Low, slow and small aircraft infrared radiation characteristic calculation method and storage medium

Abstract

The application discloses a low, slow and small aircraft infrared radiation characteristic calculation method and a storage medium. The calculation method adopts a modular design idea to realize low, slow and small aircraft infrared radiation brightness, projection area and infrared radiation intensity calculation functions. A simplified target geometric model is adopted to realize radiation component and target projection area calculation, a target environment radiation model is adopted to realize target self-radiation brightness and environment radiation brightness calculation, and a bidirectional reflection distribution function model is adopted to realize target reflection environment radiation component calculation. The low, slow and small aircraft infrared radiation characteristic calculation method is reasonable in modeling, reliable in calculation data, meets actual engineering design requirements, and can provide a theoretical basis and data support for index demonstration, project scheme design, imaging system simulation and the like of infrared thermal imaging equipment such as an infrared search and tracking system and an infrared reconnaissance system for low, slow and small aircrafts.

Description

Technical Field

[0001] This invention belongs to the field of target infrared radiation characteristic measurement and calculation technology, specifically relating to a method for calculating the infrared radiation characteristics of low-speed, small aircraft and a storage medium. Background Technology

[0002] In recent years, with the rapid development of unmanned aerial vehicle (UAV) technology, various types of UAVs have been widely used in local conflicts around the world, demonstrating a strong threat. In particular, multi-rotor aircraft, which currently dominate the market and are characterized by low altitude, slow speed, and small size—often using plastic fuselages—have small radar cross-sections, high low-altitude noise interference, and good stealth capabilities, making radar detection technology ineffective. Compared to passive radio detection technologies, which face challenges in timely acquisition or updating of radio frequency databases and communication protocols; acoustic detection technologies, which have short detection ranges and high false alarm rates in noisy environments; and visible light detection equipment, which cannot operate at night, infrared thermal imagers, with their day-and-night operation, passive imaging, and strong anti-interference capabilities, hold an advantage in low-altitude, slow-speed, and small aircraft countermeasure systems and play a crucial role.

[0003] Infrared thermal imagers generate signal images by receiving the difference in infrared radiation intensity between the target and the background. Therefore, the study of the infrared radiation characteristics of low-speed and small aircraft can provide a theoretical basis for project demonstration, imaging simulation, overall design and testing of infrared thermal imaging equipment such as infrared search and track systems and infrared reconnaissance systems.

[0004] Currently, domestic and international simulations of the infrared radiation characteristics of aircraft mainly focus on high-speed fighter jets, high-altitude attack aircraft, or missile targets. Their infrared radiation characteristics differ from those of low-speed, slow-moving, and small aircraft primarily in two aspects. First, due to their high speed, the fuselage skin emits strong radiation; second, because they employ turbofan, turbojet, or rocket engines, the infrared radiation intensity of their exhaust nozzles and plumes is high, making the influence of external environmental radiation negligible. Therefore, the infrared characteristic analysis of these targets mainly focuses on the skin, exhaust nozzle, and plume. Low-speed, slow-moving, and small aircraft, however, have low speed and low power, making their reflection of surrounding environmental radiation non-negligible. Currently, there are no domestic reports on theoretical simulations of the infrared radiation characteristics of such targets.

[0005] Chinese patent CN112215957A discloses a method for simulating the infrared characteristics of aerial targets. It optimizes the target surface with mesh using 3D model building software and achieves shape simulation through bottom-level drawing. It uses the VegaPrime platform to simulate the infrared radiation characteristics of high-altitude, high-speed aircraft with exhaust plumes.

[0006] Chinese patent CN114676379A discloses a method and apparatus for calculating the overall infrared radiation characteristics of a hypersonic cruise vehicle, comprising five parts: calculation of the temperature distribution of the vehicle body, calculation of the infrared radiation characteristics of the vehicle body, calculation of the flow field distribution of the tail flame, calculation of the infrared radiation characteristics of the tail flame, and calculation of the overall infrared radiation.

