Polarization two-channel gain ratio inversion method based on polarization laser radar

Abstract

The invention discloses a polarization two-channel gain ratio inversion method based on a polarization laser radar, and belongs to the technical field of polarization laser radar inversion methods. The method comprises the following steps: selecting the weather with cirrus clouds, and carrying out vertical detection on the atmosphere by using a polarization laser radar to obtain a first group of data and a second group of data; respectively extracting corresponding total polarization channel data from the two groups of data; performing inversion by using a Klett equation to obtain a volume cloud layer optical thickness corresponding to the second group of data; combining the volume cloud layer part of the second group of polarization channel total data with the non-volume cloud layer part of the first group of polarization channel total data to generate mixed data; the polarization two-channel gain ratio of the volume cloud layer part in the mixed data is adjusted, the optical thickness of the volume cloud layer can be changed, and when the optical thickness of the volume cloud layer obtained by inversion of the mixed data is consistent with a result obtained by using the second group of data, the polarization two-channel gain ratio of the volume cloud layer is the polarization two-channel gain ratio corresponding to the second group of data.

Description

Technical Field

[0001] This invention relates to the field of polarization lidar inversion methods, specifically to a polarization two-channel gain ratio inversion method based on polarization lidar. Background Technology

[0002] The method for determining the polarization gain ratio of polarization lidar channels is crucial to the accuracy of polarization lidar measurements of the depolarization ratio of clouds and aerosols. The depolarization ratio of clouds and aerosols directly affects the classification of the detected objects and the judgment of particle shapes. Therefore, accurately measuring or solving the polarization gain ratio of polarization lidar channels is a very important topic.

[0003] Currently, the gain ratio of the two polarization channels is mainly obtained through two methods. One method involves loading measurement devices into the transmitting or receiving optical path of the polarization lidar. For example, placing a half-wave plate or depolarizer in the receiving optical path of the polarization lidar allows for the measurement of the gain ratio of the two polarization channels. The method using a half-wave plate is called the half-wave plate method, which is further divided into the 90° method, +45° method, ±45° method, and ∆45° method. The spaceborne lidar CALIOP measures the gain ratio of the two polarization channels by placing a depolarizer in the receiving optical path; this method is called the depolarizer method. Both the half-wave plate method and the depolarizer method require placing hardware in the optical path of the polarization lidar, increasing the manufacturing cost of the polarization lidar. Since depolarizers cannot completely depolarize the beam in the optical path, using the depolarizer method introduces errors into the measurement results of the polarization channel gain ratio. Another method is the clean atmosphere measurement method. This involves selecting a clean atmosphere free of aerosols and clouds, measuring its actual depolarization ratio using a polarization lidar, and then obtaining the theoretical depolarization ratio of the clean atmosphere based on relevant atmospheric scattering theories. The polarization channel gain ratio of the polarization lidar can then be obtained from the actual and theoretical depolarization ratios of the clean atmosphere. However, because clean atmospheres often contain small amounts of aerosols and clouds in actual atmospheres, the polarization channel gain ratio obtained using the clean atmosphere measurement method often has significant errors. Some two-channel lidars lack polarization calibration devices for the polarization channels, making polarization calibration a challenge for these lidars. Summary of the Invention

[0004] The purpose of this invention is to provide a polarization two-channel gain ratio inversion method based on polarization lidar. In order to accurately polarize lidar data without a polarization calibration device, the polarization two-channel gain ratio of the polarization lidar can be accurately obtained, thereby solving the problems mentioned in the background art.

[0005] To achieve the above objectives, the present invention provides the following technical solution: a polarization two-channel gain ratio inversion method based on polarization lidar, comprising the following steps:

[0006] S1: Obtain the first and second sets of data: Select a weather with cirrus clouds. With the known gain ratio of the two polarization channels, use a polarization lidar to perform vertical detection on the atmosphere to obtain the first set of polarization parallel channel and polarization vertical channel data; with the unknown gain ratio of the two polarization channels, use a polarization lidar to perform vertical detection on the atmosphere to obtain the second set of polarization parallel channel and polarization vertical channel data.

