An automatic calibration ocean laser radar system and calibration method

Abstract

This invention relates to the field of marine environmental monitoring technology, specifically to an automatically calibrated marine lidar system and calibration method. The system includes a detection cabin, a scanning gimbal, a control cabin, and a support frame. The detection cabin houses a laser emitting unit and an optical receiving unit. The scanning gimbal is fixed below the detection cabin to control its detection direction. The bottom of the scanning gimbal is fixed above the control cabin. The control cabin integrates a data acquisition card, a controller, a network module, and signal acquisition and control software. The support frame supports the control cabin. By integrating the optical system, signal acquisition and control system, and scanning gimbal into a single unit, the system can perform automatic calibration functions such as automatic calibration of the receiving and receiving optical axes, automatic adjustment of the receiving field of view, signal saturation correction, and laser detection direction control, reducing the need for on-site personnel operation.

Description

Technical Field

[0001] This invention relates to the field of marine environmental monitoring technology, specifically to an automatically calibrated marine lidar system and calibration method. Background Technology

[0002] The ocean covers approximately 71% of the Earth's surface and is a vital strategic space for global economic and social development. It is also a crucial field for studying climate change and Earth sciences, and its role and importance in global economic and social development and ecological environmental protection are increasingly prominent. Strengthening ocean observation and developing new detection methods are of great and far-reaching significance for caring for, understanding, and managing the ocean, and further promoting my country's development into a maritime power. The three elements of ocean color—phytoplankton chlorophyll, colored dissolved organic matter (CDOM), and suspended matter—are important indicators for studying changes in the marine environment and are crucial parameters characterizing the marine environment and ocean water quality.

[0003] Marine lidar, as an advanced active remote sensing technology, has been applied in shallow sea topographic mapping, marine biomass estimation, and water optical parameter inversion due to its high spatiotemporal resolution and vertical profile detection capabilities. However, in practical applications, traditional marine lidar systems still face several technical bottlenecks. First, suspended particles and plankton in seawater cause severe multiple scattering effects, resulting in laser pulse broadening in both space and time, thus affecting measurement accuracy. Existing systems often lack effective real-time adjustment methods to suppress or compensate for this effect. Second, marine lidar is typically deployed in complex environments such as airborne, shipborne, or fixed offshore stations. Long-term vibration and environmental changes can lead to problems such as deviation of the lidar's light-receiving and receiving axes, further affecting data quality. Traditional calibration methods often rely on manual adjustment, which is inefficient, has limited accuracy, and is difficult to perform in real-time during unattended observations. In addition, strong near-field echoes can easily lead to signal saturation, causing signal distortion. Currently, the automation level of marine lidar systems on the market is generally insufficient, and they do not have automatic calibration functions. In long-term, large-scale marine monitoring missions, how to achieve automatic calibration of the receiving and receiving light axes, automatic adjustment of the receiving field of view, and signal saturation correction are the key to the development of marine lidar towards intelligence, engineering, and operationalization. Summary of the Invention

[0004] To address the problems existing in the prior art, the purpose of this invention is to provide an automatic calibration system and calibration method for marine lidar, which enables automatic calibration of the receiving and receiving light axes, automatic adjustment of the receiving field of view, signal saturation correction, and laser detection direction control, thereby improving the reliability of continuous observation, data accuracy, and personnel operation safety of marine lidar.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows: an automatically calibrated marine lidar system, comprising a detection cabin, a scanning gimbal, a control cabin, and a support frame. The detection cabin houses a laser emitting unit and an optical receiving unit. The scanning gimbal is fixed below the detection cabin to control its detection direction. The bottom of the scanning gimbal is fixed above the control cabin. The control cabin integrates a data acquisition card, a controller, a network module, and signal acquisition and control software. The support frame supports the control cabin. The laser emitting unit includes a pulsed laser, a reflector, a two-dimensional electrically controlled adjustment frame, a sampling mirror, and a photoelectric position detector. The optical receiving unit includes a transmission telescope and an electrically controlled pinhole aperture. The electrically controlled pinhole aperture is located behind the transmission telescope. The reflector is mounted on the two-dimensional electrically controlled adjustment frame to electrically control the two-dimensional angle of the reflector and adjust the direction of the emitted laser. The sampling mirror is fixed at the center of the telescope's front end. The pulsed laser emits pulsed laser light as a detection source, which passes sequentially through the reflector and the sampling mirror before being emitted into the target. The echo signal from the interaction between the laser and the target is received by the optical receiving unit.

[0006] The aforementioned automatically calibrated marine lidar system further includes a fixed frame for the laser emitting unit. The sampling mirror is mounted on the fixed frame, and the size of the fixed frame is smaller than that of the two-dimensional electronically adjustable frame.

[0007] The aforementioned automatically calibrated marine lidar system includes an optical receiving unit that further comprises a collimating lens, an electrically controlled attenuator wheel, a narrowband interference filter, a polarizing beam splitter, a converging lens, and a photomultiplier tube. The echo signal is first received by a transmission telescope, then passes sequentially through an electrically controlled pinhole aperture, a collimating lens, a narrowband interference filter, and an electrically controlled attenuator wheel. The polarizing beam splitter then splits the optical signal into parallel and perpendicular components, which are then converged by the converging lens into the photomultiplier tube, converting the optical signal into an electrical signal. This electrical signal is synchronously received by the data acquisition card, and the data is stored and displayed on the controller via signal acquisition and control software.

[0008] In the aforementioned automatically calibrated marine lidar system, the aperture D of the electrically controlled pinhole aperture is adjustable in the range of 0 mm to 10 mm. It is placed coaxially with the transmission telescope and installed at the focal position of the transmission telescope. The focal length of the transmission telescope is L, and the receiving field of view θ of the transmission telescope is θ = D / L.

