CN110717979A - Atmospheric and three-dimensional earth surface coupling radiation simulation method based on photon tracking - Google Patents
Atmospheric and three-dimensional earth surface coupling radiation simulation method based on photon tracking Download PDFInfo
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Abstract
The invention discloses an atmosphere and three-dimensional earth surface coupling radiation simulation method based on photon tracking, which comprises the following steps: (1) reading input parameters of a light source, an atmospheric scene and an earth surface scene, and calculating the atmospheric input parameters; (2) emitting a photon from a light source into a scene, simulating the traveling distance of the photon according to the calculation result of the step (1), and determining the position of a collision point; (3) updating photon position, direction and energy information at a collision point, and collecting energy contribution; (4) calculating the next photon heading according to the photon position, and continuing to track the photons until a termination condition is met; (5) and emitting new photons, repeating the tracking process, and obtaining a converged atmospheric top radiance result after the set number of photons is reached. The method simulates the radiation transmission coupling of the earth surface and the atmosphere by tracking the photon direction and position information, is closer to the actual condition, and further more accurately simulates the influence of the atmosphere on the earth surface radiation transmission signal.
Description
Technical Field
The invention relates to the field of quantitative remote sensing simulation, in particular to an atmospheric and three-dimensional earth surface coupling radiation simulation method based on photon tracking. The method has important significance in the aspects of research such as quantitative analysis of the influence of atmospheric coupling on remote sensing signals, surface vegetation ecology and the like.
Background
The remote sensing measurement is an important information acquisition tool for exploring the state of the ground object by remotely receiving electromagnetic radiation information from the ground object through a sensor, and the characteristic information such as the directional reflectivity of the ground object is acquired to obtain abundant object structure and category information. The method has the advantages that the three-dimensional earth surface scene model can be established, the reflection spectrum of the scene can be directly simulated and calculated by modifying the input scene state parameters, and the method has important significance for quantitatively researching the influence of the three-dimensional structure of the earth surface scene on the remote sensing signals.
For remote sensing observation, the influence of the earth atmosphere on the signal is a key problem which cannot be ignored. The reflectivity characteristics of a surface scene at a particular observation angle depend to some extent on the current atmospheric conditions. The method mainly adopted at present is to obtain a spectral signal at the top height of an earth surface scene by using a remote sensing signal at the top of the atmosphere through atmospheric correction. However, this requires various approximate assumptions about the atmospheric conditions, which can introduce large errors in atypical cases; on the other hand, the existing atmosphere coupling model mostly adopts the method of accumulating the contributions of the earth surface scene and the atmosphere at the bottom height of the atmosphere and simulating the coupling of the earth surface scene and the atmosphere by calculating the interaction for a limited time, so that the real physical process cannot be restored, the atmosphere proximity effect is ignored in most cases, and the obtained result has limited precision.
Therefore, the relationship between the remote sensing signals and the atmospheric properties and the three-dimensional earth surface scene is quantitatively researched, the most convenient and most developing method is to establish a model capable of really restoring the radiation transmission coupling of the three-dimensional earth surface scene and the atmosphere, and the method has important significance and application value in the aspects of better understanding and analyzing the radiation transmission process of the three-dimensional earth surface scene, actual sensor index design and development and the like.
It is important to note that this section is intended to provide a background or context to the invention that is recited in the claims. The description herein is not admitted to be prior art by inclusion in this section.
Disclosure of Invention
The invention aims to provide an atmosphere and three-dimensional earth surface coupling radiation simulation method based on photon tracking, aiming at the problems of more approximations and inaccurate simulation results in the radiation transmission simulation process of the three-dimensional earth surface scene considering the atmosphere at present.
The technical solution of the invention is as follows: the atmospheric portion and the surface portion are parametrized and described by the monte carlo concept. Tracking the direction, position and energy information of each photon in the simulation process, judging whether the photon is in the atmosphere or in a three-dimensional ground scene by using the position information of the photon, and correspondingly calculating the traveling distance and direction of the photon by using the optical parameters of the atmosphere or the optical and structural parameters of the ground scene. The energy contribution in the direction of observation of the sensor is collected and accumulated at the location where each photon collides with an element in the scene. And obtaining the convergent atmospheric layer top radiance through a simulation process of a certain photon number. The method comprises the following specific steps:
(1) reading input parameters of a light source, an atmospheric scene and an earth surface scene, and calculating the atmospheric input parameters;
(2) emitting a photon from a light source into a scene, simulating the traveling distance of the photon according to the calculation result of the step (1), and determining the position of a collision point;
(3) updating photon position, direction and energy information at the collision point, and collecting energy contribution of photons to the sensor;
(4) calculating the next photon heading according to the photon position, and continuing to track the photons until a termination condition is reached;
(5) new photons are emitted from the light source, the tracking process is repeated, and after the set number of photons is reached, a converged atmospheric top radiance result is obtained.
