Simulation method of polarization signal of ocean lidar based on semi-analytical Monte Carlo model

Abstract

The present invention discloses a method for simulating polarization signals of an ocean laser radar based on a semi-analytical Monte Carlo model, and belongs to the technical field of laser radar remote sensing detection. The method comprises: establishing a global observation geometric coordinate system according to the hardware parameters of the laser radar, and determining the environmental medium parameters of the simulation model according to the optical parameters of the water body; constructing a semi-analytical Monte Carlo model, carrying out the propagation tracking of photon packets, and calculating and recording the polarization weight signal contribution of the photon packets that meet the receiving conditions; performing statistical calculations on the polarization weight signal contribution to obtain the polarization photon number signal and depolarization ratio of the laser radar. The present invention regards a single laser pulse as multiple photon packets, tracks the random propagation process of the photon packets in the seawater, and uses the signal estimation model to simulate the polarization signal of the ocean laser radar for the photon packets that meet the receiving conditions. The present invention can achieve relatively accurate simulation of polarization signals and depolarization ratios of ocean laser radars.

Description

Technical Field

[0001] The invention relates to the technical field of laser radar remote sensing detection in ocean optics, and in particular to a method for simulating polarization signals of ocean laser radar based on a semi-analytical Monte Carlo model. Background Art

[0002] Marine LiDAR is an active remote sensing detection technology. It has the advantages of penetrating the upper ocean water, working all day, meeting polar observations, and rich echo signal polarization information. It has been used for vertical profile observation research of seawater. Marine LiDAR has a far-reaching impact on promoting research on marine ecological dynamic processes, blue carbon estimation, plankton community distribution, direct detection of marine biological signals, marine stratification effects, internal waves, etc. However, the development and detection of marine LiDAR are still full of challenges. For example, the strong reflection of the sea surface makes the dynamic detection range of LiDAR large, and the inherent strong attenuation of seawater leads to shallow detection depth. In addition, due to the lack of basic measurement research on the scattering characteristics of polarized light of water components, the interpretation of laser echo polarization signals (such as depolarization ratio) is still facing great difficulties. Therefore, for the detection of marine LiDAR, both the development of the system and the interpretation and analysis of the detection signal require simulation models to provide prior theoretical support for the exploration of its key technologies and the analysis of the detection mechanism of polarization signals.

[0003] Patent 1 (patent application number: 202111362947.3) proposes a satellite-borne ocean lidar detection simulation method. By converting the average number of photoelectrons into detection probability, the photon count distribution of the satellite-borne ocean lidar on the water surface, water body and seabed is obtained. Patent 2 (patent application number: 202111009955.X) proposes a fast simulation method for ocean lidar based on Gaussian convolution, which uses the convolution of a Gaussian laser beam and a Gaussian scattering phase function of a water body to realize the fast calculation of multiple scattered signals of the laser in the water body. Both of the above methods are aimed at the simulation of scalar detection signals of ocean lidars, and do not have the simulation function of polarization signals, and cannot further provide theoretical support for the polarization-related system design and signal interpretation of ocean lidars. Summary of the invention

[0004] In view of the fact that existing simulation methods do not have the simulation function of polarization signals, the present invention provides a simulation method of polarization signals of ocean laser radar based on a semi-analytical Monte Carlo model.

[0005] Monte Carlo technology is a numerical simulation method based on probability and statistical theory. It intuitively shows the process of light beams being absorbed and scattered by particles during transmission, defines the probability of scattering and absorption, the probability of scattering in various directions, the probability distribution of random forward distances, etc., solves complex media and boundary conditions, and can also model complex sensors. The semi-analytical Monte Carlo method regards photons as a huge photon packet. It uses the weight method to perform appropriate weight attenuation on the photon packet so that it can continue to propagate after each scattering, preventing the photon packet from being absorbed by the water and being removed from the light field. The receiving geometric conditions are used to determine whether the photon packet participates in the formation of the lidar signal each time it is scattered, and the analytical expression is used to realize the rapid calculation and construction of the lidar signal.

