An aerosol parameter inversion method based on coherent Doppler lidar

Abstract

The application discloses an aerosol parameter inversion method based on a coherent Doppler laser radar, and comprises the following steps: emitting laser pulses to the atmosphere through the coherent Doppler laser radar, and receiving echo signals reflected back; correcting the echo signal carrier-to-noise ratio of the coherent Doppler laser radar in terms of distance, focusing function and system constant to obtain an attenuation backscattering coefficient; and inverting the attenuation backscattering coefficient to obtain the extinction coefficient and the backscattering coefficient of the aerosol. The application realizes absolute calibration of the echo signal intensity of the atmospheric aerosol, and inversion of the backscattering coefficient and the extinction coefficient of the aerosol.

Description

Technical Field

[0001] This invention belongs to the field of lidar technology and relates to an aerosol parameter inversion method based on coherent Doppler lidar. Background Technology

[0002] Coherent Doppler lidar can detect the velocity of moving targets (atmospheric aerosols, clouds, hard targets) by analyzing their echo signals. It is currently widely used in fields such as aviation safety, wind power generation, and air pollution monitoring and forecasting.

[0003] In some research and application scenarios, aerosol concentration information receives the same level of attention as atmospheric wind speed. Currently, aerosol information is generally acquired directly through aerosol lidar operating on a multi-mode detection basis. However, existing aerosol lidar systems require significant pulse energy, power consumption, and are bulky, and are easily affected by solar background radiation noise during the day. Furthermore, the commonly used operating wavelengths (visible or near-infrared) pose a threat to human eye safety. Coherent lidar uses the eye-safe mid-infrared band, and its all-fiber system can ensure high stability and integration. However, it is essentially a single-mode detection system, and its detection efficiency is easily affected by factors such as laser beam focal length and turbulence, making it difficult to deduce atmospheric parameters such as aerosol extinction coefficient and backscattering coefficient. Coherent lidar echo signal strength is also relatively poor. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the prior art and provide an aerosol parameter inversion method based on coherent Doppler lidar, which realizes the absolute calibration of the intensity of atmospheric aerosol echo signals and the inversion of aerosol backscattering coefficient and extinction coefficient.

[0005] To achieve the above objectives, the present invention is implemented using the following technical solution:

[0006] A method for aerosol parameter inversion based on coherent Doppler lidar, characterized by comprising:

[0007] Receive echo signals; where the echo signals are the reflected signals of the laser pulses emitted into the atmosphere by the coherent Doppler lidar;

[0008] The carrier-to-noise ratio of the echo signal is corrected for distance, focusing function, and system constant to obtain the attenuated backscattering coefficient;

[0009] The extinction coefficient and backscattering coefficient of the aerosol are obtained by inverting the attenuated backscattering coefficient.

[0010] Optionally, the carrier-to-noise ratio expression for the echo signal is:

[0011]

[0012] Where CNR(R) is the carrier-to-noise ratio of the echo signal, and E T Let R be the single-pulse energy of the laser emitted by the coherent Doppler lidar, R be the distance between the aerosol and the coherent Doppler lidar, σ(r) be the atmospheric extinction coefficient, including atmospheric molecular extinction and aerosol extinction, β(R) be the aerosol backscattering coefficient, and A be the single-pulse energy of the laser emitted by the coherent Doppler lidar. r Let η be the effective receiving area of ​​the coherent Doppler lidar, c be the speed of light in air, and η be the velocity of light in air. h (R) is the focusing function, η o Let hν be the efficiency of the coherent Doppler lidar system, hν be the photon energy, and B be the value of B. w This represents the bandwidth of the coherent Doppler lidar.

[0013] Optionally, the expression for the attenuation backscattering coefficient is:

[0014]

[0015] Where β'(R) is the attenuation backscattering coefficient, cons. is a constant factor related to the coherent Doppler lidar system constants, and η h (R) is the focusing function, R is the distance between the aerosol and the coherent Doppler lidar, and CNR(R) is the carrier-to-noise ratio of the echo signal.

