Strict photon counting mechanism laser radar underwater sounding point coordinate calculation method

Abstract

The application discloses a rigorous photon counting mechanism laser radar underwater sounding point coordinate calculation method, mainly including the following steps: (1) analyzing the structure of the photon counting laser radar scanning system, establishing the geometric relationship model of the incidence angle, the azimuth angle and the mirror normal vector of the laser reflected light on the water surface; (2) constructing the coordinate calculation model of the laser water surface incidence point in the laser scanning reference coordinate system; (3) proposing a constant light speed light ray tracing model in the underwater layer, and constructing the coordinate calculation model of the underwater sounding point in the laser scanning reference coordinate system; (4) establishing the relationship model of the laser scanning reference coordinate system and the WGS84 space rectangular coordinate system, and returning the laser underwater sounding point coordinates to the WGS84 space rectangular coordinate system. Through the above steps, the accurate measurement of the underwater laser radar sounding point coordinates can be realized.

Description

Technical Field

[0001] This invention relates to a rigorous method for calculating the coordinates of underwater sounding points in a photon-counting lidar system, and particularly to a method for calculating the coordinates of underwater sounding points in a photon-counting lidar system based on underwater ray tracing. Background Technology

[0002] The United States was one of the first countries in the world to conduct research on airborne laser bathymetry systems. In 1968, Hickman and Hogg of Syracuse University built the world's first laser seawater measurement system, which for the first time verified the feasibility of laser water depth measurement technology and initially established the theoretical foundation for marine laser detection technology. Subsequently, the U.S. Navy successfully developed the Airborne Pulsed Laser System (PLADS) and conducted tests in 1971. The U.S. National Atmospheric and Space Administration (NASA) has successfully developed an airborne laser depth gauge (ALB). Experiments were conducted from 1971 to 1974 using a 50Hz Nd:YAG laser, achieving a depth measurement of approximately 10 meters when the water surface transparency was 5 meters. In the late 1970s, NASA developed an airborne hydrographic lidar (AOL) device with scanning and high-speed data recording capabilities. Using a 400Hz low-peak-power 2kW helium-neon laser, seabed topography at depths less than 10m was mapped. In the 1980s, the US Navy used a 500Hz fuel laser with a crystalline structure for circular scanning, achieving a signal dynamic range of 120dB. Combined with GPS positioning, a 1GHz sampling rate, and logarithmic amplifiers, differential and low-pass filters for signal processing, the results were displayed in color and manually identified and reprocessed. This achieved a processing speed of 5MIPS, but this was 1-2 orders of magnitude lower than the required real-time processing speed. Subsequently, the US adopted a new computer system, increasing the processing speed to 300MIPS to ensure that any feasible algorithm could be implemented. The system was tested off the coast of Florida in February 1990.

[0003] In the mid-1980s, the U.S. Army Corps of Engineers (USACE) initiated a development project to produce the Airborne Laser Scanning Depth Sounder (SHOALS). The project was ultimately developed with the support of Optech Corporation of Canada. Initially used for environmental measurements of navigation routes, the SHOALS system quickly evolved into a coastal mapping system. Today, SHOALS has become one of the main methods for near-shore bathymetry, and Optech has cooperative relationships with the U.S. Navy's meteorological, oceanographic command, and naval oceanographic offices. Based on these relationships, the SHOALS system undertakes a large number of tasks such as chart production and rapid environmental estimation for military exercises. Through more than thirty years of effort and technological breakthroughs, Optech has successfully developed the SHOALS 200 (1993), SHOALS 400 (1998), SHOALS 1000 (2003), and SHOALS 3000 (2006) series of products for bathymetry. Among them, the SHOALS 3000T, as its latest model, integrates Optech's years of research results and practical operational experience. It is a successfully finalized commercial airborne laser bathymetry system with simultaneous water depth and topographic measurement capabilities, and can be equipped with advanced accessories such as high-resolution digital cameras, hyperspectral remote sensing, and hyperspectral imaging. Weighing 217 kg, this equipment is generally used on large helicopter platforms and is essentially unsuitable for use on UAV platforms. Currently, its main users are the U.S. Navy and the National Oceanic and Atmospheric Administration (NOAA). Additionally, the system has been adopted by the Japanese Coast Guard and Fugro-pelagos Commercial. Table 1 compares the parameters of several shallow-sea mapping laser radar systems.

