Satellite-borne laser water depth correction and extraction method considering sea surface waves
By simulating the shape of sea waves and calculating the laser incident angle, and combining Snell's law and the law of refraction for water depth correction, the error problem caused by sea waves in spaceborne laser water depth measurement was solved, and high-precision water depth extraction was achieved.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- NANJING UNIV
- Filing Date
- 2026-03-16
- Publication Date
- 2026-04-28
AI Technical Summary
Existing spaceborne laser depth measurement methods fail to effectively account for the effects of sea surface waves, resulting in depth calculation errors, especially in shallow sea areas where accuracy is insufficient.
By simulating sea wave patterns, fitting photon points on the sea surface, calculating the laser incident angle, and combining Snell's law and the law of refraction for water depth correction, including steps such as data preprocessing, sea surface fitting, elevation matching, and water depth correction, accurate water depth values are obtained.
It improves the accuracy and reliability of spaceborne laser water depth extraction, overcomes the shortcomings of traditional methods that do not consider wave characteristics, and supports high-precision underwater topography extraction and inversion applications.
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Figure CN121934047A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of single-photon signal extraction, and more specifically, to a method for spaceborne laser depth correction and extraction that takes into account sea surface waves. Background Technology
[0002] Shallow water underwater topography data is fundamental geographic information for the marine environment, widely serving fields such as navigation safety, maritime rescue, and resource exploration. It can also assist in scientific research such as seabed sediment classification and coral reef habitat mapping. Among commonly used depth measurement methods, shipborne sonar is difficult to operate in complex terrain or extremely shallow waters, while airborne LiDAR suffers from drawbacks such as high cost and unsuitability for large-scale depth measurement. Compared to traditional depth sounding methods, satellite depth sounding methods have advantages such as low cost, wide coverage, and no geographical limitations. Therefore, research on depth extraction and underwater topography remote sensing inversion based on the ICESat-2 satellite has attracted much attention.
[0003] In 2018, NASA launched ICESat-2, the Ice, Cloud, and Land Elevation Satellite, designed to collect information on land, vegetation, sea ice, and clouds. Compared to previous satellites, ICESat-2 carries an advanced topographic laser altimeter system. Its data product, ATL03, provides high-precision geospatial information including signal photons from the sea surface and seabed. It can detect depths up to approximately 40 meters in clear water, allowing for the reconstruction of vertical ocean profiles and the inversion of water depths, providing a new satellite data source for global shallow-sea topographic mapping (Parrish et al., 2019; Zhang et al., 2019; Albright et al., 2020; Ma et al., 2020; Le Quilleuc et al., 2021; Hsu et al., 2021; Thomas et al., 2021).
[0004] During the propagation of a laser pulse from a satellite to the seabed, it must pass through two different media: air and seawater. Due to the different speeds of light in air and water, according to Snell's law, the laser refracts when passing through the air-sea interface, causing a deflection of the photon's actual propagation path. Therefore, the seabed photon coordinates directly recorded by ICESat-2 are not their true positions, but rather their apparent positions after refraction. Without correction, this will directly cause errors in water depth calculation. Furthermore, local elevation changes and refraction angle changes caused by sea surface wave fluctuations will further affect the water depth measurement results. Current research has used different methods to simulate measurement scenarios to improve measurement accuracy. Parrish et al. assumed the water surface to be flat and conducted in-depth research on satellite depth sounding using photon-counting lidar. They proposed an effective refraction effect correction method by calculating the relationship between the laser angle and propagation speed using Snell's law (Parrish et al., 2019). Xu et al. further considered the impact of dynamic water surface wave effects on water depth measurement in a lake scenario and used a method based on unique time stamps to correct the depth sounding results (Xu et al., 2019). Ma et al. considered the impact of wave height on sounding accuracy in ocean sounding, pointing out that uncorrected wave height and slope may introduce centimeter-level to meter-level errors in the horizontal and vertical accuracy of sounding (Ma et al., 2020).
[0005] No effective solutions have yet been proposed to address the problems in the relevant technologies. Summary of the Invention
[0006] To address the problems in related technologies, this invention proposes a spaceborne laser depth correction and extraction method that takes into account sea surface waves, in order to overcome the aforementioned technical problems existing in the existing related technologies.
