Airborne laser radar bidirectional detection method

Abstract

This invention belongs to the field of infrared radiation characteristic measurement and atmospheric transmission correction technology, and particularly relates to a two-way detection method for airborne lidar. The method includes: S1: emitting laser beams to both above and below the aircraft using a lidar, and acquiring the echo signals from above and below the aircraft; S2: calculating the signal-to-noise ratio (SNR) on a computer based on the corresponding values ​​of the above and below echo signals; S3: stitching together the above and below distance-squared corrected signal profiles based on the difference between the above and below SNRs to obtain a stitched distance-squared corrected signal profile; S4: inverting the stitched distance-squared corrected signal profile using the Femald method to obtain the aerosol extinction coefficient; S5: calculating the atmospheric transmittance based on the aerosol extinction coefficient. This invention enables simultaneous detection of both atmospheric extinction coefficient and atmospheric transmittance above and below the flight platform.

Description

Technical Field

[0001] This invention belongs to the field of infrared radiation characteristic measurement and atmospheric transmission correction technology, and particularly relates to a bidirectional detection method for airborne lidar. Background Technology

[0002] The accuracy of target radiation characteristic measurement depends on the accuracy of atmospheric transmittance measurement and radiation calibration. With the development of airborne platform target radiation characteristic measurement equipment, it has gradually become an important means of measuring the infrared radiation characteristics of space targets. In the process of measuring the infrared radiation characteristics of targets in the upper atmosphere, accurately obtaining the atmospheric transmittance along the path is crucial for improving the accuracy of target radiation characteristic measurement. Lidar can achieve accurate measurement of extinction coefficient and atmospheric transmittance, the core of which is obtaining a high signal-to-noise ratio echo signal at the measurement location. However, since the attenuation of the laser emission signal is proportional to the square of the detection distance, in order to reduce the attenuation of the emission signal and improve the signal-to-noise ratio of the upper-level echo signal, airborne aerosol lidar can be used for close-range emission and detection in the high atmosphere, thereby achieving high-precision measurement.

[0003] Currently available airborne atmospheric lidar systems, including airborne multi-channel aerosol-cloud particle lidar systems, airborne atmospheric environment detection lidar, and airborne hyperspectral resolution lidar, all involve mounting lidar equipment on aircraft or drones to detect the atmospheric environment. These lidar devices all detect in a single direction, limiting their measurement scope and making it impossible to simultaneously and accurately measure aerosol extinction coefficients and atmospheric transmittance above and below the airborne platform.

[0004] Traditional ground-based, airborne, and spaceborne lidar systems can only detect single-axis aerosol extinction coefficients and atmospheric transmittance. Ground-based lidar is affected by the complex atmospheric composition near the ground, and the attenuation of its echo signal is proportional to the square of the detection distance, resulting in low signal-to-noise ratios at higher altitudes and short effective detection ranges, making it difficult to accurately obtain upper-level aerosol extinction coefficients and atmospheric transmittance. Airborne lidar can only detect atmospheric transmittance above or below the platform in a single operation, and cannot perform real-time bidirectional detection, thus limiting its measurement capabilities. Spaceborne lidar is mounted on a satellite platform and relies on satellite operation, making continuous observation of the same location impossible. Due to the low signal-to-noise ratio of ground signals, it cannot accurately measure near-surface atmospheric transmittance. Summary of the Invention

[0005] In view of this, the present invention aims to provide a bidirectional detection method for airborne lidar to solve the problem that existing measurement devices all detect in a single direction, have limited measurement directions, and cannot simultaneously and accurately measure the aerosol extinction coefficient and atmospheric transmittance above and below the airborne platform. The present invention fixes the lidar to the lower part of the fuselage of the rear section of the aircraft, sets up lidar optical windows for measuring above and below the parallel platform, fixes the lidar inside the cabin, and places a beam splitter and a reflector at the front end of the lidar to achieve simultaneous detection of bidirectional atmospheric extinction coefficient and atmospheric transmittance above and below the flight platform.