[0016] Optionally, under the condition of an untruncated Gaussian beam, the expression for the focusing function is:

[0017]

[0018] Where, η h (R) is the focusing function, ω T To emit laser pulses at the coherent Doppler lidar e -2 Irradiance radius, λ is the laser pulse wavelength, R f R is the radius of curvature of the equiphase surface from which the laser pulse is emitted, ρ0 is the transverse coherence length related to turbulence, and R is the distance between the aerosol and the coherent Doppler lidar.

[0019] Optionally, the constant factor is calibrated using a non-precipitating stratocumulus target signal with known optical scattering characteristics; the echo signal of the non-precipitating stratocumulus starts from 0, and its optical thickness is expressed as:

[0020]

[0021] Where τ(z) is the optical thickness of the non-precipitating stratocumulus, z is the length of the laser pulse within the non-precipitating stratocumulus, and σ is the extinction coefficient of the cloud at a distance r from the coherent Doppler lidar. c (r) and backscattering coefficient β cThe relational expression for (r) is:

[0022] σ c (r)=ηS c β c (r) (5)

[0023] Among them, S c The lidar ratio of the cloud is given by η, where η is the multiple scattering factor and β' is the attenuation backscattering coefficient of the cloud. c The path integral expression for (z) is:

[0024]

[0025] Alternatively, the inversion method can be the slope method, the Klett-Fernald inversion method, or the iterative inversion method. The slope method is suitable for conditions where the horizontal atmosphere is homogeneous.

[0026] Optionally, in the Klett-Fernald inversion method, based on the characteristic of low near-field echo attenuation, a calibration point is selected in the near field, and the backscattering coefficient obtained by Klett-Fernald inversion is calibrated using the attenuated backscattering coefficient of the calibration point. The inversion result is then iteratively updated by updating the value of the far-field reference point.

[0027] Optional Klett-Fernald inversion methods include:

[0028] Select the maximum detection range R of the coherent Doppler lidar c As a reference point, the reference point value is set as the aerosol backscattering coefficient β0(R). c ) equals the attenuation backscattering coefficient β'(R) c );

[0029] Neglecting atmospheric scattering and extinction, the Klett-Fernald inversion method is expressed as follows:

[0030]

[0031] Among them, S a Here, R is the distance between the aerosol and the coherent Doppler lidar, β'(r) and β'(R) are the attenuated backscattering coefficients, and β... i (R c R is the maximum detection range. c At point i, the aerosol backscattering coefficient value after the i-th iteration, β i+1 (R) represents the updated backscattering coefficient curve, N max The maximum number of iterations is set.

[0032] Based on the small path attenuation of near-field echoes, and assuming near-field β'(R)≈β(R), a near-field calibration point R is set. m Determine whether ||β is satisfied. i+1 (R m )-β'(R m )||<δ, where δ is a given threshold;

[0033] If the condition is met, stop the inversion calculation; otherwise, ||β| ... i+1 (R m )-β'(R m If || < δ, update the reference point value β. i+1 (R c )=β i (R c )β'(R m ) / β i+1 (R m ), and continue to calculate using the expression of the Klett-Fernald inversion method;

[0034] The aerosol backscattering coefficient β(R) = β i+1 (R) and atmospheric extinction coefficient σ(R)=Sβ(R).

[0035] Optionally, in the iterative inversion method, the aerosol backscattering coefficient and atmospheric extinction coefficient are inverted by iteratively correcting the path attenuation based on the aerosol lidar ratio S.

