On-orbit calibration method for space-borne laser altimeter combining single-peak and multi-peak footprints

Abstract

The application relates to an on-orbit calibration method of a satellite-borne laser altimeter combined with single-peak and multi-peak footprints, which comprises the following steps: using waveform matching to obtain the reference position of the multi-peak footprint, iteratively calculating the footprint position of the single-peak according to reference terrain data, and proposing a calculation model for jointly calculating calibration parameters by using single-peak and multi-peak footprint data, so that the ranging calibration parameter is calculated by using single-peak footprint data, and the pointing angle calibration parameter is calculated by using multi-peak footprint data; finally, the calibration precision is improved by using multi-orbit joint calibration. Compared with the prior art, the algorithm and the model can effectively calibrate the ranging and angle measurement system errors of the satellite-borne laser altimeter.

Description

Technical Field

[0001] This invention relates to the field of laser altimeter calibration technology, and in particular to an on-orbit calibration method for a spaceborne laser altimeter that combines single-peak and multi-peak footprints. Background Technology

[0002] Spaceborne laser altimetry technology is widely used in various scientific fields due to its advantages such as all-weather observation, large observation scale, saving manpower and material resources, strong penetration and high measurement accuracy.

[0003] Before satellite launch, laser altimeters measure data such as the satellite's positioning matrix and laser pointing angle in a laboratory. However, after launch, due to platform flutter and changes in ambient temperature, parameters such as the satellite's pointing angle may change to some extent. Therefore, to improve ranging accuracy and provide high-quality data support for subsequent data processing, it is necessary to perform on-orbit calibration of the ranging and pointing angle system errors of the spaceborne laser altimeter. On-orbit geometric calibration methods can be divided into three categories: 1) on-orbit geometric calibration based on ocean surface attitude maneuvers; 2) on-orbit geometric calibration based on in-situ footprint measurements; and 3) on-orbit geometric calibration based on natural terrain surfaces. Ocean surface maneuver calibration methods have high requirements for the satellite platform and are not suitable for most satellite platforms. In-situ footprint measurement methods offer high accuracy but consume significant manpower and resources, making short-cycle, multiple calibrations impossible. Methods based on natural terrain surfaces do not rely on field experiments and have a wide range of terrain data sources with high selectivity. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the existing technology and provide an on-orbit calibration method for a spaceborne laser altimeter that combines single-peak and multi-peak footprints to improve the accuracy of ranging and pointing angle calibration.

[0005] The objective of this invention can be achieved through the following technical solutions:

[0006] A method for on-orbit calibration of a spaceborne laser altimeter combining single-peak and multi-peak footprints includes the following steps:

[0007] Based on the echoes obtained by the spaceborne laser altimeter, the waveform features of each footprint were extracted, and single-peak footprint data and multi-peak footprint data were selected.

[0008] For multi-peak footprint data, waveform simulation and matching are performed using reference terrain data to obtain the horizontal offset of a single footprint in the multi-peak footprint data. Then, the horizontal offset of each footprint is globally optimized according to the system error characteristics to obtain the reference position of the multi-peak footprint data.

[0009] The elevation value of the single-peak footprint is corrected using reference terrain data to obtain the initial reference position of the single-peak footprint.

[0010] The reference positions of the multi-peak footprint data and the single-peak footprint data are substituted into the joint calibration model to solve the calibration parameters. During the calculation process, the ranging calibration parameters are solved from the single-peak footprint data, and the pointing angle calibration parameters are solved from the multi-peak footprint data.

[0011] Furthermore, the process of obtaining the reference position of the multi-peak footprint data specifically includes:

[0012] In the multi-peak footprint data, a square search area is made with each laser footprint initial value as the center. The center of the laser footprint is moved within the search area with a step size not less than the reference terrain resolution. The echo inside the footprint is simulated in the reference terrain data, and the correlation coefficient is calculated with the real echo. The footprint position with the highest waveform correlation is taken as the horizontal offset of the current footprint.

[0013] Data with a correlation coefficient less than 0.8 in the waveform correlation coefficient matrix of all multi-peak footprints on the same track are removed and superimposed. The position with the highest correlation after superposition is used as the global horizontal offset.

[0014] The horizontal offset of each footprint in the multi-peak footprint data is subtracted from the global horizontal offset. For footprint data where the difference in the X or Y direction is greater than the matching accuracy, a square search area is drawn with the global horizontal offset as the center and the matching accuracy as the side length. The position with the highest waveform correlation in the search area is taken as the final horizontal offset of the current footprint.

