A method and apparatus for dynamic surface and underwater target localization based on lidar

Abstract

This invention belongs to the field of lidar detection and provides a dynamic underwater target localization method based on lidar. The main steps include: S1: The lidar simultaneously emits a blue-green beam and multiple near-infrared beams surrounding the blue-green beam; S2: Obtaining the pointing angles of the multiple near-infrared beams and the blue-green beam emitted by the lidar, and multiple water surface distance signals received by the lidar from the near-infrared beams returning via the water surface; S3: Calculating the normal vector of the wavefront cell where the blue-green beam intersects with the air-water interface, and determining the position of this intersection point; S4: Calculating the angle of refraction of the blue-green beam through the water surface; S5: Obtaining the three-dimensional coordinates of the underwater target. This invention captures dynamic water surface reflection characteristics using near-infrared beams, accurately obtains wavefront position and angle information, and simultaneously emits blue-green beams for dynamic underwater target detection, thereby achieving high-precision localization of underwater targets.

Description

Technical Field

[0001] This invention relates to the field of lidar underwater detection technology, and more specifically, to a method and apparatus for dynamic surface and underwater target localization based on lidar. Background Technology

[0002] Traditional lidar systems are susceptible to the impact of dynamic water surface fluctuations on their positioning accuracy when detecting underwater targets. Due to these fluctuations, the incident direction of the laser beam changes as it crosses the water-air interface, leading to uncertainty in the beam's underwater propagation path. This path deviation causes accumulated errors, severely affecting the accuracy of traditional lidar systems in underwater target detection, especially under dynamic water surface conditions, making it difficult for the system to accurately pinpoint the target's location.

[0003] Traditional lidar systems typically ignore the effects of air-water interface fluctuations during underwater detection, assuming the laser beam propagates in a straight line within the medium and neglecting the disturbance to the optical path caused by surface ripples. In the past, due to multiple scattering, the beam diverged rapidly underwater, forming a large spot, making the positioning error caused by surface ripples relatively insignificant, thus reducing the need for real-time dynamic positioning. However, with the development of new optical technologies such as vortex beams, multiple scattering components in the echo signal have been effectively suppressed, and the optical path deviation introduced by surface ripples has gradually become the dominant factor affecting detection accuracy. Therefore, there is an urgent need to develop methods capable of real-time positioning of the underwater optical path.

[0004] Existing correction methods for underwater lidar detection mostly employ the blue-green light band to locate the normal vectors of wavefront elements. However, the blue-green light band is difficult to use for high-precision measurement of water surface distances, making it impossible to accurately obtain the normal vector information of wavefront elements. Furthermore, the blue-green light-based scanning method cannot meet the real-time requirements of dynamic wave scenarios in terms of temporal resolution, and cannot effectively track and dynamically locate rapidly changing wavefronts. In addition, the scanning interval of such methods is usually preset by the system hardware structure and cannot be adaptively adjusted according to actual wave conditions, limiting its applicability under varying sea conditions. Therefore, this paper proposes a method and device for precise target localization in underwater lidar detection on dynamic water surfaces, which is of great significance for improving the underwater detection capabilities of lidar systems in dynamic water surface environments. Summary of the Invention

[0005] The purpose of this invention is to provide a method and apparatus for dynamic surface and underwater target localization based on lidar.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: a dynamic surface and underwater target localization method based on lidar, mainly comprising the following steps: S1: The lidar simultaneously emits a blue-green beam and multiple near-infrared beams surrounding the blue-green beam; S2: Obtain the pointing angles of multiple (at least three) near-infrared beams and blue-green beams emitted by the lidar, multiple water surface distance signals of the near-infrared beams received by the lidar returning after passing through the water surface, and the distance signals of the blue-green beams received by the lidar after being refracted by the water surface and reaching the underwater target and then scattered. S3: Based on the pointing angles of the multiple near-infrared beams and the distances to the multiple returning water surfaces, as well as the pointing angles of the blue-green beams, calculate the normal vector of the wavefront element where the intersection of the blue-green beams and the air-water interface is located, and determine the position of the intersection point. S4: Calculate the angle at which the blue-green beam is refracted through the water surface based on the intersection point of the blue-green beam and the air-water interface, the normal vector of the wavefront unit, and the pointing angle of the blue-green beam. S5: Obtain the three-dimensional coordinates of the underwater target based on the angle of refraction of the blue-green beam through the water surface and the distance of the blue-green beam returning to the underwater target.