[0007] Therefore, the specific technical solution adopted by the present invention is as follows: a spaceborne laser depth correction and extraction method taking into account sea surface waves, characterized in that the method includes the following steps:
[0008] S1. Data preprocessing: Collect spaceborne laser strip data, extract effective data, and separate sea surface photon points from seabed photon points;
[0009] S2, Sea Surface Fitting: Perform sea surface wave fitting on the extracted sea surface photon points to obtain the instantaneous wave angle and elevation values at each sea surface photon point location;
[0010] S3. Elevation matching: The incident ray path of each sea surface photon point is modeled based on the fitting results. The seabed photon point and the sea surface photon point are matched based on the modeling path of the incident ray and the initial water depth value is calculated based on the elevation difference between the two.
[0011] S4. Water depth correction: Perform refraction correction and geophysical correction on the initial water depth value to obtain the final extracted water depth value.
[0012] Furthermore, the elevation matching in S3 includes the following steps:
[0013] S31. Model the incident ray path for each sea surface photon point. Establish a coordinate system with the satellite's orbital direction as the X-axis and the elevation as the Y-axis. Calculate the incident angle based on the sea wave angle and the incident ray angle. Calculate the refraction angle of the incident ray using Snell's law to obtain the slope of the incident ray. Then, calculate the path of the incident ray based on the position of the sea surface photon point in the coordinate system.
[0014] S32. Based on the modeling path of the incident ray, start from the position of the photon point on the sea surface and search downwards to find whether there is a photon point on the seabed within the preset radius distance of the incident ray. If a photon point on the seabed is found, match the point with the photon point on the sea surface.
[0015] S33. Subtract the elevation value of the matched seabed photon point from the elevation value of the sea surface photon point in the sea surface wave fitting result to obtain the initial water depth value of the sea surface photon point location.
[0016] Furthermore, the refraction correction process in S4 during the water depth correction includes the following steps:
[0017] S41. Based on the wave angle and the incident beam elevation angle parameters in the spaceborne laser strip data, calculate the angle between the incident ray and the normal to the incident surface. , The formula is as follows:
[0018]
[0019] In the formula, The incident beam elevation angle is obtained from the ATL03 dataset. The angle between the tangent of the sea wave and the horizontal plane;
[0020] S42. Based on the law of refraction and considering marine environmental factors, calculate the angle of incidence of the light ray. and the point of incidence to the observed seabed photon point Distance from the incident point to the actual seabed photon point The formula is as follows:
[0021]
[0022]
[0023]
[0024] In the formula, This is the initial water depth value. , These are the refractive indices of the atmosphere and seawater, respectively.
[0025] S43. Calculate the correction distance using the Law of Cosines. and correction angle Thus, the water refraction correction term can be calculated. The formula is as follows:
[0026]
[0027]
[0028]
[0029]
[0030]
[0031] Solve for the water refraction correction term corresponding to each seabed photon point. It is used for refraction correction of photon points on the seabed.
[0032] The beneficial effects of this invention are as follows: The invention proposes a spaceborne laser water depth correction and extraction method that takes into account sea surface waves. It simulates the instantaneous sea surface wave morphology and calculates the laser incident angle, thereby achieving accurate spaceborne laser water depth correction that takes into account sea surface waves. This improves the accuracy and reliability of spaceborne laser water depth extraction, overcomes the shortcomings of traditional water depth extraction methods that do not consider wave characteristics, and effectively supports underwater topography extraction and inversion related applications. Attached Figure Description
[0033] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.
[0034] Figure 1 This is a flowchart of a spaceborne laser depth correction and extraction method that takes into account sea surface waves according to an embodiment of the present invention.
[0035] Figure 2 This includes an overview of the case study area and a map showing the distribution of photon points on ICESat-2.
[0036] Figure 3 This is a schematic diagram of the sea surface wave fitting results in the case study area;
[0037] Figure 4 It is a water depth refraction correction model;
[0038] Figure 5 This is a schematic diagram of the corrected photon point depth based on the water depth correction results in the example study area;
[0039] Figure 6 This is a scatter plot evaluating the accuracy of water depth extraction results in the case study area. Detailed Implementation
[0040] To further illustrate the various embodiments, the present invention provides accompanying drawings, which are part of the disclosure of the present invention. These drawings are mainly used to illustrate the embodiments and can be used in conjunction with the relevant descriptions in the specification to explain the operating principles of the embodiments. With reference to these drawings, those skilled in the art should be able to understand other possible implementation methods and the advantages of the present invention. The components in the drawings are not drawn to scale, and similar component symbols are generally used to represent similar components.
