Doppler laser radar blind area compensation method based on side backward light scintillation principle

Abstract

The invention discloses a Doppler laser radar blind area compensation method based on a side backward light scintillation principle, and belongs to the technical field of large airflow field detection. According to the method, a wind field in a dead zone range of the Doppler wind measurement laser radar is taken as a detection object, the laser light intensity on a detection path is obtained through a Sammer imaging principle, and detection distance information is obtained through a geometrical relationship between pixels and laser light. According to a Taylor freezing condition, when the time required by the development of a group of aerosols is greater than the time required by the aerosols to pass through the two laser beams, determining that the aerosols are 'frozen'. When the distance between the two detection points is small enough, the two received signals have high similarity, and the time is hysteretic due to the distance between the detection points. The wind speed information is acquired by using the change of the brightness fluctuation of the adjacent pixels on the measurement path along with time, the dead zone of the Doppler wind measurement laser radar is compensated, and the wind speed measurement precision is improved. The method also has the advantages of low cost, high convenience and high temporal-spatial resolution.

Description

Technical Field

[0001] The invention belongs to the technical field of atmospheric flow field detection, and relates to a Doppler laser radar blind area compensation method based on the principle of lateral and backward light flickering. Background Art

[0002] At present, the mainstream method for measuring atmospheric wind fields is Doppler wind laser radar. Compared with traditional methods, Doppler wind laser radar has 360° all-round unattended scanning capability all day long, higher temporal and spatial resolution, higher accuracy, and anti-electromagnetic interference. It is highly integrated and suitable for a variety of mobile observation platforms and is widely used in atmospheric detection, aerospace and other fields. However, due to the interference of mirror reflection signals and the crosstalk between transceivers and transmitters, pulse Doppler laser radar will have a detection blind area of ​​several hundred meters to one thousand meters. When detecting the vertical profile of the wind field, the area where the data is missing is exactly the bottom of the atmosphere (atmospheric boundary layer) that is most closely related to human production and life. The wind field changes in this area have an important impact on pollutant transmission, boundary layer convection, and safe takeoff and landing of aircraft. At present, most pulse laser radars are dual-axis or coaxial non-concentric structures. Such a geometric structure will make it impossible for laser radars to avoid detection blind areas. Therefore, it is of great scientific significance and broad application prospects to develop a low-cost, convenient, and high temporal and spatial resolution blind area compensation method to make up for the design limitations of the geometric structure of Doppler wind laser radar. Summary of the invention

[0003] In order to solve the problem of the blind spot of pulse Doppler laser radar in the prior art due to the interference of mirror reflection signals and the crosstalk between transmission and reception, the purpose of the present invention is to provide a Doppler laser radar blind spot compensation method based on the principle of side and rearward light flicker, which uses the brightness fluctuation of adjacent pixels on the measurement path to obtain wind speed information over time, compensates for the blind spot of Doppler wind laser radar, and improves the accuracy of wind speed measurement. The present invention also has the advantages of low cost, strong convenience, and high temporal and spatial resolution.

[0004] The purpose of the present invention is mainly achieved through the following technical solutions:

[0005] The present invention discloses a Doppler laser radar blind area compensation method based on the principle of side-rear light flickering. The wind field in the blind area of ​​the Doppler wind laser radar is used as the detection object, the laser light intensity on the detection path is obtained by the Sham imaging principle, and the detection distance information is obtained by using the geometric relationship between the pixel and the laser light. According to the Taylor freezing condition, when the time required for a cloud of aerosol to develop is greater than the time it takes for the aerosol to pass through two laser beams, the aerosol is determined to be "frozen", that is, the structure of the aerosol will not change significantly. Therefore, when the distance between the two detection points is small enough, the two received signals have a high degree of similarity, but in time they show hysteresis due to the distance between the detection points. The wind speed information is obtained by using the change in the brightness fluctuation of adjacent pixels on the measurement path over time to compensate for the blind area of ​​the Doppler wind laser radar.

[0006] The Doppler laser radar blind area compensation method based on the side and rear light flicker principle disclosed in the present invention comprises the following steps:

[0007] Step 1: Use a charge coupled device (CCD) to clearly image the detection blind area in the entire object plane. By making the observation geometry of the CCD satisfy Sham's law, the image plane, object plane and lens plane of the imaging system are adjusted to intersect in a straight line, thereby increasing the depth of field and achieving clear imaging of the detection blind area.