[0007] Chinese patent CN114219020A discloses a method for inverting spatial target parameters based on multi-angle sequence images. The method represents the spatial target using small planar units, establishes nodal equilibrium equations, solves for the surface temperature distribution of the target, and calculates the infrared radiation intensity of the spatial target using the inverse ray tracing method based on Planck's law.

[0008] The calculation methods described in the aforementioned patents all focus on high-speed, high-altitude, and high-power targets, neglecting the influence of environmental radiation reflected by low-altitude, slow-speed, and small aircraft. Low-altitude, slow-speed, and small aircraft fly at low altitudes, have low speeds, small sizes, and low power; their reflection of environmental radiation affects the simulation results of infrared systems or the signal-to-noise ratio of infrared detection equipment. Numerical calculation methods for the infrared radiation characteristics of aircraft generally require numerous specialized software programs, involve complex modeling, and involve a large amount of computation. Numerical simulation is particularly difficult when considering the reflected radiation from complex environments. Therefore, it is necessary to adopt engineering calculation methods to establish a calculation model for the infrared radiation characteristics of low-altitude, slow-speed, and small aircraft.

[0009] A study by Nicolette Fudala, Robert Short, Jennifer Hewitt, and others from the College of Photonics and Photonics at the University of Central Florida, titled "Comparison of Mid-wave and Long-wave Intensity Characteristics in Infrared Search and Tracking of Small Rotary-wing Unmanned Aerial Vehicles," explores this topic. [1] In this study, the infrared radiation intensity of the DJI Phantom 4 Pro drone in the 3μm-5μm band was measured, which can be used as a comparison object for the calculation method of this invention.

[0010] [1] Nicolette Fudala, Robert Short, Jennifer Hewitt, et al. Comparison of midwave versus longwave intensity signatures for infrared search and track of small rotorcraft unmanned aerial vehicles [J]. Optical Engineering, 2019, 58(4). Summary of the Invention

[0011] The purpose of this invention is to address the shortcomings of existing methods by proposing a method for calculating the infrared radiation characteristics of low-altitude, slow-moving, and small aircraft. This method solves the problem of accurately calculating the reflected environmental radiation intensity of low-altitude, slow-moving, and small aircraft in existing technologies, thereby providing a theoretical basis and data support for the technical specification demonstration, scheme design, and system testing of infrared thermal imaging detection equipment.

[0012] To achieve the above objectives, this invention includes three computational models: (1) target geometry model, (2) target / environment infrared radiation model, and (3) bidirectional reflectance distribution function model.

[0013] The target geometric model of the calculation method is a three-dimensional geometric structure that is approximated by a set of planar approximations of the target based on the aircraft's shape parameters.

[0014] The target / environment infrared radiation model of the calculation method includes two parts: the infrared radiation of the target itself and the infrared radiation reflected from the environment by the target.

[0015] The bidirectional reflection distribution function model of the calculation method is the ratio of the reflected radiance to the incident radiance within a small solid angle in the reflection direction. It is a function of five variables: wavelength, azimuth and zenith angle of the incident light, and azimuth and zenith angle of the reflected light.

[0016] Furthermore, preferably, a basic coordinate system based on the target is established according to the target's geometric model. The basic coordinate system is a right-handed Cartesian coordinate system, and the infrared thermal imager and the sun are determined from the origin using spherical coordinates.

[0017] Furthermore, preferably, according to the target / environment infrared radiation model, the target's own infrared radiation is mainly radiation from the fuselage skin, and the reflected environmental infrared radiation includes the target fuselage's reflection of solar radiation, ground radiation, ground-reflected solar radiation, ground-reflected sky radiation, and sky radiation. In special cases, there may be cloud-reflected radiation.

[0018] Furthermore, preferably, the commonly used bidirectional reflection distribution function model is the CooK-Torrance model based on micro-surface elements, which is divided into a specular reflection part and a diffuse reflection part, and the diffuse reflection part can be approximated as an ideal Lambertian diffuse reflection.

