Seamless integrated measurement method for intertidal zone topography based on acousto-optic remote sensing and rollers

Abstract

The present invention discloses a seamless integrated measurement method for intertidal zone topography based on acousto-optic remote sensing and rollers, which mainly includes the following steps: (1) Research the structure of the photon counting lidar scanning system, and construct a calculation model for the coordinates of the lidar underwater sounding points in the WGS84 space rectangular coordinate system; (2) Considering the attitude change of the hovercraft, on the basis of the ray tracing model, construct a calculation model for the coordinates of the single-beam sonar sounding points in the WGS84 space rectangular coordinate system; (3) Establish the geometric relationship between the measurement support placement box at the tail of the hovercraft and the mechanical roller, and construct a calculation model for the coordinates of the lowest sounding point of the mechanical roller in the WGS84 space rectangular coordinate system; (4) For different types of approximately homologous sounding point clouds in the overlapping area, propose corresponding fusion processing methods to obtain a high-precision and high-density underwater topography measurement data set for the intertidal zone. Through the above steps, efficient measurement of the underwater topography of the intertidal zone can be achieved.

Description

Technical Field

[0001] The present invention relates to an integrated measurement technology of lidar, single-beam sonar and mechanical rollers, and particularly to a seamless integrated measurement method for intertidal zone topography based on acousto-optic remote sensing and rollers. Background Art

[0002] The intertidal zone refers to the area between the shoreline and the zero-meter line of the theoretical lowest tide level (theoretical depth datum). Since the intertidal zone is at the junction of land and sea, neither bathymetric survey nor land survey can fully cover it, thus creating a blank area. [1] Due to the needs of comprehensive development and utilization and management such as aquaculture, salt field development, reclamation (road), coastal protection and treatment, harbor construction, and tourist attraction construction, it has increasingly become an urgent need to conduct coastal zone topography and underwater topography surveys to provide accurate large-scale topographic maps for coastal engineering. [2] .

[0003] The difficulties of intertidal zone mapping technology mainly include: (1) being submerged by water at high tide and exposed above the water surface at low tide, and the measurement window time is generally between 2 and 4 hours. Traditional manned aircraft aviation operation platforms are restricted by airspace application and flight preparation and cannot meet the measurement requirements; (2) the mapping objects in the intertidal zone are complex, with many tidal pools, a lot of aquatic vegetation, and low silt reflectivity, and the existing land mapping lidar technology cannot effectively map; (3) "people cannot go down and boats cannot come up", and the existing manual running measurement or shipborne measurement methods are inefficient and dangerous for mapping work and cannot meet the increasingly urgent dynamic monitoring service requirements of the intertidal zone. At present, unmanned aerial vehicle (UAV)-borne land-sea lidar can effectively measure the land in the intertidal zone or waters with good water quality, but cannot effectively measure waters with poor water quality (high turbidity), and is limited by the endurance of the UAV, so the operation time is not long. [3] The single-beam sonar can effectively measure all waters with a depth greater than 0.5 meters and less than its maximum measurement range. [4] The mechanical rollers can effectively measure waters with a depth less than 0.5 meters. By leveraging the advantages of the three, a seamless integrated measurement method for intertidal zone topography based on acousto-optic remote sensing and rollers is proposed, which can effectively measure all waters and land parts in the intertidal zone, obtain a high-precision large-scale intertidal zone topographic map, and meet the fine geospatial information requirements for coastal zone development.

[0004] Lidar is a new direction in current intertidal zone mapping technology. At present, there is no air-cushion vehicle-borne lidar system in China that can effectively carry out operational intertidal zone mapping. The detection sensitivity of the lidar sounding radar developed by foreign manufacturers cannot meet the requirements of intertidal zone mapping with low reflectivity. In addition, these lidar sounding radar systems are bulky and require a large aircraft platform for carrying, with high requirements for the use of flight airspace, and cannot meet the all-weather fast mobility requirements of intertidal zone mapping. [5] .

[0005] The United States is the country that was the first in the world to conduct research on airborne lidar bathymetry systems. In 1968, Hickman and Hogg from Syracuse University in the United States built the world's first laser sea water measurement system, verified the feasibility of laser bathymetry technology for the first time, and initially established the theoretical basis for marine laser detection technology. [6] ; Subsequently, the US Navy successfully developed an airborne pulsed laser system (PLADS) and conducted tests in 1971. [7] ; The National Aeronautics and Space Administration (NASA) of the United States successfully developed an airborne lidar bathymeter (ALB). [8] , conducted tests from 1971 to 1974. It used a 50Hz Nd:YAG laser, and when the transparency of the water quality disc was 5m, the bathymetry reached about 10m. In the late 1970s, NASA developed an airborne hydrographic lidar (AOL) device with scanning and high-speed data recording capabilities. [9] , which used a 400Hz low-peak-power 2kW helium-neon laser to map the seabed topography with a water depth of less than 10m. In the 1980s, the US Navy used a 500Hz fuel laser, achieved circular scanning with a crystal device, had a received signal dynamic range of 120dB, was combined with the Global Positioning System (GPS) for positioning, had a sampling rate of 1GHz, used a logarithmic amplifier, differential, and low-pass filters for signal processing, and the processing results were displayed in color coding for manual recognition and reprocessing, with a processing speed of 5MIPS, but it was still 1 to 2 orders of magnitude lower than the actual required real-time processing speed. Subsequently, the United States adopted a new computer system, increasing the processing speed to 300MIPS to ensure that any feasible algorithm could be implemented.