[0008] Step 2: Identify the pixel on the CCD image in step 1 that corresponds to the laser light in the detection blind area. The laser beam forms a bright line in the atmosphere, and the backscattered light of the laser beam at different distances is captured by the CCD sensor, thereby forming a group of pixels with greater brightness along the direction of laser propagation. A pair of pixels with a distance interval of L along the direction of laser light propagation is selected and recorded as x1 and y1, and the set of brightness values ​​of the pair of pixels in the time series is represented as x(t) and y(t), respectively. The greater brightness refers to the brightness greater than the preset threshold.

[0009] Step 3: Perform cross-correlation processing on x(t) and y(t) in step 2. The waveforms of the time series x(t) and y(t) of a pair of pixel brightness values ​​with a distance interval of L along the propagation direction of the laser light are highly similar as they change with time t, but there is a time delay τ, which is caused by the distance between the pixels when the turbulence passes between the two pixels; after cross-correlation processing of these two sets of time series, the transit time τ0 between the two signals can be solved more accurately. When the environment meets the Taylor turbulence freezing condition, the wind speed v0 at the corresponding point on the laser light is expressed as:

[0010]

[0011] Among them, the cross-correlation processing process uses the cross-correlation function to obtain the cross-correlation coefficient between two groups of time series. The cross-correlation function is to keep one of the two groups of time series unchanged and delay the other group by a time period τ, and solve the correlation between the two time series after processing. This process is called cross-correlation analysis. In the cross-correlation analysis process, the cross-correlation coefficient between the two groups of time series changes with the size of the time period τ. The cross-correlation function R xy The discrete expression of is:

[0012]

[0013] Where x(i·Δ-τ) and y(i·Δ) represent the sampling functions of the received light flicker signal, respectively, Δ is the sampling period, and N is the actual number of sampling times.

[0014] Step 4: Perform least squares calculation on the result of step 3 to obtain the average wind speed in the predetermined range L on the laser light and compensate for the blind spot of the Doppler wind laser radar. In actual measurement, there are very few cases that meet the Taylor turbulence freezing conditions. Therefore, the least squares calculation is performed using multiple groups of pixel pairs to obtain the average wind speed in a specific range L on the laser light. The laser light imaging pixel is an N×M matrix, where the values ​​of N and M are determined according to the field of view of the laser radar telescope and the CCD pixel resolution. The selection ensures that the value of M is greater than 2, indicating that the pixel area corresponding to the cross section of the laser light is greater than two columns. For [x1(t)…x m (t)] and [y1(t)…y m (t)] to calculate the mutual correlation coefficient and obtain the m transit times [τ1…τ m ], for [τ1…τ m ] The average value is calculated to obtain the average transit time τ0 corresponding to the distance interval L, and the average wind speed in the predetermined range L is obtained based on the average transit time τ0 corresponding to the distance interval L.

[0015] Preferably, the present invention is mainly implemented based on a charge coupled device (CCD), a wide-angle lens, a narrow-band single anti-transmission filter, and a Doppler wind laser radar. The charge coupled device (CCD) is used as a detector to obtain a side scattering grayscale image of aerosols on the laser light.

[0016] Beneficial effects:

[0017] 1. The present invention discloses a Doppler laser radar blind spot compensation method based on the principle of side and rear light flickering. The laser light intensity on the detection path is obtained by the Sham imaging principle, and the detection distance information is obtained by using the geometric relationship between the pixel and the laser light. The cross-correlation processing scheme is used to effectively solve the problem of a hundred-meter detection blind spot in the prior art caused by mirror reflection signal interference and transceiver crosstalk in the pulse Doppler laser radar.

[0018] 2. The present invention discloses a Doppler laser radar blind spot compensation method based on the principle of side and rear light flickering, which uses the solution of brightness fluctuations of adjacent pixels on the measurement path changing with time to obtain wind speed information and compensate for the blind spot of the Doppler wind measurement laser radar.

[0019] 3. The present invention discloses a Doppler laser radar blind spot compensation method based on the principle of side and rear light flickering, which uses a low-cost and highly integrated charge-coupled device solution to capture radar blind spot signals, improve the accuracy of wind speed measurement, and compensate for the design limitations of the geometric structure of the Doppler wind measurement laser radar. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a flow chart of the Doppler laser radar blind area compensation method based on the side and rear light flicker principle of the present invention;

[0021] Figure 2 It is an imaging schematic diagram of the Doppler laser radar blind area compensation method based on the side and rear light glint principle of the present invention;

[0022] Figure 3 Schematic diagram of signal collection using a charge coupled device (CCD). DETAILED DESCRIPTION

[0023] In order to better illustrate the purpose and advantages of the present invention, the invention is further described below with reference to the accompanying drawings and examples.