[0019] This invention proposes a method for calculating the infrared radiation characteristics of low-altitude, slow-moving, small aircraft, including a target geometric model, a target / environment infrared radiation model, and a two-way reflectance distribution function model. The calculation process is as follows: Figure 1As shown: First, a geometric model and basic coordinate system of the target are constructed using the target prototype parameters. Second, a mechanistic analysis of the radiation characteristics of the target / environment is conducted, and the environmental radiation factors affecting the infrared radiation characteristics of the target are calculated. Finally, based on the azimuth and zenith angles of the observer, the sun, the earth's surface, and the sky relative to the target, combined with the two-way reflectance distribution function model, the infrared radiance and projected area of ​​the target / environment are comprehensively solved. The target radiance intensity is obtained by multiplying the calculated target / environment radiance by the projected area.

[0020] This invention proposes a method for calculating the infrared radiation characteristics of low-altitude, slow-moving, small aircraft. The target geometric model is described using a set of planar approximations, and is constructed as follows: Figure 2 The simplified model of the square pyramid shown is assumed. The basic coordinate system is a right-handed Cartesian coordinate system with the Z-axis pointing vertically upwards. Its center point O is located at the center of the top surface BCDE of the square pyramid target ABCDE. The X-axis points to the front of the target, and the Y-axis points to the left side of the target. The infrared thermal imager and the sun are determined from the origin using spherical coordinates. Vector i is the unit normal vector of plane ABC (x...). i ,y i ,z i Its direction is outward. The zenith angle and azimuth angle of the infrared thermal imager in the target coordinate system are... The zenith angle and azimuth angle of the sun in the target coordinate system are: Based on the coordinate system transformation method, the unit vector of the infrared thermal imager and the sun in the target coordinate system is (x... t ,y t ,z t ), (x s ,y s ,z s The angle α between the unit normal vector i of plane ABC and the line-of-sight direction of the infrared thermal imager has the following relationship:

[0021]

[0022] The projected area of ​​the target relative to the direction of the infrared thermal imager is equal to the surface area multiplied by the cosine of the angle between the surface normal and the direction vector of the thermal imager:

[0023] S t =S ΔABC ·cosα (2)

[0024] When cosα is less than zero, there is occlusion, and its projected area is zero.

[0025] This invention proposes a method for calculating the infrared radiation characteristics of low-altitude, slow-moving, small aircraft, including a target / environment infrared radiation model such as... Figure 3As shown. When a drone is flying in the air, the infrared radiation received by the infrared thermal imager from the target includes two parts: the target's own infrared radiation and the target's reflected radiation. The target's own radiation is mainly the radiation from the fuselage, while the reflected radiation includes the target's reflection of solar radiation, ground radiation, solar radiation reflected from the ground, sky radiation reflected from the ground, and sky radiation. In special cases, cloud reflection radiation may also exist. The target's radiation brightness is the sum of all components:

[0026] L total =L sel +L rs +L re +L rsky (3)

[0027] L total L represents the total spectral radiance of the target. sel L represents the target's own radiance. rs The brightness of solar radiation reflected from the target; L re The target reflects surface radiance, including surface radiance, surface reflected solar radiance, and surface reflected sky radiance; L rsky The target reflects the brightness of the sky's radiation.

[0028] This invention proposes a method for calculating the infrared radiation characteristics of low-altitude, slow-moving, small aircraft, including a two-way reflection distribution function model, as follows: Figure 4 As shown, its expression is:

[0029]

[0030] In the formula, It is the reflected radiance. It is the incident irradiance. θ i and Let θ be the zenith angle and azimuth angle of incidence; r and λ represents the zenith angle and azimuth angle of the reflected light; λ is the wavelength. The commonly used bidirectional reflectance distribution function is the CooK-Torrance model based on micro-surface elements, which consists of specular reflection and diffuse reflection components. The diffuse reflection component can be approximated as ideal Lambertian diffuse reflection. The expression for the CooK-Torrance model is as follows:

[0031]

[0032] In the formula, k s k is the specular reflection coefficient. d denoted as D, where D is the diffuse reflectance coefficient; G is the normal distribution function; and F is the shading function.