[0006] To achieve the above object, the present invention is achieved through the following technical solutions:

[0007] A simulation method for polarization signals of ocean lidar based on a semi-analytical Monte Carlo model comprises the following steps:

[0008] Step 1: Establish a global observation geometric coordinate system based on the laser radar hardware parameters, and determine the simulation model environment medium parameters based on the water body optical parameters;

[0009] Step 2: Construct a semi-analytical Monte Carlo model to track the propagation of photon packets, and calculate and record the polarization weight signal contribution of photon packets that meet the reception conditions;

[0010] Step 3: Statistically calculate the contribution of the polarization weight signal to obtain the polarization photon number signal and depolarization ratio of the laser radar. Further, the Step 2 specifically includes:

[0011] (1) Photon packet emission: According to the laser light source type, beam incident angle, and light source coordinates, the initial photon packet is assigned starting coordinates, movement direction, and photon packet weight;

[0012] (2) Photon packet advance: The optical path length of the photon packet is sampled and converted into the geometric distance of the photon packet through the beam attenuation coefficient of the dielectric layer. The coordinates of the photon packet are updated based on the current direction of movement and the original coordinates.

[0013] (3) Determine whether the photon packet is in the water body: Determine whether the photon packet is in the water body based on the new coordinates of the photon packet. If the photon packet enters the water body from the atmosphere, a reflection and refraction correction is required. If the photon packet leaves the water body and enters the water body, the photon packet is considered dead and no longer tracked.

[0014] (4) Weight update: The weight of the photon packet is updated based on the single scattering albedo of the water body, indicating the probability that the photon packet will survive the scattering event without being absorbed;

[0015] (5) Calculation of water body polarization weight signal: Based on the new coordinates P of the current photon packet i New direction of movement and the new weight w i First, determine whether the photon packet meets the requirements of the detector. If so, calculate and record the polarization weight signal of the photon packet.

[0016] (6) Polarization scattering of photon packets: The scattering angle of the photon packet is sampled based on the scattering phase function of the particles, and the scattering direction of the current photon packet is updated;

[0017] (7) Determine whether a photon is dead: The weight of a photon decreases due to multiple scattering. When it is less than the threshold value of 10 -6 When it is detected, it is considered dead and a new photon packet is emitted to start tracking.

[0018] Furthermore, in step (5), the specific calculation method is as follows:

[0019] When the laser radar observes the sea surface vertically, the image point P′ of the detector is first obtained by using approximate geometry. det :

[0020]

[0021] n wat is the relative refractive index of the water body, from which the interface incident cosine of the photon packet before leaving the water can be obtained and the sea surface interaction point P i surfEst :

[0022]

[0023] Determine the coordinate point P of the photon packet i and its sea surface interaction point P i surfEst Do the following acceptance conditions all meet?

[0024] P est (x) 2 +P est (y) 2 ≤R det 2

[0025]

[0026] Where P est Point P i Or click P i surfEst , H is the height of the laser radar from the sea level, R tele is the radius of the telescope, θFOV is the receiving field of view.

[0027] Furthermore, in step (5), the specific calculation method is as follows:

[0028] When the laser radar observes the sea surface at an angle, the incident angle is θ v , the image point P′ of the detector det It is expressed as:

[0029]

[0030] in,

[0031]

[0032] Point P i Or click P i surfEst The following relationship should be satisfied:

[0033]

[0034] When the receiving conditions are met, the scattered contribution is calculated first, and the incident cosine before leaving the water is and point P i The incident cosine of the photon packet at Calculate the scattering contribution angle θ′ est :

[0035]

[0036] The rotation angles γ1 and γ2 in the rotation matrix are calculated as follows:

[0037]

[0038] where n z =[0,0,1]; optical thickness of water τ Wat (P i ,P i surfEst ) is calculated by the following formula:

[0039]

[0040] Then calculate the refraction part of the sea-air interface and calculate the incident angle χ in ,as follows:

[0041]