[0036] Optional iterative inversion methods include:

[0037] The initial value of the aerosol backscattering coefficient β0(R) is set to be equal to the attenuated backscattering coefficient β'(R);

[0038] Update the backscattering coefficient β according to the expression of the iterative inversion method. i+1 (R), the expression for the iterative inversion method is:

[0039]

[0040] Where, σ i (r)=S c β i (r), σ i (r) is the extinction coefficient for the i-th iteration, β i (r) is the aerosol backscattering coefficient in the i-th iteration, S is the set aerosol lidar ratio, R is the distance between the aerosol and the coherent Doppler lidar, β'(R) is the attenuated backscattering coefficient, and N max The maximum number of iterations is set.

[0041] Determine if ||β is satisfied i+1(R)-β i (R)||<δ, where δ is a given threshold;

[0042] If the condition is met, the iteration stops; if not, the iteration continues according to the expression of the iterative inversion method until the maximum number of iterations is reached.

[0043] The aerosol backscattering coefficient β(R) after path attenuation calibration is obtained. i+1 (R) and extinction coefficient σ(R) = Sβ(R).

[0044] Compared with the prior art, the beneficial effects achieved by the present invention are as follows:

[0045] This invention provides an aerosol parameter inversion method based on coherent Doppler lidar. By calibrating the distance, focusing function, and system constant of the echo signal, the absolute calibration of the intensity of atmospheric aerosol echo signal is achieved, and the aerosol attenuation backscattering coefficient is obtained.

[0046] This invention utilizes a given lidar ratio to invert the aerosol extinction coefficient and backscattering coefficient using the Collis slope method, Klett-Fernald method, or iterative method, thereby improving the reception intensity of echo signals in coherent lidar for aerosol detection and increasing detection efficiency. Attached Figure Description

[0047] Figure 1 This is the echo carrier-to-noise ratio curve of the coherent Doppler lidar provided in the embodiments of the present invention;

[0048] Figure 2 Yes Figure 1 The attenuation backscattering coefficient curve is obtained by performing the aforementioned distance correction, focusing function correction, and system constant correction on the mid-echo curve.

[0049] Figure 3 It is based on Figure 2 The backscattering coefficient curve obtained by inverting the medium-attenuation backscattering coefficient;

[0050] Figure 4 It is based on Figure 2 Extinction coefficient curve obtained by inverting the medium-attenuation backscattering coefficient;

[0051] Figure 5 This is a flowchart of an embodiment of the present invention. Detailed Implementation

[0052] The present invention will be further described below with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present invention, and should not be used to limit the scope of protection of the present invention.

[0053] Example 1

[0054] like Figures 1 to 5 As shown, a method for aerosol parameter inversion based on coherent Doppler lidar includes the following steps:

[0055] Step S1: Receive the echo signal; wherein, the echo signal is the reflected signal of the laser pulse emitted by the coherent Doppler lidar into the atmosphere;

[0056] Step S2: Correct the echo signal carrier-to-noise ratio for distance, focusing function, and system constant to obtain the attenuated backscattering coefficient;

[0057] The carrier-to-noise ratio (CNR) of the echo signal is expressed as follows:

[0058]

[0059] Where CNR(R) is the carrier-to-noise ratio of the echo signal, and E T Let R be the single-pulse energy of the laser emitted by the coherent Doppler lidar, R be the distance between the aerosol and the coherent Doppler lidar, σ(r) be the atmospheric extinction coefficient, including atmospheric molecular extinction and aerosol extinction, β(R) be the aerosol backscattering coefficient, and A be the single-pulse energy of the laser emitted by the coherent Doppler lidar. r Let η be the effective receiving area of ​​the coherent Doppler lidar, c be the speed of light in air, and η be the velocity of light in air. h (R) is the focusing function, η o Let hν be the efficiency of the coherent Doppler lidar system, hν be the photon energy, and B be the value of B. w The bandwidth of the coherent Doppler lidar;

[0060] The expression for the attenuation backscattering coefficient β'(R) is:

[0061]

[0062] Where β'(R) is the attenuation backscattering coefficient, cons. is a constant factor related to the coherent Doppler lidar system constants, and η h (R) is the focusing function, R is the distance between the aerosol and the coherent Doppler lidar, and CNR(R) is the carrier-to-noise ratio of the echo signal.