[0004] Table 1 Comparison of parameters of several shallow sea mapping lidar systems

[0005] parameter SHOALS 3000T Hawk Eye II LADS MK II Measurement frequency 3KHz 4KHz 900Hz Flight altitude 300~400m 250~500m 366~671m Water depth measurement accuracy IHO Order 1 IHO Order 1 IHO Order 1 Horizontal accuracy IHO Order 1 IHO Order 1 5m CEP 95% Minimum detection depth 0.2 0.3 0.5 Maximum detection depth 50 3x disc transparency 70m Scan width Maximum 0.75 times flight altitude 100~350m

[0006] Furthermore, RIEGL's airborne water depth laser scanning system is highly competitive. Table 2 shows a parameter comparison between its two water-land combined measurement laser scanning systems and this system. RIEGL's water-land combined measurement laser scanning systems are available in two types: the lightweight BathyCopter and the high-efficiency VQ-880-G. The BathyCopter is small but can only perform single-point scanning, resulting in low mapping efficiency. The VQ-880-G uses a linear detection system, which cannot meet the signal-to-noise ratio requirements of the low reflectivity of intertidal mudflats. The system is also bulky and consumes a lot of power, making it unsuitable for UAV platforms. This system, however, uses a photon counting system for intertidal mapping, achieving high detection efficiency while meeting the requirements of miniaturization, low power consumption, and high accuracy.

[0007] Table 2 Comparison of parameters of RIEGL land-water joint survey laser scanning system

[0008] parameter BathyCopter VQ-880-G laser wavelength 532nm 532, 1064nm Detection system Linear detection Linear detection Operational flight altitude 10-40m 600m Flight platform small rotorcraft Fixed-wing aircraft laser emission frequency 4KHz 550kHz water depth 1.5x Transparent Disc 1.5x Transparent Disc Scan width Single-point scan (measurement profile) ±20° Distance measurement accuracy 2.5cm@30m 2.5cm DEM grid mapping / / Mapping Scale / / weight 5.3Kg (GNSS inertial navigation unit, lens, and support structure required separately) 60Kg Power consumption 50W 400W

[0009] Traditional mapping lidar detection systems, whether using waveform digital sampling, multi-pulse measurement, or pulse width measurement, are essentially based on the detection of echo waveforms. This detection system cannot fully utilize the photon energy in the echo pulse, thus requiring high laser single-pulse energy and a high system optical aperture. To address the inefficiency of linear detection systems, a photon counting detection system with single-photon sensitivity has been introduced into the mapping lidar field.

[0010] The first successful application of photon-counting lidar for Earth mapping was achieved during NASA's Instrument Incubation Program (IIP) on a P-3 spacecraft. NASA's Goddard Space Flight Center was the first to conduct research in this area. Its first-generation airborne demonstrator system, called the Micro Altimeter, used a 532nm laser with a repetition rate of 10kHz and a pulse energy of 2uJ. It employed a 2×2 photomultiplier tube (PMT) operating in photon-counting mode as the echo detector, and used a 20cm off-axis telescope for both transmission and reception, along with a single optical wedge in front of the telescope, to achieve conical scanning imaging of the Earth. .

[0011] Building upon the Micro Altimeter, researchers at Goddard Space Flight Center and Sigma developed the second-generation airborne verification system, Imaging Photon-counting Altimeter (IPA). Its laser wavelength remains 532nm, but the repetition rate has been increased to 22kHz, and the pulse energy is 6.4uJ. It still uses a photon-counting PMT as the echo detector, but the number of elements has been increased to 10×10. The system employs dual optical wedge scanners to provide one-dimensional and two-dimensional scanning for different platform velocities, enabling wide-swath imaging during single flybys.

[0012] Due to its extremely high sensitivity, the photon counting detection system can penetrate a certain water depth to obtain the topography of shallow water areas. Based on this, researchers at the University of Florida have developed a prototype of the photon counting system lidar (Coastal Tactical-Mapping System, CATS). It was used for measurements in coastal areas and successfully measured the topography at water depths of up to 5 meters.

