A full-link simulation method for echo signal of space-borne atmosphere and ocean profile detection lidar

Abstract

The present invention discloses a full-link simulation method for echo signals of a space-borne atmospheric and oceanic profile detection laser radar, including: (1) laser radar hardware system modeling; (2) atmospheric environment modeling; (3) sea-air interface modeling; (4) seawater environment modeling; (5) construction of a numerical model of laser radiation transmission. The present invention models the laser radar hardware system and the atmosphere-sea-seawater environment, and uses the Monte Carlo numerical method to track the entire process of photons from emission to interaction with the atmosphere and seawater to reception by the detector; a scattering angle sampling method combining the list method and the rejection reception method is proposed to improve the problem of low efficiency of laser polarization state tracking, and a full-link simulation model for echo signals of a space-borne atmospheric and oceanic profile detection laser radar is constructed, providing a reliable tool for the study of the signal mechanism of space-borne atmospheric and oceanic laser radars.

Description

Technical Field

[0001] The present invention belongs to the technical field of laser radars, and in particular relates to a full-link simulation method for a satellite-borne atmosphere-ocean profile detection laser radar echo signal. Background Art

[0002] Spaceborne LiDAR is an active remote sensing method with vertical profile detection capability. It can achieve high-precision, large-scale, and long-period profile detection of the optical properties of the atmosphere and ocean. Compared with in-situ detection and water color remote sensing, it has irreplaceable advantages and is currently a highly-regarded atmospheric and ocean remote sensing tool. At present, there is no high-resolution spaceborne atmosphere and ocean continuous profile detection LiDAR in orbit in the world. Its R&D cycle is long and the development cost is high. In order to have an intuitive understanding of the echo signal of the spaceborne ocean LiDAR, it is very important to carry out simulation demonstration of the spaceborne ocean LiDAR.

[0003] Existing studies have shown that the Monte Carlo numerical simulation method is an important means of constructing an accurate laser radar radiation transmission model. For example, the Chinese patent document with publication number CN113361080A discloses a GPU-based semi-analytical Monte Carlo simulation method for multi-layer water photon transmission. By using GPU parallel computing for the multi-layer water photon transmission semi-analytical Monte Carlo model, the calculation speed of the semi-analytical Monte Carlo simulation of multi-layer water photon transmission is improved.

[0004] A Chinese patent document with publication number CN116430353A discloses a method for simulating water body lidar signals, which imports the measured layer-by-layer absorption coefficient, the measured layer-by-layer attenuation coefficient, the simulated layer-by-layer backscattering probability and the multiple scattering set into a Monte Carlo model for simulation, verification and correction to obtain a target correction result, and uses the target correction result to correct the water body lidar signal simulation.

[0005] However, the full-link simulation of spaceborne lidar involves multiple media layers such as the atmosphere, air-sea interface and seawater, and the optical properties of different media are complex; existing models are generally only for specific lidar systems and single atmosphere or seawater media, and lack full-link simulation models for atmosphere-seawater continuum media with different hardware parameters; in addition, the simulated laser pulse energy of the spaceborne platform is strong and the medium attenuation effect is strong, but the efficiency of Monte Carlo numerical simulation is very low, especially when tracing the laser polarization state, the simulation speed will drop significantly.

[0006] Therefore, there is an urgent need to develop a high-efficiency full-link simulation method for spaceborne lidar that can target the atmosphere-sea surface-seawater continuum, in order to provide effective technical support for a deep understanding of the lidar detection mechanism, develop effective inversion methods, and further promote the development of future spaceborne atmosphere-ocean lidar systems. Summary of the invention

[0007] In view of the limitations in the existing simulation of spaceborne atmosphere and ocean lidar echo signals, the present invention provides a full-link simulation method for spaceborne atmosphere and ocean profile detection lidar echo signals, which can perform high-efficiency numerical simulation of echo signals for different lidar system parameters and different atmosphere and seawater components.