[0033] The expression for the D function is:

[0034]

[0035] The expression for the G function is:

[0036]

[0037] In the formula, σ is the surface roughness of the target; α is the angle between the normal of the target macroscopic plane and the normal of the reflected micro-surface element; and β is the angle between the normal of the micro-surface element and the incident light and the reflected light. α represents the difference between the incident azimuth and the reflected azimuth. In the target's fundamental coordinate system, α and β can be expressed as:

[0038]

[0039]

[0040] The Fresnel reflectance of incident natural light, obtained from Fresnel's formula, is:

[0041]

[0042] In the formula, r s The reflectance of the radiation component perpendicular to the incident surface; r p Let be the reflectance of the radiation component parallel to the incident surface; its expression is as follows:

[0043]

[0044]

[0045] ε is the complex refractive index of the target surface material.

[0046] ε=n+ik (13)

[0047] n is the real part of the complex refractive index; k is the imaginary part of the complex refractive index.

[0048] The total radiance L reflected from the target in the direction of observation ref The expression is:

[0049]

[0050] This invention proposes a method for calculating the infrared radiation characteristics of low-altitude, slow-moving, and small aircraft. The target / environment infrared radiation model includes an aircraft fuselage skin temperature Tr, and the calculation formula is as follows:

[0051]

[0052] In the formula: T amb γ is the ambient air temperature; γ is the specific heat rate of air, which is 1.4 when there is no significant heating; k is the coefficient of restitution, which is generally taken as 0.82 for laminar flow; Ma is the Mach number of the aircraft.

[0053] Radiance L of aircraft skin at any wavelength sel The formula is:

[0054]

[0055] In the formula: λ1λ2 represents the response band range of the infrared detector; ε λ c1 and c2 are the emissivity of the fuselage surface material; c1 and c2 are the first and second radiation constants, respectively.

[0056] This invention proposes a method for calculating the infrared radiation characteristics of low-altitude, slow-moving, small aircraft. The solar radiation illuminance expression in the target / environment infrared radiation model is as follows:

[0057]

[0058] In the formula: A(U * ,β) is the absorption coefficient, E o ρ is the solar constant, β is the solar altitude angle, and ρ is the solar constant. g ρ represents the regional surface reflectance. o Rayleigh atmospheric reflectance;

[0059] This invention proposes a method for calculating the infrared radiation characteristics of low-altitude, slow-moving, small aircraft. The expression for sky irradiance in the target / environment infrared radiation model is as follows:

[0060]

[0061] In the formula: c1 and c2 are the first and second radiation constants; θ is the solar zenith angle; λ is the wavelength; Ta is the sky temperature; A, Bk, and Ck are empirical constants, usually A = 2.0, Bk = 0.61, and Ck = 0.05; Ea is the near-surface water vapor pressure; CAR is a coefficient related to cloud type; and CC is the cloud cover rate.

[0062] This invention proposes a method for calculating the infrared radiation characteristics of low-altitude, slow-moving, small aircraft. The surface irradiance expression in the target / environment infrared radiation model is as follows:

[0063]

[0064] In the formula: λ is the wavelength, ρ e c1 and c2 are the ground reflectivity; c1 and c2 are the first and second radiation constants, respectively; T e This refers to the Earth's surface temperature.

[0065] A computer-readable storage medium having a computer program stored thereon, the computer program being executed by a processor to implement the steps of a method for calculating the infrared radiation characteristics of a low-speed, small aircraft as described in this invention.

[0066] Beneficial effects of the present invention

[0067] 1) The calculation method proposed in this invention introduces a two-way reflection distribution function, which can effectively calculate the diffuse reflection and specular reflection radiation components of solar radiation, surface radiation, and sky radiation of the aircraft.

[0068] 2) The calculation method proposed in this invention includes a simplified design of the target geometric model, which constructs the shape of the aircraft using a set of planes, thereby reducing the amount of calculation.

[0069] 3) The calculation method proposed in this invention includes a target environment radiation model that uses simplified mathematical formulas to realize infrared radiation brightness transmission, efficiently simulates radiation transmission phenomena, and reduces computational complexity.