Reading in three-dimensional structure parameters, spectrum parameters and atmospheric spectrum parameters of a light source and a ground surface scene, and calculating and processing atmospheric input parameters:
describing a three-dimensional earth surface scene by using a method of combining geometric patches, describing the size, position and orientation information of each triangle or circle by using three-dimensional coordinates, and simultaneously inputting the hyperspectral reflectivity and transmissivity information of each patch; the three-dimensional earth surface scene with any complexity degree can be described by using the splicing of the geometric patches.
Due to the vertical heterogeneity of the earth's atmosphere, the atmosphere above the three-dimensional earth surface scene is modeled into 12 height layers with unequal intervals, and the atmosphere level in each layer is uniform and has corresponding optical parameters; for each altitude, the atmosphere is classified by type into atmospheric molecules, aerosols and clouds; respectively inputting the extinction coefficients and single scattering albedo of atmospheric molecules, aerosol and cloud at each layering position of each simulation waveband; the dual Henyey-Greenstein asymmetry factor parameters corresponding to the input aerosol and cloud describe their scattering phase function.
The sun is used as an incident light source of an atmosphere and three-dimensional earth surface coupling scene, the input geometric parameters of the sun comprise a sun zenith angle and an azimuth angle, the light source is arranged at the top of the atmosphere, and the size of the light source is kept the same as the size of the boundary of the three-dimensional earth surface scene below. The input spectral parameter is the spectral distribution of the incident solar energy.
Preprocessing input atmospheric optical parameters, and generating an atmospheric molecular scattering phase function lookup table by Rayleigh scattering; calculating a scattering phase function lookup table of cloud and aerosol by using double Henyey-Greenstein parameters corresponding to different wave bands and different height layers; adjusting the actual height of each atmosphere layer according to the atmospheric elevation; and (3) carrying out equivalent synthesis on the absorption coefficients, the single scattering albedo and the scattering phase function lookup table of different atmospheric components at each wave band and each height position, and respectively storing the results as calculation parameters directly used in the subsequent simulation process.
Emitting a photon from a light source into a scene in the step (2), simulating the traveling distance of the photon according to the calculation result in the step (1), and determining the position of a collision point: and determining the initial position, direction and energy of the photon entering the scene from the top of the atmospheric layer according to the set light source parameters.
The light source surface is arranged on the top of the atmosphere, the initial position of each photon is randomly determined on the light source surface by using a uniform random number, the direction of the photon is determined by the input zenith angle and azimuth angle of the sun, and the initial energy of each photon is determined by the input spectral parameters:
in the formula, N is the number of photons to be tracked, S represents the base area of a simulation scene, and lambda is the currently simulated wavelength; after entering a scene, photons firstly enter an atmosphere module and are transmitted along a straight line according to a set initial direction. The distance traveled by the ray in the scene is calculated based on the location of the photon.
And determining that the photons are in the atmosphere or in the three-dimensional earth surface scene according to the positions of the photons. And if the photons are in the atmosphere, finding the atmospheric equivalent extinction coefficient of the current position according to the height hierarchy, and calculating to obtain a free path. The free path S of the photon traveling in the atmosphere is determined by the random number and the equivalent extinction coefficient of the layer:
rand is a uniform random number, β, between 0 and 1atmCalculating the atmospheric equivalent extinction coefficient at the current position obtained in the step (1), wherein iz represents the atmospheric layer where the photon is currently located, and lambda is the currently simulated wavelength; after traveling the free path S, the photons collide with atmospheric components.
If the photons are in the earth surface scene, simulating the photons to propagate according to the current direction until the photons collide with the geometric patch, performing geometric intersection according to the three-dimensional coordinates of the triangle or the wafer, and calculating the number of the triangle or the wafer where the light rays collide and the position of the collision point.