[0007] Preferably, step S3, which involves calculating the normal vector of the wavefront element where the blue-green beam intersects with the air-water interface, and determining the location of that intersection point, specifically includes: Based on the pointing angles of the beams emitted by the multi-beam (at least three-beam) near-infrared lidar and their corresponding distances from the water surface, calculate the spatial coordinates of the intersection points of each beam and the water surface; Based on the spatial coordinates of multiple water surface intersection points and the pointing angle of the blue-green beam, the normal vector of the wave surface unit is calculated, and the intersection point of the blue-green beam and the air-water interface is determined.

[0008] Preferably, the calculation of the angle of refraction of the blue-green light beam through the water surface in step S4 specifically includes: Calculate the incident angle of the blue-green beam on the water surface based on the normal vector of the wavefront element and the pointing angle of the blue-green beam. The angle of refraction is calculated using Snell's law based on the angle of incidence and the refractive indices of air and water. Based on the normal vector and refraction angle of the wavefront element, the angle at which the blue-green beam propagates underwater relative to the horizontal direction is determined.

[0009] Preferably, the positioning method includes: A spherical coordinate system is established with the lidar as the origin. The pointing angle of the blue-green beam includes the elevation angle θ0 and the azimuth angle φ0 in the spherical coordinate system. The pointing angle of the near-infrared beam is the elevation angle θ and n (n≥3) azimuth angles φ0 in a new coordinate system with the same origin and the direction of the blue-green beam as the z-axis. i The distance r from the water surface where the near-infrared beam is reflected back is... i The formula for converting i=1,2,…,n from spherical coordinates to rectangular coordinates is as follows: (I) (II) (III) , , These represent the coordinates in a spatial rectangular coordinate system calculated from the water surface distance received by the near-infrared lidar, where i = 1, 2, 3, ... n; Preferably, the positioning method further includes: The , , Multi-point coordinates are ( , , ), ( , , ), ( , , ), ……( , , The normal vector The angle between the z-axis and the positive z-axis is acute. Using the aforementioned multi-point coordinates, the normal vector of the wavefront element containing the intersection point of the blue-green beam and the air-water interface is determined by the least squares method. : (Ⅳ) In the formula, λ is The smallest eigenvalue, This is the eigenvector of the matrix.

[0010] The direction vector of the blue-green beam is obtained based on its pointing angle and return distance, and is denoted as follows: The point of intersection with the air-water interface is denoted as ; The angle of incidence is: (V) Calculate the angle of refraction based on the angle of incidence and Snell's law: (VI) In the formula, The refractive index of the water body Let be the refractive index of air; determine the refraction direction vector within the incident plane. ; according to , , Coplanar and and With an included angle of γ2, we obtain the following system of equations: (VII) (VIII) Expanded to: (IX) (X) The above system of equations usually has two sets of solutions, let's call them... , The correct refraction direction can be selected using the following discriminant: (XI) when hour, That is, the direction of refraction; when hour, That is, the direction of refraction.

[0011] Preferably, in step S5, the target distance returned by the blue-green beam and the aforementioned refraction direction are... The specific spatial location of the underwater target is obtained; let D be the distance between the underwater target returned by the blue-green beam and the incident point on the water surface, then the specific spatial coordinates of the underwater target are: (XII) (XIII)) (XIV) In the formula, x, y, and z are the refraction vectors. The component is obtained through step S4.

[0012] Preferably, the positioning method is applicable to airborne platforms, shipborne platforms, or other detection platforms with dynamic water surface interference.