[0063] The focusing function is related to the size of the emitted laser pulse and the radius of curvature of the equiphase surface. Under the condition of an untruncated Gaussian beam, the focusing function η h The expression for (R) is:

[0064]

[0065] Where, η h (R) is the focusing function, ω T To emit laser pulses at the coherent Doppler lidar e-2 Irradiance radius, λ is the laser pulse wavelength, R f R is the radius of curvature of the equiphase surface that emits the laser pulse, ρ0 is the transverse coherence length related to turbulence, and R is the distance between the aerosol and the coherent Doppler lidar.

[0066] The constant factor *cons* is used for calibration with atmospheric target signals of known optical scattering characteristics, such as sufficiently thick non-precipitating stratocumulus clouds. The echo signal of non-precipitating stratocumulus clouds starts from 0, and its optical thickness is expressed as:

[0067]

[0068] Where τ(z) is the optical thickness of the non-precipitating stratocumulus, z is the length of the laser pulse within the non-precipitating stratocumulus, and σ is the extinction coefficient of the cloud at a distance r from the coherent Doppler lidar. c (r) and backscattering coefficient β c The relational expression for (r) is:

[0069] σ c (r)=ηS c β c (r) (5)

[0070] Among them, S c The lidar ratio of the cloud is given by η, where η is the multiple scattering factor and β' is the attenuation backscattering coefficient of the cloud. c The path integral expression for (z) is:

[0071]

[0072] At the 1.5-micron laser wavelength commonly used in coherent Doppler lidar, the radar ratio S of stratocumulus clouds is calculated to be approximately 20 based on Mie scattering theory. Since coherent Doppler lidar has a very narrow field of view, the multiple scattering factor is close to 1. Therefore, the constant factor in the attenuation backscattering coefficient can be determined through the above integral formula.

[0073] Step S3: The attenuation backscattering coefficient is inverted using the Collis slope method to obtain the extinction coefficient and backscattering coefficient of the aerosol.

[0074] The Collis slope method is applicable to ideal conditions where atmospheric aerosols are horizontally uniformly distributed. It can obtain the absolute extinction coefficient, and differentiating the formula yields:

[0075]

[0076] Under conditions of uniform horizontal distribution of aerosols, σ and β are both constants, therefore Therefore, the expression for the extinction coefficient

[0077]

[0078] Example 2

[0079] In this embodiment, steps S1 and S2 are the same as in Embodiment 1, and step S3 is implemented by the following method.

[0080] Step S3: The attenuation backscattering coefficient is inverted using the Klett-Fernald inversion method to obtain the extinction coefficient and backscattering coefficient of the aerosol.

[0081] In the Klett-Fernald inversion method, taking advantage of the low attenuation of near-field echoes, a calibration point is selected in the near field. The attenuated backscattering coefficient at this calibration point is used to calibrate the backscattering coefficients obtained from the Klett-Fernald inversion. The inversion results are then iteratively updated by updating the values ​​of the far-field reference point. The specific process is as follows:

[0082] Select the maximum detection range R of the coherent lidar c As a reference point, the reference point value is set as the aerosol backscattering coefficient β0(R). c ) equals the attenuation backscattering coefficient β'(R) c );

[0083] Neglecting atmospheric scattering and extinction, the Klett-Fernald inversion method is expressed as follows:

[0084]

[0085] Among them, S a Here, R is the distance between the aerosol and the coherent Doppler lidar, β'(r) and β'(R) are the attenuated backscattering coefficients, and β... i (R c R is the maximum detection range. c At point i, the aerosol backscattering coefficient value after the i-th iteration, β i+1 (R) represents the updated backscattering coefficient curve, N max The maximum number of iterations is set, and without loss of generality, it can be taken as 3;

[0086] Since the path attenuation of near-field echoes is very small, β'(R)≈β(R) is satisfied in the near field.