[0009] In the aforementioned automatically calibrated marine lidar system, the electrically controlled attenuator wheel is installed in front of the polarization beam splitter. The electrically controlled attenuator wheel is equipped with six sets of attenuators with different transmittance. The data acquisition and control software outputs commands to control the attenuators with different transmittance to enter the optical receiving unit, attenuating the received signal light to different degrees. When the received signal is saturated, the electrically controlled attenuator wheel controls the attenuators with different transmittance to enter the optical receiving unit in sequence. When the near-field signal is just unsaturated, the electrically controlled attenuator wheel stops adjusting.

[0010] A calibration method for an automatically calibrated marine lidar system as described in any of the above claims includes the following steps: Step 1: Determine the position of the emitted laser when the optical axis is collimated by the photoelectric position detector. Under clear and cloudless atmospheric conditions at night and when the observation platform is stable, the marine lidar controls the laser detection direction to enter the atmosphere through the scanning gimbal. The stable atmospheric signal is used as the detection element for the optical axis calibration process. There are no obstructions in the laser detection path. Step 2: Control the aperture of the pinhole aperture with electronic control, adjust the receiving field of view, and observe whether there is a valid signal in the signal acquisition and control software. If it is determined to be a valid signal, directly perform fine adjustment of the optical axis. If it is determined to be a non-valid signal, the emitted beam does not enter the receiving field of view, and then perform coarse and fine adjustment of the optical axis in sequence. Step 3: After completing the coarse and fine adjustment of the light-receiving axis collimation, install the photoelectric position detector behind the sampling mirror, so that the transmitted emitted light is incident on the photosensitive surface of the photoelectric position detector, and fix the photoelectric position detector after the light spot is centered and evenly distributed on its photosensitive surface. Record the position and distribution of the light spot at this time as the standard position. Step 4: When the marine lidar is observing, if the emitted beam deviates, the distribution of its spot on the photoelectric position detector will also change. According to the change in its position on the photoelectric position detector, the two-dimensional electronically controlled adjustment frame with the reflector is adjusted in the X or Y direction until the position of the spot on the photoelectric position detector is restored to the standard position. Step 5: By cooperating with the telescope through the electronically controlled aperture, the receiving field of view is calculated based on the telescope's focal length. The aperture is adjusted to automatically control the receiving field of view, adapting to different needs such as water body multiple scattering research, water body optical parameter detection, and depolarization ratio measurement. Step 6: Monitor the near-field signal of the water body. If strong saturation occurs, control the electronically controlled attenuator wheel to switch attenuators with different transmittance to attenuate the received signal until the near-field signal is just unsaturated, thus completing the saturation correction.

[0011] In the calibration method of the above-mentioned automatic calibration marine lidar system, when determining whether a signal is valid in step 2, a signal-to-noise ratio (SNR) threshold is set. If the SNR of the signal at the reference distance is lower than the SNR threshold, it is considered not a valid signal. If the SNR of the signal at the reference distance is not lower than the SNR threshold, it is considered a valid signal.

[0012] In the above-described automatic calibration method for a marine lidar system, step 2, the coarse calibration includes: Step C1: Set the coarse adjustment step size of the two-dimensional electronically controlled adjustment frame in the X and Y directions to control the angle change of the reflector and further adjust the direction of laser emission; Step C2: The two-dimensional electronically controlled adjustment frame is controlled by the signal acquisition and control software to adjust the angle of the reflector, and the emitted beam is controlled to perform a spiral scan. A signal acquisition is performed synchronously every time the angle is adjusted, and the signal accumulation time is set. Step C3: As the spiral scan proceeds, the transmitted beam gradually enters the telescope's receiving field of view. When the signal-to-noise ratio of the reference range echo signal is higher than the set signal-to-noise ratio threshold, the transmitted beam has entered the telescope's receiving field of view, achieving preliminary collimation.

[0013] In the above-mentioned automatic calibration method for a marine lidar system, step 2, the fine-tuning includes: Step J1: Set the fine-tuning step size of the two-dimensional electronically controlled adjustment frame in the X and Y directions, assuming that after the optical axis is coarsely adjusted, the emitted beam falls at point M in the receiving field of view; Step J2: Control the emitted beam to continue scanning along the original X direction at small angles according to the fine-tuning step size. Record the echo signal once for each position it moves to. Set the signal accumulation time. The signal at the reference distance will gradually increase and then decrease until the emitted beam scans to point N and leaves the telescope's field of view. Step J3: The echo signal intensity at the reference distance and the adjustment angle of the emitted optical axis X direction are approximately trapezoidal functions. The upper base of the trapezoid curve is the scanning angle range corresponding to when the emitted beam completely enters the telescope's receiving field of view in the X direction. The angle in the X direction corresponding to the center position of the upper base is the optimal position of the emitted beam in this direction, which is set as point C. Control the emitted beam to adjust to the optimal position point C in the X direction. Step J4: Adjust the two-dimensional electronically controlled adjustment frame using data acquisition and control software to position the emitted beam in the Y direction. Refer to the adjustment method in the X direction to determine the optimal position in the Y direction and complete the fine adjustment of the optical axis.