[0024] Embodiment 1:

[0025] The Doppler laser radar blind spot compensation method based on the side and rear light flicker principle disclosed in this embodiment is mainly based on a charge coupled device CCD, a wide-angle lens, a narrow-band single anti-transmission filter, and a Doppler wind laser radar. The aperture is F2.8 and the focal length is 10mm. The 1550nm single anti-transmission narrow-band single anti-transmission filter and the Doppler wind laser radar. The wide-angle lens is used to obtain a larger field of view; the 1550nm band single anti-transmission narrow-band filter is used to filter out the influence of background light and interference light.

[0026] The main factors causing the light flicker phenomenon include two aspects, one is the atmospheric turbulence field, and the other is the atmospheric particle concentration field. Since the change in particle concentration is one of the causes of light flicker. Studies have found that the low-frequency part of the light flicker phenomenon is related to the random change of particle concentration, which ultimately manifests as light flicker. The linear relationship between the standard deviation of the light intensity fluctuation caused by the low-frequency part of the light flicker and the particle concentration is used to measure the particle concentration. The present invention utilizes the high-frequency part of the flicker phenomenon that is related to the air flow velocity, especially in the presence of atmospheric turbulence. Due to the influence of turbulence, the illumination path of the light beam is refracted, and high-frequency light flicker will occur at this time, which is manifested as light intensity fluctuations on the CCD detector. According to the Taylor turbulence freezing assumption, the flow field wind speed can be obtained by using the cross-correlation algorithm of multiple groups of pixels at specific intervals in the direction of the laser light. For example Figure 1 As shown, the Doppler laser radar blind area compensation method based on the side and rear light flashing principle disclosed in this embodiment is specifically implemented as follows:

[0027] Step 1: Use a charge coupled device (CCD) to clearly image the detection blind area in the entire object plane. By making the observation geometry of the CCD satisfy Sham's law, the image plane, object plane and lens plane of the imaging system are adjusted to intersect in a straight line, thereby increasing the depth of field to achieve clear imaging of the detection blind area. Figure 2 shown.

[0028] Step 2: Identify the pixels on the CCD image in step 1 that correspond to the laser light in the detection blind area. The laser beam forms a bright line in the atmosphere, and the backscattered light of the laser beam at different distances is captured by the CCD sensor, thus forming a group of pixels with greater brightness along the direction of laser propagation. A pair of pixels with a distance interval of L along the direction of laser light propagation is selected and recorded as x1 and y1, and the set of the brightness values ​​of the pair of pixels in the time series is represented as x(t) and y(t), respectively.

[0029] Step 3: Perform cross-correlation processing on x(t) and y(t) in step 2. The waveforms of the time series x(t) and y(t) of a pair of pixel brightness values ​​separated by a distance L along the propagation direction of the laser light and changing with time t are highly similar, as shown in Figure 3As shown, there is a time delay τ, which is caused by the distance between pixels when turbulence passes between two pixels; after cross-correlating these two sets of time series, the transit time τ0 between the two signals can be solved more accurately. Assuming that the experimental environment meets the conditions of Taylor turbulence freezing assumption, the cross-correlation coefficient between the two sets of sinusoidal time series with a cross-correlation processing interval distance L changes with the size of the time period τ, and finally reaches the maximum value at a time point. The time τ0 at this time is generally called the transit time. The wind speed can be calculated by the ratio of L to τ0. When the environment meets the Taylor turbulence freezing condition, the wind speed v0 at the corresponding point on the laser light is expressed as:

[0030]

[0031] Among them, the cross-correlation processing process uses the cross-correlation function to obtain the cross-correlation coefficient between two groups of time series. The cross-correlation function is to keep one of the two groups of time series unchanged and delay the other group by a time period τ, and solve the correlation between the two time series after processing. This process is called cross-correlation analysis. In the cross-correlation analysis process, the cross-correlation coefficient between the two groups of time series changes with the size of the time period τ. The cross-correlation function R xy The discrete expression of is:

[0032]

[0033] Where x(i·Δ-τ) and y(i·Δ) represent the sampling functions of the received light flicker signal, respectively, Δ is the sampling period, and N is the actual number of sampling times.