[0013] A dynamic surface and underwater target localization device based on lidar, specifically including: A lidar transmitting module, used to simultaneously emit a blue-green beam and multiple near-infrared beams surrounding the blue-green beam; The lidar receiving module is used to acquire the water surface distance signal of multiple near-infrared beams received by the lidar and their return after passing through the water surface, as well as the distance signal of the blue-green beams received by the lidar after being refracted by the water surface and scattered after reaching the underwater target. The lidar attitude positioning module is used to obtain the pointing angles of the multiple near-infrared beams and blue-green beams emitted by the lidar. The signal processing module is used to calculate the normal vector of the wavefront element and determine the intersection point of the blue-green beam and the air-water interface; to determine the angle of refraction of the blue-green beam through the water surface; and to determine the three-dimensional coordinates of the underwater target.

[0014] Beneficial effects This invention offers at least the following advantages: It captures dynamic water surface reflection characteristics using a near-infrared beam, accurately acquires wavefront position and angle information, and simultaneously emits blue-green light beams for dynamic underwater target detection, thereby achieving high-precision positioning of underwater targets. By combining precise wavefront morphology perception with optical beam splitting elements to control the laser beam path, real-time positioning of the laser transmission path can be achieved under dynamic water surface conditions, significantly improving detection accuracy and system stability.

[0015] By employing transient wavefront rapid sensing and modeling technology, the problem of laser pointing uncertainty caused by the air-water interface is effectively solved, enabling precise beam positioning under dynamic wave interference, thereby supporting accurate underwater target detection. Near-infrared beams are used to perform high-precision measurement of water surface distance and accurately calculate the normal vector of wavefront units. Simultaneous emission of blue-green beams is used for underwater target detection, and combined with wavefront information, accurate modeling and positioning of the refracted optical path are achieved. Attached Figure Description

[0016] Figure 1 Schematic diagram of a dynamic surface and underwater target localization method based on lidar; Figure 2 : Schematic diagram for calculating the angle of refraction of blue-green light beams through water surface. Detailed Implementation

[0017] Embodiments of the present invention will now be described in detail with reference to the accompanying drawings. Figure 1 The diagram shows a schematic of an airborne lidar for precise water surface detection.

[0018] In this embodiment of the invention, the pointing angle of the near-infrared beam, the pointing angle of the blue-green beam, the target distance signal returned by the blue-green beam, and the water surface distance signal returned by the near-infrared beam can all be obtained through recorded data.

[0019] Example: Establish a spherical coordinate system with the lidar as the origin. The pointing angle of the blue and green beams includes the pitch angle θ0 in the spherical coordinate system. And azimuth φ0= The pointing angle of the near-infrared beam is the pitch angle θ in a new coordinate system with the same origin and the direction of the blue-green beam as the z-axis. And azimuth φ1= φ2= φ3= Calculations were performed under typical ocean conditions with a wave height of 4.2m and a wavelength of 50m, and under typical airborne lidar flight altitude conditions of 300m.

[0020] The distances to the water surface returned by the near-infrared beams are r1 = 313.426m, r2 = 313.165m, and r3 = 313.143m. The formula for converting from spherical coordinates to rectangular coordinates is as follows (i = 1, 2, 3): (I) (II) (III) , , (i=1,2,3) represent the coordinates in the spatial rectangular coordinate system obtained by calculating the water surface distance received by the near-infrared lidar.

[0021] In step S3, based on the pointing angles of the multiple near-infrared beams and the distances to the multiple returning water surfaces, as well as the pointing angle of the blue-green beams, the normal vector of the wavefront element where the intersection of the blue-green beams and the air-water interface is located is calculated, and the position of the intersection point is determined, including: Based on the pointing angles of the beams emitted by the multi-beam near-infrared lidar and their corresponding distances to the water surface, the spatial coordinates of the intersection points of each beam and the water surface are calculated. The coordinates of the three intersection points can be obtained using the coordinate transformation formula described above. The coordinates of the three points can be measured using coordinate transformations (Ⅰ) to (Ⅲ), denoted as (…). , , ), ( , , ), ( , , ),satisfy( ()( )-( ()( )>0, to ensure the normal vector The angle between the z-axis and the positive z-axis is acute.