[0014] In the above-mentioned calibration method for an automatically calibrated marine lidar system, in step J3, the divergence angle of the laser beam can be obtained by the rising or falling edge of the trapezoidal curve. The full width at half maximum (FWHM) of the trapezoidal curve corresponds to the receiving field of view of the receiving telescope. The aperture of the electronically controlled pinhole is adjusted to the state during observation. After the pinhole is reduced, the fine-tuning process of the transmitting optical axis in the X and Y directions can be repeated. At this time, the upper base of the trapezoidal curve becomes shorter according to the reduction of the receiving field of view. After the pinhole aperture is reduced, the collimation between the transmitting optical axis and the receiving optical axis can be further improved through the fine-tuning process.

[0015] The beneficial effects of the automatically calibrated marine lidar system and calibration method of this invention are as follows: By integrating the optical system, signal acquisition and control system, and scanning gimbal into a single design, it can automatically perform calibration functions such as automatic calibration of the receiving and transmitting optical axes, automatic adjustment of the receiving field of view, signal saturation correction, and laser detection direction control, reducing on-site personnel operation. The automatic calibration function of the receiving and transmitting optical axes uses a stable atmospheric signal as the detection element. First, a coarse adjustment of the optical axis is performed using a spiral scanning method on a two-dimensional electronically controlled adjustment frame to ensure the transmitted beam enters the receiving field of view. Then, fine adjustment is performed in the X and Y directions with small steps, and the optimal optical axis position is determined based on the trapezoidal function relationship of the echo signal intensity. The standard position of the light spot during optical axis collimation is recorded by a photoelectric position detector, and subsequent rapid calibration can be performed based on the light spot deviation, achieving rapid collimation for continuous field observation. This improves detection accuracy, reliability, and application flexibility, simplifies personnel operation procedures, and enhances operational safety. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the marine lidar system structure in an embodiment of the present invention; Figure 2 This is a structural diagram of the optical system of the marine lidar in an embodiment of the present invention; Figure 3 This is a flowchart of the software control function of the marine lidar system in an embodiment of the present invention; Figure 4 This is a flowchart of the automatic calibration process of the marine lidar system in an embodiment of the present invention; Figure 5 This is a schematic diagram of the coarse adjustment process for the light-receiving and receiving axis calibration of the marine lidar system in an embodiment of the present invention; Figure 6 This is a schematic diagram of the fine-tuning process for the light-receiving and receiving axis calibration of the marine lidar system in an embodiment of the present invention. Detailed Implementation

[0017] To enable those skilled in the art to better understand the technical solution of the present invention, the technical solution of the present invention will be described below in conjunction with specific embodiments and accompanying drawings.

[0018] This technical solution addresses the problems existing in the prior art by providing an automatically calibrated marine lidar system and calibration method. It enables automatic calibration of the receiving and transmitting optical paths, automatic adjustment of the receiving field of view, signal saturation correction, and laser emission direction control for the marine lidar. This improves the automatic calibration capability of marine lidar observations, reduces on-site personnel operation, enhances the reliability of marine lidar observations, increases the accuracy of observation data, and improves the safety of personnel during operation, thus providing a reliable technical means for marine environmental monitoring and research.

[0019] Example 1 like Figure 1 As shown, an automatically calibrated marine lidar system mainly consists of a detection cabin 1, a scanning gimbal 2, a control cabin 3, and a support frame 4, with the support frame supporting the control cabin. The structure within the detection cabin primarily includes an optical system and a signal acquisition and control system. Through the combination of the detection cabin and the scanning gimbal, under the control of the control cabin, it can achieve automatic calibration functions such as automatic calibration of the receiving and receiving optical axes, automatic adjustment of the receiving field of view, signal saturation correction, and laser detection direction control. Automatic calibration enhances the reliability of continuous marine lidar observation data, reduces the need for on-site personnel intervention in the calibration process, and improves the unattended detection capabilities and personnel safety during marine observations.

[0020] This embodiment uses a single-wavelength transmit-receive dual-channel coaxial marine lidar as an example to introduce the structure, composition and functional principle of its automatic calibration system, which can be used to detect the optical parameters of marine water and the polarization state of particulate matter.

[0021] like Figure 2 As shown, the optical system includes a laser emitting unit and an optical receiving unit. The laser emitting unit mainly includes a pulsed laser 1-1, a reflector 1-2, a sampling mirror 1-3, a photoelectric position detector 1-4, a two-dimensional electrically controlled adjustable frame 1-5, and a fixed frame 1-6. It adopts a coaxial emission structure to reduce the detection blind zone. The reflector is installed on the two-dimensional electrically controlled adjustable frame to adjust the laser emission direction. The sampling mirror is fixed at the center of the front end of the telescope and transmits light to the photoelectric position detector for spot position monitoring.

[0022] The optical receiving unit mainly includes a transmission telescope 1-7, an electrically controlled pinhole aperture 1-8, a collimating lens 1-9, an electrically controlled attenuator wheel 1-10, a narrowband interference filter 1-11, a polarizing beam splitter 1-12, a converging lens 1-13, and a photomultiplier tube 1-14, used to receive laser echo signals and convert them into electrical signals. The optical system is integrated and installed in the marine lidar detection cabin 1. The marine lidar detection cabin 1 is fixedly mounted on a scanning gimbal 2, and the scanning gimbal can control the direction of detection by the cabin 1.

[0023] like Figure 1As shown, the signal acquisition and control subsystem mainly includes a data acquisition card 3-1, a controller, a network module 3-3, and signal acquisition and control software, which are integrated and installed in the control cabin 3 of the marine lidar. The controller uses a computer 3-2. The control cabin 3 can be installed below the scanning pan-tilt unit 2 to receive, store, and display electrical signals and output control commands, realizing the integrated design of the marine lidar.