[0008] A full-link simulation method for a spaceborne atmosphere and ocean profile detection lidar echo signal, comprising:

[0009] (1) LiDAR hardware system modeling

[0010] A coordinate system is established with the laser radar location as the origin, and the initial coordinates and direction of the photon are determined by the off-axis distance of the laser and telescope and the emission tilt angle;

[0011] Calculate the number of simulated photons according to the laser pulse energy, set the initial Stokes vector of the photons according to the polarization state, and set the statistical time interval of the received photon signal according to the pulse width; determine the receiving range of photons according to the working altitude, telescope aperture and field of view;

[0012] (2) Layered atmospheric environment modeling

[0013] The entire atmosphere is constructed as a medium that is uniform in the horizontal direction and layered in the vertical direction. The types and distribution heights of atmospheric clouds and aerosols are set. The optical characteristic parameters of molecules, aerosols and cloud particles in the atmosphere are calculated. The optical parameters of different atmospheric components are added to obtain the optical characteristic parameters of the atmosphere as a whole.

[0014] (3) Air-sea interface modeling

[0015] The Cox-Munk rough sea surface model related to wind speed is used to simulate the propagation of light beams at the sea-air interface. The normal vector of each point on the sea surface is calculated according to the mean square slope of the sea surface related to wind speed. Then, the direction vectors of refracted and reflected photons are calculated according to the direction vector of the incident photon.

[0016] (4) Layered seawater environment modeling

[0017] The seawater is divided into two types: Class I water and Class II water. The components of Class I water are: pure seawater, phytoplankton and colored dissolved organic matter, and Class II water also includes non-algae particles. The total optical characteristic parameters of seawater, including scattering and absorption characteristics, are obtained according to the concentration of different components and the bio-optical model. The scattering phase function of seawater is set, and its polarization characteristics are described by the normalized seawater Mueller matrix. The maximum depth of the simulation is set.

[0018] (5) Construction of numerical model of laser radiation transmission

[0019] The Monte Carlo method is used to construct the entire process of photons being emitted from the laser to the atmosphere, sea surface and seawater, and then returning to the detector, as follows:

[0020] The absorption and scattering of the photons set in step (1) in the atmosphere and seawater media set in steps (2) to (4) are equivalent to probabilistic events; the scattering probability is determined according to the optical properties of the atmosphere and seawater set in steps (2) and (4), the scattering direction is determined according to the scattering phase function, and the polarization state is tracked using the meridian plane method;

[0021] In order to improve the simulation efficiency and make any form of scattering phase function applicable, a sampling method combining the list method and the rejection acceptance method is used to determine the scattering angle; the maximum tracing depth of the photon is determined according to the simulation maximum depth set in step (4), and the tracing is stopped when the photon exceeds the maximum depth;

[0022] Finally, the number of photons reaching the detector is counted according to the photon receiving range set in step (1). After the number of photons that meet the laser pulse energy is accumulated, the laser radar echo profile with distance resolution information is obtained according to the time-of-flight method.

[0023] The simulation method of the present invention models the hardware parameters of the laser radar, including the transmitting and receiving systems, the atmospheric medium, the air-sea interface and the seawater medium, simulates the atmospheric and seawater medium as stratified media with horizontal and vertical layers, calculates the absorption and scattering characteristics of atmospheric molecules, aerosols and cloud particles, describes the wind-induced rough sea surface with the Cox-Munk model, divides the seawater into Class I and Class II water bodies, and calculates the optical characteristics of different seawater components with the bio-optical model. The Monte Carlo numerical simulation method is used to construct a laser radiation transmission model, and the scattering angle sampling method combined with the list method and the rejection reception method is used to improve the simulation efficiency of the polarization state, and finally realizes the high-efficiency simulation of the echo signal of the space-borne atmosphere-ocean laser radar.

[0024] Furthermore, in step (1), the number of simulated photons is calculated according to the energy of the emitted laser pulse; the offset of the initial coordinates of the photons represents the off-axis amount of emission and reception, and the emission direction represents the divergence angle of the light beam; the polarization state of the laser is represented by the Stokes vector [I, Q, U, V].

