A method for predicting the maximum underwater measurement depth of airborne LiDAR

Abstract

The present invention discloses a method for predicting the maximum underwater measurement depth of an airborne LiDAR. The method is based on the empirical relationship between the maximum underwater measurement depth of a traditional airborne LiDAR and the system attenuation coefficient, improves and couples related parameters such as the airborne LiDAR sensor system parameters, the airborne platform flight parameters, and the transmission path medium environment parameters that affect the maximum underwater measurement depth of the airborne LiDAR, and constructs a more systematic and complete prediction model for the maximum underwater measurement depth of the airborne LiDAR. The maximum underwater measurement depth of the LiDAR is predicted using the constructed model and parameter input. The invention is simple to operate for predicting the maximum underwater measurement depth of the airborne LiDAR. Through actual field verification, the accuracy is about 0.7 meters, the reliability is high, and the cost waste caused by unknown detectability is greatly saved.

Description

Technical Field

[0001] The present invention relates to the field of underwater detection by laser radar, and in particular to a prediction method for predicting the maximum underwater measurement depth of an airborne LiDAR. Technical Background

[0002] In recent years, airborne LiDAR has gradually developed into a major active underwater detection technology, which performs underwater detection by emitting blue-green band lasers that can penetrate water bodies. Airborne LiDAR underwater detection technology can quickly and accurately obtain nearshore water depth and underwater terrain, especially for shallow seas, islands and reefs and other areas that cannot be reached by ships. It has significant advantages. It has the characteristics of high precision, high resolution, flexibility, high speed and high efficiency, so the maximum underwater detectable depth of airborne LiDAR is a very important parameter.

[0003] For airborne LiDAR underwater detection technology, the differences in atmospheric parameters in different weather conditions, the differences in sea surface waves in different climates, the differences in water quality in different sea areas and the differences in seabed quality can all cause the laser to attenuate to varying degrees during transmission. Therefore, the detectable depth of airborne LiDAR in different sea areas is unknown.

[0004] The maximum underwater measurement depth value of the airborne LiDAR predicted by the method of the present invention can be used to predetermine the detectable range of the radar, and provide route design and detectable range reference for the flight of the airborne LiDAR aircraft in combination with the mission requirements, saving the cost waste caused by unknown detectability. At the same time, it is conducive to the parameter selection when integrating related equipment at the beginning of the design, and provides a basis for the design of radar system parameters. Summary of the invention

[0005] The present invention proposes a method for predicting the maximum underwater measurement depth of airborne LiDAR. Specifically, based on the empirical relationship between the maximum underwater measurement depth of traditional airborne LiDAR and the system attenuation coefficient, the maximum measurement depth is coupled and improved with parameters such as LiDAR sensor system parameters, airborne platform flight parameters, transmission path medium parameters, and transmission environment parameters, and a more systematic and complete prediction model for the maximum underwater measurement depth of airborne LiDAR is constructed to predict the maximum underwater measurement depth of airborne LiDAR.

[0006] 1. Including the coupling and improvement of related parameters of LiDAR sensor system. Among them, the algorithm for calculating the effective area of ​​the laser receiver using the receiving aperture is coupled; the algorithm for calculating the background noise power using the effective receiving area is improved; and the algorithm for calculating the effective receiving power using the receiving field of view angle is innovatively established.

[0007] 2. Including the coupling of laser transmission marine environment attenuation and empirical relations. The algorithm for calculating the beam attenuation coefficient and single scattering albedo using the scattering coefficient and absorption coefficient is coupled to the algorithm for underwater transmission attenuation. Further, the relationship between the obtained beam attenuation coefficient and single scattering albedo and the effective attenuation coefficient of the system is used to obtain the algorithm for the effective attenuation coefficient β of the system.

[0008] 3. Including the coupling of attenuation and empirical relationship caused by laser pulses during atmospheric transmission. Innovative algorithms are added to calculate the first atmospheric attenuation before the incident sea surface and the second atmospheric attenuation after the exit of the sea surface. The model is based on the Beer-Lambert law, and integrates the algorithm for calculating the atmospheric attenuation coefficient using atmospheric visibility; the algorithm for calculating the first atmospheric attenuation before the incident sea surface using peak power, aircraft flight altitude and atmospheric attenuation coefficient; the algorithm for calculating the actual atmospheric transmission distance using the scanning zenith angle and aircraft flight altitude; and the algorithm for calculating the second atmospheric attenuation after the exit of the sea surface using the actual atmospheric transmission distance, atmospheric attenuation coefficient and the sensitivity of the lidar detector.