[0042] Calculate the water refraction angle χ tra ,as follows:

[0043] χ tra =arcsin(nwat sinx in )

[0044] Finally, underwater point P i The polarization weight signal contribution S at Wat Calculated by the following formula:

[0045]

[0046] A tele is the aperture area of ​​the detector telescope, S in is the incident Stokes vector carried by the photon packet at the i-th scattering; point P i The polarization weight signal at the detector corresponds to the distance z recorded by the detector det for:

[0047]

[0048] Furthermore, in Step 3, the laser radar single scattering weighted signal profile S is calculated. single , the part of the scattering number i=1 in the recorded water body polarization weighted signal contribution is calculated according to the distance z det The accumulation is as follows:

[0049]

[0050] Where N P is the total number of simulated photon packets; calculate the total scattering weighted signal profile S of the lidar total as follows:

[0051]

[0052] Calculate the number of photons N actually emitted by the laser L It can be calculated based on:

[0053]

[0054] Where E L is the laser pulse energy, λ L is the wavelength, They refer to Planck's constant 6.626×10 -34 Js and the speed of light in the atmosphere is 2.998×10 8 m / s;

[0055] The calculated polarized photoelectron number signal is:

[0056]

[0057] Among them, PP Num Represents the parallel channel photoelectron number signal, CPNum is the photoelectron number signal of the orthogonal channel, I f and Q f They are the profile signal contributions S single or S total The I and Q components, η oe and η qe is the optical efficiency and quantum efficiency of the detector;

[0058] The depolarization ratio Dp of the simulated signal is calculated as:

[0059]

[0060] Compared with the prior art, the present invention has the following beneficial effects:

[0061] The present invention regards a single laser pulse as multiple photon packets, tracks the random propagation process of photon packets in seawater, and uses a signal estimation model to simulate the polarization signal of the ocean lidar for photon packets that meet the receiving conditions. The present invention simulates the random propagation process of laser in seawater based on a semi-analytical Monte Carlo model, and can achieve a relatively accurate simulation of the polarization signal and depolarization ratio of the ocean lidar. BRIEF DESCRIPTION OF THE DRAWINGS

[0062] Figure 1 It is a calculation flow chart of the ocean lidar polarization signal simulation method based on semi-analytical Monte Carlo simulation proposed in the present invention.

[0063] Figure 2 Schematic diagram of vertical and tilt detection geometry when calculating and recording the polarization weight signal contribution.

[0064] Figure 3 It is the single scattered polarized light electron number signal and depolarization ratio of this embodiment 2.

[0065] Figure 4 It is the total polarized photoelectron number signal and depolarization ratio of this embodiment 2. DETAILED DESCRIPTION

[0066] The technical solution of the present invention is further described below in conjunction with embodiments.

[0067] Example 1

[0068] A simulation method for polarization signals of ocean lidar based on a semi-analytical Monte Carlo model comprises the following steps:

[0069] Step 1: Establish a global observation geometric coordinate system based on the laser radar hardware parameters, and determine the simulation model environment medium parameters based on the water body optical parameters;

[0070] Step 2: Construct a semi-analytical Monte Carlo model to track the propagation of photon packets, and calculate and record the polarization weight signal contribution of photon packets that meet the receiving conditions, such as Figure 1 As shown, specifically:

[0071] (1) Photon packet emission: According to the laser light source type, beam incident angle, and light source coordinates, the initial photon packet is assigned starting coordinates, movement direction, and photon packet weight;

[0072] (2) Photon packet advance: The optical path length of the photon packet is sampled and converted into the geometric distance of the photon packet through the beam attenuation coefficient of the dielectric layer. The coordinates of the photon packet are updated based on the current direction of movement and the original coordinates.

[0073] (3) Determine whether the photon packet is in the water body: Determine whether the photon packet is in the water body based on the new coordinates of the photon packet. If the photon packet enters the water body from the atmosphere, a reflection and refraction correction is required. If the photon packet leaves the water body and enters the water body, the photon packet is considered dead and no longer tracked.