[0070] 4) The calculation method proposed in this invention adopts a dual coordinate system: the aircraft target adopts a Cartesian rectangular coordinate system, and the infrared thermal imager and environmental radiation source adopt a spherical coordinate system, which effectively solves the target projection problem and the calculation problem of radiation components of various radiation sources.

[0071] In summary, commonly used numerical calculation methods typically involve first establishing a geometric model of the target using 3D software, then meshing the model using software such as ICEM, Gambit, or GridPro, and finally using software like Fluent to perform numerical simulations based on fluid dynamics laws to obtain the target's temperature field distribution. Finally, the reverse Monte Carlo method is used for line tracing, tracking, and statistical analysis to obtain the target's infrared radiation intensity data. These methods require numerous specialized software programs, involve complex modeling, and are computationally intensive, especially when considering the target's reflection radiation from complex environments, making numerical simulation difficult. The calculation method of this invention features a simple geometric structure, efficient radiation transfer process, and concise calculation of the bidirectional reflection distribution function. The program developed using this method is fast and can quickly and efficiently calculate the projected area and infrared radiation brightness of the target relative to the scene and infrared thermal imager in various relative directions and positions. Attached Figure Description

[0072] Figure 1 : Flowchart of the calculation process of this invention.

[0073] Figure 2 The target geometric model of this invention.

[0074] Figure 3 The target / environmental radiation model of this invention.

[0075] Figure 4 This invention relates to a bidirectional reflection distribution function model.

[0076] Figure 5 Calculation results of mid-wave infrared radiation characteristics of a low-speed, small aircraft in Embodiment 3 of this invention.

[0077] Figure 6 Calculation results of long-wave infrared radiation characteristics of low-speed small aircraft in Embodiment 4 of the present invention.

[0078] Figure 7 Image of mid-wave infrared test of low-speed small aircraft in Embodiment 3 of this invention.

[0079] Figure 8 : Long-wave infrared test image of a low-speed, small aircraft in Embodiment 4 of this invention. Detailed Implementation

[0080] The present invention will be further described in detail below with reference to embodiments.

[0081] Those skilled in the art will understand that the following embodiments are for illustrative purposes only and should not be construed as limiting the scope of the invention. Where specific techniques or conditions are not specified in the embodiments, they should be performed in accordance with the techniques or conditions described in the literature in the field or according to the product manual.

[0082] This invention discloses a method for calculating the infrared radiation characteristics of low-altitude, slow-moving, small aircraft, comprising a target geometric model, a target / environment infrared radiation model, and a two-way reflectance distribution function model. The calculation process is as follows: Figure 1 As shown: First, a geometric model and basic coordinate system of the target are constructed using the target prototype parameters. Second, a mechanistic analysis of the radiation characteristics of the target / environment is conducted, and the environmental radiation factors affecting the infrared radiation characteristics of the target are calculated. Finally, based on the azimuth and zenith angles of the observer, the sun, the earth's surface, and the sky relative to the target, combined with the two-way reflectance distribution function model, the infrared radiance and projected area of ​​the target / environment are comprehensively solved. The target radiance intensity is obtained by multiplying the calculated target / environment radiance by the projected area.

[0083] Example 1

[0084] like Figure 1 As shown, a method for calculating the infrared radiation characteristics of a low-speed small aircraft includes the following three calculation models: (1) target geometry model, (2) target / environment infrared radiation model, and (3) bidirectional reflection distribution function model.

[0085] The target geometric model of the calculation method is a three-dimensional geometric structure that is approximated by a set of planar approximations of the target based on the aircraft's shape parameters.

[0086] The target / environment infrared radiation model of the calculation method includes two parts: the infrared radiation of the target itself and the infrared radiation reflected from the environment by the target.

[0087] The bidirectional reflection distribution function model of the calculation method is the ratio of the reflected radiance to the incident radiance within a small solid angle in the reflection direction. It is a function of five variables: wavelength, azimuth and zenith angle of the incident light, and azimuth and zenith angle of the reflected light.

[0088] Example 2

[0089] Based on Example 1, it also includes:

[0090] Based on the target's geometric model, a basic coordinate system is established. The basic coordinate system is a right-handed Cartesian coordinate system, and the infrared thermal imager and the sun are determined from the origin using spherical coordinates.