[0007] S2: Synthesize the total data of the first group of polarization channels: The gain ratio k1 of the two polarization channels in the first group is known. Based on k1, the total data P1(r') of the first group of polarization channels is synthesized using the data of the parallel polarization channel and the perpendicular polarization channel in the first group. The synthesis formula is as follows:

[0008] (1)

[0009] Where k1 is the gain ratio of the two polarization channels during the measurement of the first set of data, P p1 (r') and P s1 (r') represents the data of the first group of polarization parallel channel and polarization perpendicular channel, respectively, and P1(r') represents the total data of the first group of polarization channels after synthesis;

[0010] S3: Synthesizing the total data of the second group of polarization channels: The gain ratio k2 of the two polarization channels in the second group is unknown. Based on the gain ratio k1 of the two polarization channels in the first group, the total data P2(r') of the second group of polarization channels is synthesized using the data of the parallel polarization channel and the perpendicular polarization channel in the second group. The formula used is as follows:

[0011] (2)

[0012] Among them, P p2 (r') and P s2 (r') represents the polarization parallel channel and polarization perpendicular channel data of the second group, respectively, and P2(r') represents the total data of the second group of polarization channels after synthesis;

[0013] S4: Obtain the true value of the optical thickness of the cirrus cloud layer in the second group: According to P2(r'), the extinction coefficient profile of the cirrus cloud layer corresponding to the total data of the polarization channel in the second group is obtained by inversion using the Klett equation. According to the optical thickness formula, the optical thickness of the cirrus cloud layer is solved by using the extinction coefficient profile of the cirrus cloud layer.

[0014] The Klett equation used is as follows:

[0015] Klett's forward equation:

[0016] (3)

[0017] Klett's backward equation:

[0018] (4)

[0019] Where, S(r')=ln[(r') 2 [P(r')], where the subscript c represents the calibration point, P(r') is the polarization lidar echo signal at height r', and P1(r') and P2(r') represent the total data of the first and second polarization channels, respectively, and P is the mixed data synthesized from the first group of non-ciricular cloud data and the second group of cirrus cloud data. hn (r'), where n is an integer, n = 0, 1, 2, ..., m, ...; σ c =σ(r' c ), S c =S(r' c ), σ(r' c B is the extinction coefficient of the aerosol or cirrus cloud at the calibration point, and B = B(r') is the extinction backscattering ratio of the aerosol or cirrus cloud at height r'. c =B(r' c ), B(r' c ) represents the aerosol or cirrus cloud extinction backscattering ratio at the calibration point height, and σ(r') represents the aerosol or cirrus cloud extinction coefficient at height r'.

[0020] The formula for calculating the optical thickness of cirrus clouds is as follows:

[0021] (5)

[0022] Wherein, σ(r') is the extinction coefficient of aerosol or cirrus cloud at height r'. In formula (5), since τ is the optical thickness of cirrus cloud, σ(r') only represents the extinction coefficient of cirrus cloud. r1 and r2 are the heights of the bottom and top of cirrus cloud, respectively.

[0023] The total polarization channel data P2(r') synthesized using formula (2), the cirrus extinction coefficient profile obtained by inversion from the Klett equation, and the cirrus optical thickness calculated from the cirrus extinction coefficient profile using formula (5) are all true values. The cirrus optical thickness calculated using P2(r') synthesized using formula (2) is called the true value of the cirrus optical thickness, denoted as τ. t ;

[0024] S5: Combine the total polarization channel data of the non-cirrus cloud layer in Group 1 and the cirrus cloud layer in Group 2 into mixed data, and use the mixed data to obtain the polarization channel gain ratio when the data of Group 2 is detected.

[0025] Furthermore, the specific steps of S5 are as follows:

[0026] The polarization gain ratio for the two channels of the data detected outside the cirrus layer in Group 1 is k1, and the true value of the polarization gain ratio for the two channels of the cirrus layer data in Group 2 is k2. The cirrus layer polarization gain ratio k used in the mixed data synthesis is... xn It can be obtained using the following formula:

[0027] (6)

[0028] Where a is a constant assigned based on the parameters of the two polarization channels of the lidar, g is the set step size, for example g=0.01, and n is an integer, n=0, 1, 2, ..., m, ...;

[0029] Total data P of cirrus cloud polarization channel in mixed data 2n The synthesis formula for (r') is:

[0030] (7)