Wherein, step (3) updates photon position, direction and energy information at the collision point, and collects the energy contribution of the photon to the sensor: at the collision point, the energy carried by the photon will decay at the collision point:
Qout(λ)＝Qin(λ)·ω(λ)
wherein QoutAnd QinRepresenting the photon energy after and before the collision, respectively, and ω is the single scattering albedo, the equivalent atmospheric albedo for the current location in the atmosphere, and the sum of the reflectivity and transmissivity of the current collision triangle or disc in the surface scene.
The new scattering direction distribution of the photons after collision in the atmosphere meets the distribution of the scattering phase function, and the new transmission direction is extracted and calculated by using the scattering phase function lookup table at the current position calculated in the step (1); the scattering of photons at the surface of a patch of the earth's surface scene assumes that it satisfies the lambert's law, and a new scattering direction is calculated from the patch orientation information.
Calculating the energy contribution of the accumulated collision event to the set observation sensor by using a photon diffusion method at the position of each collision; if the path from the collision point position to the sensor is not shielded, calculating the probability of light scattering to the sensor direction according to the incident direction of photons and the position information of the collision point, and collecting energy according to the proportion:
Qcollect(Ω,λ)＝Qout(λ)·P(Ω,λ)·exp(-La·βatm)
p is the probability of light scattering towards the sensor, LaRepresenting the path length of the atmosphere passing from the point of impact to the sensor location, and the last exponential term in the formula describes the extinction of the atmosphere over this distance.
And (4) calculating the next photon heading according to the photon position, and continuing to track the photons until the termination condition is reached: the photons will leave the collision point in the new direction, continue to travel in a straight line and repeat the collision process. In the whole tracking process, the photons carry position, direction and energy information, the position information of the photons is used for judging whether the photons are in the atmosphere or a ground surface scene currently, when the photons are in the atmosphere, a large gas phase function lookup table is used for randomly extracting a new direction of the photons in the subsequent transmission process, the photons are continuously tracked by using a path length calculation method and a collision processing method in the atmosphere, the information carried by the photons is updated, and the sensor contribution is collected; when the photons are in the earth surface scene, randomly extracting new directions of the photons according to the normal direction of the current collided patch and the Lambert law, continuously tracking the photons by using a path length calculation method and a collision processing method of the earth surface scene, updating information carried by the photons and collecting sensor contributions.
After a certain number of collisions, the energy carried on the photon will become smaller, once the photon energy is less than the threshold, a "russian wheel" is used to conserve energy and unbiased to terminate the photon tracking process.
The photons have specific direction and position information when entering the earth surface scene from the bottom of the atmosphere, and the non-isotropy of the sky diffused light after atmospheric scattering can be well simulated. Once the photons leave the upper boundary of the earth surface scene through collision and scattering in the earth surface scene, the photons can return to the atmosphere to be continuously tracked by using the height information and the specific propagation direction of the photons, so that the cyclic reciprocating action of the photons at the boundary of the atmosphere and the earth surface scene between the two parts is well reproduced, and the complete coupling of the atmosphere and the earth surface scene under the atmosphere is realized.
And (3) emitting new photons in the step (5), repeating the tracking process, and obtaining a converged atmospheric top radiance result after the set number of photons is reached: the Monte Carlo thought is used, in the simulation process of a photon, each uncertain physical process is described as a probability process according to the physical distribution, a random number sampling method is used for determining specific events occurring in the simulation, and the position described by each random method strictly follows the physical distribution rule.
Through a large number of repeated simulation (namely, a certain number of simulated photon numbers) each position described by using a random event is fully sampled according to the distribution, and finally, the radiance signal received by the observation direction of the atmospheric dome sensor in the final atmosphere can be obtained through the accumulated energy contribution to the observation sensor in each time:
the meanings of the symbols in the above formula are as defined above.
Compared with the prior art, the invention has the advantages that: the transmission process of photons in a scene containing atmosphere is restored based on a physical principle, the cyclic reciprocating action process of the photons at the junction of the atmosphere and the earth surface scene is truly reproduced, the radiation process coupling of the atmosphere and the earth surface scene is achieved, the assumption and approximation in the process are reduced, and therefore the simulation error of the top radiance of the atmosphere layer is reduced.