[0034] Step 4: Perform least squares calculation on the result of step 3 to obtain the average wind speed in the predetermined range L on the laser light and compensate for the blind spot of the Doppler wind laser radar. In actual measurement, there are very few cases that meet the Taylor turbulence freezing conditions. Therefore, the least squares calculation is performed using multiple groups of pixel pairs to obtain the average wind speed in a specific range L on the laser light. The laser light imaging pixel is an N×M matrix, where the values ​​of N and M are determined according to the field of view of the laser radar telescope and the CCD pixel resolution. The selection ensures that the value of M is greater than 2, indicating that the pixel area corresponding to the cross section of the laser light is greater than two columns. For [x1(t)…x m (t)] and [y1(t)…y m (t)] to calculate the mutual correlation coefficient and obtain the m transit times [τ1…τ m ], for [τ1…τ m ] The average value is calculated to obtain the average transit time τ0 corresponding to the distance interval L, and the average wind speed in the predetermined range L is obtained based on the average transit time τ0 corresponding to the distance interval L.

[0035] The specific description above further illustrates the purpose, technical solutions and beneficial effects of the invention in detail. It should be understood that the above is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A Doppler laser radar blind area compensation method based on the principle of lateral and backward light flashing, characterized in that: The following steps are included: Step 1: Use a charge coupled device (CCD) to clearly image the detection blind area in the entire object plane; by making the observation geometry of the CCD satisfy Sham's law, adjust the image plane, object plane and lens plane of the imaging system to intersect in a straight line, thereby increasing the depth of field and achieving clear imaging of the detection blind area; Step 2: Identify the pixel on the CCD image corresponding to the laser light in the detection blind area in step 1; the laser beam forms a bright line in the atmosphere, and the backscattered light of the laser beam at different distances is captured by the CCD sensor, thereby forming a group of pixels with relatively high brightness along the propagation direction of the laser; select a pair of pixels with a distance interval of L along the propagation direction of the laser light and record them as x1 and y1, and the set of brightness values ​​of the pair of pixels in the time series is respectively expressed as x(t) and y(t); the relatively high brightness refers to the brightness greater than a preset threshold; Step 3: Perform cross-correlation processing on x(t) and y(t) in step 2; the waveforms of the time series x(t) and y(t) of a pair of pixel brightness values ​​with a distance interval of L along the propagation direction of the laser light are highly similar, but there is a time delay τ, which is caused by the distance between the pixels when the turbulence passes between the two pixels; after cross-correlation processing of these two sets of time series, the transit time τ0 between the two signals can be solved more accurately; when the environment meets the Taylor turbulence freezing condition, the wind speed v0 at the corresponding point on the laser light is expressed as: The cross-correlation process uses the cross-correlation function to obtain the cross-correlation coefficient between two time series. The cross-correlation function is to keep one of the two time series unchanged and delay the other by a time period τ, and solve the correlation between the two time series after processing. This process is called cross-correlation analysis. In the cross-correlation analysis process, the cross-correlation coefficient between the two time series changes with the size of the time period τ. The cross-correlation function R xy The discrete expression of is: Where x(i·Δ-τ) and y(i·Δ) represent the sampling functions of the received light flicker signal, respectively, where Δ is the sampling period; N is the actual number of sampling times; Step 4: Perform least square calculation on the result of step 3 to obtain the average wind speed in a predetermined range L on the laser light and compensate for the blind area of ​​the Doppler wind laser radar. Perform least square calculation on multiple groups of pixel pairs to obtain the average wind speed in a specific range L on the laser light. The laser light imaging pixel is an N×M matrix, where the values ​​of N and M are determined according to the field of view of the laser radar telescope and the CCD pixel resolution. The selection ensures that the value of M is greater than 2, indicating that the pixel area corresponding to the cross section of the laser light is greater than two columns. m (t)] and [y1(t)…y m (t)] to calculate the mutual correlation coefficient and obtain the m transit times [τ1…τ m ], for [τ1…τ m ] The average value is calculated to obtain the average transit time τ0 corresponding to the distance interval L, and the average wind speed in the predetermined range L is obtained based on the average transit time τ0 corresponding to the distance interval L.

2. The Doppler laser radar blind area compensation method based on the side and rear light flicker principle as claimed in claim 1, wherein The features are: based on charge coupled device CCD, wide-angle lens, narrow-band single anti-reflection filter, and Doppler wind laser radar; The charge coupled device (CCD) is used as a detector to obtain the side scattering grayscale image of the aerosol on the laser light.

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