[0009] 4. Including the coupling of laser transmission sea surface reflection attenuation. Innovative and added algorithms to calculate the power attenuation caused by the sea surface when the laser enters the water and the power attenuation caused by the sea surface when the laser leaves the water. Based on Snell's law, Fresnel's formula and the law of refraction, the algorithm uses the scanning zenith angle to inversely calculate the refraction angle, and then calculates the water surface reflectivity. The algorithm further uses the water surface reflectivity, laser peak power, aircraft flight altitude and atmospheric attenuation coefficient to calculate the power attenuation caused by the sea surface when the laser enters the water; and further uses the water surface reflectivity and the sensitivity of the laser radar detector to calculate the power attenuation caused by the sea surface when the laser leaves the water.

[0010] 5. Integration of the prediction model for the maximum underwater measurement depth of airborne LiDAR. According to the invention contents 1-4, the coupling of the LiDAR sensor system related parameters, laser transmission ocean environment attenuation, laser transmission atmospheric attenuation, and laser transmission sea surface reflection attenuation in the traditional empirical relationship formula is improved. A complete prediction model for the maximum underwater measurement depth of airborne LiDAR is constructed.

[0011] The beneficial effect of the present invention is that after obtaining the predicted value of the maximum underwater detectable depth of the airborne laser radar, the detectable range can be pre-determined, providing route design and detectable range reference for the airborne laser radar aircraft flight, saving the cost waste caused by unknown detectability. At the same time, it can provide prior data for the unknown airborne laser radar system to measure the water area, providing a basis for the design of radar system parameters. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 It is an overall structural framework diagram of the real-time prediction method according to an embodiment of the present invention.

[0013] Figure 2 It is a parameter diagram that needs to be input to the real-time prediction method of the embodiment of the present invention.

[0014] Figure 3 This is an output display diagram of the real-time prediction method according to an embodiment of the present invention.

[0015] Figure 4 It is an image output reference image under the influence of real-time combination parameters in an embodiment of the present invention. DETAILED DESCRIPTION

[0016] In order to have a clearer understanding of the technical features, purposes and effects of the present invention, the specific implementation methods of the present invention are now described with reference to the accompanying drawings. The system parameters of the laser radar system specifications adopt the parameters of the CZMIL system of Optech. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments performed by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0017] In specific implementation, the technical solution of the present invention can be implemented by computer programming and automatic operation. The present invention proposes a method for predicting the maximum underwater measurement depth of airborne LiDAR, specifically as follows Figure 1 The overall structural framework of the real-time prediction method is shown in the figure, which includes three parts: model establishment, parameter input and output display.

[0018] The specific workflow is:

[0019] Step 1: Input parameters

[0020] The real-time prediction method requires input parameters such as Figure 2 As shown in the figure, the parameters that need to be input include LiDAR system parameters: laser peak transmission power, receiving aperture, laser scanning zenith angle, receiving system efficiency, spectral receiving bandwidth, receiving field angle, receiver sensitivity; atmospheric and sea surface environmental parameters: atmospheric visibility; water quality and seabed environmental parameters: bottom reflectivity, water body scattering coefficient, water body absorption coefficient; aircraft flight parameters: aircraft flight altitude. Parameters can be fully input autonomously, some parameters can be input internally, and the relationship in the algorithm can be used to output the relative changes of any two parameters to the predicted value of the maximum underwater detectable depth for design reference. After the system-related parameters are determined in advance, they are entered into the program in advance. The environmental-related parameters are measured and input on the spot through absorption attenuation measuring instruments, atmospheric visibility measuring instruments, etc.

[0021] Step 2: Coupling and improvement of the airborne LiDAR underwater maximum measurement depth prediction model.