[0006] To achieve the above objectives, the technical solution created by this invention is implemented as follows: A bidirectional detection method for airborne lidar specifically includes the following steps: S1: Use lidar to emit laser beams onto and off the aircraft respectively, and acquire the echo signals from the aircraft and the aircraft. S2: Based on the signal-to-noise ratio (SNR) on the computer and the signal-to-noise ratio (SNR) on the ground, corresponding to the onboard and offboard echo signals; S3: Based on the difference between the on-board signal-to-noise ratio and the off-board signal-to-noise ratio, the on-board distance squared correction signal profile and the off-board distance squared correction signal profile are spliced ​​together to obtain the spliced ​​distance squared correction signal profile; S4: The aerosol extinction coefficient is obtained by inverting the splicing distance squared correction signal profile based on the Femald method. S5: Calculate atmospheric transmittance based on aerosol extinction coefficient.

[0007] Furthermore, in step S1, the lidar is mounted on a lidar mounting base, which is installed in the rear equipment bay of the aircraft. An upper protective window and a lower protective window are provided on the skin of the rear equipment bay. The laser beam emitted by the lidar is split by a beam splitter to obtain a first laser beam and a second laser beam. The first laser beam is emitted through the upper protective window and is used to detect information on board the aircraft. The second laser beam is emitted through the lower protective window and is used to detect information below the aircraft.

[0008] Furthermore, the centers of the upper and lower protection windows are not collinear.

[0009] Furthermore, in step S2, the onboard signal-to-noise ratio The calculation formula is: ; Signal-to-noise ratio The calculation formula is: ; in, For onboard signal-to-noise ratio, Signal-to-noise ratio under machine conditions. This is the onboard echo signal. The echo signal is from the machine. and These represent the attenuation of the aircraft echo signal after passing through the upper atmosphere and the attenuation of the aircraft echo signal after passing through the near-ground atmosphere, respectively, at the same distance. This is the background noise signal.

[0010] Furthermore, step S3 specifically includes the following steps: S31: If the difference between the on-board signal-to-noise ratio and the off-board signal-to-noise ratio is less than a preset threshold, then the on-board distance squared correction signal profile and the off-board distance squared correction signal profile are dynamically spliced; otherwise, proceed to step S32. S32: Forcefully constrain and splice the on-plane distance squared correction signal profile and the off-plane distance squared correction signal profile.

[0011] Furthermore, in step S31, the calculation formula for dynamically splicing the on-board distance squared correction signal profile and the off-board distance squared correction signal profile is as follows: ; ; Where H is the aircraft's flight altitude. This refers to the echo signal received by the lidar at altitude H of the aircraft. This refers to the echo signal received by the lidar at an altitude H above the aircraft. This refers to the echo signal received by the lidar at an altitude H below the aircraft. To adjust the weight of the on-board distance squared correction signal profile, To adjust the weight of the squared distance correction signal profile, The onboard signal-to-noise ratio. The signal-to-noise ratio under machine conditions.

[0012] Furthermore, in step S32, the calculation process for forcibly constraining and splicing the on-board distance squared correction signal profile and the off-board distance squared correction signal profile is as follows: make The gain coefficient of the on-board echo signal or the off-board echo signal is adjusted using the least squares method. : ; Based on gain coefficient The signal profile is corrected by splicing the squared distance using the following formula: ; in, This refers to the aircraft's flight altitude.

[0013] Furthermore, in step S4, the splicing distance squared correction signal profile is inverted based on the Femald method to obtain the calculation formula for the aerosol extinction coefficient below height H: ; in, The extinction coefficient of aerosols below height H. To splice the distance squared correction signal profile. For the lidar to receive the atmospheric backscattered echo signal at a distance H, The height represents the boundary value of the aerosol extinction coefficient. This represents the boundary value of the aerosol extinction coefficient at Hc. Let h be the extinction coefficient of the gas molecules at a distance h. The extinction coefficient of the gas molecules at a distance H is... The extinction backscattering ratio of atmospheric molecules. is the ratio of lidar, h is the distance variable, and h' is the derivative of h; Based on the Femald method, the splicing distance squared correction signal profile is inverted, and the formula for calculating the aerosol extinction coefficient above height H is obtained as follows: ; in, The extinction coefficient of aerosols above height H.