[0091] According to the target / environment infrared radiation model, the target's own infrared radiation is mainly radiation from the fuselage skin. The reflected infrared radiation from the environment includes the target fuselage reflecting solar radiation, ground radiation, ground-reflected solar radiation, ground-reflected sky radiation, and sky radiation. In special cases, there is also radiation reflected by clouds.

[0092] The commonly used bidirectional reflectance distribution function model is the CooK-Torrance model based on micro-surface elements, which is divided into specular reflection and diffuse reflection components. The diffuse reflection component can be approximated as an ideal Lambertian diffuse reflection.

[0093] Example 3

[0094] like Figure 1 As shown, a method for calculating the infrared radiation characteristics of low-altitude, slow-moving, small aircraft includes a target geometric model, a target / environment infrared radiation model, and a two-way reflectance distribution function model. The calculation process is as follows: Figure 1 As shown: First, a geometric model and basic coordinate system of the target are constructed using the target prototype parameters. Second, a mechanistic analysis of the radiation characteristics of the target / environment is conducted, and the environmental radiation factors affecting the infrared radiation characteristics of the target are calculated. Finally, based on the azimuth and zenith angles of the observer, the sun, the earth's surface, and the sky relative to the target, combined with the two-way reflectance distribution function model, the infrared radiance and projected area of ​​the target / environment are comprehensively solved. The target radiance intensity is obtained by multiplying the calculated target / environment radiance by the projected area.

[0095] A program for calculating the infrared radiation characteristics of low-speed, small aircraft was developed according to the calculation method of this invention. The program interface is as follows: Figure 5 As shown, the system consists of a model parameter setting area and a calculation result display area. The model parameter settings mainly use data such as target shape parameters, relative position parameters between the target and the sun, and between the target and the thermal imager, as well as climate conditions and the thermal imager's response bands as input conditions. The calculation results include the target's fuselage radiance on its five end faces; the target's diffuse and specular reflections of solar, surface, and sky radiance; the projected area along the line of sight; and the total target radiance.

[0096] Input the following parameters for a typical low-altitude, slow-moving, small drone: DJI Phantom 4 Pro drone, dimensions: length 247mm, width 247mm, height 135mm; position parameters: thermal imager observation zenith angle 90°, thermal imager azimuth angle 0°, solar zenith angle 50°, solar azimuth angle 0°; aircraft status: flight altitude 2m, flight speed 0, surface emissivity 0.9; weather conditions: atmospheric transmittance 0.98 (close-range), ground temperature 303.5K; thermal imager response bands: initial band 3μm, termination band 5μm. The program calculation results are as follows: Figure 5 As shown, the thermal imager observed a total radiance of 2.476 W / sr / m² for the aircraft. 2 The total projected area is 0.017m². 2 The total radiation intensity is 0.041 W / sr.

[0097] Under the above parameters, the Institute of Photonics and Photonics at the University of Central Florida measured the infrared radiation intensity of a DJI Phantom 4 Pro drone in the 3μm-5μm band to be 0.0415 W / sr (as described in [1] in the background art). The difference between this result and the calculation result obtained by the method proposed in this invention is 0.0005, and the difference percentage is 1.2%. The mid-wave infrared test image of the drone is shown below. Figure 7 As shown.

[0098] Example 4

[0099] A method for calculating the infrared radiation characteristics of low-altitude, slow-moving, small aircraft includes a target geometric model, a target / environment infrared radiation model, and a two-way reflectance distribution function model. The calculation process is as follows: Figure 1 As shown: First, a geometric model and basic coordinate system of the target are constructed using the target prototype parameters. Second, a mechanistic analysis of the radiation characteristics of the target / environment is conducted, and the environmental radiation factors affecting the infrared radiation characteristics of the target are calculated. Finally, based on the azimuth and zenith angles of the observer, the sun, the earth's surface, and the sky relative to the target, combined with the two-way reflectance distribution function model, the infrared radiance and projected area of ​​the target / environment are comprehensively solved. The target radiance intensity is obtained by multiplying the calculated target / environment radiance by the projected area.