It has the following advantages: (1) through computer simulation, under the limited calculation time, remote sensing interested quantities such as directional reflectivity factors and the like at different heights, at any observation angle, in an optional wavelength range and under different atmospheric and surface parameter setting conditions can be calculated. The functions are various, and modifiable parameters are abundant and representative. (2) The three-dimensional ground object scene with any complexity can be described by utilizing a three-dimensional coordinate geometric patch mode, the construction of the three-dimensional scene and the radiation simulation process are independent, the three-dimensional scene can be introduced into the latest development result of computer graphics, and the three-dimensional ground object scene has wide application scenes. (3) The model established by the method is a computer simulation model, and compared with other common modeling methods of remote sensing analytical models and geometric optical models, the method has higher precision, and simultaneously introduces various acceleration algorithms in the simulation process, and is effectively controlled in the calculation time. Along with the improvement of the performance of the computer, the calculation efficiency is further improved.
Drawings
Fig. 1 is a flowchart of an atmospheric and three-dimensional earth surface coupled radiation simulation method based on photon tracking according to the present invention.
Detailed Description
In order to better explain the photon tracking-based three-dimensional earth surface and atmosphere radiation coupling simulation method, the radiation transmission simulation is carried out by utilizing hyperspectral three-dimensional vegetation canopy data and atmosphere data in the wavelength range from visible light to short wave infrared, and the atmosphere layer top reflectivity in the vertical observation direction is obtained through calculation. The method comprises the following concrete steps:
(1) reading input parameters of a light source, an atmospheric scene and a ground surface scene, and calculating the atmospheric input parameters: the input parameters of the three-dimensional earth surface scene comprise the structural parameters and the spectral parameters, and the structural parameter file and the reflectivity and transmissivity spectral file of the corn canopy scene (described by splicing triangular patches with different sizes) in the jointing stage are read in. And reading in a soil structure parameter file and a reflection spectrum file which describe soil.
Reading in atmospheric spectral parameters in a corresponding wavelength range, using standard mid-latitude summer atmospheric type as atmospheric input parameters, dividing the atmospheric spectral parameters into 12 layered height layers with dense lower ends and sparse upper ends according to a height interval, wherein the waveband interval can be 400-2500 nm, but is not limited to the above;
the sun is used as an incident light source of the coupled scene of the atmosphere and the three-dimensional earth surface, the zenith angle of the incident sun is set to be 30 degrees, and the azimuth angle is set to be 0 degree. The light source is arranged at the top of the atmosphere, and the size of the light source is kept the same as the size of the boundary of the three-dimensional scene of the earth surface below the light source. The input spectral parameter is the spectral distribution of the incident solar energy.
Preprocessing input atmospheric optical parameters, and generating an atmospheric molecular scattering phase function by Rayleigh scattering; calculating a scattering phase function lookup table of cloud and aerosol by using double Henyey-Greenstein parameters corresponding to different wave bands and different height layers; adjusting the actual height of each atmosphere layer according to the atmospheric elevation; performing equivalent synthesis on the absorption coefficients, single scattering albedo and scattering phase function lookup tables of different atmospheric components at each wave band and each height position; and respectively store the calculation parameters directly used in the subsequent simulation process.
(2) Emitting a photon from a light source into a scene, simulating the traveling distance of the photon according to the calculation result of the step (1), and determining the position of a collision point: the light source surface is arranged at the top of the atmosphere, a random position is randomly determined on the light source surface by using a uniform random number and is used as a photon starting point, and the light source surface enters a scene according to a zenith angle of 30 degrees and an azimuth angle of 0 degree. The initial energy of each photon is determined by the input spectral parameters:
in the formula, N is the number of photons to be tracked, S represents the base area of a simulation scene, and lambda is the currently simulated wavelength; after entering a scene, photons firstly enter an atmosphere module and are transmitted along a straight line according to a set initial direction. The distance traveled by the ray in the scene is calculated based on the location of the photon.
And determining that the photons are in the atmosphere or in the corn canopy according to the positions of the photons. And if the photons are in the atmosphere, finding the atmospheric equivalent extinction coefficient of the current position according to the height hierarchy, and calculating to obtain a free path.
The free path S of the photon travelling in the atmosphere is determined by the random number and the extinction coefficient of the layer:
rand is a uniform random number, β, between 0 and 1atmCalculating the atmospheric equivalent extinction coefficient at the current position obtained in the step (1), wherein iz represents the atmospheric layer where the photon is currently located, and lambda is the currently simulated wavelength; after traveling the free path S, the photons collide with atmospheric components.