[10] . This system was tested off the coast of Florida in February 1990.

[0006] In the mid-1980s, the United States Army Corps of Engineers (USACE) launched a development project to produce an airborne laser scanning bathymetry system (SHOALS).

[11] , this project was finally developed with the support of Optech Canada Inc. The SHOALS system was initially applied to the measurement of navigation line environments and soon evolved into a coastal area mapping system. Today, SHOALS has become one of the main means of offshore bathymetry. Optech has cooperative relationships with the U.S. Navy Meteorology and Oceanography Command and the Naval Oceanographic Office. Based on these relationships, the SHOALS system undertakes a large number of tasks such as chart production and rapid environmental estimation for military exercises. After more than 30 years of efforts and technological breakthroughs, Optech has successively developed a series of products for bathymetry, including SHOALS 200 (1993), SHOALS 400 (1998), SHOALS 1000 (2003), and SHOALS 3000 (2006).

[12] . Among them, the SHOALS 3000T, as its latest model, incorporates Optech's years of research results and practical operation experience. It is a commercially available airborne laser bathymetry system that has been successfully finalized. It has the function of synchronous measurement of water depth and topography and can be equipped with advanced accessories such as high-resolution digital cameras, hyperspectral remote sensing, and ultraspectral. This device weighs 217 kg and is generally used on large helicopter platforms and is basically inapplicable to unmanned aerial vehicle platforms. Currently, its main users are the U.S. Navy and the National Oceanic and Atmospheric Administration. In addition, this system has been promoted and applied in the Japan Coast Guard and the Fugro-pelagos commercial company. Table 1 shows the parameter comparison of several shallow sea mapping lidar systems.

[0007] Table 1 Parameter Comparison of Several Shallow Sea Mapping Lidar Systems

[0008] 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 Order1 IHO Order1 IHO Order1 Horizontal accuracy IHO Order1 IHO Order1 5m CEP 95% Minimum detection depth 0.2 0.3 0.5 Maximum detection depth 50 3 times disc transparency 70m Scan width Maximum 0.75 times flight altitude 100～350m

[0009] In addition, the airborne bathymetric lidar system developed by RIEGL has relatively strong competitiveness. The parameter comparison between the two waterway joint measurement lidar systems developed by RIEGL and this system is shown in Table 2. The waterway joint measurement lidar systems developed by RIEGL are divided into two types: the lightweight BathyCopter and the high-efficiency VQ-880-G.

[13] , the BathyCopter is small in mass but can only perform single-point scanning, resulting in low mapping efficiency; the VQ-880-G uses a linear detection system and cannot meet the signal-to-noise ratio requirements under the current situation of low reflectivity of tidal flats in intertidal zone mapping. The system is bulky and has high power consumption and cannot be applied to unmanned aerial vehicle platforms. This system uses a photon counting system for intertidal zone mapping, meeting the requirements of high detection efficiency while achieving the requirements of miniaturization, low power consumption, and high precision for the system.

[0010] Table 2 Parameter Comparison of RIEGL Waterway Joint Measurement Lidar Systems

[0011]

[0012] For traditional surveying and mapping lidar detection systems, whether it is waveform digital sampling, multi-pulse measurement or pulse width measurement, their essence is the detection of echo waveforms. This detection system cannot make full use of the photon energy in the echo pulses, thus requiring relatively high laser single-pulse energy and system optical aperture. To solve the low-efficiency problem of the linear detection system, a photon counting detection system with single-photon sensitivity has been introduced into the field of surveying and mapping lidar.

[0013] The first use of a lidar with a photon counting system for ground surveying was successfully achieved in the Instrument Incubator Program (IIP) of the NASA P-3 aircraft. The NASA Goddard Space Flight Center was the first to conduct research work in this area. Its first-generation airborne verification system was called the Micro Altimeter, with a laser wavelength of 532 nm, a repetition rate of 10 KHz, a pulse energy of 2 μJ, and a 2×2 element photomultiplier tube (PMT) operating in photon counting mode as the echo detector. The transmitter and receiver shared a 20 cm aperture off-axis telescope and were combined with a single optical wedge in front of the telescope to achieve ground conical scanning imaging.

[14] 。

[0014] Based on the Micro Altimeter, researchers at the NASA Goddard Space Flight Center and Sigma Corporation developed the second-generation airborne verification system, the Imaging Photon-counting Altimeter (IPA).