[0014] Furthermore, in step S5, the formula for calculating the atmospheric transmittance T based on the aerosol extinction coefficient is as follows: ; in, , The distance that the aircraft can detect upwards or downwards. To detect the aircraft's upward or downward movement The height reached later.

[0015] Compared with the prior art, the present invention can achieve the following beneficial effects: (1) The airborne lidar bidirectional detection method described in this invention, by fixing the lidar to the lower part of the fuselage of the rear section of the aircraft, sets up lidar optical windows for measuring above and below the parallel platform, fixes the lidar inside the cabin, and by placing a beam splitter and a reflector at the front end of the lidar, achieves simultaneous detection of the atmospheric extinction coefficient and atmospheric transmittance in both directions above and below the flight platform. This invention uses a single laser emission system for both on-board and off-board measurements, has high integration, low cost, and, compared with existing technologies, increases the detection range and improves the detection capability.

[0016] (2) The airborne lidar bidirectional detection method described in this invention employs close-range transmission and reception in the upper atmosphere, effectively improving the signal-to-noise ratio of lidar echo signals in both the lower and upper atmospheres, which is more conducive to improving the accuracy of transmitting power measurement across the entire atmosphere. The airborne platform can fly at different altitudes, making it more flexible in use. When detecting the upper atmosphere, it can avoid interference from the lower atmosphere, resulting in a higher measurement range and more accurate high-altitude measurement data. When measuring the lower atmosphere, because the ground atmosphere is relatively dense, a good signal-to-noise ratio echo signal can still be obtained even when the laser energy is low, ensuring more accurate measurement data of ground aerosols. Attached Figure Description

[0017] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments and descriptions of the invention are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings: Figure 1 A schematic flowchart of the airborne lidar bidirectional detection method described in the embodiments of the present invention; Figure 2 A schematic diagram of the structure of the airborne lidar bidirectional detection method described in the embodiments of the present invention; Figure 3 A schematic diagram of the structure of the airborne lidar bidirectional detection described in the embodiment of the present invention; Figure 4 This is a schematic diagram comparing the distance squared correction signal before and after splicing processing, as described in an embodiment of the present invention.

[0018] Figure descriptions: 1. LiDAR; 2. Beam splitter; 3. Reflector; 4. Upper protective window; 5. Lower protective window. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and do not constitute a limitation thereof.

[0020] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other.

[0021] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer," etc., indicating orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation on this invention. Furthermore, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, features defined with "first," "second," etc., may explicitly or implicitly include one or more of that feature. In the description of this invention, unless otherwise stated, "a plurality of" means two or more.

[0022] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art will understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0023] The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0024] like Figures 1-2 As shown, this invention proposes a bidirectional detection method for an airborne lidar 1, specifically including the following steps: S1: The lidar 1 emits laser beams to both the upper and lower sides of the aircraft, and acquires the upper and lower echo signals; S2: Based on the signal-to-noise ratio (SNR) of the upper and lower echo signals, the corresponding signals are calculated on a computer; S3: Based on the difference between the upper and lower SNR, the upper and lower distance squared correction signal profiles are stitched together to obtain a stitched distance squared correction signal profile; S4: The stitched distance squared correction signal profile is inverted using the Femald method to obtain the aerosol extinction coefficient; S5: The atmospheric transmittance is calculated based on the aerosol extinction coefficient.

[0025] It should be noted that this invention, by adding a beam splitter 2 and a reflector 3 to the optical path of lidar 1, enables the transmission and reception of laser signals and echo signals through the optical protection window reserved in the aircraft skin, allowing for simultaneous bidirectional detection of atmospheric transmittance above and below the airborne platform. Before performing aerosol extinction coefficient inversion, it is necessary to stitch together the two echo signal detection profiles (one above and one below the aircraft) based on the echo characteristics of lidar 1 signals from the airborne platform, making them a complete detection profile. The bidirectional detection data is processed using a dual-profile stitching strategy to achieve aerosol extinction coefficient inversion and atmospheric transmittance calculation.