If the photons are in the earth surface scene, simulating the photons to propagate according to the current direction until the photons collide with the geometric patch, performing geometric intersection according to the three-dimensional coordinates of the triangle or the wafer, and calculating the number of the triangle or the wafer where the light rays collide and the position of the collision point.
(3) Photon position, direction and energy information are updated at the collision point, and the energy contribution of the photons to the sensor is collected: at the collision point, the energy carried by the photon will decay at the collision point:
Qout(λ)＝Qin(λ)·ω(λ)
wherein QoutAnd QinRepresenting the photon energy after and before the collision, respectively, and ω is the single scattering albedo, the equivalent atmospheric albedo for the current location in the atmosphere, and the sum of the reflectivity and transmissivity of the current collision triangle or disc in the surface scene.
The new scattering direction distribution of the photons after collision in the atmosphere meets the distribution of the scattering phase function, and the new transmission direction is extracted and calculated by using the scattering phase function lookup table at the current position calculated in the step (1); the scattering of photons at the surface of a patch of the earth's surface scene assumes that it satisfies the lambert's law, and a new scattering direction is calculated from the patch orientation information.
Calculating the energy contribution of the accumulated collision event to the set observation sensor by using a photon diffusion method at the position of each collision; if the path from the collision point position to the sensor is not shielded, calculating the probability of light scattering to the sensor direction according to the incident direction of photons and the position information of the collision point, and collecting energy according to the proportion:
Qcollect(Ω,λ)＝Qout(λ)·P(Ω,λ)·exp(-La·βatm)
p is the probability of light scattering towards the sensor, LaRepresenting the path length of the atmosphere passing from the point of impact to the sensor location, and the last exponential term in the equation describes the extinction of the atmosphere over this distance.
(4) And calculating the next photon direction according to the photon position, and continuing to track the photon until a termination condition is reached: the photons leave the collision point in a new direction, continue to propagate along a straight line and repeat the collision process; in the whole tracking process, the photons carry position, direction and energy information, the position information of the photons is used for judging whether the photons are in the atmosphere or a ground surface scene currently, when the photons are in the atmosphere, a large gas phase function lookup table is used for randomly extracting a new direction of the photons in the subsequent transmission process, the photons are continuously tracked by using a path length calculation method and a collision processing method in the atmosphere, the information carried by the photons is updated, and the sensor contribution is collected; when the photons are in the earth surface scene, randomly extracting new directions of the photons according to the normal direction of the current collided patch and the Lambert law, continuously tracking the photons by using a path length calculation method and a collision processing method of the earth surface scene, updating information carried by the photons and collecting sensor contributions.
After a certain number of collisions, the energy carried on the photon will become smaller, once the photon energy is less than the threshold, a "russian wheel" is used to conserve energy and unbiased to terminate the photon tracking process.
The photons have specific direction and position information when entering the earth surface scene from the bottom of the atmosphere, and the non-isotropy of the sky diffused light after atmospheric scattering can be well simulated. Once the photons leave the upper boundary of the earth surface scene through collision and scattering in the earth surface scene, the photons can return to the atmosphere to be continuously tracked by using the height information and the specific propagation direction of the photons, so that the cyclic reciprocating action of the photons at the boundary of the atmosphere and the earth surface scene between the two parts is well reproduced, and the complete coupling of the atmosphere and the earth surface scene under the atmosphere is realized.
(5) New photons are emitted, the tracking process is repeated, and after the set number of photons is reached, a convergent atmospheric layer top radiance result is obtained: by using Monte Carlo thought, in the simulation process of a photon, each uncertain physical process (the distance traveled by the photon and the direction of scattering after collision) is described as a probability process according to the physical distribution, and a random number sampling method is used for determining specific events occurring in the simulation, wherein each position described by using the random method strictly follows the physical distribution rule.
Through simulation of N-10 ten thousand photon number, each position described by using a random event is fully sampled according to the distribution, and finally, a final coupling result of the whole three-dimensional earth surface scene and the atmosphere, namely a radiance signal received in the observation direction of the atmospheric layer top sensor, can be obtained through the accumulated energy contribution to the observation sensor each time:
the meanings of the symbols in the above formula are as defined above. Without multithread optimization, the computation time on a personal notebook computer (CPU Intel Core i7-7500U @2.70GHz) is approximately 5.2 minutes.