[15] Its laser wavelength remained 532 nm, the repetition rate was increased to 22 KHz, and the pulse energy was 6.4 μJ. At the same time, a PMT operating in photon counting mode was still selected as the echo detector, but the number of elements increased to 10×10. The system used a double optical wedge scanner to provide one-dimensional and two-dimensional scans for different platform speeds, enabling wide-area imaging for a single overflight of airspace.

[0015] Due to its extremely high sensitivity, the photon counting detection system can penetrate a certain water depth to obtain the topography of the shallow water area. On this basis, researchers at the University of Florida in the United States developed a lidar prototype with a photon counting system (Coastal Tactical-Mapping System, CATS).

[16] It was used for measuring in coastal areas and successfully measured the topography below a water depth of 5 m.

[0016] ICESat-1, launched by NASA in 2003, was the world's first satellite equipped with a lidar altimeter. Its main payload was the Geoscience Laser Altimeter System (GLAS). As the successor to ICESat-1, ICESat-2's main payload is the Advanced Topographic Laser Altimeter System (ATLAS).

[17] This payload achieved the multi-beam push-broom function that ICESat-1 did not have. Due to the use of a high repetition rate (10 kHz) photon counting detection system, the laser energy required by the system was significantly reduced. The total pulse energy before beam splitting was only 400 μJ, yet it achieved a measurement accuracy of approximately 10 cm and a horizontal resolution of approximately 70 cm.

[0017] In shallow water bathymetry, single-beam sonar is the most widely used depth sounding device. Traditional underwater bathymetry obtains the water level at the sounding point by means of a water level correction model, and then subtracts the observed water depth to obtain the elevation of the underwater terrain point. The calculation process does not consider the influence of the ship's attitude on sounding, and the accurate water level at the sounding point cannot be obtained, resulting in low accuracy of the underwater terrain. [18-20] Research and practice have confirmed that the two factors of water level correction effect and sounding effect severely restrict the improvement of underwater bathymetry accuracy.

[21] Combining single-beam sonar with GNSS, GPS-RTK technology, and attitude sensors, etc., can achieve high-precision tide-free mode underwater bathymetry.

[22] Zhao Jianhu believed in his research in 2006 and 2008 that the sounding accuracy of current sounding equipment can reach 0.3% of the water depth under ideal conditions, which can fully meet the requirements of IHOS-44 bathymetry accuracy, but the comprehensive water depth accuracy of the measurement results is far lower than this index.

[23] The reason is that a precise instantaneous three-dimensional reference has not been provided for the transducer of the sounding system and the influence of the sounding effect has not been strictly considered. Wang Shuanlin believes that the main error sources affecting single-beam sounding include the installation deviation of the transducer, sound velocity error, delay effect of sonar, GPS error, ship speed and ship attitude, etc.

[24] Hao Bingyi summarized the error sources of single-beam sounding, simply analyzed the influence of errors on measurement accuracy and proposed several control measures. Combining error factor analysis and corresponding accuracy control theory, a series of practical demonstration studies were carried out in inland waterway engineering.

[25] Aiming at the difficulty of effectively obtaining water depth data at the same location in different periods affected by the dynamic marine environment, Liu Zhihao et al. proposed a multi-period water depth profile analysis method of combined uncertainty.

[26] Through steps such as initial uncertainty calculation, water depth and uncertainty transfer, and Kalman filter update, a time-series topographic profile is constructed, and the analysis of topographic evolution of the profile is completed based on hypothesis testing. The results show that the method in this paper obtains a water depth profile that can be effectively compared with the actual topographic profile and can accurately evaluate the area with sudden topographic changes.

[0018] In summary, although there are many advantages in using airborne LiDAR to measure the intertidal zone and single-beam sonar measurement is also simple and convenient, the current technology still has the following defects:

[0019] (1) Using airborne LiDAR alone to measure the intertidal zone, especially in shallow water areas with high turbidity, LiDAR still cannot completely penetrate the water body to reach the bottom.

[0020] (2) Airborne LiDAR is easily affected by wind, resulting in too much jitter of the attitude sensor, causing a large attitude error, and ultimately affecting the mapping accuracy of LiDAR.

[0021] (3) The sounding accuracy of single-beam sonar is easily affected by installation error, beam emission angle error, and near-field environmental noise. It is difficult to completely eliminate the error, and due to its emission mechanism limitation, the resolution of underwater measurement points is not high.

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[30] He Linbang, Zhao Jianhu, Zhang Hongmei, et al. Precise multi-beam ray tracing method considering attitude angles [J]. Journal of Harbin Engineering University, 2015, (1): 46-50. Summary of the Invention

[0053] The technical problem to be solved by the present invention is to address the issues that marine lidar fails to effectively measure underwater terrain in turbid waters of the intertidal zone and the low measurement efficiency of single-beam sonar. By integrating an air-cushion vehicle-mounted lidar, single-beam sonar, and mechanical rollers, the underwater terrain of the intertidal zone is measured, a calculation model for the coordinates of three types of bathymetric point clouds in the WGS84 space rectangular coordinate system is constructed, and finally, different types of bathymetric points of approximate homologous points are fused to obtain high-precision and high-density bathymetric points of the underwater terrain in the intertidal zone that meet the measurement requirements.