[0026] In some embodiments, in step S1, the lidar 1 is mounted on a lidar 1 mounting base, which is installed in the rear equipment bay of the aircraft. An upper protective window 4 and a lower protective window 5 are provided on the skin of the rear equipment bay. The laser beam emitted by the lidar 1 is split by a beam splitter 2 to obtain a first laser beam and a second laser beam. The first laser beam exits through the upper protective window 4 for detecting onboard information, and the second laser beam exits through the lower protective window 5 for detecting offboard information. The beam splitting can use a beam splitter with a continuously variable splitting ratio or a beam splitter with preset splitting ratios and switchable settings.

[0027] In some embodiments, the centers of the upper protection window 4 and the lower protection window 5 are not collinear.

[0028] It should be noted that, as Figure 3 As shown, the center of the beam splitter 2 at the front end of the optical path of the lidar 1 coincides with the center of the lower protective window 5. A reflector 3 is provided in the optical path between the beam splitter 2 and the upper protective window 4, with the center of the reflector 3 coinciding with the center of the upper protective window 4. The beam splitter 2, through a special optical thin film (dielectric or metal film), utilizes the transmission, reflection, and interference characteristics of light to distribute the incident laser energy in a 7:3 ratio. Specific wavelength splitting is achieved by precisely controlling the thickness and refractive index of the dielectric film. Metal film splitting relies on a semi-transparent, semi-reflective coating to achieve energy division. The splitting ratio is affected by the incident angle, polarization state, and wavelength. The metal film is designed with a 45° incident angle to reduce deviation. The reflector 3 uses a highly reflective coating to reduce the attenuation of the transmitted light from the beam splitter 2 transmitted downwards.

[0029] Furthermore, this invention has bidirectional detection capability, with effective detection ranges for both above-ground and below-ground detection. and That is, the total effective detection range of airborne lidar 1 is .

[0030] In some embodiments, in step S2, the on-board signal-to-noise ratio The calculation formula is: ; Signal-to-noise ratio The calculation formula is: ; in, For onboard signal-to-noise ratio, Signal-to-noise ratio under machine conditions. This is the onboard echo signal. The echo signal is from the machine. and These represent the attenuation of the aircraft echo signal after passing through the upper atmosphere and the attenuation of the aircraft echo signal after passing through the near-ground atmosphere, respectively, at the same distance. This is the background noise signal.

[0031] It should be noted that the signal-to-noise ratio (SNR) is the ratio of the signal received by lidar 1 to the total noise generated. If we use... This indicates the number of signal photoelectrons collected within a certain time period (i.e., the target scattered echo signal). Let S represent the total number of noise photoelectrons collected within the same time period. Then, the signal-to-noise ratio (SNR) can be expressed as: ; The target scattered echo signal received by lidar 1 It should be the original echo signal obtained from lidar 1 minus the background noise signal: ; in, The raw echo signal of lidar 1 is obtained directly from the detector of lidar 1, including the target scattered echo signal and the background noise signal; As the detection distance increases, the target scattered echo signal of lidar 1 becomes increasingly weak. Once a certain distance is reached, the signal collected by lidar 1 is considered to be merely a background noise signal. Assume... Let be the standard deviation of the original echo signal received by lidar 1, and let be the uncertainty of the signal. Then we have: ; at this time: ; Theories such as the quantum properties of photons and photoelectric detection models indicate that the number of electrons received by lidar 1 per unit time follows a Poisson distribution. Without considering other noise effects, we have... At this point, the above equation becomes: ; Assume that the original echo signal of lidar 1 attenuates differently when traveling the same distance through the upper atmosphere and the near-ground atmosphere, respectively. and ,and Then we have: ; Similarly, we have: ; The difference between the two is: ; Known ,therefore In other words, when detecting targets at the same distance, the signal-to-noise ratio at high altitudes is higher than that at near-ground altitudes, resulting in stronger detection capabilities.