[0087] Select near-field calibration point R m Determine whether ||β is satisfied. i+1 (R m )-β'(R m If || < δ, where δ is a given threshold; if this condition is not met, the reference point value β is updated. i+1 (Rc )=β i (R c )β'(R m ) / β i+1 (R m ), and continue to calculate using the expression of the Klett-Fernald inversion method;

[0088] The aerosol backscattering coefficient β(R) = β i+1 (R) and extinction coefficient σ(R) = Sβ(R).

[0089] Example 3

[0090] In this embodiment, steps S1 and S2 are the same as in Embodiment 1, and step S3 is implemented by the following method.

[0091] Step S3: The attenuation backscattering coefficient is inverted using an iterative inversion method to obtain the extinction coefficient and backscattering coefficient of the aerosol.

[0092] In the iterative inversion method, the aerosol backscattering coefficient and extinction coefficient are inverted by iteratively correcting the path attenuation based on the aerosol lidar ratio S. The iterative inversion method includes:

[0093] The initial value of the aerosol backscattering coefficient β0(R) is set to be equal to the attenuated backscattering coefficient β'(R);

[0094] Update the backscattering coefficient β according to the expression of the iterative inversion method. i+1 The expression for the iterative inversion method is:

[0095]

[0096] Where, σ i (r)=S c β i (r), σ i (r) is the extinction coefficient for the i-th iteration, β i (r) is the aerosol backscattering coefficient in the i-th iteration, S is the set aerosol lidar ratio, R is the distance between the aerosol and the coherent Doppler lidar, β'(R) is the attenuated backscattering coefficient, and N max The maximum number of iterations is set, and without loss of generality, it can be taken as 3;

[0097] Determine if ||β is satisfied i+1 (R)-β i (R)||<δ, where δ is a given threshold;

[0098] If the condition is met, the iteration stops; if not, the iteration continues according to the expression of the iterative inversion method until the maximum number of iterations is reached.

[0099] The aerosol backscattering coefficient β(R) after path attenuation calibration is obtained. i+1 (R) and extinction coefficient σ(R) = Sβ(R).

[0100] Those skilled in the art will understand that embodiments of this application can be provided as methods, systems, or computer program products. Therefore, this application can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, this application can take the form of a computer program product embodied on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0101] This application is described with reference to flowchart illustrations and / or block diagrams of methods, systems, and computer program products according to embodiments of this application. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart... Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0102] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0103] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0104] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for aerosol parameter inversion based on coherent Doppler lidar, characterized in that, include: Receive echo signals; where the echo signals are the reflected signals of the laser pulses emitted into the atmosphere by the coherent Doppler lidar; The carrier-to-noise ratio of the echo signal is corrected for distance, focusing function, and system constant to obtain the attenuated backscattering coefficient; The extinction coefficient and backscattering coefficient of the aerosol are obtained by inverting the attenuation backscattering coefficient. The inversion methods include the slope method, the Klett-Fernald inversion method, or the iterative inversion method. The slope method is suitable for conditions where the horizontal atmosphere is homogeneous. In the Klett-Fernald inversion method, based on the characteristic of small attenuation of near-field echo, a calibration point is selected in the near field. The attenuated backscattering coefficient at the calibration point is used to calibrate the backscattering coefficient obtained by Klett-Fernald inversion. The inversion result is iteratively updated by updating the value of the far-field reference point. The Klett-Fernald inversion method includes: Select the maximum detection range of the coherent Doppler lidar As a reference point, the value at the reference point is set as the aerosol backscattering coefficient. Equal to the attenuation backscattering coefficient ; Neglecting atmospheric scattering and extinction, the Klett-Fernald inversion method is expressed as follows: （9）； in, For the set aerosol lidar ratio, The distance between the aerosol and the coherent Doppler lidar. and To attenuate the backscattering coefficient, Maximum detection range At point i, the aerosol backscattering coefficient value after the i-th iteration. To update the backscattering coefficient curve, The maximum number of iterations is set. Based on the fact that near-field echoes have low path attenuation, in the near field... Set near-field calibration points Determine whether it satisfies ,in For a given threshold; If the conditions are met, stop the inversion calculation; otherwise... Update reference point values Continue to calculate using the expression obtained through the Klett-Fernald inversion method; Obtain the aerosol backscattering coefficient and atmospheric extinction coefficient .