[0022] In the embodiment, the relationship between the maximum depth measurement of a conventional laser radar and the system attenuation coefficient is as follows:

[0023]

[0024] L max is the maximum measurement depth of the system, β is the effective attenuation coefficient of the system, P A is the effective received power of the system, P B is the background noise power, P T is the peak power or maximum emission power of the green laser, ρ represents the bottom reflectivity, A r is the effective area of ​​the laser receiver, η is the efficiency of the receiving system, and H is the height of the aircraft from the sea surface when flying. This algorithm only considers the attenuation of simple water bodies through the effective attenuation coefficient β of the system. Theoretically, this may not be correct, but it is sufficient to describe the system performance.

[0025] Since the attenuation of the atmosphere and sea surface is not considered, in order to make further accurate calculations, the influence of factors such as the atmosphere and sea surface should be taken into account, and the relationship between the maximum sounding depth and system parameters and environmental impact should be integrated and improved.

[0026] Step 3: Integrate the LiDAR system parameter algorithm with the receiving aperture D, which is easier to obtain directly. r .

[0027] For the effective area of ​​the receiving system, it is directly calculated from the input receiving aperture, and

[0028]

[0029] Where D r For receiving caliber.

[0030] Then it is brought into the main model for further calculation.

[0031] Step 4: Fusion background noise power P in the main model in algorithm improvement B The algorithm:

[0032] For the calculation of sunlight background noise power, the background noise power P can be calculated by the following formula: B , and its calculation formula is:

[0033] P B =I s A r D s Ω (3)

[0034] Where: A r is the effective area of ​​the receiving system, calculated from the receiving aperture. sis the background irradiance, and its value in light haze weather (under normal weather conditions) is:

[0035]

[0036] Ω is the receiving solid angle, and its value is:

[0037]

[0038] Where θ is the receiving field of view, D s Spectral receiving bandwidth. The background noise power P can be calculated from the input parameters B , and finally the main model is integrated for further calculation.

[0039] Step 5: Fusion and improvement of underwater transmission attenuation in algorithm improvement

[0040] Using the relationship between the beam attenuation coefficient c(λ) and the effective attenuation coefficient β of the system, Chen Wenge et al. established the following relationship using the Monte Carlo method:

[0041] β=c(λ)(1-0.832w 0 ) (6)

[0042] Where c(λ) represents the beam attenuation coefficient, w 0 represents the single scattering albedo, which is the ratio of the scattering coefficient to the beam attenuation coefficient. 0 The calculation formula is as follows:

[0043]

[0044] Among them, a(λ) is the absorption coefficient and b(λ) is the scattering coefficient.

[0045] Therefore, for the underwater attenuation part, the beam attenuation coefficient c(λ) and the single scattering albedo w are determined by inputting the scattering coefficient and absorption coefficient of the water body. 0 , and then the effective attenuation coefficient β of the system is obtained. Finally, further calculations can be made through the relationship between β and the maximum sounding depth and the attenuation coefficient in the main model.

[0046] Step 6: Integration of atmospheric transmission attenuation in algorithm improvement

[0047] Since the attenuation of laser energy is closely related to the wavelength of the incident laser, its attenuation law follows the Beer-Lambert law:

[0048] I x (λ)=I 0 (λ)exp[-c 气 (λ)x] (8)

[0049] In the formula, I 0 (λ) is the initial laser irradiance or laser power before transmission, I x (λ) is the laser irradiance after transmitting x distance in the medium, c 气 (λ) is the atmospheric attenuation coefficient.

[0050] If the Beer-Lambert law is expressed in the form of power:

[0051] P x =P T exp[-c 气 (λ)x] (9)

[0052] P T is the peak power, and Px is the laser power after transmission x distance. The following is a further understanding of the Beer-Lambert law:

[0053]

[0054] P T is the peak power, H is the aircraft flight altitude, F is the water surface reflectivity, P 最小灵敏度 is the minimum power that the laser radar can receive, c 气 (λ) is the atmospheric attenuation coefficient, P 射入水面前 is the laser power when it reaches the sea surface after being attenuated by the atmosphere, P 射出水面后 It is the power of the laser when it is just emitted from the water surface after being attenuated by the atmosphere and by the water body and reflected from the seabed.