[0054] To achieve the above object, the seamless integrated measurement method for intertidal zone terrain based on acousto-optic remote sensing and rollers provided by the present invention includes the following steps:

[0055] (1) Study the scanning structure of photon-counting lidar and construct a calculation model for the coordinates of lidar underwater bathymetric points in the WGS84 space rectangular coordinate system, that is, first deduce the coordinate calculation formula for underwater bathymetric points in the lidar scanning reference coordinate system, and then, according to the geometric position relationship among the center of the lidar scanning reference coordinate system, the center of the GPS antenna, and the center of the inertial measurement unit, the coordinates of underwater bathymetric points are mapped to the WGS84 space rectangular coordinate system;

[0056] (2) Construct a calculation model for the coordinates of single-beam sonar sounding points in the WGS84 space rectangular coordinate system. That is, first consider the attitude change of the hovercraft to calculate the initial incident angle of the single-beam sonar beam, and then derive the coordinates of the seabed sounding points in the transducer coordinate system according to ray tracing. Similarly, the single-beam underwater sounding points are mapped back to the WGS84 space rectangular coordinate system;

[0057] (3) Construct a calculation model for the coordinates of the lowest sounding point of the mechanical roller in the WGS84 space rectangular coordinate system. That is, fix a measurement support installation box at the left rear of the hovercraft, and derive the coordinate formula of the lowest point of the roller according to the geometric relationship of the roller fixedly connected to it. Similarly, the coordinates of the lowest sounding point of the roller are mapped back to the WGS84 space rectangular coordinate system;

[0058] (4) For the fusion processing of different types of sounding point clouds in the overlapping area, according to the approximate homologous point rule, consider two points that meet the conditions as homologous points, merge them into one sounding point, reassign values, and take the average values of their X, Y, and Z coordinates respectively. Obtain a high-precision and high-density underwater topographic measurement data set for the intertidal zone.

[0059] In an embodiment of the present invention, constructing a calculation model for the coordinates of lidar underwater sounding points in the WGS84 space rectangular coordinate system mainly includes the following steps:

[0060] (1) Establish the lidar scanning reference coordinate system and its transition coordinate system;

[0061] (2) Calculate the normal vector of the mirror in the lidar scanning reference coordinate system;

[0062] (3) Calculate the nadir angle and azimuth angle of the reflected light in the lidar scanning reference coordinate system;

[0063] (4) Calculate the coordinates of the underwater sounding points in the lidar scanning reference coordinate system;

[0064] (5) Calculate the mapping of the underwater sounding points to the WGS84 space rectangular coordinate system.

[0065] In an embodiment of the present invention, constructing a calculation model for the coordinates of single-beam sonar sounding points in the WGS84 space rectangular coordinate system mainly includes the following steps:

[0066] (1) Consider the change of the hovercraft attitude and calculate the initial incident angle of the single-beam sonar beam;

[0067] (2) Construct a ray-tracing model in the transducer coordinate system;

[0068] (3) After obtaining the underwater beam footprint coordinates, construct a calculation model for mapping the coordinates to the WGS84 space rectangular coordinate system.

[0069] In an embodiment of the present invention, a calculation model for the coordinates of the lowest sounding point of a mechanical roller in the WGS84 space rectangular coordinate system is constructed, mainly including the following steps:

[0070] (1) Establish a space rectangular coordinate system with the center of the measurement support placement box at the left stern of the hovercraft as the origin, and calculate the coordinates of the lowest sounding point of the mechanical roller in this coordinate system;

[0071] (2) Construct a calculation model for converting these coordinates to the WGS84 space rectangular coordinate system.

[0072] In an embodiment of the present invention, the fusion processing of different types of sounding points with approximately the same name in the overlapping area mainly includes the following steps:

[0073] (1) Set the conditions for the rules of different types of approximately the same name points;

[0074] (2) Merge different types of sounding points that meet the conditions of approximately the same name points into a new sounding point, reassign values, and respectively take the average values of their X, Y, and Z coordinates.

[0075] The technical problems to be solved by the present invention mainly include the following aspects:

[0076] (1) Construct a calculation model for the coordinates of the underwater sounding points of lidar in the WGS84 space rectangular coordinate system;

[0077] (2) Construct a calculation model for the coordinates of the sounding points of a single-beam sonar in the WGS84 space rectangular coordinate system;

[0078] (3) Construct a calculation model for the coordinates of the lowest sounding point of a mechanical roller in the WGS84 space rectangular coordinate system;

[0079] (4) Make corresponding fusion processing for different types of sounding points with approximately the same name in the overlapping area.