[0013] Launched by NASA in 2003, ICESat-1 was the world's first satellite to carry a laser altimeter radar. Its primary payload was the Geoscience Laser Altimeter System (GLAS). ICESat-2, as the successor to ICESat-1, has the Advanced Topographic Laser Altimeter System (ATLAS) as its primary payload. This payload achieves the multi-beam pushbroom function that ICESat-1 did not achieve. Furthermore, due to the use of a high repetition rate (10 kHz) photon counting detection system, the laser energy required by the system is greatly reduced. The total energy of the pre-splitting pulse is only 400 μJ, yet it achieves a measurement accuracy of about 10 cm and a horizontal resolution of about 70 cm.

[0014] my country's research on lidar depth sounding technology began in the 1980s, with related technical research and system development underway. Currently, the newly developed Mapper5000 system has completed multiple flight tests in the waters near some islands in the South China Sea, obtaining three-dimensional topographic data of the islands and reefs. Its maximum measured depth is 51.00 m, its shallowest depth is 0.25 m, and its depth sounding accuracy is 0.23 m, laying a solid technical foundation for the development of spaceborne marine exploration lidar in my country. .

[0015] In the field of lidar depth measurement algorithms, scholars both domestically and internationally have made significant contributions. Guenther, through statistical analysis, discovered that the intensity of water surface echoes in the received waveforms of the blue-green channels can exhibit substantial deviations due to environmental factors. He further pointed out that sometimes the detected water surface echoes may be a mixture of backscattered water waves or simply a backscattered water waveform. He termed this problem the "water surface uncertainty" problem, arguing that determining the water surface position solely using blue-green waveforms is inaccurate. Allouis proposed an algorithm that combines near-infrared and blue-green lasers to improve shallow water depth extraction accuracy. First, amplitude and time offset corrections are applied to the red and blue-green bands. Then, the amplitude of the near-infrared band is adjusted to correspond with the green band. The adjusted near-infrared signal is then subtracted from the green band signal to obtain the bottom signal. Finally, the distance corresponding to the peak difference between the near-infrared and bottom signals is taken as the shallow water depth. Allouis et al. used two Gaussian functions to fit the echo signals from the sea surface and seabed, respectively. The fitted waveforms showed that the fitting effect for the echo signals was good. However, this method is ineffective at processing backscattered echo signals from seabeds, potentially leading to the seabed echo signal being overwhelmed by backscattered interference. Wang et al. validated six algorithms—peak detection, mean squared difference function, Gaussian decomposition, quadrilateral fitting, RL deconvolution, and Wiener filtering—using simulated data and real data collected by Optech Aquarius. Experimental results showed that the RL deconvolution algorithm has significant advantages. Wong and Antoniou proposed using an exponentially modified Gaussian (EMG) function to decompose the waveform into surface echo and bottom echo, thereby automatically calculating the water depth. They claimed that this method can accurately estimate the water depth even when the two echo components almost completely overlap. Cheng et al. believed that backscattering from water bodies would shift the peak positions of surface and bottom echoes. Therefore, they proposed using EMG to decompose the waveform into three parts: surface echo, backscattering from water bodies, and bottom echo. Liu et al. developed a refraction correction method that simultaneously considers sea surface waves and beam incidence angle. This model uses sea surface wave theory and Snell's law to determine the propagation distance of photons in water and performs position correction through geometric relationships. The proposed model was practically validated at selected locations, demonstrating that the proposed refraction correction method can more accurately and effectively correct water depth errors. .

[0016] In summary, despite the many advantages of UAV-borne LiDAR, current technology still has the following shortcomings:

[0017] (1) The UAV-borne lidar is easily affected by wind, which can cause the attitude sensor to jitter too much, resulting in a large attitude error and ultimately affecting the mapping accuracy of the lidar.

[0018] (2) Since the water body is composed of water masses with different temperatures, salinity and density, light will be emitted and refracted at the interface of these different water masses. However, the current underwater laser sounding point coordinate calculation model does not take this problem into account, which will affect the calculation accuracy of the underwater sounding point coordinate.