[0100] A program for calculating the infrared radiation characteristics of low-altitude, slow-moving, and small aircraft was developed according to the calculation method of this invention. The program interface is shown below. Figure 6 As shown, the system consists of a model parameter setting area and a calculation result display area. The model parameter settings mainly use data such as target shape parameters, relative position parameters between the target and the sun, and between the target and the thermal imager, as well as climate conditions and the thermal imager's response bands as input conditions. The calculation results include the target's fuselage radiance on its five end faces; the target's diffuse and specular reflections of solar, surface, and sky radiance; the projected area along the line of sight; and the total target radiance.

[0101] Input the following external parameters for a typical low-altitude, slow-moving, small drone: DJI Phantom 4 Pro drone, dimensions: length 247mm, width 247mm, height 135mm; position parameters: thermal imager observation zenith angle 96.5°, thermal imager azimuth angle 0°, solar zenith angle 50°, solar azimuth angle 0°; drone status: flight altitude 2m, flight speed 0, surface emissivity 0.9; weather conditions: atmospheric transmittance 0.995 (close-range), ground temperature 303.5K; thermal imager response bands: initial band 8μm, termination band 14μm. The program calculation results are as follows: Figure 6 As shown, the thermal imager observed a total radiance of 64.449 W / sr / m² for the aircraft. 2 The total projected area is 0.022m². 2 The total radiation intensity is 1.401 W / sr.

[0102] Under the aforementioned parameters, the Institute of Photonics and Photonics at the University of Central Florida measured the infrared radiation intensity of a DJI Phantom 4 Pro drone in the 8μm-14μm band to be 1.4194 W / sr (as described in the background art). [1] The difference between the calculation result obtained and the calculation result obtained by the method proposed in this invention is 0.0184, and the percentage difference is 1.3%. The long-wave infrared test image of the UAV is shown below. Figure 8 As shown.

[0103] Finally, it should be noted that the above embodiments are only used to illustrate and not limit the technical solutions of the present invention. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the present invention without departing from the spirit and scope of the present invention. Any modifications or partial substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for calculating infrared radiation characteristics of a low, slow and small aircraft, characterized in that, The method comprises three models of a target geometric model, a target / environment infrared radiation model and a bidirectional reflectance distribution function model; the target geometric model is a three-dimensional geometric structure of the target which is described by a set of planes according to aircraft shape parameters; the target / environment infrared radiation model comprises two parts of infrared radiation of the target itself and infrared radiation of the target reflecting the environment; the bidirectional reflectance distribution function model is a ratio of radiation brightness of reflection within a small solid angle of a reflection direction to incident radiation irradiance; The method comprises the following steps: Step 1, constructing a target geometric model and a basic coordinate system by using target prototype parameters; Step 2, carrying out mechanism analysis on radiation characteristics of the target / environment, and respectively calculating environmental radiation factors affecting infrared radiation characteristics of the target; Step 3, according to azimuth angles and zenith angles of the observer, the sun, the ground and the sky relative to the target, and combining the bidirectional reflectance distribution function model, comprehensively solving infrared radiation brightness and projected area of the target / environment; Step 4, obtaining target radiation intensity by multiplying the calculated target / environment radiation brightness by the projected area; The infrared radiation of the target itself is fuselage skin radiation, and the infrared radiation of the target reflecting the environment comprises reflection of the target fuselage to the sun radiation, the ground radiation, the ground reflected sun radiation, the ground reflected sky radiation and the sky radiation; The basic coordinate system is a right-hand Cartesian rectangular coordinate system with a Z-axis vertically upward, and the center O point is located at the center of the top surface BCDE of the quadrangular pyramid target ABCDE, the X-axis points to the front end of the target, and the Y-axis points to the left side of the target; the infrared thermal imager and the sun are determined by spherical coordinates from the origin; a vector i is a unit normal vector of the plane ABC (x i ,y i ,z i ), and the direction is outward; the zenith angle and the azimuth angle of the infrared thermal imager in the target coordinate system are , and the zenith angle and the azimuth angle of the sun in the target coordinate system are ; according to a coordinate system conversion method, unit vectors of the infrared thermal imager and the sun in the target coordinate system are (x t ,y t ,z t ) and (x s ,y s ,z s ); and an included angle between the unit normal vector i of the plane ABC and the line-of-sight direction of the infrared thermal imager exists the following relationship: (1)， The projected area of the target relative to the direction of the infrared thermal imager is equal to the surface area multiplied by the cosine of the included angle between the face normal and the direction vector of the thermal imager: (2)， When If less than zero, then there is an occlusion, and the projected area is zero. The target radiation brightness is the sum of various components: （3）， wherein: is the total spectral radiance of the target, is the self-radiance of the target, is the solar-radiance of the target, is the ground-radiance of the target, including the ground-radiance, the ground-solar-radiance, and the ground-sky-radiance, is the sky-radiance of the target.