Claims (6)
1. An atmospheric and three-dimensional earth surface coupling radiation simulation method based on photon tracking is characterized by comprising the following steps:
(1) reading input parameters of a light source, an atmospheric scene and an earth surface scene, and calculating the atmospheric input parameters;
(2) emitting a photon from a light source into a scene, simulating the traveling distance of the photon according to the calculation result of the step (1), and determining the position of a collision point;
(3) updating photon position, direction and energy information at the collision point, and collecting energy contribution of photons to the sensor;
(4) calculating the next photon heading according to the photon position, and continuing to track the photons until a termination condition is reached;
(5) new photons are emitted from the light source, the tracking process is repeated, and after the set number of photons is reached, a converged atmospheric top radiance result is obtained.
2. The atmospheric and three-dimensional earth surface coupled radiation simulation method based on photon tracking according to claim 1, characterized in that: the simulation method comprises the following steps of (1) reading input parameters of a light source, an atmospheric scene and an earth surface scene, and calculating the input parameters:
describing a three-dimensional earth surface scene by using a method of combining geometric patches, describing the size, position and orientation information of each triangle or circle by using three-dimensional coordinates, and simultaneously inputting the hyperspectral reflectivity and transmissivity information of each patch; three-dimensional earth surface scenes with any complexity can be described by splicing the geometric patches;
due to the vertical heterogeneity of the earth's atmosphere, the atmosphere above the three-dimensional earth surface scene is modeled into 12 height layers with unequal intervals, and the atmosphere level in each layer is uniform and has corresponding optical parameters; for each altitude, the atmosphere is classified by type into atmospheric molecules, aerosols and clouds; respectively inputting the extinction coefficients and single scattering albedo of atmospheric molecules, aerosol and cloud at each layering position of each simulation waveband; the double Henyey-Greenstein asymmetric parameters corresponding to the input aerosol and cloud describe scattering phase functions of the aerosol and cloud;
the sun is used as an incident light source of an atmosphere and three-dimensional earth surface coupling scene, the input geometric parameters comprise a sun zenith angle and an azimuth angle, the light source is arranged at the top of the atmosphere, and the size of the light source is the same as the size of the boundary of the three-dimensional earth surface scene below; the input spectral parameters are spectral distribution of incident solar energy;
preprocessing input atmospheric optical parameters; generating an atmospheric molecular scattering phase function lookup table by using Rayleigh scattering; calculating a scattering phase function lookup table of cloud and aerosol by using double Henyey-Greenstein parameters corresponding to different wave bands and different height layers;
adjusting the actual layering height of each atmosphere layer according to the atmospheric elevation;
performing equivalent synthesis on the absorption coefficients, single scattering albedo and scattering phase function lookup tables of different atmospheric components at each wave band and each height position; and respectively store the calculation parameters directly used in the subsequent simulation process.
3. The atmospheric and three-dimensional earth surface coupled radiation simulation method based on photon tracking according to claim 1, characterized in that: emitting a photon from a light source into a scene, simulating the traveling distance of the photon according to the calculation result of the step (1), and determining the position of a collision point:
the light source surface is arranged on the top of the atmosphere, the initial position of each photon is randomly determined on the light source surface by using a uniform random number, the direction of the photon is determined by the input zenith angle and azimuth angle of the sun, and the initial energy of each photon is determined by the input spectral parameters:
in the formula, N is the number of photons to be tracked, S represents the base area of a simulation scene, and lambda is the currently simulated wavelength;
after entering a scene, photons firstly enter an atmosphere module and are transmitted along a straight line according to a set initial direction; the calculation method of the distance traveled by the ray in the scene is determined according to the position of the photon;
in the subsequent propagation process, if the photons are in the atmosphere, the extinction coefficients of the current position are found according to the height hierarchy, and a free path is obtained through calculation; the free path S of the photon travelling in the atmosphere is determined by the random number and the extinction coefficient of the layer:
rand is a uniform random number, β, between 0 and 1atmCalculating the atmospheric equivalent extinction coefficient at the current position obtained in the step (1), wherein iz represents the atmospheric layer where the photon is currently located, and lambda is the currently simulated wavelength; after traveling the free path S, the photons collide with atmospheric components;
if the photons are in the earth surface scene, simulating the photons to propagate according to the current direction until the photons collide with the geometric patch, performing geometric intersection according to the three-dimensional coordinates of the triangle or the wafer, and calculating the number of the triangle or the wafer where the light rays collide and the position of the collision point.