[0025] In step (2), the optical properties of atmospheric molecules are calculated using the U.S. Standard Atmospheric Model; the optical properties of aerosols and clouds are calculated using Mie scattering theory, or obtained from in-situ experimental data provided by satellite-borne, airborne, or ground-based instruments;

[0026] When using Mie scattering theory for calculations, the microphysical properties of clouds and aerosol particles in the atmosphere are derived from the optical properties software package of aerosols and clouds, and the particle size distribution functions used are gamma distribution and log-normal distribution. The stratified thickness of the atmosphere in the simulation is 30m, and the vertical resolution of the atmospheric echo signal is also 30m.

[0027] In step (3), the normal vector of the rough sea surface is determined by the azimuth angle α and the tilt angle β. The azimuth angle α is uniformly distributed between 0 and 2π, and the expression of the tilt angle β is:

[0028] β=tan -1 {[-σ 2 ln(1-R)] 1 / 2}=tan -1 [(-σ 2 ln R) 1 / 2 ]

[0029] Where R is a random number uniformly distributed between 0 and 1, and σ is the root mean square slope of the sea surface related to the wind speed V, expressed as: σ = (0.003 + 0.00512V) 1 / 2 ; Assume that the normal direction of the sea surface is The normal vector to the incident plane is Get the direction vector of the refracted photon for:

[0030]

[0031] Among them, parameters A, B and C are expressed as:

[0032]

[0033] Among them, the parameters p and q are expressed as:

[0034] Get the direction vector of the reflected photon for:

[0035]

[0036] The expressions of parameters D, E and F are:

[0037]

[0038] In step (4), the absorption and scattering coefficients of a type of water body are:

[0039] a(λ)=a w (λ)+a ph (λ)+a g (λ)

[0040] b(λ)=b w (λ)+b ph (λ)

[0041] Among them, the subscript w represents pure seawater, ph represents phytoplankton, g represents colored dissolved organic matter, and Class II water bodies are based on Class I water bodies plus non-algae particles NAP; the absorption and scattering coefficients of pure seawater at different wavelengths are obtained by looking up the table method; the absorption and scattering coefficients of phytoplankton are calculated from the chlorophyll concentration according to the bio-optical model, and the absorption coefficients of colored dissolved organic matter at different wavelengths are calculated; the absorption and scattering coefficients of non-algae particles are calculated according to their concentrations combined with the bio-optical model and Mie scattering theory.

[0042] In step (5), a sampling method combining the list method and the rejection reception method is used to determine the scattering angle. The specific process is as follows:

[0043] First, establish the cumulative distribution function lookup table of the scattering phase function:

[0044]

[0045] Where Δθ represents the sampling angle interval, K represents the sequence in the probability distribution table; the value of P is greater than 0 and less than or equal to 1;

[0046] Generate two random numbers R and α r Among them, 0≤R<1, 0≤α r <2π, the range of scattering angle [KΔθ, (K+1)Δθ] is determined according to the lookup table;

[0047] In polarized Monte Carlo radiative transfer simulation, for the Stokes vector S 0 =[I 0 ,Q 0 ,U 0 ,V 0 ] for incident light, the scattering phase function is expressed as

[0048]

[0049] Among them, M 11 and M 12 are the 1,1 and 1,2 elements in the scattering phase matrix; compare and The larger one is taken as the reference distribution of the phase function; then two random numbers R and θ are generated. r , where 0 <R≤1,θ r In the interval [KΔθ,(K+1)Δθ]; if Then confirm θ r and α r is the scattering angle and rotation angle, otherwise repeat the above sampling process.

[0050] Compared with the prior art, the present invention has the following beneficial effects:

[0051] 1. The present invention constructs a full-link satellite-borne atmosphere-ocean profile detection lidar signal simulation model, models the lidar hardware system and the atmosphere-sea surface-seawater environment, and tracks the entire process of photons from emission, atmospheric scattering and absorption, sea surface reflection and refraction, water scattering and absorption to reception by the detector.