[0074] (4) Weight update: The weight of the photon packet is updated based on the single scattering albedo of the water body, indicating the probability that the photon packet will survive the scattering event without being absorbed;

[0075] (5) Calculation of water body polarization weight signal: Based on the new coordinates P of the current photon packet i New direction of movement and the new weight w i First, determine whether the photon packet meets the requirements of the detector. If so, calculate and record the polarization weight signal of the photon packet. Figure 2 The specific calculation method is as follows:

[0076] When the laser radar observes the sea surface vertically, the image point P′ of the detector is first obtained by using approximate geometry. det :

[0077]

[0078] n wat is the relative refractive index of the water body, from which the interface incident cosine of the photon packet before leaving the water can be obtained and the sea surface interaction point P i surfEst :

[0079]

[0080] Determine the coordinate point P of the photon packet i and its sea surface interaction point P i surfEst Do the following acceptance conditions all meet?

[0081] P est (x) 2 +P est (y) 2 ≤R det 2

[0082]

[0083] Where P est Point P i Or click P i surfEst , H is the height of the laser radar from the sea level, R tele is the radius of the telescope, θ FOV is the receiving field of view.

[0084] When the laser radar observes the sea surface at an angle, the incident angle is θ v , the image point P′ of the detector det It is expressed as:

[0085]

[0086] in,

[0087]

[0088] Click P i Or click P i surfEst The following relationship should be satisfied:

[0089]

[0090] When the receiving conditions are met, the scattered contribution is calculated first, and the incident cosine before leaving the water is and point P i The incident cosine of the photon packet at Calculate the scattering contribution angle θ′ est :

[0091]

[0092] The rotation angles γ1 and γ2 in the rotation matrix are calculated as follows:

[0093]

[0094] where n z =[0,0,1]; optical thickness of water τ Wat (P i ,P i surfEst ) is calculated by the following formula:

[0095]

[0096] Then calculate the refraction part of the sea-air interface and calculate the incident angle χ in ,as follows:

[0097]

[0098] Calculate the water refraction angle χ tra ,as follows:

[0099] χ tra =arcsin(n wat sinx in )

[0100] Finally, underwater point P i The polarization weight signal contribution S at Wat Calculated by the following formula:

[0101]

[0102] A tele is the aperture area of ​​the detector telescope, S in is the incident Stokes vector carried by the photon packet at the i-th scattering; point P i The polarization weight signal at the detector corresponds to the distance z recorded by the detector det for:

[0103]

[0104] (6) Polarization scattering of photon packets: The scattering angle of the photon packet is sampled based on the scattering phase function of the particles, and the scattering direction of the current photon packet is updated;

[0105] (7) Determine whether a photon is dead: The weight of a photon decreases due to multiple scattering. When it is less than the threshold value of 10 -6 When it is detected, it is considered dead and a new photon packet is emitted to start tracking.

[0106] Step 3: Statistically calculate the contribution of the polarization weight signal to obtain the polarization photon number signal and depolarization ratio of the laser radar. Calculate the laser radar single scattering weight signal profile S single , the part of the scattering number i=1 in the recorded water body polarization weighted signal contribution is calculated according to the distance z det The accumulation is as follows:

[0107]

[0108] Where N P is the total number of simulated photon packets; calculate the total scattering weighted signal profile S of the lidar total as follows:

[0109]

[0110] Calculate the number of photons N actually emitted by the laser L It can be calculated based on:

[0111]

[0112] Where E L is the laser pulse energy, λ L is the wavelength, They refer to Planck's constant 6.626×10 -34 Js and the speed of light in the atmosphere is 2.998×10 8 m / s;

[0113] The calculated polarized photoelectron number signal is:

[0114]

[0115] Among them, PP Num Represents the parallel channel photoelectron number signal, CP Num is the photoelectron number signal of the orthogonal channel, I f and Q f They are the profile signal contributions S single or S total The I and Q components, η oe and η qe is the optical efficiency and quantum efficiency of the detector;