[0019]

[0020]

[0021]

[0022]

[0023] Summary of the Invention

[0024] The technical problem this invention aims to solve is the low accuracy of underwater lidar depth sounding point coordinate calculations due to the failure to consider the refraction of light at the interfaces of different water masses. Based on the analysis of the structure of a photon-counting lidar scanning system, this invention establishes the geometric relationship between the incident angle and azimuth angle of the reflected light at the water surface and the normal vector of the reflecting mirror, constructing a coordinate calculation model for the laser incident point at the water surface in the lidar scanning reference coordinate system. Furthermore, it proposes a constant light speed tracking model within the underwater layer to construct a coordinate calculation model for the underwater depth sounding point in the lidar scanning reference coordinate system. Finally, through a geometric relationship model between the lidar reference coordinate system and the WGS84 spatial rectangular coordinate system, the underwater laser depth sounding point is repositioned into the WGS84 spatial rectangular coordinate system.

[0025] To achieve the above objectives, the present invention provides a rigorous method for calculating the coordinates of underwater depth sounding points using a photon counting mechanism lidar, comprising the following steps:

[0026] (1) Analyze the structure of the photon counting lidar scanning system and establish the geometric relationship between the incident angle and azimuth angle of the laser reflected light on the water surface and the normal vector of the reflector;

[0027] (2) Construct a coordinate calculation model of the laser water surface incident point in the laser scanning reference coordinate system;

[0028] (3) A constant light speed ray tracing model is proposed for the underwater layer, and a coordinate calculation model for the underwater depth sounding point in the laser scanning reference coordinate system is constructed;

[0029] (4) Establish a relationship model between the laser scanning reference coordinate system and the WGS84 spatial rectangular coordinate system, and relocate the coordinates of the laser underwater sounding point to the WGS84 spatial rectangular coordinate system.

[0030] In one embodiment of the present invention, establishing a geometric relationship model between the incident angle and azimuth angle of the laser reflected light on the water surface and the normal vector of the reflecting mirror mainly includes the following steps:

[0031] (1) Analyze the structure of the photon counting lidar scanning system;

[0032] (2) Establishment of the lidar scanning reference coordinate system and its transition coordinate system;

[0033] (3) Calculation of the normal vector of the reflector in the laser radar scanning reference coordinate system;

[0034] (4) Establishment of a geometric relationship model between the incident angle and azimuth angle of the reflected light on the water surface and the normal vector of the reflector in the laser radar scanning reference coordinate system.

[0035] In one embodiment of the present invention, constructing a coordinate calculation model of the laser water surface incident point in a laser scanning reference coordinate system mainly includes the following steps:

[0036] (1) Decompose the components of the incident angle of the laser reflected light on the water surface along the X-axis and Y-axis, and establish the relationship between the components and the normal vector of the reflecting mirror respectively;

[0037] (2) Calculate the three-dimensional coordinates of the laser incident point on the water surface;

[0038] In one embodiment of the present invention, a constant light speed ray tracing model for underwater layers is proposed, and a coordinate calculation model for underwater depth sounding points in a laser scanning reference coordinate system is constructed, mainly including the following steps:

[0039] (1) Construct an underwater light speed profile;

[0040] (2) A constant light speed ray tracing model for underwater layers is proposed;

[0041] (3) Construct a coordinate calculation model of the underwater sounding point in the laser scanning reference coordinate system.

[0042] In one embodiment of the present invention, a relationship model between the laser scanning reference coordinate system and the WGS84 spatial rectangular coordinate system is established, and the coordinates of the laser underwater depth sounding points are normalized to the WGS84 spatial rectangular coordinate system. This mainly includes the following steps:

[0043] (1) Based on the construction principles of the laser scanning reference coordinate system and the WGS84 spatial rectangular coordinate system, establish a relationship model between the two;

[0044] (2) Relocate the coordinates of the laser underwater sounding point to the WGS84 spatial rectangular coordinate system.

[0045] The technical problems to be solved by this invention mainly include the following aspects:

[0046] (1) Analyze the structure of the single-photon lidar scanning system and establish a geometric relationship model between the laser reflected ray and the normal vector of the mirror;

[0047] (2) Construct a coordinate calculation model of the laser radar incident point on the water surface in the laser scanning reference coordinate system;

[0048] (3) A constant light speed ray tracing model for underwater layers is proposed;

[0049] (4) Establish a coordinate calculation model for underwater laser depth sounding points in the laser scanning reference coordinate system;

[0050] (5) Reset the coordinates of the underwater sounding points to the WGS84 spatial rectangular coordinate system.