[0052] 2. The present invention proposes for the first time a scattering angle sampling method combining the list method and the rejection reception method, which solves the problem of low efficiency of numerical simulation of polarized laser radar, makes numerical simulation of satellite-borne atmospheric and oceanic polarized laser radar signals possible, and can provide a reliable tool for the signal mechanism of satellite-borne atmospheric and oceanic laser radar. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Figure 1 This is a flow chart of a full-link simulation method for a space-borne atmosphere and ocean profile detection lidar echo signal of the present invention;

[0054] Figure 2 A distribution diagram of atmospheric components in a CALIOP detection scene used in an embodiment of the present invention;

[0055] Figure 3 The atmospheric extinction coefficient distribution diagram used in the embodiment of the present invention;

[0056] Figure 4 The scattering phase functions of atmospheric aerosols and clouds used in the embodiments of the present invention;

[0057] Figure 5 A water body chlorophyll concentration distribution diagram used in the embodiment of the present invention;

[0058] Figure 6 A distribution diagram of water absorption and scattering coefficients used in the embodiments of the present invention;

[0059] Figure 7 The atmospheric attenuation backscatter echo signal obtained by simulation in the embodiment of the present invention is

[0060] Figure 8 It is the atmospheric attenuated backscatter echo signal actually detected by CALIOP;

[0061] Fig. 9 This is a comparison chart between the simulated atmospheric echo signal and the measured signal;

[0062] Fig.10 This is a cross-sectional diagram of the seawater lidar echo signal obtained by simulation according to an embodiment of the present invention. DETAILED DESCRIPTION

[0063] The present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be pointed out that the embodiments described below are intended to facilitate the understanding of the present invention and do not have any limiting effect on the present invention.

[0064] In order to demonstrate the implementation process of the present invention and verify the accuracy of the method, the atmospheric echo signal of the space-borne atmospheric laser radar CALIOP is simulated, and clean ocean water is selected to simulate the seawater signal.

[0065] like Figure 1 As shown, a full-link simulation method for a spaceborne atmosphere and ocean profile detection lidar echo signal includes:

[0066] Step 1: Set the hardware parameters of the space-borne laser radar to be simulated, as shown in Table 1. In order to obtain a stable echo signal and increase the simulation speed, the number of simulated photons is set to 10. 9 The coordinate system is constructed with the laser radar transmitting end as the coordinate origin, and the initial position of the photon packet is set at the coordinate origin and the initial movement direction is perpendicular to the z-axis and emitted downward.

[0067] Table 1 LiDAR system parameters used in simulation

[0068]

[0069] Step 2: Construct the atmospheric and seawater environment model. The waters in the center of the South Pacific Gyre and the air above it are selected as the detection objects of the lidar. A satellite track of CALIOP near the South Pacific Gyre in November 2006 is selected. The distribution of atmospheric components in the scene corresponding to this track is as follows: Figure 2 As shown in Figure 1, it mainly includes four types: atmospheric molecules, marine aerosols, ice clouds and water clouds. The absorption coefficient and scattering coefficient of atmospheric molecules are calculated using the US standard atmospheric model based on Rayleigh scattering theory. Aerosols and clouds are non-absorbing, and their scattering coefficients, i.e., extinction coefficients, come from the Level 2 data products of CALIOP, such as Figure 3 The scattering phase functions of aerosols and clouds are calculated according to Mie scattering theory, as shown in Figure 4 shown.

[0070] For the modeling of the seawater environment, the South Pacific Gyre Center area was selected. The annual average chlorophyll a concentration in this area is only 0.019 mg / m 3 , which is the lowest value in the world's high seas, so it is called the world's "cleanest" water. The extremely low chlorophyll a concentration allows the laser to penetrate deep enough water, which is of great value for ocean exploration. The water chlorophyll concentration data used in the simulation is as follows Figure 5As shown in the figure, the data comes from the BIOSOPE (Biogeochemistry and Optics South Pacific Experiment) cruise, which detected the spatial distribution and structure of phytoplankton pigments on a 8000km long transect from the mesotrophic waters near the Marquesas Islands (141°W, 8°S) to the eutrophic waters of the Chilean upwelling area (73°W, 34°S). The absorption and scattering coefficient distribution of the water body is calculated according to a type of water body bio-optical model, as shown in the figure below. Figure 6 As shown. The FF phase function is used in the simulation to describe the scattering characteristics of the water body. The maximum water depth of the simulation is 80m, assuming that the water surface is calm and windless.