4. The atmospheric and three-dimensional surface-coupled radiation simulation method based on photon tracking according to claim 1, wherein the step (3) updates photon position, direction and energy information at the collision point and collects energy contribution of photons to the sensor: the energy carried by the photons will decay at the collision point:
Qout(λ)＝Qin(λ)·ω(λ)
wherein QoutAnd QinRespectively representing photon energy after collision and photon energy before collision, wherein omega is single scattering albedo, equivalent atmospheric albedo of the current position in the atmosphere, and the sum of reflectivity and transmissivity of the current collision geometric patch in a ground scene;
the new scattering direction distribution of the photons after collision in the atmosphere meets the distribution of the scattering phase function, and the new transmission direction is extracted and calculated by using the scattering phase function lookup table at the current position calculated in the step (1); the scattering of photons on the surface of a surface patch of the earth surface scene assumes that the photons satisfy the Lambert's law, and a new scattering direction is calculated according to the normal orientation information of the current surface patch;
calculating the energy contribution of the accumulated collision event to the set observation sensor by using a photon diffusion method at the position of each collision; if the path from the collision point position to the sensor is not shielded, calculating the probability of light scattering to the sensor direction according to the incident direction of photons and the position information of the collision point, and collecting energy according to the proportion:
Qcollect(Ω,λ)＝Qout(λ)·P(Ω,λ)·exp(-La·βatm)
p is the probability of light scattering towards the sensor, LaRepresenting the path length of the atmosphere passing from the point of impact to the sensor location, and the last exponential term in the equation describes the extinction of the atmosphere over this distance.
5. The atmospheric and three-dimensional earth surface coupled radiation simulation method based on photon tracking according to claim 1, characterized in that: and (4) calculating the next photon heading according to the photon position, and continuously tracking the photons until the termination condition is reached:
the photons leave the collision point in a new direction, continue to propagate along a straight line and repeat the collision process; in the whole tracking process, the photons carry position, direction and energy information, the position information of the photons is used for judging whether the photons are in the atmosphere or a ground surface scene currently, when the photons are in the atmosphere, a large gas phase function lookup table is used for randomly extracting a new direction of the photons in the subsequent transmission process, the photons are continuously tracked by using a path length calculation method and a collision processing method in the atmosphere, the information carried by the photons is updated, and the sensor contribution is collected; when the photons are in the earth surface scene, randomly extracting new directions of the photons according to the normal direction of a current collision patch and the Lambert law, continuously tracking the photons by using a path length calculation method and a collision processing method of the earth surface scene, updating information carried by the photons and collecting sensor contributions;
after a certain number of collisions, the energy carried by the photon becomes smaller, and once the photon energy is smaller than a threshold value, the tracking process of the photon is terminated in an energy conservation and unbiased manner by using a Russian wheel;
the photons have specific direction and position information when entering a ground surface scene from the bottom of the atmosphere, and the non-isotropy of sky diffused light after atmospheric scattering can be well simulated;
once the photons leave the upper boundary of the earth surface scene through collision and scattering in the earth surface scene, the photons can return to the atmosphere again to continue tracking by using the height information and the specific propagation direction of the photons, so that the cyclic reciprocating action of the photons at the boundary of the atmosphere and the earth surface scene between the two parts is well reproduced, and the complete coupling of the atmosphere and the earth surface scene under the atmosphere is realized.
6. The atmospheric and three-dimensional earth surface coupled radiation simulation method based on photon tracking according to claim 1, characterized in that: the step (5) of the simulation method is to emit new photons, repeat the tracking process, and obtain a convergent simulation result after the set number of photons is reached:
describing each uncertain physical process into a probability process according to the physical distribution in the simulation process of a photon by using the Monte Carlo thought, and determining a specific event occurring in the simulation by using a random number sampling method; each position described by using a random method strictly follows the physical distribution rule thereof;
through a large number of repeated simulation processes, namely a certain number of simulated photon numbers, each position described by using a random event is fully sampled according to the distribution, and finally, a final coupling result of the whole three-dimensional earth surface scene and the atmosphere, namely a radiance signal received in the observation direction of the atmospheric layer top sensor, can be obtained through the accumulated energy contribution to the observation sensor each time:
the meanings of the symbols in the above formula are as defined above.
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