[0041] According to embodiments of the present invention, a method for spaceborne laser water depth correction and extraction that takes into account sea surface waves is provided. Compared with traditional methods, the present invention achieves accurate correction of spaceborne laser water depth that takes into account sea surface waves by simulating instantaneous sea surface wave morphology and calculating laser incident angle, thereby improving the accuracy and reliability of spaceborne laser water depth extraction. It overcomes the shortcomings of traditional water depth extraction methods that do not consider wave characteristics and effectively supports underwater topography extraction and inversion related applications.
[0042] The present invention will now be further described in conjunction with the accompanying drawings and specific embodiments, such as... Figure 1 As shown, the spaceborne laser depth correction and extraction method considering sea surface waves according to an embodiment of the present invention includes the following steps:
[0043] S1. Data preprocessing: Collect ICESat-2 strip data, extract valid data, and separate surface photon points from seabed photon points.
[0044] like Figure 2 As shown, this example uses Wufang Reef in a certain sea area as the experimental area. Wufang Reef consists of five reef blocks, located between 10.46° and 10.55° north latitude and 115.70° and 115.81° east longitude, with a reef area of approximately 80 square kilometers. The ICESat-2 data collection date in this example is November 22, 2021.
[0045] During ICESat-2 data preprocessing, effective seabed photon points that reflect seabed topography are separated through visual interpretation, and sea surface photon points corresponding to their geographical locations are also separated to ensure that water depth values can be extracted.
[0046] S2. Sea Surface Fitting: The sea surface photon point extraction results are fitted with sea surface waves based on a smooth spline function to obtain the instantaneous wave angle and elevation values of each sea surface photon point position.
[0047] The sea surface wave fitting process includes the following steps:
[0048] S21. Separate the sea surface photon points and segment the data using a preset along-track distance interval as the size of the smoothing window. In this example, the preset along-track distance interval is 500 meters. Generally, the recommended segmentation range is 400-600 meters.
[0049] S22. Apply a smooth spline function to the sea surface photon points within each smoothing window to obtain the sea surface wave curve (see...). Figure 3 ), calculate the instantaneous wave angle and elevation value at the location corresponding to each photon point on the sea surface.
[0050] The formula for fitting sea surface waves using a smooth spline function is:
[0051]
[0052] The goal of smooth spline functions is to find a function Fit the water surface data points. Among them, The sum of squares of the elevation residuals for all sea surface photon points is given. The smaller the sum of squares of the elevation residuals, the higher the fitting accuracy. A smoothness penalty term is applied to all sea surface photon points, measured by the integral of the square of the second derivative; the smaller the integral value, the smoother the curve. The goodness of fit and the smoothness penalty term are combined to obtain the fitting result. For smoothing parameters, smoothing parameters Controlling the intensity of punishment, when When the penalty term disappears, the optimal solution will pass through every data point, making the fitting result highly susceptible to small amounts of noise; when When the penalty term becomes infinitely large, the optimal solution becomes 0, and the fitting result will be infinitely smooth; when Choosing a moderately positive value strikes a balance between goodness of fit and smoothness, yielding the best fit result. In the formula above, This indicates the number of photon points on the sea surface within the smoothing window. Indicates the first [number] within the smooth window The orbital distance of a single photon point on the sea surface Indicates the first [number] within the smooth window Elevation of a single photon point on the sea surface Let be the fitting function for the distance along the orbit of the photon point on the sea surface and the elevation of the photon point on the sea surface. Indicates the first Elevation fitting values at photon points on the sea surface. It is from the first sea surface photon point to the second The interval of a sea surface photon point The square of the second derivative of the fitted elevation value of each photon point on the sea surface is integrated. The smaller the integral value, the smoother the overall function curve; the larger the integral value, the more turbulent the function curve.
[0053] S3. Elevation Matching: Based on the sea surface wave fitting results, the incident ray path of each sea surface photon point is modeled, the seabed photon point is matched with the sea surface photon point, and the initial water depth value at the corresponding position is calculated based on the elevation difference between the two.
[0054] Elevation matching includes the following steps:
[0055] S31. Model the incident ray path for each sea surface photon point. Establish a coordinate system with the satellite's orbital direction as the X-axis and the elevation as the Y-axis. Calculate the incident angle based on the sea wave angle and the incident ray angle. Calculate the refraction angle of the incident ray using Snell's law to obtain the slope of the incident ray. Then, calculate the path of the incident ray based on the position of the sea surface photon point in the coordinate system.
[0056] S32. Based on the modeling path of the incident ray, ensure the reliability of the matching between surface photon points and seabed photon points using ray tracing. Starting from the position of the surface photon point, search downwards to find whether there is a seabed photon point within a preset radius distance of the incident ray (set to 1.5 meters in this example, but values of 1.3-1.7 meters are also feasible). If a seabed photon point is found, match it with the surface photon point for subsequent water depth extraction.