[0024] The scanning gimbal is mainly used to control the direction of marine lidar detection. The detection cabin is fixed on the scanning gimbal, and the elevation angle (-90°~90°) and azimuth angle (0~360°) of the detection cabin can be controlled to realize automatic control of the laser detection direction and detection mode, thereby improving the detection flexibility.

[0025] Furthermore, such as Figure 2 As shown, the laser emitting unit emits blue-green pulsed laser light as a detection source through pulsed laser 1-1, which then passes sequentially through reflector 1-2 and sampling mirror 1-3 before being emitted into the target. The reflector is mounted on a two-dimensional electrically controlled adjustment mount 1-5, which can electrically control the two-dimensional angle (X and Y directions) of the reflector to further adjust the direction of the emitted laser. The marine lidar adopts a coaxial emission structure, that is, the sampling mirror 1-3 is fixed at the center of the front end of the telescope 1-7. The coaxial emission structure can reduce the blind zone of the system. The sampling mirror 1-3 is mounted on a fixed mount 1-6, the size of which is smaller than the two-dimensional electrically controlled adjustment mount, which can reduce the obstruction to the telescope.

[0026] Preferably, the reflector 1-2 has a reflectivity greater than 99%, the sampling mirror has a reflectivity ≥95%, and a transmittance ≤5%. The transmitted light is incident on the photosensitive surface of the photoelectric position detector 1-4 to record the position of the light spot. Preferably, the photoelectric position detector 1-4 is mainly used for monitoring the position of the light spot, and commonly used detectors include CCD charge-coupled devices and four-quadrant photodetectors.

[0027] The echo signal from the interaction between the laser and the target is received by the optical receiving unit. The optical echo signal is first received by the transmission telescope 1-7, then sequentially passes through the electrically controlled pinhole aperture 1-8, collimating lens 1-9, narrowband interference filter 1-11, and electrically controlled attenuator wheel 1-10. A polarizing beam splitter 1-12 then separates the optical signal into parallel and perpendicular components, which are then converged by a converging lens 1-13 and focused into a photomultiplier tube 1-14, converting the optical signal into an electrical signal. The converted electrical signal is synchronously received by the data acquisition card, and the data is stored and displayed on a computer using signal acquisition and control software. The electrically controlled pinhole aperture 1-8 is preferably selected with an aperture D in the range of 0 mm to 10 mm, allowing for automatic control of the telescope's receiving field of view.

[0028] like Figure 3As shown, the main functions of the signal acquisition and control software consist of system control and signal acquisition. System control includes laser control, two-dimensional electrically controlled adjustment of the lens mount, electrically controlled pinhole aperture control, electrically controlled attenuator wheel control, scanning pan-tilt control, and network module control. Signal acquisition includes echo signals and system status.

[0029] The electrically controlled attenuator wheel is installed in front of the polarizing beam splitter. The electrically controlled attenuator wheel is equipped with 6 sets of attenuators with different transmittance. The data acquisition and control software outputs commands to control the attenuators with different transmittance to enter the optical receiving unit, and attenuate the received signal light to different degrees. When the received signal is saturated, the electrically controlled attenuator wheel controls the attenuators with different transmittance to enter the optical receiving unit in sequence. When the near-field signal is just unsaturated, the electrically controlled attenuator wheel stops adjusting.

[0030] The automatic calibration function of the marine lidar is mainly achieved through two-dimensional electronically controlled adjustment of the mirror frame 1-5, photoelectric position detector 1-4, electronically controlled pinhole aperture 1-8, scanning gimbal 2, and electronically controlled attenuator wheel 1-10.

[0031] Example 2 like Figures 3-6 As shown, this embodiment describes the calibration method for the automatically calibrated marine lidar system in Embodiment 1, including the following steps: Step 1: Determine the position of the emitted laser when the optical axis is collimated by the photoelectric position detector. Under clear and cloudless atmospheric conditions at night and when the observation platform is stable, the marine lidar controls the laser detection direction to enter the atmosphere through the scanning gimbal. The stable atmospheric signal is used as the detection element for the optical axis calibration process, and there are no obstructions in the laser detection path.

[0032] Step 2: Control the aperture of the electronically controlled pinhole aperture, adjust the receiving field of view, and observe whether there is a valid signal in the signal acquisition and control software. If it is determined to be a valid signal, directly perform fine adjustment of the optical axis. If it is determined to be a non-valid signal, the emitted beam does not enter the receiving field of view, and then perform coarse and fine adjustment of the optical axis in sequence.

[0033] Step 3: After completing the coarse and fine adjustment of the light-receiving axis collimation, install the photoelectric position detector behind the sampling mirror, so that the transmitted emitted light is incident on the photosensitive surface of the photoelectric position detector, and then fix the photoelectric position detector after the light spot is centered and evenly distributed on its photosensitive surface. Record the position and distribution of the light spot at this time as the standard position.

[0034] Step 4: When the marine lidar is observing, if the emitted beam deviates, the distribution of its spot on the photoelectric position detector will also change. Based on the change in its position on the photoelectric position detector, the two-dimensional electronically controlled adjustment frame with the reflector is adjusted accordingly in the X or Y direction until the position of the spot on the photoelectric position detector returns to the standard position.

[0035] Step 5: By cooperating with the telescope through an electrically controlled aperture, the receiving field of view is calculated based on the telescope's focal length. The aperture is adjusted to automatically control the receiving field of view, adapting to different needs such as water body multiple scattering research, water body optical parameter detection, and depolarization ratio measurement.

[0036] Step 6: Monitor the near-field signal of the water body. If strong saturation occurs, control the electronically controlled attenuator wheel to switch attenuators with different transmittance to attenuate the received signal until the near-field signal is just unsaturated, thus completing the saturation correction.