[0031] P p2 (r') and P s2 The physical meaning of (r') is the same as that of formula (2); the mixed data synthesized from the first group of non-cirrus cloud data and the second group of cirrus cloud data is denoted as P. hn (r'), the value of n is consistent with that in formula (6); according to the polarization two-channel parameter settings of the second set of data, assign a value to a in formula (6), a=0.75, and the initial value of n is 0; according to formula (7) and the second set of polarization parallel channel and polarization vertical channel data P p2 (r') and P s2 (r'), to obtain the synthesized second set of polarization channel total data P 20 (r'), with P 20 The cirrus cloud data in (r') replaces the cirrus cloud data in P1(r'), and P... 20 The total polarization channel data of the cirrus cloud layer in (r') and the total polarization channel data of the first group outside the cirrus cloud layer are used to obtain the mixed data P. h0 (r');

[0032] Using mixed data P hn The optical thickness of the cirrus layer obtained by inversion (r') is denoted as τ. n n = 0, 1, 2, ..., m, ...; according to P h0 (r') and Klett's equation are used to obtain the extinction coefficient profile of the cirrus cloud layer in the mixed data; then the optical thickness τ0 of the cirrus cloud layer is calculated using formula (5), and τ0 is compared with the true value of the optical thickness τ of the cirrus cloud layer obtained in S4. t Comparison, if τ0 = τ t Then k2=k x0Since a is known, k2=a is the gain ratio of the two polarization channels corresponding to the second set of data;

[0033] If τ0≠τ t The value of n is taken sequentially according to the cyclic formula n=n+1, which is 1, 2, 3, ..., m, ... For each value of n, the above steps are repeated, and a series of polarization two-channel gain ratios k are obtained using formula (6). x1 k x2 k x3 , ..., k xm Substituting into formula (7), we obtain the corresponding total data P of the polarization channel of the second group of cirrus clouds. 21 (r'), P 22 (r'), P 23 (r'), ..., P 2m (r'), ..., using the total data of the cirrus polarization channel in group 2 and the total data of the non-cirrus polarization channel in group 1 to synthesize mixed data P h1 (r'), P h2 (r'), P h3 (r'), ..., P hm (r'), ..., using mixed data and the Klett equation, a series of cirrus extinction coefficient profiles in the mixed data are calculated. According to formula (5), the corresponding optical thickness of the cirrus in the mixed data is obtained and compared with the true value of the optical thickness of the cirrus obtained in S4; when n takes a certain value m, using the mixed data P hm The optical thickness τ of cirrus clouds is obtained by using the Klett equation and formula (5). m , and τ m And the true value τ of the cirrus cloud optical thickness obtained using P2(r') in step S4. t They are equal, at which point k2 = k xm =a+mg is the gain ratio of the two polarization channels when the polarization lidar detects the second set of data.

[0034] Furthermore, in step S1, the first set of data is measured when the gain ratio k1 of the two polarization channels is known. Then, the voltage and other parameters on the photomultiplier tubes of the two polarization channels are adjusted to change the gain ratio of the two polarization channels. The second set of data is obtained by atmospheric detection using polarization lidar. The change in the optical thickness of aerosols outside the cirrus cloud layer between the first and second sets of data is less than 5%, and the measurement time interval between the two sets of data must be less than 30 minutes.

[0035] Furthermore, in S5, when the mixed data is composed of the cirrus cloud layer of the second group and the non-cirrus cloud layer of the first group, the thickness of the cirrus cloud layer in the total polarization channel data of the second group is significantly different from that in the total polarization channel data of the first group. The height range of the cirrus cloud layer with the greater thickness in the two groups of data is defined as the height range of the cirrus cloud layer. The mixed data is composed of the total polarization channel data of the cirrus cloud layer of the second group and the total polarization channel data of the non-cirrus cloud layer of the first group.

[0036] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0037] This invention can accurately obtain the gain ratio of the two polarization channels after adjusting the polarization channel parameters simply by continuously measuring two sets of data in weather with cirrus clouds. It has two major advantages: first, it eliminates the need to place a polarization calibration device on the polarization channel, simplifying the optical path of the polarization lidar and saving the development cost of the polarization lidar; second, the relative error of the calculation result of the polarization channel gain ratio using this method can usually be controlled within 2%. Attached Figure Description

[0038] Figure 1 This is an overall flowchart of the method of the present invention.