[0022] ( , , =(68.854,68.854,297.917) ( , , =(67.911,68.580,297.922) ( , , =(68.575,67.906,297.901) Based on the spatial coordinates of the multiple water surface intersection points, the normal vector of the wave surface unit is calculated, and the intersection point of the blue-green beam and the air-water interface is determined.

[0023] Horizontal plane normal vector with waves: (Ⅳ) (0.00904, -0.01597, 0.81811) The direction vector of the blue-green beam is denoted as: The point of intersection with the air-water interface is denoted as . (68.446, 68.447, 297.913) In step S4, based on the intersection of the blue-green beam and the air-water interface, the normal vector of the wavefront element, and the pointing angle of the blue-green beam, the angle of refraction of the blue-green beam through the water surface is calculated, including: Calculate the incident angle and refraction angle of the blue-green beam on the water surface based on the normal vector of the wavefront element and the pointing angle of the blue-green beam.

[0024] The angle of incidence is: (V) 18.384° Calculate the angle of refraction based on the angle of incidence and Snell's law: (VI) 24.793° In the formula, The refractive index of the water body 1.00029 is the refractive index of air. Determine the refraction direction vector within the incident plane. .

[0025] according to , , Coplanar and and With an included angle of γ2, we obtain the following system of equations: (VII) (VIII) Expanded to: (IX) (X) The above system of equations usually has two sets of solutions, let's call them... , The correct refraction direction can be selected using the following discriminant: (XI) when hour, That is, the direction of refraction; when hour, That is, the direction of refraction.

[0026] =(-0.2230, -0.2863, 1) In step S5, the target distance returned by the blue-green beam and the aforementioned refraction direction are determined. This yields the specific spatial location of the underwater target. Let the distance between the underwater target returned by the blue-green beam and the incident point on the water surface be... 12.426m The specific spatial coordinates of the underwater target are: (XII) (XIII) (XIV) In the formula, x and y are the refraction vectors. The component is obtained through step S4.

[0027] (65.841, 65.103, 309.594) This invention calculates the coordinates of the intersection points of three near-infrared beams with the dynamic water surface to determine the intersection points of the blue-green beams with the dynamic water surface and their normal vectors. Then, it combines the direction of the blue-green beams to accurately calculate the angle of refraction through the water surface, thereby effectively avoiding the change in the refraction path caused by the dynamic water surface fluctuations and improving the detection accuracy.

[0028] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A dynamic surface and underwater target localization method based on lidar, characterized in that, Includes the following steps: S1: The lidar simultaneously emits a blue-green beam and multiple near-infrared beams surrounding the blue-green beam; S2: Obtain the pointing angles of multiple near-infrared beams and blue-green beams emitted by the lidar, multiple water surface distance signals returned by the near-infrared beams received by the lidar after passing through the water surface, and the distance signals scattered by the blue-green beams received by the lidar after being refracted by the water surface to reach the underwater target. S3: Based on the pointing angles of the multiple near-infrared beams and the distances to the multiple returning water surfaces, as well as the pointing angle of the blue-green beam, calculate the normal vector of the wavefront cell where the intersection of the blue-green beam and the air-water interface is located, and determine the position of the intersection point. S4: Calculate the angle at which the blue-green beam is refracted through the water surface based on the intersection point of the blue-green beam and the air-water interface, the normal vector of the wavefront unit, and the pointing angle of the blue-green beam. S5: Obtain the three-dimensional coordinates of the underwater target based on the angle of refraction of the blue-green beam through the water surface and the distance of the blue-green beam returning to the underwater target.

2. The dynamic surface and underwater target localization method based on lidar according to claim 1, characterized in that, Step S3 involves calculating the normal vector of the wavefront element where the blue-green beam intersects with the air-water interface, and determining the location of that intersection point. Specifically, this includes: Based on the pointing angle of the beams emitted by the multi-beam near-infrared lidar and their corresponding distances from the water surface, calculate the spatial coordinates of the intersection points of each beam and the water surface; Based on the spatial coordinates of multiple water surface intersection points and the pointing angle of the blue-green beam, the normal vector of the wave surface unit is calculated, and the intersection point of the blue-green beam and the air-water interface is determined.