[0037] Specifically, the automatic calibration function of the receiving and transmitting optical axes of marine lidar is crucial for ensuring data accuracy and reliability during continuous operation. To guarantee the continuity of maritime observations, higher demands are placed on the efficiency and effectiveness of its automatic calibration. This invention designs a rapid automatic calibration system and method for the receiving and transmitting optical axes.

[0038] The automatic calibration function of the receiving and transmitting light axis of the marine lidar is mainly achieved through the coordinated operation of the two-dimensional electronically controlled adjustment frame 1-5, sampling mirror 1-3, photoelectric position detector 1-4, electronically controlled pinhole aperture 1-8, and scanning gimbal 2. Because factors such as water surface waves, platform sway, and changes in water parameters affect the strength of the echo signal during marine lidar parameter detection, which further influences the signal judgment for receiving and transmitting light axis calibration, stable detection elements are required for calibration.

[0039] First, before continuous observation, the position of the emitted laser for optical axis alignment must be determined using photoelectric position detectors 1-4. Calibration is performed on a clear, cloudless night, ensuring stable atmospheric conditions and the observation platform, and eliminating interference from daytime background light. The marine lidar uses scanning gimbal 2 to control the laser detection direction into the atmosphere, using a stable atmospheric signal as the detection element for the optical axis calibration process, and ensuring there are no obstructions in the laser detection path.

[0040] Furthermore, the electronically controlled pinhole aperture 1-8 controls the aperture diameter to be 10 mm, resulting in a larger receiving field of view, making it easier for the received signal to enter the receiving field of view. At this time, observe whether there is a valid signal in the signal acquisition and control software. For example, select a signal at a reference distance of 100 m. The selected reference distance is not within the system's blind zone and there is no signal saturation. Set the signal accumulation time to 5 s and set a reasonable signal-to-noise ratio threshold (such as a signal-to-noise ratio threshold of 2). If the signal-to-noise ratio of the signal at the reference distance is lower than the signal-to-noise ratio threshold, it is considered not a valid signal, indicating that the transmitted beam has not entered the receiving field of view. It is necessary to perform coarse and fine adjustments to the optical axis in sequence.

[0041] The automatic optical axis calibration function uses a stable atmospheric signal as the detection element. First, the optical axis is coarsely adjusted by spiral scanning through a two-dimensional electronically controlled adjustment frame to bring the emitted beam into the receiving field of view. Then, fine adjustment is performed in the X and Y directions with small steps, and the optimal optical axis position is determined according to the trapezoidal function relationship of the echo signal intensity. The standard position of the light spot when the optical axis is collimated is recorded by a photoelectric position detector. In the later stage, it can be quickly calibrated according to the light spot deviation, so as to realize rapid collimation for continuous observation in the field.

[0042] The automatic field-of-view adjustment function works with the telescope through an electrically controlled aperture diaphragm (aperture adjustable from 0 mm to 10 mm). It calculates the receiving field of view angle based on the telescope's focal length and automatically controls the receiving field of view angle by adjusting the aperture diaphragm diameter. This adapts to different needs such as water body multiple scattering research, water body optical parameter detection, and depolarization ratio measurement.

[0043] The aforementioned signal saturation correction function monitors the water body to detect near-field signals. If strong saturation occurs, it controls the electronically controlled attenuator wheel to switch attenuators with different transmittance to attenuate the received signal until the near-field signal is just unsaturated, thus completing the saturation correction. This function primarily addresses the signal saturation problem in the Mie scattering channel.

[0044] The laser detection direction control function automatically adjusts the pitch and azimuth angles of the detection chamber through the scanning gimbal, realizing flexible control of the laser detection direction and adapting to scenarios such as optical path debugging, signal testing, and detection at different water entry angles.

[0045] Coarse and fine adjustment of the optical axis, such as Figure 5 and Figure 6As shown, if there is no valid signal at the reference distance, coarse adjustment of the emitted optical axis is performed first. The coarse adjustment step size (e.g., 100 μrad) in the X and Y directions is set for the two-dimensional electronically controlled adjustment frame 1-5 to control the angle change of the reflector and further adjust the direction of the emitted laser. To allow the emitted beam to enter the receiving field of view as quickly as possible, this invention uses a spiral scanning method for coarse optical axis adjustment. That is, the two-dimensional electronically controlled adjustment frame is controlled by signal acquisition and control software to adjust the reflector angle, controlling the emitted beam to perform a spiral scan. Signal acquisition is performed synchronously after each angle adjustment. The signal accumulation time can be set; here, a preferred signal accumulation time is 5 s. As the spiral scan proceeds, the emitted beam gradually enters the telescope's receiving field of view. When the signal-to-noise ratio of the reference distance echo signal is higher than the set signal-to-noise ratio threshold, it indicates that the emitted beam has entered the telescope's receiving field of view, achieving preliminary collimation.

[0046] After initial collimation through coarse optical axis adjustment, the signal is not yet guaranteed to be optimal, requiring further fine-tuning of the optical axis. Two-dimensional electronically controlled adjustment frames 1-5 are set with fine-tuning step sizes (e.g., 20 μrad) in the X and Y directions. Assuming the emitted beam falls at point M in the receiving field of view after coarse optical axis adjustment, the emitted beam is then controlled to continue scanning along the original X direction (assumed to be the X+ direction) at small angles according to the fine-tuning step size. An echo signal is recorded at each position moved. Preferably, the signal accumulation time is set to 5 s. The signal at the reference distance will gradually increase and then decrease until the emitted beam scans to point N and leaves the telescope's field of view. During the emitted beam scanning process, the echo signal intensity at the reference distance and the adjustment angle of the emitted optical axis in the X direction approximately follow a trapezoidal function relationship. The upper base of the trapezoidal curve represents the scanning angle range corresponding to the emitted beam fully entering the telescope's receiving field of view in the X direction. The angle in the X direction corresponding to the center of the upper base is the optimal position of the emitted beam in this direction, denoted as point C. The emitted beam is then controlled to adjust to the optimal position C in the X direction.