[0015] The reference position of each footprint is calculated based on the final horizontal offset of each footprint in the multi-peak footprint and the reference terrain data.

[0016] Furthermore, the method also includes:

[0017] After subtracting the horizontal offset of each footprint in the multi-peak footprint data from the global horizontal offset, the horizontal offset of the footprint data whose difference in the X and Y directions is less than or equal to the matching accuracy is taken as the final horizontal offset.

[0018] Furthermore, the side length of the square search area is determined based on the coarse calibration accuracy and the accuracy of the laser altimeter.

[0019] Furthermore, the process of calculating the elevation reference value of single-peak footprint data is as follows:

[0020] Using the corrected footprint plane position as the center, select reference terrain data within the radius of the single-peak footprint, and calculate the average elevation of all terrain points within the selected area as the current footprint elevation reference value.

[0021] Furthermore, the expression for the joint calibration model is:

[0022] V = BX + CPt - L

[0023] In the formula, R is the rotation matrix from the laser coordinate system to the station center coordinate system, and ρ is the distance measured. Represents calibration parameters. ω is the pitch angle, Δρ1 is the distance correction, C represents the partial derivative with respect to the station center coordinates; P is the weight matrix, and t is the correction for the lower coordinates of the station center coordinate system. V is the difference between the estimated position and the reference position of the footprint, and V is the footprint position residual.

[0024] Furthermore, through derivation using the least squares principle, the formula for solving the unknowns of the joint calibration model is as follows:

[0025]

[0026] After continuous optimization through global iteration, the final solution is obtained.

[0027] Furthermore, the method also includes calibrating the system deviation of the ranging and pointing of the spaceborne laser altimeter based on the calculated calibration parameters.

[0028] Furthermore, the method performs on-orbit calibration of the laser altimeter based on all single-peak footprint data and multi-peak footprint data of multiple orbits involved in the calculation.

[0029] Furthermore, the reference terrain data is high-precision reference terrain data.

[0030] Compared with the prior art, the present invention has the following advantages:

[0031] (1) This invention considers that single-peak data with low terrain heterogeneity and small ground undulation has higher accuracy in calculating distance values. Conversely, multi-peak data with high terrain heterogeneity and significant ground features or obvious undulations has lower accuracy in calculating distance values ​​due to errors caused by multi-peaks and slope effects. Therefore, single-peak data is more suitable for calculating distance errors during calibration. However, in the geometric calibration method based on waveform matching, multi-peak data with high terrain heterogeneity has more features and higher matching accuracy than single-peak data, thus achieving higher accuracy in angle calibration values. Therefore, this invention proposes a calculation model that uses single-peak and multi-peak footprint data to jointly calculate calibration parameters. This allows the distance calibration parameters to be calculated from single-peak footprint data and the pointing angle calibration parameters to be calculated from multi-peak footprint data, effectively calibrating the distance and angle measurement system errors of the spaceborne laser altimeter.

[0032] (2) The proposed geometric calibration method was verified using high-precision airborne point cloud data and simulated laser altimetry data. Experimental results show that the proposed method can achieve on-orbit geometric calibration of full-waveform spaceborne laser altimetry data, with a calibration accuracy of 0.1m for ranging error and 2.0″ for laser pointing angle error. Attached Figure Description

[0033] Figure 1 This is a schematic diagram of the terrain and waveform within a single-peak footprint provided in an embodiment of the present invention;

[0034] Figure 2 This is a schematic diagram of the terrain and waveform within a multi-peak footprint provided in an embodiment of the present invention;

[0035] Figure 3 This is a flowchart illustrating an on-orbit calibration method for a spaceborne laser altimeter that combines single-peak and multi-peak footprints, as provided in an embodiment of the present invention. Detailed Implementation

[0036] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0037] Therefore, the following detailed description of the embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention.