2. The method of claim 1, wherein, In step 3: The expression of the bidirectional reflectance distribution function model is: （4）， where: is the reflected radiance, is the incident irradiance, and are the incident zenith and azimuth angles, and are the reflected zenith and azimuth angles, is the wavelength.

3. The low, slow and small aircraft infrared radiation characteristic calculation method according to claim 2, characterized in that: The commonly used bidirectional reflectance distribution function is the CooK-Torrance model based on a micro facet, and the expression is as follows: （5）， wherein: is the specular reflection coefficient, is the diffuse reflection coefficient, D is the normal distribution function, G is the masking function, and F is the Fresnel reflection ratio; The expression of the D function is: （6）， The expression of the G function is: （7）， wherein: is the target surface roughness; is the target macro plane normal and the reflected micro facet normal angle, is the micro facet normal and the incident light, reflected light angle; is the difference between the incident azimuthal angle and the reflected azimuthal angle; In the target basic coordinate system With may be expressed as: （8）， （9）， The Fresnel reflection ratio of natural light incidence is obtained from the Fresnel formula, and the expression is as follows: （10）， wherein: Rvis the reflectance of the radiation component normal to the plane of incidence, R / / is the reflectance of the radiation component parallel to the plane of incidence; The expression is as follows: （11）， （12）， Refractive index of target surface material; （13）， n is the real part of the complex refractive index; k is the imaginary part of the complex refractive index; The total radiance L of the target reflected to the observation direction ref The expression is: （14）。 4. The low, slow and small aircraft infrared radiation characteristic calculation method according to claim 1, characterized in that: The fuselage skin radiation temperature is Tr, and the calculation formula is as follows: （15）， where: T is the ambient air temperature; Cp is the air specific heat ratio; k is the recovery coefficient; Ma is the aircraft Mach number; The skin is bright in the radiation of any wavelength The formula is: （16）， wherein: is the emissivity of the body surface material, is the emissivity of the body surface material, are the first and second radiation constants.

5. The low, slow and small aircraft infrared radiation characteristic calculation method according to any one of claims 1-4, characterized in that: The expression of the sun radiation irradiance in the target / environment infrared radiation model is as follows: （17）， where: is the absorption coefficient, is the solar constant, is the solar elevation angle, is the regional surface albedo; is the Rayleigh atmospheric albedo; The expression of the sky radiation irradiance in the target / environment infrared radiation model is as follows: （18）， wherein: is the first, second radiation constant; is the solar zenith angle; is the wavelength; Ta is the sky temperature; A, Bk and Ck are empirical constants, and are usually taken as A = 2.0, Bk = 0.61 and Ck = 0.05; Ea is the near-ground water vapor pressure; CAR is a coefficient related to the type of cloud; CC is the cloud coverage; The expression of the ground radiation irradiance in the target / environment infrared radiation model is as follows: （19）， wherein: is the wavelength, is the ground reflectivity; are the first and second radiation constants; is the ground temperature.

6. A computer-readable storage medium having stored thereon a computer program, characterized in that, The computer program is executed by a processor to realize the steps of the low, slow and small aircraft infrared radiation characteristic calculation method according to any one of claims 1-5.

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