[0047] Furthermore, the optimal position in the Y direction is determined by referring to the adjustment method in the X direction. First, the two-dimensional electronically controlled adjustment frame 1-5 is adjusted using data acquisition and control software to fine-tune the optical axis of the emitted beam in the Y direction. The emitted beam is first moved out of the telescope's receiving field of view. To shorten the time, the scanning direction is determined when scanning begins at point C. Using point C as the center, the beam is first moved in one direction along the Y axis (let's say the Y+ direction). If the signal at the reference distance weakens, the beam continues to move along this direction with coarse adjustment steps. If the signal at the reference distance strengthens, the beam moves in the opposite direction with coarse adjustment steps. When the signal-to-noise ratio at the reference distance is lower than the signal-to-noise ratio threshold, the emitted beam has essentially moved out of the telescope's receiving field of view. Then, the two-dimensional electronically controlled adjustment frame 1-5 is used to move the emitted beam in the Y direction in the opposite direction to where it was previously moved out of the telescope's receiving field of view, with fine-tuning steps. An echo signal is recorded at each position until the emitted beam moves out of the telescope's receiving field of view again. During this scanning process in the Y direction, the echo signal intensity at the reference distance and the adjustment angle of the emitted optical axis in the Y direction approximately form a trapezoidal function relationship. At this point, the center position of the upper base of the trapezoidal curve corresponds to the center position O of the telescope's field of view. The two-dimensional electronically controlled adjustment frame 1-5 is then used to move the emitted beam to point O, which is essentially the position with the strongest signal, thus completing the fine-tuning of the optical axis.

[0048] Furthermore, the divergence angle of the laser beam can be obtained from the rising or falling edge of the trapezoidal curve, and the full width at half maximum (FWHM) of the trapezoidal curve corresponds to the receiving field of view of the receiving telescope.

[0049] Furthermore, the aperture of the electronically controlled pinhole aperture 1-8 is adjusted to the observation state. After the aperture is reduced, the fine adjustment process of the transmitting optical axis in the X and Y directions can be repeated. At this time, the upper base of the trapezoidal curve becomes shorter according to the reduction of the receiving field of view. After the aperture of the pinhole aperture is reduced, the collimation of the transmitting optical axis and the receiving optical axis can be further improved through the fine adjustment process.

[0050] Furthermore, after completing the coarse and fine adjustments for the optical axis alignment, the emitted optical axis should now be in the optimal position. At this point, install the photoelectric position detector 1-4 behind the sampling mirror 1-3, allowing the transmitted emitted light to strike the photosensitive surface of the photoelectric position detector 1-4. After ensuring the light spot is centered and evenly distributed on the photosensitive surface, fix the photoelectric position detector 1-4 and record the position and distribution of the light spot as a standard position. This completes the optical axis alignment process and determines the position of the emitted light spot on the photoelectric position detector 1-4 when the signal is optimal. This facilitates rapid calibration to the optimal signal position if the emitted optical axis deviates during subsequent marine lidar observations.

[0051] When the marine lidar is used for observation, if the emitted beam deviates, the distribution of its spot on the photoelectric position detector 1-4 will also change. Based on this change in position on the photoelectric position detector, the two-dimensional electronically controlled adjustment frame 1-5, which houses the reflector, is adjusted accordingly in the X or Y direction until the spot's position on the photoelectric position detector 1-4 returns to the standard position. This allows for rapid alignment of the emission and reception axes during continuous field observations, preventing data quality degradation due to axis deviation.

[0052] Furthermore, if the signal-to-noise ratio of the signal at the reference distance is higher than the signal-to-noise ratio threshold before the optical axis collimation adjustment, it is considered that the marine lidar system has been able to obtain an effective signal, and there is no need to perform a coarse optical axis adjustment process; instead, fine optical axis adjustment can be performed directly.

[0053] Furthermore, the step size, rotation speed, distance of the reference signal, signal accumulation time, and signal-to-noise ratio threshold settings for coarse and fine adjustment of the optical axis are all very important, directly affecting the collimation time and accuracy of the optical axis.

[0054] The coarse and fine adjustments of the optical axis of a marine lidar are more suitable for calibration of the receiving and transmitting optical axes before the lidar leaves the factory or when the optical axis deviates significantly. After the receiving and transmitting optical axes are calibrated, the standard position of the transmitted beam is located using photoelectric position detectors 1-4. During marine lidar observation, the receiving and transmitting optical axes can be quickly calibrated using the standard position. This embodiment not only provides a calibration scheme for cases with large deviations in the receiving and transmitting optical axes during automatic calibration, but also provides a method for rapid receiving and transmitting optical axis calibration in marine lidar field applications by locating the standard position of the receiving and transmitting optical axes using photoelectric position detectors.