[0032] In some embodiments, step S3 specifically includes the following steps: S31: If the difference between the on-board signal-to-noise ratio and the off-board signal-to-noise ratio is less than the preset threshold (set according to actual needs), then the on-board distance squared correction signal profile and the off-board distance squared correction signal profile are dynamically spliced; otherwise, proceed to step S32. S32: Forcefully constrain and splice the on-plane distance squared correction signal profile and the off-plane distance squared correction signal profile.

[0033] It should be noted that the attenuation of laser emission energy is proportional to the square of the distance. In the initial stage of launch from an airborne platform, the laser has high power, thus enabling the acquisition of high signal-to-noise ratio echo energy from upper atmospheric aerosol scattering. When detecting aerosols near the ground, although the detection location is farther from the launch point and laser attenuation is greater, the denser ground aerosols possess strong backscattering capabilities, still allowing for the acquisition of substantial backscattered signals and a relatively good signal-to-noise ratio.

[0034] Before inverting the aerosol extinction coefficient, a continuous range-squared corrected signal obtained by preprocessing the original echo signal is needed. The Fernald method is then used to invert this continuous range-squared corrected signal to obtain the aerosol extinction coefficient. However, due to the unique nature of bidirectional detection on an airborne platform, two range-squared corrected signal profiles (upward and downward) are obtained during measurement. Therefore, these two profiles need to be stitched together before the Fernald method inversion. Since the two range-squared corrected signal profiles have no overlapping area, commonly used stitching methods are not applicable. This invention proposes a stitching strategy to address this issue.

[0035] Since the echo signal is multiplied by the square of the distance to obtain the corrected signal profile, and multiplying by the square of the distance does not affect the continuity of the profile, for the sake of calculation convenience, the echo signal p(H) is used for splicing calculation here, and the calculation effect is equivalent to the profile splicing calculation.

[0036] In some embodiments, the calculation formula for dynamically splicing the on-board distance squared correction signal profile and the off-board distance squared correction signal profile in step S31 is as follows: ; ; Where H is the aircraft's flight altitude. This refers to the echo signal received by lidar 1 at altitude H of the aircraft. The echo signal received by lidar 1 at an altitude H above the aircraft. This refers to the echo signal received by lidar 1 at an altitude H below the aircraft. To adjust the weight of the on-board distance squared correction signal profile, To adjust the weight of the squared distance correction signal profile, The onboard signal-to-noise ratio. The signal-to-noise ratio under machine conditions.

[0037] In some embodiments, the calculation process for forcibly constraining and splicing the on-board distance squared correction signal profile and the off-board distance squared correction signal profile in step S32 is as follows: make The gain coefficient of the on-board echo signal or the off-board echo signal is adjusted using the least squares method. : ; Based on gain coefficient The signal profile is corrected by splicing the squared distance using the following formula: ; in, This refers to the aircraft's flight altitude.

[0038] It should be noted that if the signal-to-noise ratios of the two are similar, then the dynamic weighting method should be selected: Assuming the aircraft is at an altitude In the flight area, the aerosol is continuous. When the signal-to-noise ratios of the two-way echo signals are close, that is, when their signal-to-noise ratios are less than a certain selected threshold... At that time, weights are assigned to the on-board distance squared corrected signal profile and the off-board distance squared corrected signal profile according to the signal-to-noise ratio, as follows: ; Let the echo signal received by lidar 1 at altitude H of the aircraft be... ,but The splicing formula at the location is: ; If the signal-to-noise ratio difference between the two is too large, exceeding a certain threshold, then a forced constraint splicing method will be selected: Assuming the aircraft's flight altitude remains at The aerosols in the flight area are continuous. When the signal-to-noise ratio difference between the two-way echo signals is large and exceeds the selected threshold, At that time, enforce a continuity constraint at height H, let: ; Then, the gain coefficient of the on-board echo signal or the off-board echo signal is adjusted using the least squares algorithm. : ; The splicing is achieved using the adjusted signal: ; After stitching using the above method, the signal profile is corrected by the square of the stitching distance, and the aerosol extinction coefficient is inverted based on the Fernald method.