[0071] Step 3: The echo signal is simulated according to the LiDAR Monte Carlo radiation transfer model and the above atmospheric and ocean environmental parameters, and the detected photon packet signal is counted according to the time-of-flight method. When counting, since the thickness of the entire atmosphere is 30km and the depth of the water body is 80m, in order to intuitively display the echo signal, the vertical resolution of the signal in the atmosphere and seawater is set to 30m and 0.3m respectively.

[0072] Figure 7 and Figure 8 They are respectively the simulated atmospheric echo signal and the signal actually detected by CALIOP. It can be seen that the simulation method proposed in the present invention reproduces the spatial distribution of the cloud layer and the aerosol layer in the scene well. Fig. 9 This is a comparison chart of the simulated signal and the measured signal. The root mean square error (RMSE) between the CALIOP measured signal and the simulated signal is 0.0075, which can well illustrate the accuracy of the simulation method. Fig.10 This is the simulated seawater lidar echo signal profile. It can be seen that the echo signal of the water body drops by 3 orders of magnitude near 60m, from which it can be inferred that the maximum detection depth of the lidar in this area is about 60m. The simulation results of the ocean echo signal can provide a good basis for evaluating the detection performance of the space-borne lidar.

[0073] The embodiments described above provide a detailed description of the technical solutions and beneficial effects of the present invention. It should be understood that the above are only specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, supplements and equivalent substitutions made within the scope of the principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A full-link simulation method for echo signal of space-borne atmosphere and ocean profile detection lidar, characterized in that: include: (1) LiDAR hardware system modeling A coordinate system is established with the laser radar location as the origin, and the initial coordinates and direction of the photon are determined by the off-axis distance of the laser and telescope and the emission tilt angle; Calculate the number of simulated photons according to the laser pulse energy, set the initial Stokes vector of the photons according to the polarization state, and set the statistical time interval of the received photon signal according to the pulse width; determine the receiving range of photons according to the working altitude, telescope aperture and field of view; (2) Layered atmospheric environment modeling The entire atmosphere is constructed as a medium that is uniform in the horizontal direction and layered in the vertical direction. The types and distribution heights of atmospheric clouds and aerosols are set. The optical characteristic parameters of molecules, aerosols and cloud particles in the atmosphere are calculated. The optical parameters of different atmospheric components are added to obtain the optical characteristic parameters of the atmosphere as a whole. (3) Air-sea interface modeling The Cox-Munk rough sea surface model related to wind speed is used to simulate the propagation of light beams at the sea-air interface. The normal vector of each point on the sea surface is calculated according to the mean square slope of the sea surface related to wind speed. Then, the direction vectors of refracted and reflected photons are calculated according to the direction vector of the incident photon. (4) Layered seawater environment modeling The seawater is divided into two types: Class I water bodies and Class II water bodies. The components of Class I water bodies are: pure seawater, phytoplankton and colored dissolved organic matter, and Class II water bodies also include non-algae particles. The total optical characteristic parameters of seawater, including scattering and absorption characteristics, are obtained according to the concentration of different components and the bio-optical model. The scattering phase function of seawater is set, and its polarization characteristics are described by the normalized seawater Mueller matrix. The maximum depth of the simulation is set. (5) Construction of numerical model of laser radiation transmission The Monte Carlo method is used to construct the entire process of photons being emitted from the laser to the atmosphere, sea surface and seawater, and then returning to the detector, as follows: The absorption and scattering of the photons set in step (1) in the atmosphere and seawater media set in steps (2) to (4) are equivalent to probabilistic events; the scattering probability is determined according to the optical properties of the atmosphere and seawater set in steps (2) and (4), the scattering direction is determined according to the scattering phase function, and the polarization state is tracked using the meridian plane method; The scattering angle is determined by using a sampling method combining the list method and the rejection acceptance method; the maximum tracing depth of the photon is determined according to the simulation maximum depth set in step (4), and the tracing is stopped when the photon exceeds the maximum depth; the scattering angle is determined by using a sampling method combining the list method and the rejection acceptance method, and the specific process is as follows: First, establish the cumulative distribution function lookup table of the scattering phase function: Where Δθ represents the sampling angle interval, K represents the sequence in the probability distribution table; the value of P is greater than 0 and less than or equal to 1; Generate two random numbers R and α r Among them, 0≤R<1, 0≤α r <2π, the range of scattering angle [KΔθ, (K+1)Δθ] is determined according to the lookup table; In polarized Monte Carlo radiative transfer simulation, for incident light with Stokes vector S0 = [I0, Q0, U0, V0], the scattering phase function is expressed as Among them, M 11 and M 12 are the 1,1 and 1,2 elements in the scattering phase matrix; compare and The larger one is taken as the reference distribution of the phase function; then two random numbers R and θ are generated. r , where 0 <R≤1,θ r In the interval [KΔθ,(K+1)Δθ]; if Then confirm θ r and α r is the scattering angle and rotation angle, otherwise repeat the above sampling process; Finally, the number of photons reaching the detector is counted according to the photon receiving range set in step (1). After the number of photons that meet the laser pulse energy is accumulated, the laser radar echo profile with distance resolution information is obtained according to the time-of-flight method.