[0051] The beneficial effects of the present invention through the above technical solution are:

[0052] (1) By studying the scanning structure of photon counting lidar, a geometric relationship model between the laser reflected ray and the normal vector of the mirror was constructed, which can accurately calculate the coordinates of the laser incident point on the water surface;

[0053] (2) A constant light speed ray tracing model based on the light speed profile is proposed, and a coordinate calculation model of the underwater laser sounding point in the laser scanning reference coordinate system is established, which can greatly improve the measurement accuracy of the underwater sounding point. Attached Figure Description

[0054] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art 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.

[0055] Figure 1 This is a schematic diagram of the elliptical scanning system of the photon counting lidar of the present invention;

[0056] Figure 2 These are the two Cartesian coordinate systems of the normal direction vector of the reflector in this invention;

[0057] Figure 3 This refers to the geometric angle of the reflected light rays in the sensor coordinate system according to the present invention;

[0058] Figure 4 This is a schematic diagram of the geometric structure of the emitted laser calculated from the change of normal in this invention;

[0059] Figure 5 This is a schematic diagram of the laser incident point on the sea surface according to the present invention;

[0060] Figure 6 This is a schematic diagram of the intralayer constant light speed ray tracing of the present invention;

[0061] Figure 7 This is a schematic diagram of the underwater optical path of the lidar of the present invention;

[0062] Figure 8 It is a flowchart of a rigorous photon counting mechanism for calculating the coordinates of underwater depth sounding points in lidar. Detailed Implementation

[0063] To make the technical means, creative features, objectives and effects of this invention easier to understand, the invention will be further described below with reference to specific illustrations.

[0064] First, this invention relates to the following technical terms:

[0065] Photon counting lidar

[0066] Photon-counting lidar, also known as single-photon lidar, is a type of lidar with high sensitivity and high temporal resolution. It uses a single-photon detector—a photodetector capable of detecting weak echo signals down to the level of a single photon—as the photoelectric conversion device. Combined with high-precision time-correlated single-photon counting (TCSPC) technology, it can achieve high-precision detection of weak signals, making it suitable for scenarios with limited echo intensity, such as long-distance targets and low-reflectivity targets. .

[0067] Inertial navigation system

[0068] An inertial navigation system (INS) is an autonomous navigation system that does not rely on external information and does not radiate energy to the outside. Its working environment includes not only the air and ground, but also underwater. The basic working principle of inertial navigation is based on Newton's laws of motion. By measuring the acceleration of the carrier in the inertial reference frame, integrating it over time, and transforming it into the navigation coordinate system, information such as velocity, yaw angle, and position in the navigation coordinate system can be obtained.

[0069] Underwater light tracing method

[0070] Ray tracing is a method based on underwater light speed profiles for calculating the coordinates of underwater laser depth sounding points (projection points) within a lidar scanning reference coordinate system. Assuming that photons in each water column move at a constant speed, the incident angle and refraction angle of the light at the interface of each water column are calculated according to Snell's law. The travel time and horizontal displacement of the light in each water column are then calculated until the light disappears at some point on the interface or within the layer.

[0071] The rigorous photon counting mechanism lidar method for calculating underwater depth sounding point coordinates, as described in this invention, mainly includes the following steps:

[0072] (1) Analyze the structure of the photon counting lidar scanning system and establish a geometric relationship model between the incident angle and azimuth angle of the laser reflected light on the water surface and the normal vector of the reflector;

[0073] (2) Construct a coordinate calculation model of the laser water surface incident point in the laser scanning reference coordinate system;

[0074] (3) A constant light speed ray tracing model is proposed for the underwater layer, and a coordinate calculation model for the underwater depth sounding point in the laser scanning reference coordinate system is constructed;

[0075] (4) Establish a relationship model between the laser scanning reference coordinate system and the WGS84 spatial rectangular coordinate system, and relocate the coordinates of the laser underwater sounding point to the WGS84 spatial rectangular coordinate system.

[0076] See Figures 1 to 7 As shown, the specific embodiments of the present invention will now be described in detail below:

[0077] (1) Overall technical solution

[0078] First, the structure of the photon-counting lidar scanning system is analyzed, and a geometric relationship model is established between the incident angle and azimuth angle of the laser reflected light on the water surface and the normal vector of the reflector. Second, a coordinate calculation model of the laser incident point on the water surface in the laser scanning reference coordinate system is constructed. Third, a constant light speed ray tracking model is proposed for the underwater layer, and a coordinate calculation model of the underwater sounding point in the laser scanning reference coordinate system is constructed. Finally, a relationship model between the laser scanning reference coordinate system and the WGS84 spatial rectangular coordinate system is established, and the coordinates of the laser underwater sounding point are normalized to the WGS84 spatial rectangular coordinate system.