[0039] In some embodiments, in step S4, the splicing distance squared correction signal profile is inverted based on the Femald method to obtain the calculation formula for the aerosol extinction coefficient below height H: ; in, The extinction coefficient of aerosols below height H. To splice the distance squared correction signal profile. For lidar 1 to receive the atmospheric backscattered echo signal at a distance H, The height represents the boundary value of the aerosol extinction coefficient. This represents the boundary value of the aerosol extinction coefficient at Hc. Let h be the extinction coefficient of the gas molecules at a distance h. The extinction coefficient of the gas molecules at a distance H is... The extinction backscattering ratio of atmospheric molecules. Let h be the ratio of the lidar sensor to h, h be the distance variable, and h' be the derivative of h. Based on the Femald method, the splicing distance squared correction signal profile is inverted, and the formula for calculating the aerosol extinction coefficient above height H is obtained as follows: ; in, The extinction coefficient of aerosols above height H.

[0040] It should be noted that by substituting the squared correction signal profile of the stitched distance into the F method, the extinction coefficient of aerosol particles can be obtained. The extinction coefficient of aerosols below height H can be expressed using back integral as follows: ; The extinction coefficient of aerosols above altitude H is obtained by forward integration: ; in, This represents the distance squared correction signal. This indicates that lidar 1 received an atmospheric backscattered echo signal at a distance H. The height of the boundary value of the aerosol extinction coefficient is generally selected in a cleanroom where there are virtually no aerosol particles. This represents the boundary value of the aerosol extinction coefficient at Rc. Let h be the extinction coefficient of the gas molecules at a distance h. The extinction backscattering ratio of atmospheric molecules. Let h be the ratio of the lidar sensor to h, h' be the distance variable, and h' be the derivative of h. The lidar ratio is represented by the laser wavelength, which is usually related to the scale distribution and refractive index of the aerosol particles.

[0041] In some embodiments, in step S5, the formula for calculating the atmospheric transmittance T based on the aerosol extinction coefficient is as follows: ; in, , The distance that the aircraft can detect upwards or downwards. To detect the aircraft's upward or downward movement The height reached later.

[0042] Raw photon echo data acquired by airborne lidar 1 at the local location was obtained through simulation. Signal correction was performed using the squared distance method, resulting in a discontinuous curve with the discontinuity point at the flight altitude. Due to the significant difference in uplink and downlink signal-to-noise ratios at the flight altitude in this simulation, forced constraint stitching was performed, and the result is as follows. Figure 4 As shown, through splicing, the distance squared correction signal becomes a continuous curve. At this point, the aerosol extinction coefficient can be obtained by inversion using the Fernald method, thus realizing the calculation of atmospheric transmittance.

[0043] It should be understood that the various forms of processes shown above can be used to reorder, add, or delete steps. For example, the steps described in this invention disclosure can be executed in parallel, sequentially, or in different orders, as long as the desired result of the technical solution disclosed in this invention can be achieved, and this is not limited herein.

[0044] The specific embodiments described above do not constitute a limitation on the scope of protection of this invention. Those skilled in the art should understand that various modifications, combinations, sub-combinations, and substitutions can be made according to design requirements and other factors. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this invention should be included within the scope of protection of this invention.

Claims

1. An airborne laser radar bidirectional detection method, characterized in that: Specifically, the steps include the following: S1: Use lidar to emit laser beams onto and off the aircraft respectively, and acquire the echo signals from the aircraft and the aircraft. S2: Based on the signal-to-noise ratio (SNR) on the computer and the signal-to-noise ratio (SNR) on the ground, corresponding to the onboard and offboard echo signals; S3: Based on the difference between the on-board signal-to-noise ratio and the off-board signal-to-noise ratio, the on-board distance squared correction signal profile and the off-board distance squared correction signal profile are spliced ​​together to obtain the spliced ​​distance squared correction signal profile; S4: The aerosol extinction coefficient is obtained by inverting the splicing distance squared correction signal profile based on the Femald method. S5: Calculate atmospheric transmittance based on aerosol extinction coefficient.