2. The full-link simulation method for the spaceborne atmosphere and ocean profile detection laser radar echo signal according to claim 1 is characterized in that: In step (1), the number of simulated photons is calculated according to the energy of the emitted laser pulse; the offset of the initial coordinates of the photons represents the off-axis amount of emission and reception, and the emission direction represents the divergence angle of the light beam; the polarization state of the laser is represented by the Stokes vector [I, Q, U, V].

3. The full-link simulation method for the spaceborne atmosphere and ocean profile detection laser radar echo signal according to claim 1 is characterized in that: In step (2), the optical properties of atmospheric molecules are calculated using the U.S. Standard Atmospheric Model; the optical properties of aerosols and clouds are calculated using Mie scattering theory, or obtained from in-situ experimental data provided by satellite-borne, airborne, or ground-based instruments; When using Mie scattering theory for calculations, the microphysical properties of clouds and aerosol particles in the atmosphere are derived from the optical properties software package of aerosols and clouds, and the particle size distribution functions used are gamma distribution and log-normal distribution. The stratified thickness of the atmosphere in the simulation is 30m, and the vertical resolution of the atmospheric echo signal is also 30m.

4. The full-link simulation method for the spaceborne atmosphere and ocean profile detection laser radar echo signal according to claim 1 is characterized in that: In step (3), the normal vector of the rough sea surface is determined by the azimuth angle α and the tilt angle β. The azimuth angle α is uniformly distributed between 0 and 2π, and the expression of the tilt angle β is: βϼtan -1 {[−σ 2 ln(1-R)] 1 / 2 }=tan -1 [(−σ 2 ln R) 1 / 2 ] Where R is a random number uniformly distributed between 0 and 1, and σ is the root mean square slope of the sea surface related to the wind speed V, expressed as: σ = (0.003 + 0.00512V) 1 / 2 ; Assume that the normal direction of the sea surface is The normal vector to the incident plane is Get the direction vector of the refracted photon for: Among them, parameters A, B and C are expressed as: Among them, the parameters p and q are expressed as: Get the direction vector of the reflected photon for: The expressions of parameters D, E and F are:

5. The full-link simulation method for the spaceborne atmosphere and ocean profile detection laser radar echo signal according to claim 1 is characterized in that: In step (4), the absorption and scattering coefficients of a type of water body are: a(λ)=a w (λ)+a ph (λ)+a g (l) b(λ)=b w (λ)+b ph (l) Among them, the subscript w represents pure seawater, ph represents phytoplankton, g represents colored dissolved organic matter, and Class II water bodies are based on Class I water bodies plus non-algae particles NAP; the absorption and scattering coefficients of pure seawater at different wavelengths are obtained by looking up the table method; the absorption and scattering coefficients of phytoplankton are calculated from the chlorophyll concentration according to the bio-optical model, and the absorption coefficients of colored dissolved organic matter at different wavelengths are calculated; the absorption and scattering coefficients of non-algae particles are calculated according to their concentrations combined with the bio-optical model and Mie scattering theory.

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