[0039] Figure 2 This is a stitched diagram of the total data of the first and second groups of polarization channels in this invention.

[0040] Figure 3 This is a graph showing the variation of the extinction coefficient profile of cirrus clouds when calculating the polarization two-channel gain ratio according to the present invention. Detailed Implementation

[0041] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0042] Please see Figure 1-3 This embodiment provides the following technical solution: a polarization two-channel gain ratio inversion method based on polarization lidar, comprising the following steps:

[0043] S1: Obtain the data from group 1 and group 2: Specifically, this includes:

[0044] S11: Select a day with cirrus clouds and adjust the gain ratio of the two channels of the polarization lidar to the known value k1;

[0045] S12: Using polarization lidar to perform vertical atmospheric detection, obtain the first set of polarization parallel channel and polarization vertical channel data P containing cirrus cloud layers. p1 (r') and P s1 (r');

[0046] S13: Adjust parameters that affect the gain ratio of the two polarization channels, such as the voltage on the photomultiplier tube of the polarization lidar. The gain ratio k2 of the two polarization channels of the polarization lidar after adjustment is unknown.

[0047] S14: When the gain ratio of the two polarization channels is k2, a polarization lidar is used to perform vertical atmospheric detection to obtain the second set of polarization parallel channel and polarization vertical channel data P containing cirrus clouds. p2 (r') and P s2 (r');

[0048] When using the data from Group 1 and Group 2 to calculate the gain ratio of the polarization channels of a polarization lidar, the change in the optical thickness of the aerosol outside the cirrus cloud layer should be less than 5%, and the measurement time interval between the two sets of data should be less than 30 minutes.

[0049] S2: Synthesize the total data of the first group of polarization channels: The gain ratio k1 of the two polarization channels in the first group is known. Based on k1, the total data P1(r') of the first group of polarization channels is synthesized using the data of the parallel polarization channel and the perpendicular polarization channel in the first group. The specific formula is as follows:

[0050] (1)

[0051] Where k1 is the gain ratio of the two polarization channels during the measurement of the first set of data, P1(r') is the total data of the first set of polarization channels after synthesis, and P p1 (r') represents the polarization parallel channel data of the first group, P s1 (r') represents the polarization vertical channel data of the first group;

[0052] S3: Synthesize the total data of the second polarization channel: The gain ratio k2 of the two polarization channels in the second group of data is unknown. Based on the gain ratio k1 of the two polarization channels in the first group of data, synthesize the total data P2(r') of the second group of polarization channels using the data of the parallel polarization channel and the perpendicular polarization channel in the second group. The synthesis formula is as follows:

[0053] (2)

[0054] Among them, P p2 (r') and P s2 (r') represents the polarization parallel channel and polarization perpendicular channel data detected in the second group, respectively, and P2(r') represents the total polarization channel data of the second group after synthesis;

[0055] S4: Based on P2(r'), the extinction coefficient profile of the cirrus cloud layer corresponding to the total data of the second polarization channel is obtained by inversion using the Klett equation. According to the optical thickness formula, the optical thickness of the cirrus cloud layer is solved using the extinction coefficient profile. The specific steps are as follows:

[0056] S41: The Klett equation used is as follows:

[0057] Klett's forward equation:

[0058] (3)

[0059] Klett's backward equation:

[0060] (4)

[0061] Where, S(r')=ln[(r') 2 [P(r')], where the subscript c represents the calibration point, and P(r') is the polarization lidar echo signal at height r'.

[0062] P1(r') and P2(r') represent the total polarization channel data of Group 1 and Group 2, respectively, and P represents the hybrid data synthesized from Group 1 non-ciricular cloud data and Group 2 cirrus cloud data. hn (r'), where n is an integer, n = 0, 1, 2, ..., m, ...; σ c =σ(r' c ), S c =S(r' c ), σ(r' c B is the extinction coefficient of the aerosol or cirrus cloud at the calibration point, and B = B(r') is the extinction backscattering ratio of the aerosol or cirrus cloud at height r'. c =B(r' c ), B(r' c ) represents the aerosol or cirrus cloud extinction backscattering ratio at the calibration point height, and σ(r') represents the aerosol or cirrus cloud extinction coefficient at height r'.