[0055] The automatic adjustment function of the receiving field of view of the marine lidar is mainly controlled by the electrically controlled pinhole aperture 1-8. The aperture D of the electrically controlled pinhole aperture 1-8 is adjustable within the range of 0 mm to 10 mm. It is coaxially placed with the telescope 1-7 and installed at the focal position of the telescope. The focal length of the telescope is L, and the receiving field of view angle θ = D / L (rad). Thus, the automatic adjustment of the receiving field of view angle is achieved by automatically controlling the size of the aperture of the electrically controlled pinhole aperture 1-8. Through automatic adjustment of the receiving field of view angle, the marine lidar can study multiple scattering in water and detect optical parameters of water bodies. When the receiving field of view angle is small, the attenuation coefficient detected by the marine lidar mainly describes the total degree of attenuation of the collimated beam in the water body due to absorption and scattering, reflecting the attenuation of light by the water body. When the receiving field of view angle is large, the attenuation coefficient detected by the marine lidar is close to the diffuse attenuation coefficient, which can be used for water color remote sensing and primary productivity research. At the same time, multiple scattering can also affect the measurement of the depolarization ratio of water bodies. Automatic adjustment of the receiving field of view can prevent the influence of multiple scattering on the measurement of the depolarization ratio.

[0056] The marine lidar signal saturation correction function primarily utilizes the signal from the marine lidar during water body detection. If strong saturation occurs in the near field (e.g., the near-field signal reaches its maximum value and persists for a period of time), the electronically controlled attenuator wheel is adjusted to regulate the attenuation of the received signal. Because Raman and fluorescence signals are much weaker than Mie scattering signals, marine lidar signal saturation mainly occurs in the Mie scattering channel. The electronically controlled attenuator wheel 1-10 is installed before the polarization beam splitter 1-12 in the receiving optical path. Preferably, the electronically controlled attenuator wheel 1-10 is equipped with six sets of attenuators with different transmittances. Commands from the data acquisition and control software control the attenuators of different transmittances to enter the receiving optical path, attenuating the received signal light to varying degrees. When the received signal is saturated, the electronically controlled attenuator wheel 1-10 controls the attenuators of different transmittances to sequentially enter the receiving optical path. When the near-field signal is just desaturated, the electronically controlled attenuator wheel 1-10 stops adjusting, thus achieving the signal saturation correction function.

[0057] The laser detection direction control of the marine lidar is mainly achieved through the scanning gimbal 2. The elevation angle control range of the scanning gimbal 2 is -90° to 90°, and the azimuth angle control range is 0 to 360°. The detection cabin 1 of the marine lidar is installed on the scanning gimbal 2. The detection direction and detection mode of the marine lidar detection cabin 1 can be automatically controlled by the scanning gimbal, which improves the flexible detection capability of the marine lidar and plays an important role in optical path debugging, signal testing, and detection at different water entry angles.

[0058] The advantage of this invention is that it provides an automatic calibration function for marine lidar, which improves the accuracy, reliability and application flexibility of marine lidar detection, enhances the ease of operation and safety for personnel, and realizes unattended automatic calibration and detection of marine lidar.

[0059] The above embodiments are merely illustrative of the structural concept and features of the present invention, intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly, and should not be construed as limiting the scope of protection of the present invention. All equivalent changes or modifications made based on the essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. An automatically calibrated marine lidar system, characterized in that: The system includes a detection cabin, a scanning gimbal, a control cabin, and a support frame. The detection cabin houses a laser emitting unit and an optical receiving unit. The scanning gimbal is fixed below the detection cabin to control its detection direction. The bottom of the scanning gimbal is fixed above the control cabin. The control cabin integrates a data acquisition card, a controller, a network module, and signal acquisition and control software. The support frame supports the control cabin. The laser emitting unit includes a pulsed laser, a reflector, a two-dimensional electrically controlled adjustment frame, a sampling mirror, and a photoelectric position detector. The optical receiving unit includes a transmission telescope and an electrically controlled pinhole aperture. The electrically controlled pinhole aperture is located behind the transmission telescope. The reflector is mounted on the two-dimensional electrically controlled adjustment frame and used to electrically control the two-dimensional angle of the reflector, adjusting the direction of the emitted laser. The sampling mirror is fixed at the center of the telescope's front end. The pulsed laser emits pulsed laser light as a detection source, which passes sequentially through the reflector and the sampling mirror before being emitted into the target. The echo signal from the interaction between the laser and the target is received by the optical receiving unit.

2. The automatically calibrated marine lidar system according to claim 1, characterized in that: The laser emitting unit also includes a fixed frame, on which the sampling mirror is mounted. The size of the fixed frame is smaller than that of the two-dimensional electronically adjustable frame.

3. The automatically calibrated marine lidar system according to claim 1, characterized in that: The optical receiving unit also includes a collimating lens, an electrically controlled attenuator wheel, a narrowband interference filter, a polarizing beam splitter, a converging lens, and a photomultiplier tube. The echo signal is first received by a transmission telescope, and then passes sequentially through an electrically controlled pinhole aperture, a collimating lens, a narrowband interference filter, and an electrically controlled attenuator wheel. The polarizing beam splitter then splits the optical signal into parallel and perpendicular components, which are then converged by the converging lens into the photomultiplier tube, converting the optical signal into an electrical signal. The electrical signal is synchronously received by the data acquisition card, and the data is stored and displayed on the controller through signal acquisition and control software.

4. The automatically calibrated marine lidar system according to claim 3, characterized in that: The aperture D of the electrically controlled pinhole aperture is adjustable in the range of 0 mm to 10 mm. It is placed coaxially with the transmission telescope and installed at the focal position of the transmission telescope. The focal length of the transmission telescope is L, and the receiving field of view θ of the transmission telescope is θ = D / L.

5. The automatically calibrated marine lidar system according to claim 3, characterized in that: The electrically controlled attenuator wheel is installed in front of the polarizing beam splitter. The electrically controlled attenuator wheel is equipped with 6 sets of attenuators with different transmittance. The data acquisition and control software outputs commands to control the attenuators with different transmittance to enter the optical receiving unit, so as to attenuate the received signal light to different degrees. When the received signal is saturated, the electrically controlled attenuator wheel controls the attenuators with different transmittance to enter the optical receiving unit in sequence. When the near-field signal is just unsaturated, the electrically controlled attenuator wheel stops adjusting.