[0079] (2) Calculation of the coordinates of the laser spot on the water surface in the lidar scanning reference coordinate system

[0080] 1) Structure of the photon counting lidar elliptical scanning system

[0081] The marine lidar described in this article is a conventional elliptical scanning structure. Figure 1 As shown, a prism that can rotate around a rotation axis is used as a reflector to control the direction of the emitted laser beam. The emitted laser is reflected by the prism and points towards the sea surface. The angle between the normal direction of the prism and the rotation axis is 7.5°. When the prism surface rotates around the rotation axis, the laser traces a path on the sea surface with an incident angle of approximately 15°. Since the incident angle is not always equal to 15° during one scan (depending on the normal direction), the final laser point trajectory on the sea surface when the aircraft is hovering is approximately elliptical and oval. Therefore, this scanning structure is also called an oval scanning structure.

[0082] 2) LiDAR scanning reference coordinate system

[0083] Definition of the lidar scanning reference coordinate system: with the center point of the reflector as the origin O, X... s The axis points in the negative direction of the emitted laser beam, Y. s The axis points in the direction of flight, Z s Axis and X s Y s Construct a right-handed coordinate system with the axis pointing vertically upwards. The incident laser and the motor axis are in the same plane (X). s Z s (surface), laser incident horizontally (along the X-ray) s(In the negative direction of the axis), the incident point of the laser beam on the mirror is the center of the mirror. For ease of understanding, as... Figure 2 As shown, the original X s Y s Z s Coordinate system around Y s Rotating the axis counterclockwise by 45° yields a new coordinate system X. s 'Y ’ 'Z', at this time Z s The rotation directions of the shaft and the motor coincide. The normal to the reflector is in the X direction. s Z s Projection of the surface and Z s Angle between axes In Y s Z s Projection of the surface and Z s Angle between axes .

[0084] like Figure 3 As shown, the reflected light rays in X s Z s Y s Z s The angles between the projection of the plane and the Z-axis are respectively , Its nadir is Because the laser travels along the X-ray... s Incident along the negative direction of the axis, with the normal at Y. s Z s Projection of the surface and Z s Angle between axes Equal to the reflected laser in Y s Z s Projection of the surface and Z s Angle between axes (Because the incident laser line, the mirror normal, and the reflected laser line are coplanar, and the incident laser line is perpendicular to Y) s Z s According to the theorem that if one plane passes through a perpendicular line to another plane, the two planes are orthogonal, therefore, in Y... s Z s On a plane, the angle of rotation of the normal is synchronized with the angle of rotation of the reflected ray (i.e., the normal rotates by an angle θ, and the reflected ray also rotates by an angle θ). However, in the X... s Z s On a plane, when the mirror rotates (i.e., the normal) by an angle θ, the reflected ray rotates by an angle 2θ. When the normal angle changes, the included angle... And so it changed, from Easy to solve And then calculate the nadir angle of the beam. and azimuth Therefore, the change in the angle of the normal is the key.

[0085] 3) Direction vector of the mirror normal

[0086] exist Figure 2 In the middle, the normal of the mirror is in X s 'Y s 'Z s 'Normal vector of the coordinate system ( , , ):

[0087]

[0088] Then by going around Y s Rotating the coordinate axes 45° clockwise will yield the X coordinate. s Y s Z s The normal vector of the mirror in the coordinate system ( , , ):

[0089]

[0090] 4) The relevant angles of the reflected light rays in the laser scanning reference coordinate system

[0091] Depend on Figure 2 , Figure 4 According to geometric relationships, =arctan(F x / |F z |), therefore we have:

[0092]

[0093] Depend on Figure 2 According to geometric relationships, =arctan(F y / |F z |), and = ,so:

[0094]

[0095] Depend on Figure 3 The geometric relationship yields the nadir angle. and azimuth for:

[0096]

[0097]

[0098] 5) Coordinates of water surface light spots and footprints in the lidar scanning reference coordinate system