[0063] S42: Based on the extinction coefficient profile of cirrus clouds obtained by inversion from the Klett equation, the optical thickness of cirrus clouds is calculated. The formula for solving the optical thickness of cirrus clouds is as follows:

[0064] (5)

[0065] Wherein, σ(r') is the extinction coefficient of aerosol or cirrus cloud at height r'. In formula (5), since τ is the optical thickness of cirrus cloud, σ(r') only represents the extinction coefficient of cirrus cloud. r1 and r2 are the heights of the bottom and top of cirrus cloud, respectively.

[0066] k1 and k2 are the polarization channel gain ratios corresponding to the detection of the first and second sets of data, respectively, but k2 is unknown; although k1 is not the polarization channel gain ratio when measuring the second set of data, due to the polarization parallel channel and polarization perpendicular channel data P p2 (r'), P s2 (r') comes from the same polarization lidar detection, and when synthesizing the second set of polarization channel total data P2(r'), the gain ratio of the two polarization channels used in the entire height range is k1. In this way, the cirrus extinction coefficient profile and the cirrus optical thickness obtained by using P2(r') are not affected by the gain ratio of the two polarization channels used. That is to say, the polarization channel total data P2(r') synthesized by formula (2), the cirrus extinction coefficient profile obtained by inversion of the Klett equation, and the cirrus optical thickness calculated by formula (5) from the cirrus extinction coefficient profile are all true values. For ease of expression, the cirrus optical thickness calculated by using P2(r') synthesized by formula (2) is called the true value of cirrus optical thickness, denoted as τ. t ;

[0067] S5: Combine the total polarization channel data of the non-cirrus cloud layer in Group 1 and the cirrus cloud layer in Group 2 into mixed data. Use the mixed data to obtain the gain ratio of the two polarization channels during the detection of the data in Group 2. The specific steps are as follows:

[0068] The polarization gain ratio for the two channels of the data detected outside the cirrus layer in Group 1 is k1, and the true value of the polarization gain ratio for the two channels of the cirrus layer data in Group 2 is k2; the cirrus layer polarization gain ratio k used in the mixed data synthesis is... xn It can be obtained from the following formula:

[0069] (6)

[0070] Where a is a constant assigned based on the parameters of the two polarization channels of the lidar, g is the set step size, with a value of 0.01, and n is an integer, n=0, 1, 2, ..., m, ...;

[0071] Total data P of cirrus cloud polarization channel in mixed data 2n The synthesis formula for (r') is:

[0072] (7)

[0073] P p2 (r') and P s2 The physical meaning of (r') is the same as that of formula (2); the mixed data synthesized from the first group of non-cirrus cloud data and the second group of cirrus cloud data is denoted as P. hn(r'), the value of n is consistent with that of n in formula (6); when the mixed data is composed of the cirrus layer of the second group and the non-cirrus layer of the first group, the thickness of the cirrus layer of the total polarization channel data of the second group is much larger than that of the cirrus layer of the total polarization channel data of the first group. The height range of the cirrus layer with the larger thickness in the two groups of data is defined as the height range of the cirrus layer. The mixed data is composed of the total polarization channel data of the cirrus layer of the second group and the total polarization channel data of the non-cirrus layer of the first group.

[0074] Based on the polarization two-channel parameter settings of the second set of data, assign a value to a in formula (6): a=0.75, and the initial value of n is 0; based on formula (7) and the second set of polarization parallel channel and polarization perpendicular channel data P p2 (r') and P s2 (r'), to obtain the synthesized second set of polarization channel total data P 20 (r'), with P 20 The cirrus cloud data in (r') replaces the cirrus cloud data in P1(r'), and P... 20 The total polarization channel data of the cirrus cloud layer in P1(r') and the total polarization channel data outside the cirrus cloud layer in P1(r') are combined to form a mixed data P. h0 (r');

[0075] Using mixed data P hn The optical thickness of the cirrus layer obtained by inversion (r') is denoted as τ. n n=0, 1, 2, ..., m, ...; according to P h0 (r') and Klett's equation are used to obtain the extinction coefficient profile of the cirrus cloud layer in the mixed data; then, the corresponding optical thickness τ0 of the cirrus cloud layer is calculated using formula (5), and τ0 is compared with the true value of the optical thickness τ of the cirrus cloud layer obtained in S4. t Comparison, if τ0 = τ t Then k2=k x0 Since a is known, k2=a is the gain ratio of the two polarization channels corresponding to the second set of data;