6. A calibration method for an automatically calibrated marine lidar system as described in any one of claims 1-5, characterized in that, Includes the following steps: Step 1: Determine the position of the emitted laser when the optical axis is collimated by the photoelectric position detector. Under clear and cloudless atmospheric conditions at night and when the observation platform is stable, the marine lidar controls the laser detection direction to enter the atmosphere through the scanning gimbal. The stable atmospheric signal is used as the detection element for the optical axis calibration process. There are no obstructions in the laser detection path. Step 2: Control the aperture of the pinhole aperture with electronic control, adjust the receiving field of view, and observe whether there is a valid signal in the signal acquisition and control software. If it is determined to be a valid signal, directly perform fine adjustment of the optical axis. If it is determined to be a non-valid signal, the emitted beam does not enter the receiving field of view, and then perform coarse and fine adjustment of the optical axis in sequence. Step 3: After completing the coarse and fine adjustment of the light-receiving axis collimation, install the photoelectric position detector behind the sampling mirror, so that the transmitted emitted light is incident on the photosensitive surface of the photoelectric position detector, and fix the photoelectric position detector after the light spot is centered and evenly distributed on its photosensitive surface. Record the position and distribution of the light spot at this time as the standard position. Step 4: When the marine lidar is observing, if the emitted beam deviates, the distribution of its spot on the photoelectric position detector will also change. According to the change in its position on the photoelectric position detector, the two-dimensional electronically controlled adjustment frame with the reflector is adjusted in the X or Y direction until the position of the spot on the photoelectric position detector is restored to the standard position. Step 5: By cooperating with the telescope through the electronically controlled aperture, the receiving field of view is calculated based on the telescope's focal length. The aperture is adjusted to automatically control the receiving field of view, adapting to different needs such as water body multiple scattering research, water body optical parameter detection, and depolarization ratio measurement. Step 6: Monitor the near-field signal of the water body. If strong saturation occurs, control the electronically controlled attenuator wheel to switch attenuators with different transmittance to attenuate the received signal until the near-field signal is just unsaturated, thus completing the saturation correction.

7. The calibration method for an automatically calibrated marine lidar system according to claim 6, characterized in that, In step 2, when determining whether a signal is valid, a signal-to-noise ratio (SNR) threshold is set. If the SNR of the signal at the reference distance is lower than the SNR threshold, it is considered not a valid signal. If the SNR of the signal at the reference distance is not lower than the SNR threshold, it is considered a valid signal.

8. The calibration method for an automatically calibrated marine lidar system according to claim 7, characterized in that, In step 2, the coarse adjustment includes: Step C1: Set the coarse adjustment step size of the two-dimensional electronically controlled adjustment frame in the X and Y directions to control the angle change of the reflector and further adjust the direction of laser emission; Step C2: The two-dimensional electronically controlled adjustment frame is controlled by the signal acquisition and control software to adjust the angle of the reflector, and the emitted beam is controlled to perform a spiral scan. A signal acquisition is performed synchronously every time the angle is adjusted, and the signal accumulation time is set. Step C3: As the spiral scan proceeds, the transmitted beam gradually enters the telescope's receiving field of view. When the signal-to-noise ratio of the reference range echo signal is higher than the set signal-to-noise ratio threshold, the transmitted beam has entered the telescope's receiving field of view, achieving preliminary collimation.

9. The calibration method for an automatically calibrated marine lidar system according to claim 8, characterized in that, In step 2, the fine-tuning includes: Step J1: Set the fine-tuning step size of the two-dimensional electronically controlled adjustment frame in the X and Y directions, assuming that after the optical axis is coarsely adjusted, the emitted beam falls at point M in the receiving field of view; Step J2: Control the emitted beam to continue scanning along the original X direction at small angles according to the fine-tuning step size. Record the echo signal once for each position it moves to. Set the signal accumulation time. The signal at the reference distance will gradually increase and then decrease until the emitted beam scans to point N and leaves the telescope's field of view. Step J3: The echo signal intensity at the reference distance and the adjustment angle of the emitted optical axis X direction are approximately trapezoidal functions. The upper base of the trapezoid curve is the scanning angle range corresponding to when the emitted beam completely enters the telescope's receiving field of view in the X direction. The angle in the X direction corresponding to the center position of the upper base is the optimal position of the emitted beam in this direction, which is set as point C. Control the emitted beam to adjust to the optimal position point C in the X direction. Step J4: Adjust the two-dimensional electronically controlled adjustment frame using data acquisition and control software to position the emitted beam in the Y direction. Refer to the adjustment method in the X direction to determine the optimal position in the Y direction and complete the fine adjustment of the optical axis.

10. The calibration method for an automatically calibrated marine lidar system according to claim 9, characterized in that, In step J3, the divergence angle of the laser beam can be obtained by the rising or falling edge of the trapezoidal curve. The full width at half maximum (FWHM) of the trapezoidal curve corresponds to the receiving field of view of the receiving telescope. The aperture of the electronically controlled pinhole is adjusted to the state during observation. After the pinhole is reduced, the fine-tuning process of the transmitting optical axis in the X and Y directions can be repeated. At this time, the upper base of the trapezoidal curve becomes shorter according to the reduction of the receiving field of view. After the pinhole aperture is reduced, the collimation between the transmitting and receiving optical axes can be further improved through the fine-tuning process.

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