing calibration area according to the satellite orbit parameters; Step S501-3: After the initial calibration of the Amazon rainforest is completed, a shooting of the pointing calibration area is arranged immediately as the first calibration observation of the on-orbit calibration, providing a reference initial value for the subsequent business data pointing calibration; Step S501-4: In the subsequent business operation process, the same wave position is used to periodically observe the area, and auxiliary information including satellite attitude and orbit parameters are recorded each time.

5. The on-orbit calibration method of a spaceborne synthetic aperture radar according to claim 4, characterized in that: The pointing calibration area is a flat area with stable backscattering characteristics, including the Gobi Desert, which is not affected by seasonal climate changes; The calibration observation period pointing to the calibration area is an integer multiple of the satellite revisit period determined by the satellite orbit parameters, and each time the calibration observation is performed, the incident angle to the calibration area is kept the same.

6. The on-orbit calibration method of a spaceborne synthetic aperture radar according to claim 3, characterized in that: Step S502 includes: Step S502-1, performing channel merging and decompression on the observed SAR echo data to generate L0 level data of the area; Step S502-2: construct a matched filter based on the L0 level auxiliary data to perform range-direction pulse compression on the echo data; Step S502-3, accumulating the range-compressed data along the azimuth direction, and calculating the amplitude mean of each range gate to obtain a range direction pattern; Step S502-4: normalize the range direction pattern and store it in the calibration database as a reference for the first service data calibration.

7. The on-orbit calibration method of a spaceborne synthetic aperture radar according to claim 3, characterized in that: Step S503 includes: Step S503-1, based on the pointing calibration area and the calibration observation period of the pointing calibration area, periodically obtain corresponding L0 level data from the business data; Step S503-2, performing range-direction processing on the acquired data to obtain range-direction patterns of the calibration data of the second, third, ..., nth periods; Step S503-3, using a cross-correlation algorithm to successively calculate the offset of the range direction pattern of the current observation processing and the previous observation processing; Step S503-4: invert the slant distance corresponding to the offset, and then calculate the corresponding range-direction deviation angle according to the slant distance of the scene center during shooting.

8. The on-orbit calibration method for spaceborne synthetic aperture radar according to claim 7, characterized in that: The cross-correlation function expression is: Among them, x(n) represents the directional pattern after the previous observation distance is compressed, y(n) represents the directional pattern after the current observation distance is compressed, and m is the offset; when the cross-correlation function R xy When (m) takes the 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 parameter is used as the initial value of x(n); A positive offset indicates that the beam is offset away from the sub-satellite point, and a negative offset indicates that the beam is offset toward the sub-satellite point.

9. The on-orbit calibration method of a spaceborne synthetic aperture radar according to claim 7, characterized in that: According to the inversion of the offset m, the corresponding slope distance d is expressed as: Where c is the speed of light; F s is the sampling rate of the two shots before and after; Obtain the center slant distance R from the satellite attitude data by converting the Euler angles c , the conversion formula is; Among them, R x (α), R y (β), R z (γ) are the rotation angles of the satellite’s x, y, and z axes, respectively. is the initial position of the sight vector [0,0,1] t ; The satellite line of sight vector The straight line where the satellite is located intersects the earth's surface. The straight line distance between the satellite and the intersection is the slant distance R of the center of the scene. c ; And then calculate the deviation angle: A positive value indicates that the distance pointing is larger than the calibration value, and a negative value indicates that the distance pointing is smaller than the calibration value.

10. A system using the on-orbit calibration method of a spaceborne synthetic aperture radar according to any one of claims 1 to 9, characterized in that: include: Single on-orbit calibration module and successive on-orbit calibration module; among them, The single on-orbit calibration module is used to select a uniform scene target including the Amazon rainforest to perform a calibration task before the satellite performs routine observation tasks after entering orbit, and obtain the initial value of the range beam pointing calibration parameter; The sequential on-orbit calibration module is used in routine observation tasks to determine the pointing calibration area and the calibration observation period for the pointing calibration area; Based on the business data of routine observation tasks, the range direction pattern observed in the pointing calibration area in each calibration observation period is calculated; the observed range direction patterns are compared one by one, and the range pointing deviation of the current observation relative to the previous observation is inverted, the current range direction pattern is corrected, and the current range beam pointing calibration parameters are updated to ensure the accuracy of the business data of routine observation tasks in each calibration observation period.

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