[0116] The depolarization ratio Dp of the simulated signal is calculated as:

[0117]

[0118] Example 2

[0119] In this embodiment, the laser pulse energy E is set L =20mJ, wavelength λ L =532nm, the height of the laser radar from the sea level is H = 425km, the incident angle is θ v =0°, beam divergence angle θ beam =0.1mrad, receiving field angle θ FOV = 0.15mrad, telescope radius R tele =1.0m, sampling resolution is 0.11m, the optical efficiency of the detector is η oe and quantum efficiency η qe The water body is a clean ocean uniform water body with a depth of 50m, and the total attenuation coefficient of the water body is c = 0.151m -1 , total scattering coefficient b = 0.037m-1 , using the scattering phase function of Petzold's average particle and pure seawater mixture and Voss&Fry's measured seawater Mueller matrix. Set the number of simulated photon packets to 10 8 .

[0120] Based on the method provided in Example 1 and according to the above parameters, the specific simulation method includes the following steps:

[0121] Step 1: Establish a global observation geometric coordinate system based on the laser radar hardware parameters, and determine the simulation model environment medium parameters based on the water body optical parameters. Take the direction downward as the positive direction of the z-axis, and the coordinate P of the detector det =[0,0,-425000], the upper boundary of the water body is z=0m, and the lower boundary is z=200m.

[0122] Step 2: Based on the semi-analytical Monte Carlo model, the propagation of the photon packet is tracked, and the polarization weight signal contribution of the photon packet that meets the receiving conditions is calculated and recorded to obtain the polarization weight signal contribution S of the water body. Wat .

[0123] Step 3: Contribute the polarization weighted signal S Wat Statistical calculations are performed to obtain the laser radar polarization electron number signals PP for single scattering and total scattering respectively. Num (Parallel Channels) and CP Num (orthogonal channels) and depolarization ratio Dp.

[0124] from Figure 3 and Figure 4 It can be seen that the polarized photoelectron number signal of the lidar in water decays rapidly with the increase of water depth, and the single scattered signal is weaker than the total scattered signal. The depolarization ratio of the single scattered polarization signal is a constant value that does not change with depth, while the depolarization ratio of the total scattered signal gradually increases with the increase of water depth.

[0125] The polarization signal and depolarization ratio simulated by the present invention were compared and verified with the measured results of the ship-borne laser radar in the northwest Pacific Ocean. The simulated polarization signal is consistent with the measured signal. The maximum average relative errors of the parallel channel and the orthogonal channel within the detection depth of 50m are 33.29% and 22.37% respectively, and the maximum average relative error of the simulated depolarization ratio and the measured depolarization ratio is 24.13%. It shows that the simulation method provided by the present invention can realize the effective simulation of polarization signals.

[0126] Finally, although this specification is described according to implementation methods, not every implementation method contains only one independent technical solution. This narrative method of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment can also be appropriately combined to form other implementation methods that can be understood by those skilled in the art.

Claims

1. A simulation method for polarization signals of ocean lidar based on a semi-analytical Monte Carlo model, characterized in that: The following steps are involved: Step 1: Establish a global observation geometric coordinate system based on the laser radar hardware parameters, and determine the simulation model environment medium parameters based on the water body optical parameters; Step 2: Construct a semi-analytical Monte Carlo model to track the propagation of photon packets, and calculate and record the polarization weight signal contribution of photon packets that meet the receiving conditions; specifically, it includes: (1) Photon packet emission: According to the laser light source type, beam incident angle, and light source coordinates, the initial photon packet is assigned starting coordinates, movement direction, and photon packet weight; (2) Photon packet advance: The optical path length of the photon packet is sampled and converted into the geometric distance of the photon packet through the beam attenuation coefficient of the dielectric layer. The coordinates of the photon packet are updated based on the current direction of movement and the original coordinates. (3) Determine whether the photon packet is in the water body: Determine whether the photon packet is in the water body based on the new coordinates of the photon packet. If the photon packet enters the water body from the atmosphere, a reflection and refraction correction is required. If the photon packet leaves the water body and enters the water body, the photon packet is considered dead and no longer tracked. (4) Weight update: The weight of the photon packet is updated based on the single scattering albedo of the water body, indicating the probability that the photon packet will survive the scattering event without being absorbed; (5) Calculation of water body polarization weight signal: Based on the new coordinates P of the current photon packet i New direction of movement and the new weight w i First, determine whether the photon packet meets the requirements of the detector. If so, calculate and record the polarization weight signal of the photon packet. (6) Polarization scattering of photon packets: The scattering angle of the photon packet is sampled based on the scattering phase function of the particles, and the scattering direction of the current photon packet is updated; (7) Determine whether a photon is dead: The weight of a photon decreases due to multiple scattering. When it is less than the threshold value of 10 -6 When , it is considered dead, and a new photon packet is re-emitted to start tracking; Step 3: Perform statistical calculation on the contribution of polarization weight signal to obtain the polarization photon number signal and depolarization ratio of the lidar.