[0038] It should be noted that similar labels and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

[0039] Example 1

[0040] Depending on the heterogeneity of the terrain surface illuminated by the laser footprint, the laser echo received by the laser altimeter can exhibit single-peak or multi-peak patterns. When the terrain heterogeneity within the laser footprint is low and there are no obvious features, the received echo will only have a single peak on the ground, such as... Figure 1 As shown; when the internal terrain of a laser footprint is highly heterogeneous, with significant topographic undulations or prominent features, the received echo will exhibit two or more multi-peak patterns, such as... Figure 2As shown in the diagram. Based on the principles of laser altimetry and the methods for calculating distance measurements, single-peak data with low terrain heterogeneity and minimal ground undulation yields higher accuracy in distance measurement calculations. Conversely, multi-peak data with high terrain heterogeneity and significant ground features or undulations results in lower accuracy due to errors introduced by the multi-peaks and slope effects. Therefore, single-peak data is more suitable for calculating distance measurement errors during calibration. However, in geometric calibration methods based on waveform matching, multi-peak data with high terrain heterogeneity exhibits more features and higher matching accuracy than single-peak data, thus achieving higher accuracy in angle measurement calibration values. Therefore, during on-orbit geometric calibration of the altimeter, it is necessary to fully utilize both single and multi-peak data to calculate distance and angle measurements separately.

[0041] Spaceborne laser altimetry technology is widely used in 3D Earth surface data acquisition tasks due to its high precision and large-scale capabilities. After a laser altimetry satellite is launched, the laser ranging and pointing angles will change to some extent compared to pre-launch laboratory measurements due to changes in the laser payload environment. To obtain accurate ranging and laser pointing values ​​and produce high-precision altimetry products, geometric calibration of the laser payload is necessary during the satellite's on-orbit operation to calibrate systematic deviations in ranging and pointing. For example... Figure 3 As shown, this embodiment, based on the characteristics of domestically produced laser altimeter satellite payloads, provides an on-orbit calibration method for a spaceborne laser altimeter that combines single-peak and multi-peak footprints, including the following steps:

[0042] S1: Based on the echoes obtained by the spaceborne laser altimeter, extract the waveform features of each footprint and filter out single-peak footprint data and multi-peak footprint data.

[0043] S2: For multi-peak footprint data, waveform simulation and matching are performed using reference terrain data to obtain the horizontal offset of a single footprint in the multi-peak footprint data. Then, the horizontal offset of each footprint is globally optimized according to the system error characteristics to obtain the reference position of the multi-peak footprint data.

[0044] S3: Correct the elevation value of the single-peak footprint using reference terrain data to obtain the initial reference position of the single-peak footprint;

[0045] S4: Substitute the reference positions of the multi-peak footprint data and the single-peak footprint data into the joint calibration model to solve the calibration parameters. During the calculation process, the ranging calibration parameters are solved from the single-peak footprint data, and the pointing angle calibration parameters are solved from the multi-peak footprint data.

[0046] The following is a detailed description of each step:

[0047] In step S2, the specific scheme for calculating the reference position of the multi-peak footprint data is as follows:

[0048] For multi-peak footprint data, waveform simulation and matching need to be performed in conjunction with high-precision reference terrain data to obtain the horizontal offset of a single footprint. Then, the offset is globally optimized based on the system error characteristics to reduce the impact of mismatches on the calculation of calibration parameters. The specific process is as follows:

[0049] (1) A square search area is formed with the initial value of the laser footprint as the center. The side length of the area is determined according to factors such as the coarse calibration accuracy and the accuracy of the laser altimeter. The center of the laser footprint is moved within the search area with a step size not less than the reference terrain resolution. The echo inside the footprint is simulated and the correlation coefficient is calculated with the real echo. The footprint position with the highest waveform correlation is taken as the current horizontal offset of the footprint.

[0050] (2) After removing data with less than 0.8 from the waveform correlation coefficient matrix of all multi-peak footprints on the same track, the matrix is ​​superimposed, and the position with the highest correlation after superposition is taken as the global horizontal offset.

[0051] (3) Subtract the horizontal offset of each footprint from the global horizontal offset. For footprint data whose difference in the X and Y directions is less than or equal to the matching accuracy, no global optimization is performed, and the corresponding horizontal offset is the final horizontal offset. For footprint data whose difference in the X or Y directions is greater than the matching accuracy, a square search area is drawn with the global horizontal offset as the center and the matching accuracy as the side length. The position with the highest waveform correlation in the search area is taken as the final horizontal offset of the current footprint.

[0052] (4) Calculate the reference position of each footprint based on the final horizontal offset of each multi-peak footprint and the reference terrain data.

[0053] In step S3, the specific calculation process for the reference position of the single-peak footprint is as follows:

[0054] The planar position of a single-peak footprint is initially corrected based on the global matching results of the multi-peak footprint data. Then, with the corrected planar position as the center, reference high-precision terrain data within the radius of the footprint is selected, and the average elevation of all terrain points is calculated as the elevation reference value of the current footprint.