3. The dynamic surface and underwater target localization method based on lidar according to claim 2, characterized in that, The calculation of the angle of refraction of the blue-green beam through the water surface in step S4 specifically includes: Calculate the incident angle of the blue-green beam on the water surface based on the normal vector of the wavefront element and the pointing angle of the blue-green beam. The angle of refraction is calculated using Snell's law based on the angle of incidence and the refractive indices of air and water. Based on the normal vector and refraction angle of the wavefront element, the angle at which the blue-green beam propagates underwater relative to the horizontal direction is determined.

4. The dynamic surface and underwater target localization method based on lidar according to claim 3, characterized in that, The positioning method includes establishing a spherical coordinate system with the lidar as the origin. The pointing angle of the blue-green beam includes the elevation angle θ0 and the azimuth angle φ0 in the spherical coordinate system. The pointing angles of the multiple near-infrared beams are the elevation angle θ and n (n≥3) azimuth angles φ0 in a new coordinate system with the same origin and the direction of the blue-green beam as the z-axis. i The distance of the near-infrared beam returning from the water surface is r. i Where i = 1, 2, ..., n; the formula for converting spherical coordinates to rectangular coordinates is as follows: (Ⅰ) (Ⅱ) (Ⅲ) , , These represent the coordinates in a spatial rectangular coordinate system calculated from the water surface distance received by the lidar.

5. The dynamic surface and underwater target localization method based on lidar according to claim 4, characterized in that, The positioning method further includes: The , , Multi-point coordinates are ( , , ), ( , , ), ( , , ), ……( , , The normal vector of the wavefront element where the blue-green beam intersects with the air-water interface is calculated using multi-point coordinates. ; The direction vector of the blue-green beam is obtained based on its pointing angle and return distance, and is denoted as follows: The point of intersection with the air-water interface is denoted as ; The angle of incidence is: (Ⅳ) Calculate the angle of refraction based on the angle of incidence and Snell's law: (Ⅴ) In the formula, The refractive index of the water body Let be the refractive index of air; determine the refraction direction vector within the incident plane. ; according to , , Coplanar and and With an included angle of γ2, we obtain the following system of equations: (Ⅵ) (Ⅶ) Expanded to: （Ⅷ） （Ⅸ） The above system of equations usually has two sets of solutions, let's call them... , The correct refraction direction can be selected using the following discriminant: (Ⅹ) when hour, That is, the direction of refraction; when hour, That is, the direction of refraction.

6. The dynamic surface and underwater target localization method based on lidar according to claim 5, characterized in that, In step S5, the target distance returned by the blue-green beam and the refraction direction are... The specific spatial location of the underwater target is obtained; let D be the distance between the underwater target returned by the blue-green beam and the incident point on the water surface, then the specific spatial coordinates of the underwater target are: (Ⅺ) (Ⅻ) (XIII) In the formula, x, y, and z are the refraction vectors. The component is obtained through step S4.

7. The dynamic surface and underwater target localization method based on lidar according to claim 1, characterized in that, The positioning method is applicable to airborne platforms, shipborne platforms, or detection platforms with dynamic water surface interference.

8. A dynamic surface and underwater target positioning device based on lidar, characterized in that, Specifically, it includes: A lidar transmitting module, used to simultaneously emit a blue-green beam and multiple near-infrared beams surrounding the blue-green beam; The lidar receiving module is used to acquire the water surface distance signal of multiple near-infrared beams received by the lidar and their return after passing through the water surface, as well as the distance signal of the blue-green beams received by the lidar after being refracted by the water surface and scattered after reaching the underwater target. The lidar attitude positioning module is used to obtain the pointing angles of the multiple near-infrared beams and blue-green beams emitted by the lidar. The signal processing module is used to calculate the normal vector of the wavefront element and determine the intersection point of the blue-green beam and the air-water interface; to determine the angle of refraction of the blue-green beam through the water surface; and to determine the three-dimensional coordinates of the underwater target.

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