[0057] S33. Subtract the elevation value of the matched seabed photon point from the elevation value of the sea surface photon point in the sea surface wave fitting result to obtain the initial water depth value of the sea surface photon point location.
[0058] S4. Water depth correction: Perform refraction correction and geophysical correction on the initial water depth value to obtain the final extracted water depth value.
[0059] The water depth refraction correction model can be found in [reference needed]. Figure 4 Refraction correction includes the following steps:
[0060] S41. Calculate the angle between the incident ray and the normal to the incident surface based on the wave angle and the incident beam elevation angle parameters in the ICESat-2 dataset.
[0061] S42. Based on the law of refraction and combined with marine environmental factors, calculate the angle of incidence of the light ray and the distance from the incident point to the observed photon point and the actual photon point.
[0062] S43. Calculate the water depth correction value using the cosine theorem, and then correct the water depth value for refraction.
[0063] The calculation formula is as follows:
[0064]
[0065]
[0066]
[0067]
[0068]
[0069]
[0070]
[0071]
[0072]
[0073] In the formula, The incident beam elevation angle is obtained from the ATL03 dataset. Given the angle between the tangent of the sea wave and the horizontal plane, calculate the angle between the incident beam and the normal to the incident surface. . This is the initial water depth value. , These are the refractive indices of the atmosphere and seawater, respectively. Calculate the exit angle of the light beam using Snell's law of refraction. Distance from the incident point to the observed photon point Distance between the incident point and the actual photon point Subsequently, the correction distance was calculated using the law of cosines. and correction angle Solve for the water refraction correction term corresponding to each seabed photon point. It is used for refraction correction of photon points on the seabed.
[0074] Geophysical corrections include the following steps:
[0075] S44. Calculate the difference between the elevation of each sea surface photon position and the average elevation of all sea surface photons to eliminate water depth errors caused by wave undulations. The calculation formula is as follows:
[0076]
[0077] In the formula, This represents the average elevation of all photon points on the sea surface. The elevation of each sea surface photon point.
[0078] S45. After refraction correction, wave height and ocean tide corrections are applied to the water depth value, ultimately restoring the water depth to the mean sea level datum. The calculation formula is as follows:
[0079]
[0080] In the formula, To ultimately calculate the extracted water depth value based on the local mean sea level, This is the initial water depth value. For water refraction correction, , These are ocean tide and equilibrium tide correction terms obtained from the ICESat-2 dataset, respectively.
[0081] like Figure 5 The figure shows a schematic diagram of the corrected photon point depth in the example study area. As can be seen from the figure, the elevation of the corrected seabed photon point is higher than that of the uncorrected seabed photon point.
[0082] The final calibration is completed, yielding water depth values based on mean sea level. For example... Figure 6 As shown in the scatter plot of the accuracy evaluation, the ICESat-2 water depth extraction method proposed in this invention has high accuracy and reliable results.
[0083] In summary, this invention simulates instantaneous sea surface wave morphology and calculates the laser incident angle, overcoming the problem that traditional methods fail to fully consider the impact of sea surface wave angle on water depth extraction. It achieves accurate ICESat-2 water depth correction that takes sea surface waves into account, providing data and technical support for high-precision shallow water underwater topography extraction.
[0084] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.
Claims
1. A spaceborne laser depth correction and extraction method considering sea surface waves, characterized in that, The method includes the following steps: S1. Data preprocessing: Collect spaceborne laser strip data, extract effective data, and separate sea surface photon points from seabed photon points; S2, Sea Surface Fitting: Perform sea surface wave fitting on the extracted sea surface photon points to obtain the instantaneous wave angle and elevation values at each sea surface photon point location; S3. Elevation matching: The incident ray path of each sea surface photon point is modeled based on the fitting results. The seabed photon point and the sea surface photon point are matched based on the modeling path of the incident ray and the initial water depth value is calculated based on the elevation difference between the two. S4. Water depth correction: Perform refraction correction and geophysical correction on the initial water depth value to obtain the final extracted water depth value.
2. The method for spaceborne laser depth correction and extraction considering sea surface waves according to claim 1, characterized in that, In S1, during the preprocessing of spaceborne laser data, effective seabed photon points that reflect seabed topography are separated by visual interpretation, and sea surface photon points corresponding to their geographical locations are separated to ensure that water depth values can be extracted.