[0076] If τ0≠τ t The value of n is taken sequentially according to the cyclic formula n=n+1, which is 1, 2, 3, ..., m, ... For each value of n, the above steps are repeated, and a series of polarization two-channel gain ratios k are obtained using formula (6). x1 k x2 k x3 , ..., k xm Substituting into formula (7), we obtain the corresponding total data P of the polarization channel of the second group of cirrus clouds. 21 (r'), P 22 (r'), P 23 (r'), ..., P 2m(r'), ..., using the total cirrus polarization channel data of group 2 and the total non-cirrus polarization channel data of P1(r') to synthesize mixed data P h1 (r'), P h2 (r'), P h3 (r'), ..., P hm (r'), ..., using the mixed data and Klett equation, a series of cirrus extinction coefficient profiles in the mixed data are calculated. According to formula (5), the corresponding optical thickness of the cirrus in the mixed data is obtained, and compared with the true value τ of the optical thickness of the cirrus obtained in step S4. t Comparison; when n takes the value m, use mixed data P hm The optical thickness τ of cirrus clouds is obtained from (r'), Klett's equation, and formula (5). m And the true value τ of the cirrus cloud optical thickness obtained using P2(r') in step S4. t They are equal, at which point k2 = k xm =a+mg is the gain ratio of the two polarization channels when the polarization lidar detects the second set of data.

[0077] Calculation example: Figure 3 This displays the change in the extinction coefficient of cirrus clouds during the process of solving the gain ratio of two channels with unknown polarization. Figure 3 The "result" in the table refers to the cirrus extinction coefficient profile obtained by inversion using the second set of data P2(r'). These are the true values ​​of the second set of cirrus extinction coefficient profiles. "1.0", "1.05", "1.10", and "1.15" represent k, respectively. xn For values ​​of 1.0, 1.05, 1.10, and 1.15, the extinction coefficient profiles of cirrus clouds obtained by inversion using mixed data are shown in the figure. It can be seen from the figure that when k... xn When k = 1.15, the cirrus extinction coefficient profile obtained by inversion using P2(r') coincides with the cirrus extinction coefficient profile obtained by inversion using mixed data. According to the formula for the optical thickness of the cirrus layer, the optical thickness of the cirrus layer obtained by inversion using P2(r') and the optical thickness of the cirrus layer obtained by inversion using mixed data are equal. Therefore, k2 = 1.15, which is the polarization gain ratio of the polarization two channels of the polarization lidar when detecting the second set of data.

[0078] In summary, this invention replaces the cirrus data of the first group with the cirrus data of the second group, while keeping the other data of the first group unchanged, to form mixed data. Utilizing the different polarization gain ratios of the cirrus and other detection data outside the cirrus in the mixed data, accurate cirrus optical thickness can only be obtained using the mixed data when the polarization data of the cirrus is correct. Although the total polarization data of the second group is obtained based on the polarization gain ratio of the first group, the Klett equation can still accurately yield the cirrus optical thickness corresponding to the total polarization data of the second group. Using the true value of the cirrus optical thickness obtained from the total polarization data of the second group as a reference, changing the polarization gain ratio during cirrus synthesis in the mixed data allows obtaining the corresponding cirrus optical thickness in the mixed data. When the cirrus optical thickness in the mixed data is equal to the result obtained using the total polarization data of the second group, the adopted polarization gain ratio is the desired result. This method only requires two sets of data to be continuously measured by a polarization lidar in weather with cirrus clouds. It can accurately obtain the gain ratio of the two polarization channels after the polarization channel parameters are adjusted. It has two major advantages: First, it eliminates the need to place a polarization calibration device on the polarization channel, which simplifies the optical path of the polarization lidar and saves the development cost of the polarization lidar. Second, the relative error of the calculation result of the polarization channel gain ratio using this method can usually be controlled within 2%.

[0079] The present invention and its embodiments have been described above illustratively. This description is not restrictive, and the figures shown are only one embodiment of the present invention; the actual structure is not limited thereto. Therefore, if those skilled in the art are inspired by this description and design similar structures and embodiments without departing from the spirit of the present invention, such designs should fall within the protection scope of the present invention.