[0080] Through the above technical solutions, the beneficial effects of the present invention are:

[0081] (1) By studying the scanning structure of photon-counting lidar, a calculation model for the coordinates of underwater sounding points of lidar in the WGS84 space rectangular coordinate system is constructed. By scanning the underwater terrain with high precision and high density, the mapping efficiency of the intertidal zone can be greatly improved;

[0082] (2) In the turbid waters of the intertidal zone, lidar often fails to detect the bottom. Therefore, by combining single-beam sonar and mechanical roller measurement methods, the problem of topographic measurement in the blank area of lidar measurement in very shallow turbid waters can be solved. Description of the Drawings

[0083] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0084] Figure 1 is a schematic structural diagram of the lidar elliptical scanning system of the present invention;

[0085] Figure 2 are two Cartesian rectangular coordinate systems of the normal direction vector of the mirror of the present invention;

[0086] Figure 3 is the geometric angle of the reflected light of the present invention in the sensor coordinate system;

[0087] Figure 4 is a schematic structural diagram of the geometric structure for calculating the outgoing laser from the normal change of the present invention;

[0088] Figure 5 is a relationship diagram between the actual measurement points and the laser sea surface incident points of the present invention;

[0089] Figure 6 is a schematic diagram of a hovercraft-mounted mechanical roller for measuring the seabed topography of the present invention.

[0090] Figure 7 is a block diagram of a seamless integrated measurement method for intertidal zone topography based on acousto-optic remote sensing and rollers. Detailed implementation manners

[0091] In order to make the technical means, creative features, achieved purposes and effects of the present invention easy to understand, the present invention will be further described below with reference to specific illustrations.

[0092] First of all, the present invention involves the following technical terms:

[0093] Marine lidar bathymetric system

[0094] The marine lidar bathymetric system is a complex active bathymetric system integrating various technologies such as laser ranging, attitude determination, computer, environmental parameter correction, and measurement data processing

[27] .

[0095] Inertial navigation system

[0096] The inertial navigation system (INS, hereinafter referred to as the inertial navigation) is an autonomous navigation system that does not rely on external information and does not radiate energy to the outside

[28] 。Its working environment includes not only the air and the 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 an inertial reference frame, integrating it with respect to 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.

[0097] Single-beam sonar sounding technology

[0098] A short-pulse acoustic wave (beam) with a certain spatial directivity is emitted into the water by a transducer. The acoustic wave propagates in the water. When it encounters the seabed, reflection, transmission, and scattering occur. The reflected echo is received by the transducer. Given the time interval t between the transducer's emission and reception of the echo and the average propagation speed V of the acoustic wave in the water body, the one-way travel distance Z of the acoustic wave, that is, the distance from the transducer to the seabed, can be calculated.

[29] 。

[0099] Ray tracing method

[0100] Ray tracing is a calculation method for the coordinates of the beam footprint (projection point) relative to the hull coordinate system based on the sound velocity profile.

[30] 。Ray tracing usually adopts the layer addition method, that is, two adjacent sound velocity sampling points in the sound velocity profile are divided into a layer. The sound velocity change within the layer can be assumed to be a constant value (zero gradient) or a constant gradient. For the former, the calculation idea and process are simple, and the latter is relatively complex. In the ray tracing calculation process of the latter, the sound velocity change function adopts the Harmonic mean sound velocity.

[0101] The seamless integrated measurement method for intertidal zone topography based on acou-optical remote sensing and rollers of the present invention mainly includes the following steps:

[0102] (1) Study the scanning structure of the photon-counting lidar, and construct a calculation model for the coordinates of the underwater sounding points of the lidar in the WGS84 space rectangular coordinate system, that is, first deduce the coordinate calculation formula for the underwater sounding points in the lidar scanning reference coordinate system, and then according to the geometric position relationship among the center of the lidar scanning reference coordinate system, the center of the GPS antenna, and the center of the inertial measurement unit, the coordinates of the underwater sounding points are restored to the WGS84 space rectangular coordinate system;

[0103] (2) Construct a calculation model for the coordinates of the single-beam sonar sounding points in the WGS84 space rectangular coordinate system, that is, first consider the attitude change of the hovercraft, calculate the initial incident angle of the beam of the single-beam sonar, and then deduce the coordinates of the seabed sounding points in the transducer coordinate system according to ray tracing. Similarly, the single-beam underwater sounding points are restored to the WGS84 space rectangular coordinate system;

[0104] (3) Establish a calculation model for the coordinates of the lowest sounding point of the mechanical roller in the WGS84 space rectangular coordinate system. That is, fix a measurement support placement box at the left rear of the hovercraft. According to the geometric relationship of the roller fixedly connected to it, derive the coordinate formula for the lowest point of the roller. Similarly, return the coordinates of the lowest sounding point of the roller to the WGS84 space rectangular coordinate system;

[0105] (4) For the fusion processing of different types of sounding point clouds, according to the approximate homologous point rule, consider two points that meet the conditions as homologous points, merge them into one sounding point, reassign values, and respectively take the average values of their X, Y, and Z coordinates to obtain a high-precision and high-density underwater terrain measurement data set for the intertidal zone.