[0055] Where P 射出水面后 for:

[0056] P 射出水面后 =P 射出水面前 (1-F) (11)

[0057] The first atmospheric attenuation of the laser before entering the water can be directly calculated using the Beer-Lambert law:

[0058] P 大气衰减1 =P T (1-exp[-c 气 (λ)H]) (12)

[0059] It can be seen from formula 10 that when the atmospheric attenuation coefficient is constant and the aircraft flight altitude is constant, the left side of the formula is a constant, that is, it follows the linear function law. Therefore, according to formulas 10 and 11, the power after being ejected from the water surface is obtained:

[0060]

[0061] The aircraft flight altitude H at this time is the actual distance in the atmospheric transmission after conversion using the scanning zenith angle:

[0062]

[0063] θ 1 is the scanning zenith angle.

[0064] Therefore, the second atmospheric attenuation formula is as follows:

[0065]

[0066] So the total attenuation of the atmospheric part is:

[0067] P 大气衰减 =P 大气衰减1 +P 大气衰减2 (16)

[0068] The atmospheric attenuation coefficient of the laser is c 气 (λ), atmospheric visibility is V (unit km), laser wavelength is λ, and the following empirical formula is used:

[0069]

[0070] It is generally believed that the constant coefficient q is closely related to the atmospheric visibility V. The relationship table is shown in Table 2.

[0071] q coefficient value VVisibility range 1.6 V>50KM 1.3 6<V<50KM 0.16V+0.34 0.5<V<1KM 0 V<0.5KM

[0072] Therefore, the attenuation of the two-stage transmission in the atmosphere can be calculated by knowing only the atmospheric visibility through the input and the sensitivity of the receiver.

[0073] Step 7: Fusion of attenuation caused by sea surface reflection in algorithm improvement

[0074] The attenuation formula caused by sea surface reflection is as follows:

[0075]

[0076] Among them, P 射入时衰减 is the power attenuation caused by the sea surface when the laser enters the water, P 射出时衰减 is the power attenuation caused by the sea surface when the laser leaves the water, F is the water surface reflectivity, P 海面衰减 is the total attenuation caused by the sea surface, P 射入水面前 is the power before it hits the water surface, P 射出水面前 The power of the laser before it returns from the seabed to the surface of the water.

[0077] According to the Beer-Lambert law, the power attenuation caused by the sea surface when the laser enters the water is:

[0078] P 射入时衰减 =PT *exp[-c 气 (λ)*H]*F (19)

[0079] Among them, P T is the laser peak power, H is the aircraft flight altitude, c 气 (λ) is the attenuation coefficient of the atmosphere. At the same time, the power emitted from the water surface is obtained from formula 13:

[0080]

[0081] Therefore, the power attenuation caused by the sea surface when the laser comes out of the water is:

[0082]

[0083] P 最小灵敏度 The minimum power that the laser radar can receive, H is the flight altitude of the aircraft, c 气 (λ) is the attenuation coefficient of the atmosphere, and F is the reflectivity of the water surface.

[0084] For the sea surface reflectivity F is the incident angle θ 1 The function of the scanning angle (also called the scanning zenith angle) is known from Snell's law and Fresnel's formula:

[0085] When the incident angle θ 1 When the incidence is vertical, that is, when the incident angle is 0°, the sea surface reflectivity F is:

[0086]

[0087] When the incident angle θ 1 When the incident light is at a normal angle, the sea surface reflectivity F is:

[0088]

[0089] Among them, θ 1 is the incident angle, i.e. the scanning angle, θ 2 is the refraction angle, which can be calculated from the law of refraction.

[0090]

[0091] Where n(λ) 气 The refractive index of air for visible light under standard conditions is about 1.00029, n(λ) 水 Under standard conditions, the refractive index of seawater for visible light is about 1.33.

[0092] Therefore, the attenuation caused by sea surface reflection can be calculated by inputting the water refractive index and the laser scanning zenith angle (i.e., the incident angle).

[0093] Step 8: Model establishment: Laser peak emission power P T Improvements

[0094] In summary, since the main model incorporates the attenuation of the atmosphere and the attenuation of the sea surface (reflection), the laser peak emission power P in Formula 1 can be T Improved to:

[0095] P T =P T(峰值) -P 大气衰减 -P 海面衰减 (25)

[0096] Then calculate.