3. The method for spaceborne laser depth correction and extraction considering sea surface waves according to claim 1, characterized in that, The sea surface fitting in S2 includes the following steps: S21. Separate the sea surface photon points and segment the data using a preset distance interval along the track as the size of the smooth window. S22. Perform smooth spline function fitting on the sea surface photon points within each smooth window to obtain the sea surface wave curve, and calculate the instantaneous wave angle and elevation value at the corresponding position of each sea surface photon point.
4. The method for spaceborne laser depth correction and extraction considering sea surface waves according to claim 3, characterized in that, The formula for the smooth spline function in S22 is: In the formula, The sum of squared elevation residuals for all sea surface photon points. This indicates the number of photon points on the sea surface within the smoothing window. Indicates the first [number] within the smooth window The orbital distance of a single photon point on the sea surface Indicates the first [number] within the smooth window Elevation of a single photon point on the sea surface Let be the fitting function for the distance along the orbit of the photon point on the sea surface and the elevation of the photon point on the sea surface. Indicates the first Elevation fitting values at individual photon points on the sea surface; For the smoothness penalty of all sea surface photon points, For smoothing parameters, It is from the first sea surface photon point to the second The interval of a sea surface photon point The square of the second derivative of the fitted elevation value of each sea surface photon point is integrated.
5. The method for spaceborne laser depth correction and extraction considering sea surface waves according to claim 1, characterized in that, The elevation matching in S3 includes the following steps: S31. Model the incident ray path for each sea surface photon point. Establish a coordinate system with the satellite's orbital direction as the X-axis and the elevation as the Y-axis. Calculate the incident angle based on the sea wave angle and the incident ray angle. Calculate the refraction angle of the incident ray using Snell's law to obtain the slope of the incident ray. Then, calculate the path of the incident ray based on the position of the sea surface photon point in the coordinate system. S32. Based on the modeling path of the incident ray, start from the position of the photon point on the sea surface and search downwards to find whether there is a photon point on the seabed within the preset radius distance of the incident ray. If a photon point on the seabed is found, match the point with the photon point on the sea surface. S33. Subtract the elevation value of the matched seabed photon point from the elevation value of the sea surface photon point in the sea surface wave fitting result to obtain the initial water depth value of the sea surface photon point location.
6. The method for spaceborne laser depth correction and extraction considering sea surface waves according to claim 1, characterized in that, The refraction correction process in S4 during the water depth correction includes the following steps: S41. Based on the wave angle and the incident beam elevation angle parameters in the spaceborne laser strip data, calculate the angle between the incident ray and the normal to the incident surface. , The formula is as follows: In the formula, The incident beam elevation angle is obtained from the ATL03 dataset. The angle between the tangent of the sea wave and the horizontal plane; S42. Based on the law of refraction and considering marine environmental factors, calculate the angle of incidence of the light ray. and the point of incidence to the observed seabed photon point Distance from the incident point to the actual seabed photon point The formula is as follows: In the formula, This is the initial water depth value. , These are the refractive indices of the atmosphere and seawater, respectively. S43. Calculate the correction distance using the Law of Cosines. and correction angle Thus, the water refraction correction term can be calculated. The formula is as follows: Solve for the water refraction correction term corresponding to each seabed photon point. It is used for refraction correction of photon points on the seabed.
7. The method for spaceborne laser depth correction and extraction considering sea surface waves according to claim 6, characterized in that, The geophysical correction process in S4 also includes the following steps: S43. Calculate the difference between the elevation of each sea surface photon position and the average elevation of all sea surface photons to eliminate the water depth error caused by wave fluctuations. The calculation formula is as follows: In the formula, This represents the average elevation of all photon points on the sea surface. Elevation of each sea surface photon position; S45. After refraction correction, the water depth value is corrected for wave height and ocean tide, and finally the water depth is reduced to the mean sea level reference. In the formula, To ultimately calculate the extracted water depth value based on the local mean sea level, This is the initial water depth value. For water refraction correction, , These are ocean tide and isotropic tide correction terms obtained from spaceborne laser strip data, respectively.
8. The method for spaceborne laser depth correction and extraction considering sea surface waves according to claim 1, characterized in that, The spaceborne laser strip data is ICESat-2 data.
9. A method for spaceborne laser depth correction and extraction considering sea surface waves according to claim 3, characterized in that, In S21, the preset track spacing is 400-600 meters.
10. A method for spaceborne laser depth correction and extraction considering sea surface waves according to claim 5, characterized in that, In S32, the preset radius distance range is 1.3-1.7 meters.