[0106] See Figures 1 to 6 As shown below, the specific implementation manners of the present invention will be described in detail as follows:

[0107] (1) Overall technical solution

[0108] First, study the structure of the lidar scanning system and establish a calculation model for the coordinates of the lidar underwater sounding points in the WGS84 space rectangular coordinate system; second, considering the change of the ship's attitude and based on the constant gradient ray tracing model, establish a calculation model for the coordinates of the single-beam sonar sounding points in the WGS84 space rectangular coordinate system; third, establish the geometric relationship between the measurement placement box at the left rear of the ship and the mechanical roller, and establish a calculation model for the coordinates of the lowest sounding point of the mechanical roller in the WGS84 space rectangular coordinate system; finally, perform fusion processing on three types of approximately homologous sounding points in the overlapping area to obtain a high-precision and high-density underwater terrain measurement data set for the intertidal zone.

[0109] (2) Calculation of the coordinates of the lidar scanning footprint points in the WGS84 space rectangular coordinate system

[0110] 1) Structure of the lidar elliptical scanning system

[0111] The marine lidar introduced in this article has a conventional elliptical scanning structure ( Figure 1 as shown). A prism that can rotate around the 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 to 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 draws a trajectory on the sea surface with an approximate incident angle of 15°. Since the incident angle is not equal to 15° during the scanning process (related to the normal direction), the laser point trajectory on the sea surface when the aircraft is hovering is an oval shape similar to an ellipse. Therefore, this scanning structure is also called an oval scanning structure.

[0112] 2) Lidar scanning reference coordinate system

[0113] Definition of the reference coordinate system for lidar scanning: The center point of the mirror is the coordinate origin O, the X s axis points in the negative direction of the outgoing laser, the Y s axis points in the flight direction, and the Z s axis and the X s and Y s axes form a right-handed coordinate system with the direction perpendicular to the upward. The incident laser and the motor shaft are in the same plane (X s Z s plane), the laser is incident horizontally (along the negative direction of the X s axis), and the incident point of the laser on the mirror is the center of the mirror. For ease of understanding, as Figure 2 shown, the original X s Y s Z s coordinate system is rotated counterclockwise by 45° around the Y s axis to obtain a new coordinate system X s ′Y s ′Z s ′. At this time, the Z s ′axis coincides with the rotation direction of the motor. The projection of the mirror normal on the X s Z s plane and the angle s with the Z axis in the projection on the Y s Z s plane and the angle s with the Z

[0114] As Figure 3 shown, the angles between the projection of the reflected light on the X s Z s and Y s Z s planes and the Z axis are φ x and φ y respectively, and its nadir angle is φ. Since the laser is incident along the negative direction of the X s axis, the angle s Z s between the projection of the normal on the Y s axis and the Z axis is equal to the angle between the projection of the reflected laser on the Y s Z s plane and the Z s axis φ y (because the incident laser line, the mirror normal, and the reflected laser line are coplanar, and the incident laser line is perpendicular to the Y s Z s plane. According to the theorem that when a plane passes through the perpendicular of another plane, the two planes are orthogonal at this time), therefore, in the Y s Z sOn a plane, the rotation angle of the normal is synchronized with the rotation angle of the reflected light (i.e., when the normal rotates by an angle θ, the reflected light also rotates by an angle θ). And in the X s Z s plane, when the mirror rotates (i.e., the normal) by an angle θ, the reflected light rotates by an angle of 2θ. When the normal angle changes, the included angle also changes accordingly, from it is easy to solve for φ x , and then calculate the nadir angle φ and azimuth angle of the light beam Therefore, the change in the angle of the normal is the key point.

[0115] 3) Direction vector of the mirror normal

[0116] In Figure 2 , the normal vector of the mirror normal in the X s ′Y s ′Z s ′ coordinate system (F x′ , F y′ , F z′ ):

[0117]

[0118] Then, by rotating clockwise by 45° around the Y s ′ axis, the normal vector of the mirror normal in the X s Y s Z s coordinate system (F x , F y , F z ):

[0119]

[0120] 4) Relevant angles of the reflected light in the laser scanning reference coordinate system

[0121] From Figure 2 , Figure 4 geometric relationships, Therefore, there is:

[0122] φ x (θ) = 2 arctan(F x / |F z |) - 90° (3)

[0123] From Figure 2 geometric relationships, And So:

[0124] φ y (θ) = arctan(F y / |Fz ) (4)

[0125] From Figure 3 the geometric relationship, the nadir angle φ and the azimuth angle are as follows:

[0126]

[0127]

[0128] 5) Coordinates of the underwater sounding point in the lidar scanning reference coordinate system

[0129] If the measured height of the center of the reflector is H, the position coordinates of the laser sea surface incidence point are:

[0130] x s = H tan(φ x ) (7)

[0131] y s = H tan(φ y ) (8)

[0132] z s = -H (9)

[0133] As Figure 5 shown, if the sea surface is a plane, the laser sea surface incidence point and the underwater sounding point are P1 and P2 respectively, the center of the laser is represented as S, the aircraft flight altitude is H, the water depth is h, the oblique distances of the laser beam in the air and underwater are L1 and L2 respectively, and the refractive index of seawater is n, and the azimuth angle is then the formulas related to the underwater sounding point are as follows:

[0134] α(θ) = arcsin(sinφ / n) (10)

[0135] h = L2cosα (11)

[0136]

[0137]

[0138] Z s = -H - h (14)

[0139] 6) Coordinates of the underwater sounding point in the WGS84 space rectangular coordinate system

[0140]

[0141] And (X GPS , Y GPS , Z GPS ) are as follows:

[0142]

[0143] In equations (15) and (16), (X s-wgs84 , Y s-wgs84 , Z s-wgs84 ) are the coordinates of the underwater sounding point of the lidar in the WGS84 space rectangular coordinate system; (X GPS , Y GPS , Z GPS ) are the coordinates of the center of the GPS antenna on the hovercraft in the WGS84 space rectangular coordinate system; R(yaw, pitch, roll) is the rotation matrix for converting the body coordinate system to the local navigation coordinate system; It includes two parts, namely 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; is the installation declination of the laser scanning reference coordinate system relative to the IMU body coordinate system.

[0144] (3) Calculation of the coordinates of the single-beam sounding point in the WGS84 space rectangular coordinate system

[0145] According to the literature of He Linbang et al., the initial incident angle and azimuth angle of the beam are respectively:

[0146]

[0147]

[0148] Here, θ i ′ is the actual incident angle of the beam, θ i is the initial allocation angle of the beam, α is the angle of rotation around the OX axis, β is the angle of rotation around the OY axis, is the azimuth angle of the beam.

[0149] After calculating the initial incident angle of the beam, the method of constant gradient sound speed tracking is used to calculate the coordinates of the beam footprint in the hull coordinate system. Assume that the Harmonic mean sound speed of the sound ray propagation in the i-th layer is Then the horizontal displacement Δy i and the time t i are:

[0150]

[0151]

[0152] When performing layered ray tracing, in addition to calculating the vertical displacement, horizontal displacement, and propagation time of the entire layer, it is also necessary to calculate the vertical displacement and horizontal displacement of the remaining layers based on the remaining propagation time. Assume that when the ray propagates in the $i$-th layer and ends at point $r$ within this layer, the remaining time $t$ r is equal to the one-way travel time $t$ all of the beam minus the cumulative propagation time before the $i$-th layer. Then, the vertical displacement $\Delta z$ r and horizontal displacement $\Delta y$ r of the ray in the remaining layers are:

[0153]

[0154] Then, the total vertical displacement $z$ and horizontal displacement $y$ of the ray propagation are:

[0155]

[0156] According to the obtained horizontal displacement and vertical displacement, combined with the azimuth angle of the beam, the coordinates $(X$ sg , $Y$ sg , $Z$ sg ) of the beam footprint in the transducer coordinate system can be obtained:

[0157]

[0158] After obtaining the coordinates of the beam footprint in the transducer coordinate system, through homing calculation, the coordinates of the beam footprint are homed to the WGS84 space rectangular coordinate system.

[0159]

[0160] $(X$ sg-wgs84 , $Y$ sg-wgs84 , $Z$ sg-wgs84 ) are the coordinates of the beam footprint in the WGS84 space rectangular coordinate system; $(X$ GPS , $Y$ GPS , $Z$ GPS ) are the coordinates of the center of the hovercraft-mounted GPS antenna in the WGS84 space rectangular coordinate system; $R(yaw, pitch, roll)$ is the rotation matrix for converting the body coordinate system to the local navigation coordinate system; It includes two parts, namely the eccentricity difference between the center of the transducer 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; is the installation declination angle of the transducer coordinate system relative to the IMU body coordinate system.

[0161] (4) Calculation of the coordinates of the mechanical roller sounding points in the WGS84 space rectangular coordinate system

[0162] Such as Figure 6As shown in the figure, a roller measurement support installation box is fixed at the starboard tail. Taking the center of this support installation box as the origin, the ship's navigation direction as the X-axis, vertically downward perpendicular to the X-axis as the Z-axis, and the Y-axis, X-axis, and Z-axis form a right-handed coordinate system. The length of the fixed rod from the center of the measurement support installation box on the ship to the center of the roller is L, and the radius of the roller is r. When using the roller to measure the seabed, the angle between the fixed rod and the vertical direction of the center point of the measurement support installation box is θ, and the angle between the fixed rod and the positive direction of the X-axis is The angle is provided by the angle measuring instrument. Then, the X, Y, and Z coordinates of the lowest point of the roller in the roller coordinate system are respectively:

[0163]

[0164]

[0165] H w = L cosθ + r (27)

[0166] The coordinates of the lowest sounding point of the roller in the WGS84 space rectangular coordinate system are:

[0167]

[0168] (X w-wgs84 ,Y w-wgs84 ,Z w-wgs84 ) are the coordinates of the lowest sounding point of the roller in the WGS84 space rectangular coordinate system; (X GPS ,Y GPS ,Z GPS ) are the coordinates of the center of the GPS antenna on the hovercraft in the WGS84 space rectangular coordinate system; R(yaw, pitch, roll) is the rotation matrix for converting the body coordinate system to the local navigation coordinate system; It includes two parts, namely the eccentricity difference between the center of the measurement support installation box 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; is the installation declination of the measurement support coordinate system relative to the IMU body coordinate system.