[0099] like Figure 5 As shown, if the sea surface is a plane, the laser incident point on the sea surface is P1, the laser center is denoted as S, the slant range of the laser beam in the air is L1, and the azimuth angle is... If the measured height of the center of the reflector is H, then the coordinates of the laser incident point P1 on the sea surface are:

[0100]

[0101]

[0102]

[0103] (3) Calculation of the coordinates of underwater light spot footprints in the lidar scanning reference coordinate system

[0104] 1) Ray tracing algorithm based on the assumption of constant light speed within the water layer

[0105] Because the temperature, salinity, and density of each water mass differ along the vertical direction, the speed of light traveling within each mass also varies. Furthermore, light refracts at the interfaces between different water masses. Therefore, it is necessary to use an ocean light speed profiler to obtain the vertical sequence of water depth and light speed values, and then precisely track the light rays to obtain high-precision underwater light spot footprint coordinates.

[0106] Assuming the laser beam travels through a column of water consisting of N layers, the speed of light propagates at the constant speed of light within each layer. Figure 6 According to Snell's Law, we have:

[0107]

[0108] like Figure 6 As shown, let the thickness of the water column be Δz. i (Δz) i = z i+1 -z i Then the horizontal displacement Δy of the beam within the i-th layer is... i and propagation time Δt i for:

[0109]

[0110]

[0111] According to the formula Japanese style The horizontal distance y that the light beam travels through the entire water column and the propagation time t are respectively:

[0112]

[0113]

[0114] Assume the beam does not pass through all the water column layers, but only through the Nth layer, Z... r The beam disappears at this point, and at this time the horizontal displacement of the beam in that layer is Δy. r The vertical displacement is Δz r The time it takes for the light beam to travel through the entire water column is t. all The time spent at this layer is t. r The optical path length ΔS experienced by the beam in this layer is then... r Horizontal displacement Δy r Vertical displacement Δz r They are respectively:

[0115]

[0116]

[0117]

[0118] Therefore, the total horizontal and vertical displacements of the light beam in the water are respectively:

[0119]

[0120]

[0121] 2) Coordinates of underwater laser footprints in the laser scanning reference coordinate system

[0122] like Figure 7 As shown, the laser has an incident angle of... The light is incident at point P1 on the water surface, with a refraction angle of θ0. It passes through different water layers P2 and P3 in sequence, finally reaching point P4 where the light energy disappears. s y sLet OP4' be the planar coordinates of the water surface light spot footprint in the lidar scanning reference coordinate system, and OP4' be the distance L from the projection point of the underwater lidar depth sounding point on the water surface to the origin of the coordinate system. s Then its value is:

[0123]

[0124] Then the X of the underwater depth sounding point of the lidar s Y s Z s The coordinate values ​​are as follows:

[0125]

[0126]

[0127]

[0128] (4) Coordinates of underwater sounding points in the WGS84 rectangular coordinate system

[0129]

[0130] And (X) GPS ,Y GPS Z GPS )for:

[0131]

[0132] Mode , In the middle, (X) s-wgs84 ,Y s-wgs84 Z s-wgs84 (X) represents the coordinates of the underwater depth sounding point of the lidar in the WGS84 rectangular coordinate system; GPS ,Y GPS Z GPS R(yaw,pitch,roll) represents the coordinates of the airborne GPS antenna center in the WGS84 Cartesian coordinate system; R(yaw,pitch,roll) is the rotation matrix for transforming the body coordinate system to the local navigation coordinate system. It includes two parts: the eccentricity difference between the center of the laser scanning reference coordinate system and the center of the IMU body coordinate system, and the eccentricity difference between the center of the GPS antenna and the center of the IMU body coordinate system. The offset angle of the laser scanning reference coordinate system relative to the IMU body coordinate system.

[0133] Therefore, the present invention solves the following technical problems:

[0134] (1) The structure of the photon counting lidar scanning system was analyzed, and a geometric relationship model between the laser reflected ray and the normal vector of the mirror was established;

[0135] (2) A coordinate calculation model of the laser radar incident point on the water surface in the laser scanning reference coordinate system was constructed;

[0136] (3) A constant light speed ray tracing model for underwater layers was proposed;

[0137] (4) A coordinate calculation model for underwater lidar footprints in the laser scanning reference coordinate system was established;

[0138] (5) Reset the coordinates of the underwater sounding points to the WGS84 spatial rectangular coordinate system.