2. The airborne lidar bi-directional probing method of claim 1, wherein: In step S1, the lidar is mounted on a lidar mounting base, which is installed in the rear equipment bay of the aircraft. An upper protective window and a lower protective window are provided on the skin of the rear equipment bay. The laser beam emitted by the lidar is split by a beam splitter to obtain a first laser beam and a second laser beam. The first laser beam is emitted through the upper protective window to detect information on board the aircraft, and the second laser beam is emitted through the lower protective window to detect information below the aircraft.

3. The airborne lidar bidirectional detection method according to claim 2, characterized in that: The centers of the upper protection window and the lower protection window are not collinear.

4. The airborne lidar bidirectional detection method according to claim 1, characterized in that: In step S2, the onboard signal-to-noise ratio The calculation formula is: ； Signal-to-noise ratio The calculation formula is: ； in, For onboard signal-to-noise ratio, Signal-to-noise ratio under machine conditions. This is the onboard echo signal. The echo signal is from the machine. and These represent the attenuation of the aircraft echo signal after passing through the upper atmosphere and the attenuation of the aircraft echo signal after passing through the near-ground atmosphere, respectively, at the same distance. This is the background noise signal.

5. The airborne lidar bidirectional detection method according to claim 4, characterized in that: Step S3 specifically includes the following steps: S31: If the difference between the on-board signal-to-noise ratio and the off-board signal-to-noise ratio is less than a preset threshold, then the on-board distance squared correction signal profile and the off-board distance squared correction signal profile are dynamically spliced; otherwise, proceed to step S32. S32: Forcefully constrain and splice the on-plane distance squared correction signal profile and the off-plane distance squared correction signal profile.

6. The airborne lidar bidirectional detection method according to claim 5, characterized in that: In step S31, the calculation formula for dynamically splicing the on-board distance squared correction signal profile and the off-board distance squared correction signal profile is as follows: ； ； Where H is the aircraft's flight altitude. This refers to the echo signal received by the lidar at altitude H of the aircraft. This refers to the echo signal received by the lidar at an altitude H above the aircraft. This refers to the echo signal received by the lidar at an altitude H below the aircraft. To adjust the weight of the on-board distance squared correction signal profile, To adjust the weight of the squared distance correction signal profile, The onboard signal-to-noise ratio. The signal-to-noise ratio under machine conditions.

7. The airborne lidar bidirectional detection method according to claim 6, characterized in that: In step S32, the calculation process for forcibly constraining and splicing the on-board distance squared correction signal profile and the off-board distance squared correction signal profile is as follows: make The gain coefficient of the on-board echo signal or the off-board echo signal is adjusted using the least squares method. : ； Based on gain coefficient The signal profile is corrected by splicing the squared distance using the following formula: ； in, This refers to the aircraft's flight altitude.

8. The airborne lidar bidirectional detection method according to claim 7, characterized in that: In step S4, the splicing distance squared correction signal profile is inverted based on the Femald method, and the calculation formula for the aerosol extinction coefficient below height H is obtained as follows: ； in, for aerosol extinction coefficient below altitude To splice the distance squared correction signal profile. For the distance received by the lidar Atmospheric backscattered echo signal at that location The height represents the boundary value of the aerosol extinction coefficient. for The boundary value of the aerosol extinction coefficient. Distance The extinction coefficient of gas molecules at that point, The extinction coefficient of the gas molecules at a distance H is... The extinction backscattering ratio of atmospheric molecules. LiDAR ratio, For distance variables, for The derivative; The splicing distance squared correction signal profile is inverted based on the Femald method to obtain... The formula for calculating the extinction coefficient of aerosols above a certain altitude is: ； in, for Extinction coefficient of aerosols above a certain height.

9. The airborne lidar bidirectional detection method according to claim 8, characterized in that: In step S5, the formula for calculating the atmospheric transmittance T based on the aerosol extinction coefficient is as follows: ； in, , The distance that the aircraft can detect upwards or downwards. To detect the aircraft's upward or downward movement The height reached later.

Images