[0169] (5) Fusion processing of three types of sounding points in the overlapping area

[0170] After obtaining the data of the lidar sounding points, single-beam sonar sounding points, and roller sounding points, since their coordinates have been normalized to the WGS84 space rectangular coordinate system, their sounding points form a full-coverage strip measurement. When two sounding points of different types meet the conditions of Equation (29), these two points are considered approximately homologous points, and then the coordinates of the sounding points need to be re-assigned, as shown in Equation (30).

[0171]

[0172]

[0173] Thus, the following technical problems are solved in the present invention:

[0174] (1) A calculation model for the coordinates of the underwater sounding points of the lidar in the WGS84 space rectangular coordinate system is constructed;

[0175] (2) A calculation model for the coordinates of the underwater sounding points of the single-beam sonar in the WGS84 space rectangular coordinate system is constructed;

[0176] (3) A calculation model for the coordinates of the lowest sounding point of the mechanical roller in the WGS84 space rectangular coordinate system is constructed;

[0177] (4) A fusion processing method for different types of approximately homologous sounding point clouds in the overlapping area is proposed;

[0178] (5) The problem of full coverage measurement of the high-precision and high-density underwater terrain in the intertidal zone is solved.

[0179] In addition, the technical features of the present invention are as follows:

[0180] (1) Calculation models for the underwater sounding point coordinates of the lidar, single-beam sonar, and mechanical roller in the WGS84 space rectangular coordinate system are respectively constructed;

[0181] (2) For the fusion problem of three types of approximately homologous sounding points in the overlapping area, corresponding fusion processing methods are proposed.

Claims

1. A seamless integrated measurement method for intertidal zone topography based on acousto-optic remote sensing and rollers, characterized in that, It includes the following steps: (1) Study the structure of the photon counting lidar scanning system, and construct a calculation model for the coordinates of the lidar underwater sounding points in the WGS84 space rectangular coordinate system; (2) Considering the attitude change of the hovercraft, based on the ray tracing model, construct a calculation model for the coordinates of the single-beam sonar sounding points in the WGS84 space rectangular coordinate system; (3) Establish the geometric relationship between the measurement support placement box at the tail of the hovercraft and the mechanical roller, and construct a calculation model for the coordinates of the lowest sounding point of the mechanical roller in the WGS84 space rectangular coordinate system; (4) For the fusion processing of different types of sounding point clouds in the overlapping area, according to the approximate homologous point rule, consider two points that meet the conditions as homologous points, merge them into one sounding point, reassign values, and respectively take the average values of their X, Y, and Z coordinates; obtain a high-precision and high-density underwater terrain measurement dataset for the intertidal zone; The calculation model for the coordinates of the lidar underwater sounding points in the WGS84 space rectangular coordinate system described in step (1) includes the following steps: 1) The calculation model includes the analysis of the lidar scanning system structure; 2) Establish the lidar scanning reference coordinate system and its transition coordinate system; 3) Establish the relationship between the reflected light and the normal vector of the mirror in the lidar scanning reference coordinate system; 4) Calculate the coordinates of the underwater sounding points in the lidar scanning reference coordinate system; 5) Locate the underwater sounding points in the WGS84 space rectangular coordinate system; The calculation model for the coordinates of the single-beam sonar sounding points in the WGS84 space rectangular coordinate system described in step (2) includes the following steps: 1) Considering the attitude change of the hovercraft, calculate the initial incident angle of the single-beam sonar beam; 2) Establish a ray tracing model; 3) Calculate the coordinates of the beam footprint in the transducer coordinate system; 4) Locate the beam footprint coordinates in the WGS84 space rectangular coordinate system; The calculation model for the coordinates of the lowest sounding point of the mechanical roller in the WGS84 space rectangular coordinate system described in step (3) includes the following steps: 1) Establish a space rectangular coordinate system with the center of the measurement support placement box at the left stern of the hovercraft as the origin; 2) Calculate the coordinates of the lowest sounding point of the mechanical roller in this coordinate system; 3) Locate the coordinates of the lowest sounding point of the mechanical roller in the WGS84 space rectangular coordinate system; The fusion processing method for different types of approximately homologous sounding point clouds in the overlapping area described in step (4) includes the following steps: 1) Set the conditions for approximately homologous points of different types of sounding point clouds in the overlapping area. If point A(x1, y1) and point B(x2, y2) meet the following conditions, then point A and point B are approximately homologous points; 2) Make corresponding fusion processing for different types of approximately homologous sounding points that meet the conditions; 3) Obtain a high-precision and high-density underwater terrain dataset for the intertidal zone.

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