[0097] Step 9: Display the output using a computer software program

[0098] like Figure 3 As shown in , when we input the system parameters required for prediction in advance and obtain and input the environment-related parameters in real time, we can use the improved main model to output the predicted value of the maximum underwater detectable depth of the airborne laser radar using the computer as a carrier. Figure 4 As shown in the figure, when we have built-in some environmental parameters and need to understand the influence of two of the system parameters on the predicted value of the maximum underwater detectable depth of the airborne lidar for radar design, the relevant parameters can be output according to their mode and displayed in image form.

Claims

1. A method for predicting the maximum underwater measurement depth of airborne LiDAR, characterized in that: The specific steps are: Step 1: Parameters need to be input. LiDAR system parameters include laser peak transmission power, receiving aperture, laser scanning zenith angle, receiving system efficiency, spectral receiving bandwidth, receiving field of view, and receiver sensitivity; atmospheric and sea surface environmental parameters include atmospheric visibility; water quality and seabed environmental parameters include bottom reflectivity, water scattering coefficient, and water absorption coefficient; aircraft flight parameters include aircraft flight altitude; Step 2: The effective area A of the receiving system in the LiDAR sensor system parameters r , calculated by the input receiving aperture Dr, Sunlight background noise power P B The calculation can be calculated by the following formula: P B =I s A r D s Ω D s Spectral receiving bandwidth, Ω is the receiving solid angle, I s is the background irradiance, and its value is 0.0147 in light haze weather (under normal weather conditions). The receiving solid angle Ω can be calculated by the following formula: θ is the receiving field angle; Step 3: The calculation formula of the effective attenuation coefficient β in underwater transmission attenuation is as follows: β=c(λ)(1-0.832w0) c(λ) is the beam attenuation coefficient, w0 is the single scattering albedo, and the calculation formula is as follows: a(λ) is the absorption coefficient, b(λ) is the scattering coefficient; Step 4: Laser peak emission power P T The calculation formula is as follows, P T =P T(峰值) -P 大气衰减 -P 海面衰减 P T(峰值) is the original laser peak power, P 大气衰减 is the attenuation of the laser in the atmosphere, P 海面衰减 The laser is attenuated on the sea surface; Step 5: The formula for calculating the laser attenuation in the atmosphere is as follows: P 大气衰减 =P 大气衰减1 +P 大气衰减2 P 大气衰减1 is the first atmospheric attenuation of the laser before it enters the water. The calculation formula is as follows: P 大气衰减1 =P T (1-exp[-c 气 (λ)H]) Among them, c 气 (λ) is the atmospheric attenuation coefficient, H is the aircraft flight altitude, and the calculation formula is as follows: The constant coefficient q is closely related to the atmospheric visibility V and can be calculated through the relationship table. θ1 is the scanning zenith angle, H 实际航高 is the actual altitude of the aircraft. P 大气衰减2 is the second atmospheric attenuation of the laser after it leaves the water. The calculation formula is as follows: P 最小灵敏度 is the minimum power that the laser radar can receive; Step 6: The attenuation calculation formula caused by sea surface reflection is as follows: P 海面衰减 is the total attenuation caused by the sea surface, P 射入时衰减 is the power attenuation caused by the sea surface when the laser enters the water, P 射出时衰减 is the power attenuation caused by the sea surface when the laser leaves the water, F is the water surface reflectivity, P 射入水面前 is the power before it hits the water surface, P 射出水面前 is the power of the laser before it returns from the seabed and exits the water surface. The power attenuation caused by the sea surface when the laser enters the water is calculated as follows: P 射入时衰减 =P T *exp[-c 气 (λ)*H]*F The power attenuation formula caused by the sea surface when the laser comes out of water is calculated as follows: Among them, the sea surface reflectivity F is a function of the incident angle θ1 (the scanning angle is also called the scanning zenith angle), and its functional relationship is known from Snell's law and Fresnel's formula. When the incident angle θ1 is vertical, that is, when the incident angle is 0°, the sea surface reflectivity F is: When the incident angle θ1 is a general angle of incidence, the sea surface reflectivity F is: Among them, θ1 is the incident angle, i.e. the scanning angle, and θ2 is the refraction angle, which can be calculated by the law of refraction. Where n(λ) 气 The refractive index of air for visible light under standard conditions is about 1.00029, n(λ) 水 The refractive index of seawater to visible light under standard conditions is about 1.33; Step 7: Use a computer software program to calculate and output the prediction results.

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