Claims

1. A polarization two-channel gain ratio inversion method based on polarization lidar, characterized in that: Includes the following steps: S1: Obtain the first and second sets of data: Select a weather with cirrus clouds. With the known gain ratio of the two polarization channels, use a polarization lidar to perform vertical detection on the atmosphere to obtain the first set of polarization parallel channel and polarization vertical channel data; with the unknown gain ratio of the two polarization channels, use a polarization lidar to perform vertical detection on the atmosphere to obtain the second set of polarization parallel channel and polarization vertical channel data. S2: Synthesize the total data of the first group of polarization channels: The gain ratio k1 of the two polarization channels in the first group is known. Based on k1, the total data P1(r') of the first group of polarization channels is synthesized using the data of the parallel polarization channel and the perpendicular polarization channel in the first group. The synthesis formula is as follows: （1） Where k1 is the gain ratio of the two polarization channels during the measurement of the first set of data, P p1 (r') and P s1 (r') represents the data of the first group of polarization parallel channel and polarization perpendicular channel, respectively, and P1(r') represents the total data of the first group of polarization channels after synthesis; S3: Synthesize the total data of the second polarization channel: The gain ratio k2 of the two polarization channels in the second group is unknown. Based on the gain ratio k1 of the two polarization channels in the first group, the total data P2(r') of the second polarization channel is synthesized using the data of the parallel polarization channel and the perpendicular polarization channel in the second group. The synthesis formula is as follows: （2） Among them, P p2 (r') and P s2 (r') represents the polarization parallel channel and polarization perpendicular channel data detected in the second group, respectively, and P2(r') represents the total polarization channel data of the second group after synthesis; S4: Obtain the true value of the optical thickness of the cirrus cloud layer in the second group: According to P2(r'), the extinction coefficient profile of the cirrus cloud layer corresponding to the total data of the polarization channel in the second group is obtained by inversion using the Klett equation. According to the optical thickness formula, the optical thickness of the cirrus cloud layer is solved by using the extinction coefficient profile of the cirrus cloud layer. The Klett equation used is as follows: Klett's forward equation: （3） Klett's backward equation: （4） Where, S(r')=ln[(r') 2 [P(r')], where the subscript c represents the calibration point, P(r') is the polarization lidar echo signal at height r', and P1(r') and P2(r') represent the total data of the first and second polarization channels, respectively, and P is the mixed data synthesized from the first group of non-ciricular cloud data and the second group of cirrus cloud data. hn (r'), where n is an integer, n = 0, 1, 2, ..., m; σ c =σ(r' c ), S c =S(r' c ), σ(r' c B is the extinction coefficient of the aerosol or cirrus cloud at the calibration point, and B = B(r') is the extinction backscattering ratio of the aerosol or cirrus cloud at height r'. c =B(r' c ), B(r' c ) represents the aerosol or cirrus cloud extinction backscattering ratio at the calibration point height, and σ(r') represents the aerosol or cirrus cloud extinction coefficient at height r'. The formula for calculating the optical thickness of cirrus clouds is as follows: （5） Wherein, σ(r') is the extinction coefficient of aerosol or cirrus cloud at height r'. In formula (5), since τ is the optical thickness of cirrus cloud, σ(r') only represents the extinction coefficient of cirrus cloud. r1 and r2 are the heights of the bottom and top of cirrus cloud, respectively. The total polarization channel data P2(r') synthesized using formula (2), the cirrus extinction coefficient profile obtained by inversion from the Klett equation, and the cirrus optical thickness calculated from the cirrus extinction coefficient profile using formula (5) are all true values. The cirrus optical thickness calculated using P2(r') synthesized using formula (2) is called the true value of the cirrus optical thickness, denoted as τ. t ; S5: Combine the total polarization channel data of the non-cirrus cloud layer in Group 1 and the cirrus cloud layer in Group 2 into mixed data, and use the mixed data to obtain the gain ratio of the two polarization channels when detecting the data of Group 2.