[0139] In addition, the technical features of the present invention are as follows:

[0140] (1) The structure of the single-photon lidar scanning system was analyzed, and a geometric relationship model between the incident angle and azimuth angle of the laser reflected light and the normal vector of the reflector was established;

[0141] (2) A constant light speed ray tracking model for underwater layers was proposed, which accurately tracked underwater laser footprints and improved the coordinate accuracy of underwater laser depth sounding points.

Claims

1. A method for calculating the coordinates of a point of underwater sounding by a laser radar with a strict photon counting mechanism, characterized in that Includes the following steps: (1) Analyze the structure of the photon counting lidar scanning system and establish a geometric relationship model between the incident angle and azimuth angle of the laser reflected light on the water surface and the normal vector of the reflector; (2) Construct a coordinate calculation model of the laser water surface incident point in the laser scanning reference coordinate system; (3) A constant light speed ray tracing model is proposed for the underwater layer, and a coordinate calculation model for the underwater depth sounding point in the laser scanning reference coordinate system is constructed; Assuming a laser beam travels through a column of water consisting of N layers, with the light traveling at its constant speed within each layer, according to Snell's law: (10) Let the water column layer thickness be Δz i ,Δz i = z i+1 -z i , then the horizontal displacement Δy i of the light beam in the i-th layer and the propagation time Δt i are: (11) (12) According to equations (11) and (12), the horizontal distance y and propagation time t of the beam traveling through the entire water column are respectively: (13) (14) Assume the beam does not pass through all the water column layers, but only through the Nth layer, Z... r The beam disappears at this point, and at this time the horizontal displacement of the beam within that layer is Δy. r The vertical displacement is Δz r The time it takes for the light beam to travel through the entire water column is t. all The time spent at this layer is t. r The optical path length ΔS experienced by the beam in this layer is then... r Horizontal displacement Δy r Vertical displacement Δz r They are respectively: (15) (16) (17) Therefore, the total horizontal and vertical displacements of the light beam in the water are respectively: (18) (19) Let x s , y s be the plane coordinates of the water surface light spot footprint in the laser radar scanning reference coordinate system, L s be the distance from the projection point of the underwater laser radar sounding point on the water surface to the coordinate origin, then the value of L s is: (20) The X, Y, Z coordinate values of the laser radar underwater sounding point are respectively: s s s ​​​ (21) (22) (23) (4) Establish a relationship model between the laser scanning reference coordinate system and the WGS84 spatial rectangular coordinate system, and relocate the coordinates of the laser underwater sounding point to the WGS84 spatial rectangular coordinate system.

2. The method of claim 1, wherein the method is a rigorous photon counting mechanism lidar bathymetric point coordinate calculation method. The geometric relationship model established in step (1) for the incident angle and azimuth angle of the laser reflected light on the water surface and the normal vector of the reflecting mirror includes the following steps: 1) Analyze the structure of the photon counting lidar scanning system; 2) Establishment of the laser scanning reference coordinate system and its transition coordinate system; 3) Establishment of a model relating the incident angle and azimuth angle of the reflected light on the water surface to the normal vector of the mirror in the laser scanning reference coordinate system.

3. The method of claim 1, wherein the method is a rigorous photon counting mechanism lidar bathymetric point coordinate calculation method. The construction of the coordinate calculation model of the laser water surface incident point in the laser scanning reference coordinate system in step (2) includes the following steps: 1) Decompose the components of the incident angle of the laser reflected light on the water surface along the X and Y axes, and establish the relationship between the components and the normal vector of the reflecting mirror respectively; 2) Calculate the three-dimensional coordinates of the laser incident point on the water surface.

4. The method of claim 1, wherein the method is a rigorous photon counting mechanism lidar bathymetric point coordinate calculation method. Step (4) involves establishing a relationship model between the laser scanning reference coordinate system and the WGS84 spatial rectangular coordinate system, and repositioning the laser underwater sounding point coordinates into the WGS84 spatial rectangular coordinate system. This includes the following steps: 1) Based on the construction principles of the laser scanning reference coordinate system and the WGS84 spatial rectangular coordinate system, establish a relationship model between the two; 2) Relocate the coordinates of the laser underwater sounding points to the WGS84 spatial rectangular coordinate system.

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