[0055] In step S4, the specific process of the parameter calculation model combining single and multi-peak footprint data is as follows:

[0056] By combining the advantages of solving multi-peak pair angle measurement error and single-peak pair distance measurement error, a joint adjustment model is constructed, and the method for jointly solving the calibration parameters is as follows:

[0057] Substituting single and multi-peak footprint data into the joint calibration model, the calibration parameters are calculated. The calibration model is as follows:

[0058] V = BX + CPt - L (1)

[0059] in, R is the rotation matrix from the laser coordinate system to the station center coordinate system, and ρ is the distance measurement value; represents the calibration parameters, namely the corrections for angle and distance measurements; C represents the partial derivative with respect to the ENU coordinates; P is the weight matrix, and the model mainly distinguishes between multi-peak footprints as plane control points and single-peak footprints as elevation control points; t is the coordinate correction of the lower point in the ENU coordinate system.

[0060] The formula for solving the unknown can be derived using the least squares principle as follows:

[0061]

[0062] The solution X is the scaling correction, and the final solution can be obtained after continuous optimization through global iteration.

[0063] Preferably, this solution also includes step S5: performing multi-track joint calibration, specifically:

[0064] To fully utilize the acquired laser calibration data and address the issue of sparse multi-peak laser data caused by adverse weather conditions and small reference terrain undulations, a multi-track data joint calibration method is adopted to improve the on-orbit calibration accuracy of the laser altimeter. The specific implementation steps are as follows:

[0065] (1) After removing data with a correlation matrix less than 0.8 from the waveform correlation matrix of all multi-peak footprint data participating in the calculation of the track, the data are superimposed, and the position with the highest correlation after superposition is used as the global offset.

[0066] (2) Subtract the horizontal offset of each multi-peak footprint from the global offset. For footprint data with a difference in X and Y directions greater than or equal to the plane matching threshold, no global optimization is performed, and the corresponding horizontal offset is the final horizontal offset. For footprint data with a difference in X or Y directions greater than the matching accuracy, take the global horizontal offset as the center and the plane matching threshold as the side length to form a search area. Take the position with the highest waveform correlation in the search area as the final horizontal offset of the current footprint.

[0067] (3) The planar position of the single-peak footprint is initially corrected based on the global matching results of the multi-peak footprint data. Then, the reference high-precision terrain data within the radius of the footprint is selected with the corrected planar position as the center, and the average elevation of all terrain points is calculated as the elevation reference value of the current footprint.

[0068] (4) Calculate the reference position of each footprint based on the final horizontal offset of each multi-peak footprint and the reference terrain data; calculate the average elevation of all terrain points within the footprint radius based on the planar position of the single-peak footprint and the reference high-precision terrain data, and use it as the elevation reference value of the current footprint.

[0069] (5) Use Equation 1 to solve the overall calibration parameters.

[0070] Experimental Results and Discussion

[0071] This paper uses high-precision airborne point cloud data to simulate satellite laser altimetry data and experimentally verifies the proposed geometric calibration method. High-precision airborne point cloud data from the test area is used to simulate multi-track laser altimetry data, and joint calibration is performed using data from 36 adjacent tracks. The following random errors are added to the simulated laser altimetry data: orbit determination error 0.1m, attitude determination errors 3″ and 24″ respectively, ranging random error 0.3m, and pointing random error 1″. The calibration results are shown in Table 1.

[0072] Table 1. Results of joint calibration of track data

[0073]

[0074]

[0075] in conclusion

[0076] Based on the characteristics of domestically produced full-waveform spaceborne laser altimetry payloads, this paper proposes an on-orbit geometric calibration method combining single and multi-peak footprint data with multi-track integration. The proposed method is validated using high-precision airborne point cloud data and simulated laser altimetry data. Experimental results show that the proposed method can achieve on-orbit geometric calibration of full-waveform spaceborne laser altimetry data, with a calibration accuracy of 0.1m for ranging error and 2.0″ for laser pointing angle error.

[0077] The preferred embodiments of the present invention have been described in detail above. It should be understood that those skilled in the art can make numerous modifications and variations based on the concept of the present invention without creative effort. Therefore, all technical solutions that can be obtained by those skilled in the art based on the concept of the present invention through logical analysis, reasoning, or limited experimentation on the basis of existing technology should be within the scope of protection defined by the claims.