2. The method for simulating polarization signals of an ocean laser radar as claimed in claim 1, characterized in that: In step (5), the specific calculation method is as follows: When the laser radar observes the sea surface vertically, the image point P′ of the detector is first obtained by using approximate geometry. det : n wat is the relative refractive index of the water body, from which the interface incident cosine of the photon packet before leaving the water can be obtained and the sea surface interaction point P i surfEst : Determine the coordinate point P of the photon packet i and its sea surface interaction point P i surfEst Do the following acceptance conditions apply? P est (x) 2 +P est (y) 2 ≤R det 2 Where P est Point P i Or click P i surfEst , H is the height of the laser radar from the sea level, R tele is the radius of the telescope, θ FOV is the receiving field of view.

3. The method for simulating polarization signals of an ocean laser radar as claimed in claim 1, characterized in that: In step (5), the specific calculation method is as follows: When the laser radar observes the sea surface at an angle, the incident angle is θ v , the image point P′ of the detector det It is expressed as: in, Point P i Or click P i surfEst The following relationship should be satisfied: When the receiving conditions are met, the scattered contribution is calculated first, and the incident cosine before leaving the water is and point P i The incident cosine of the photon packet at Calculate the scattering contribution angle θ′ est : The rotation angles γ1 and γ2 in the rotation matrix are calculated as follows: Where n z =[0,0,1]; optical thickness of water τ Wat (P i ,P i surfEst ) is calculated by the following formula: Then calculate the refraction part of the sea-air interface and calculate the incident angle χ in ,as follows: Calculate the water refraction angle χ tra ,as follows: x tra =arcsin(n wat sinx in ) Finally, underwater point P i The polarization weight signal contribution S at Wat Calculated by the following formula: A tele is the aperture area of ​​the detector telescope, S in is the incident Stokes vector carried by the photon packet at the i-th scattering; point P i The polarization weight signal at the detector corresponds to the distance z recorded by the detector det for:

4. The method for simulating polarization signals of an ocean laser radar as claimed in claim 1, characterized in that: In Step 3, the laser radar single scattering weighted signal profile S is calculated. single , the part of the scattering number i=1 in the recorded water body polarization weighted signal contribution is calculated according to the distance z det The accumulation is as follows: Where N P is the total number of simulated photon packets; calculate the total scattering weighted signal profile S of the lidar total as follows: Calculate the number of photons N actually emitted by the laser L It can be calculated based on: Where E L is the laser pulse energy, λ L is the wavelength, They refer to Planck's constant 6.626×10 -34 Js and the speed of light in the atmosphere is 2.998×10 8 m / s; The calculated polarized photoelectron number signal is: Among them, PP Num Represents the parallel channel photoelectron number signal, CP Num is the photoelectron number signal of the orthogonal channel, I f and Q f They are the profile signal contributions S single or S total The I and Q components, η oe and η qe is the optical efficiency and quantum efficiency of the detector; The depolarization ratio Dp of the simulated signal is calculated as:

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