2. The polarization two-channel gain ratio inversion method based on polarization lidar according to claim 1, characterized in that: The specific steps of S5 are as follows: The polarization gain ratio for the two channels of the data detected outside the cirrus layer in Group 1 is k1, and the true value of the polarization gain ratio for the two channels of the cirrus layer data in Group 2 is k2. The cirrus layer polarization gain ratio k used in the mixed data synthesis is... xn Expressed using the following formula: （6） Where a is a constant assigned based on the parameters of the two polarization channels of the lidar, g is the set step size, for example g=0.01, and n is an integer, n=0, 1, 2, ..., m, ...; Total data P of cirrus cloud polarization channel in mixed data 2n The synthesis formula for (r') is: （7） P p2 (r') and P s2 The physical meaning of (r') is the same as that of formula (2); the mixed data synthesized from the first group of non-cirrus cloud data and the second group of cirrus cloud data is denoted as P. hn (r'), the value of n is consistent with that of n in formula (6); according to the polarization two-channel parameter settings of the second set of data, assign a value to a in formula (6), a=0.75, and the initial value of n is 0; according to formula (7) and the second set of polarization parallel channel and polarization vertical channel data P p2 (r') and P s2 (r'), to obtain the synthesized second set of polarization channel total data P 20 (r'), with P 20 The cirrus cloud data in (r') replaces the cirrus cloud data in P1(r'), and P... 20 The total polarization channel data of the cirrus cloud layer in (r') and the total polarization channel data of the first group outside the cirrus cloud layer are used to obtain the mixed data P. h0 (r'); Using mixed data P hn The optical thickness of the cirrus layer obtained by inversion (r') is denoted as τ. n n = 0, 1, 2, ..., m, ...; according to P h0 (r') and Klett's equation are used to obtain the extinction coefficient profile of the cirrus cloud layer in the mixed data; then the optical thickness τ0 of the cirrus cloud layer is calculated using formula (5), and τ0 is compared with the true value of the optical thickness τ of the cirrus cloud layer obtained in S4. t Comparison, if τ0 = τ t Then k2=k x0 Since a is known, then k2 = a, which is the gain ratio of the two polarization channels corresponding to the second set of data; If τ0≠τ t The value of n is taken sequentially according to the cyclic formula n=n+1, which is 1, 2, 3, ..., m, ... For each value of n, the above steps are repeated, and a series of polarization two-channel gain ratios k are obtained using formula (6). x1 k x2 k x3 , ..., k xm Substituting into formula (7), we obtain the corresponding total data P of the polarization channel of the second group of cirrus clouds. 21 (r'), P 22 (r'), P 23 (r'), ..., P 2m (r'), ..., using the total data of the cirrus polarization channel in group 2 and the total data of the non-cirrus polarization channel in group 1 to synthesize mixed data P h1 (r'), P h2 (r'), P h3 (r'), ..., P hm (r'), ..., using mixed data and the Klett equation, a series of cirrus extinction coefficient profiles in the mixed data are calculated. According to formula (5), the corresponding cirrus optical thickness is obtained and compared with the true value of cirrus optical thickness obtained in S4; when n takes a certain value m, using the mixed data P hm The optical thickness of the cirrus cloud layer obtained by (r'), Klett's equation, and formula (5) is τ. m If τ m And the true value τ of the cirrus cloud optical thickness obtained using P2(r') in step S4. t They are equal, at which point k2 = k xm =a+mg is the gain ratio of the two polarization channels when the polarization lidar detects the second set of data.

3. The polarization two-channel gain ratio inversion method based on polarization lidar according to claim 1, characterized in that: In step S1, the first set of data is measured when the gain ratio k1 of the two polarization channels is known. Then, the voltage and other parameters on the photomultiplier tubes of the two polarization channels are adjusted to change the gain ratio of the two polarization channels. The second set of data is obtained by using a polarization lidar to detect the atmosphere. The change in the optical thickness of aerosols outside the cirrus cloud layer between the first and second sets of data is less than 5%, and the measurement time interval between the two sets of data must be less than 30 minutes.

4. The polarization two-channel gain ratio inversion method based on polarization lidar as described in claim 1, characterized in that: In S5, when the mixed data consists of cirrus clouds from the second group and non-cirrus clouds from the first group, the thickness of the cirrus clouds in the total polarization channel data of the second group varies more than that of the cirrus clouds in the total polarization channel data of the first group. The height range of the cirrus clouds with the greater thickness in the two groups of data is defined as the height range of the cirrus clouds. The mixed data is composed of the total polarization channel data of the cirrus clouds from the second group and the total polarization channel data of the non-cirrus clouds from the first group.

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