2. The aerosol parameter inversion method based on coherent Doppler lidar according to claim 1, characterized in that: The carrier-to-noise ratio expression for the echo signal is: （1）； in, The carrier-to-noise ratio of the echo signal. The single-pulse energy of the laser emitted by a coherent Doppler lidar. The distance between the aerosol and the coherent Doppler lidar. This is the atmospheric extinction coefficient, which includes atmospheric molecular extinction and aerosol extinction. The aerosol backscattering coefficient is... The effective receiving area of ​​the coherent Doppler lidar. The speed of light in air. For the focusing function, For the efficiency of coherent Doppler lidar systems, Photon energy, This represents the bandwidth of the coherent Doppler lidar.

3. The aerosol parameter inversion method based on coherent Doppler lidar according to claim 1, characterized in that: The expression for the attenuation backscattering coefficient is: （2）； in, To attenuate the backscattering coefficient, A constant factor related to the constants of a coherent Doppler lidar system. For the focusing function, The distance between the aerosol and the coherent Doppler lidar. This represents the carrier-to-noise ratio of the echo signal.

4. The aerosol parameter inversion method based on coherent Doppler lidar according to claim 1, characterized in that: Under the condition of an untruncated Gaussian beam, the expression for the focusing function is: （3）； in, For the focusing function, To emit laser pulses at the coherent Doppler lidar Irradiance radius, The wavelength of the laser pulse. The radius of curvature of the isophase surface emitting the laser pulse. The transverse coherence length associated with turbulence, This represents the distance between the aerosol and the coherent Doppler lidar.

5. The aerosol parameter inversion method based on coherent Doppler lidar according to claim 3, characterized in that: The constant factor is calibrated using a non-precipitating stratocumulus target signal with known optical scattering characteristics; the echo signal of the non-precipitating stratocumulus starts from 0, and its optical thickness is expressed as: （4）； in, The optical thickness of non-precipitating stratocumulus clouds. It is the length of the laser pulse within non-precipitating stratocumulus clouds, at a distance from the coherent Doppler lidar. Extinction coefficient of clouds at that location With backscattering coefficient The relational expression is: （5）； in, The ratio of LiDAR to cloud computing, The multiple scattering factor is the attenuated backscattering coefficient of the cloud. The path integral expression is: （6）。 6. The aerosol parameter inversion method based on coherent Doppler lidar according to claim 1, characterized in that: In the iterative inversion method, based on the aerosol lidar ratio The aerosol backscattering coefficient and atmospheric extinction coefficient are retrieved by iteratively correcting the path attenuation.

7. The aerosol parameter inversion method based on coherent Doppler lidar according to claim 6, characterized in that, Iterative inversion methods include: Set the initial value for the aerosol backscattering coefficient during iteration. Equal to the attenuation backscattering coefficient ; Update the backscattering coefficient values ​​according to the expression of the iterative inversion method. The expression for the iterative inversion method is: （10）； in, , Let be the extinction coefficient for the i-th iteration. Let be the aerosol backscattering coefficient in the i-th iteration. For the set aerosol lidar ratio, The distance between the aerosol and the coherent Doppler lidar. To attenuate the backscattering coefficient, The maximum number of iterations is set. Determine if it satisfies ,in For a given threshold; If the condition is met, the iteration stops; if not, the iteration continues according to the expression of the iterative inversion method until the maximum number of iterations is reached. Obtain the aerosol backscattering coefficient after path attenuation calibration and extinction coefficient .

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