Claims

1. A method for on-orbit calibration of a spaceborne laser altimeter combining single-peak and multi-peak footprints, characterized in that, Includes the following steps: Based on the echoes obtained by the spaceborne laser altimeter, the waveform features of each footprint were extracted, and single-peak footprint data and multi-peak footprint data were selected. For multi-peak footprint data, waveform simulation and matching are performed using reference terrain data to obtain the horizontal offset of a single footprint in the multi-peak footprint data. Then, the horizontal offset of each footprint is globally optimized according to the system error characteristics to correct the reference position of the multi-peak footprint. The elevation value of the single-peak footprint is corrected using reference terrain data to obtain the initial reference position of the single-peak footprint. Substitute the reference positions of the multi-peak footprint data and the single-peak footprint data into the joint calibration model to iteratively calculate the calibration parameters. During the calculation process, the ranging calibration parameters are calculated from the single-peak footprint data, and the pointing angle calibration parameters are calculated from the multi-peak footprint data. The expression for the joint calibration model is: In the formula, , This is a rotation matrix from the laser coordinate system to the station center coordinate system. This is the distance measurement value. , representing calibration parameters, The pitch angle, This is the roll angle. This is the distance correction amount. This represents the partial derivative with respect to the coordinates of the station center; This is the weight matrix. This is the correction value for the coordinates of the lower foot point in the station-centered coordinate system. The difference between the estimated location and the reference location of the footprint. For the footprint position residual; The formula for solving the unknowns of the joint calibration model is derived using the least squares principle: After continuous optimization through global iteration, the final solution is obtained.

2. The on-orbit calibration method for a spaceborne laser altimeter combining single-peak and multi-peak footprints according to claim 1, characterized in that, The process of obtaining the reference position of the multi-peak footprint data specifically includes: In the multi-peak footprint data, a square search area is made with each laser footprint initial value as the center. The center of the laser footprint is moved within the search area with a step size not less than the reference terrain resolution. The footprint echo at each search position is simulated using the reference terrain data, and the correlation coefficient is calculated with the real echo. The footprint position with the highest waveform correlation is taken as the horizontal offset of the current footprint. Data with a correlation coefficient less than 0.8 in the waveform correlation coefficient matrix of all multi-peak footprints on the same track are removed and superimposed. The position with the highest correlation after superposition is used as the global horizontal offset. The horizontal offset of each footprint in the multi-peak footprint data is subtracted from the global horizontal offset. For footprint data where the difference in the X or Y direction is greater than the matching accuracy, a square search area is drawn with the global horizontal offset as the center and the matching accuracy as the side length. The position with the highest waveform correlation in the search area is taken as the final horizontal offset of the current footprint. The reference position of each footprint is calculated based on the final horizontal offset of each footprint in the multi-peak footprint and the reference terrain data.

3. The on-orbit calibration method for a spaceborne laser altimeter combining single-peak and multi-peak footprints according to claim 2, characterized in that, The method further includes: After subtracting the horizontal offset of each footprint in the multi-peak footprint data from the global horizontal offset, the horizontal offset of the footprint data whose difference in the X and Y directions is less than or equal to the matching accuracy is taken as the final horizontal offset.

4. The on-orbit calibration method for a spaceborne laser altimeter combining single-peak and multi-peak footprints according to claim 2, characterized in that, The side length of the square search area is determined based on the coarse calibration accuracy and the accuracy of the laser altimeter.

5. The on-orbit calibration method for a spaceborne laser altimeter combining single-peak and multi-peak footprints according to claim 1, characterized in that, The specific process for calculating the elevation reference value of single-peak footprint data is as follows: Using the corrected footprint plane position as the center, select reference terrain data within the radius of the single-peak footprint, and calculate the average elevation of all terrain points within the selected area as the current footprint elevation reference value.

6. The on-orbit calibration method for a spaceborne laser altimeter combining single-peak and multi-peak footprints according to claim 1, characterized in that, The method also includes calibrating the system deviation of the ranging and pointing of the spaceborne laser altimeter based on the calculated calibration parameters.

7. The on-orbit calibration method for a spaceborne laser altimeter combining single-peak and multi-peak footprints according to claim 1, characterized in that, The method describes on-orbit calibration of the laser altimeter based on all single-peak footprint data and multi-peak footprint data of multiple orbits involved in the calculation.

8. The on-orbit calibration method for a spaceborne laser altimeter combining single-peak and multi-peak footprints according to claim 1, characterized in that, The reference terrain data is high-precision reference terrain data.

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