Multi-path atmospheric coherence length measurement method and system for satellite-ground laser communication

Abstract

The invention discloses a multi-path atmospheric coherence length measurement method and system for satellite-ground laser communication. The system is composed of a binocular image acquisition module, a control module, a data processing module and a storage module. The binocular image acquisition module is used for acquiring paired star light spot images, the control module is used for controlling the binocular image acquisition module to effectively acquire, and the data processing module is used for extracting mass centers of star light spots from two frames of synchronously acquired images and calculating atmospheric optical turbulence intensity on a path where the light spots are located by using a differential image motion method; the storage module stores the data result. In the specific measurement method, the detection capability of the measurement system is calculated by adopting a photon flow density method; determining the number of detectable fixed star light spots and the probability thereof according to the size of the field of view of the equipment; realizing estimation of pixel motion and tracking of a star point trajectory by using an optical flow method; the atmospheric coherence length and seeing are calculated according to the mass center motion variance of the fixed star light spot, and the intensity of atmospheric optical turbulence at different positions is revealed.

Description

Technical Field

[0001] The present invention relates to the technical field of satellite-to-ground laser communication, and in particular to a multi-path atmospheric coherence length measurement method and system for satellite-to-ground laser communication. Background Art

[0002] Satellite-to-ground laser communication is a key link in the integration of space, air and space. In atmospheric optical turbulence, the atmospheric refractive index structure constant is usually used. To characterize the turbulence intensity, while the distribution of atmospheric turbulence has a vertical structure. In astronomical observations, the atmospheric coherence length is often used to describe the turbulence intensity along the entire path. In the 1990s, Sarazin and F. Roddier proposed the concept of Differential Image Motion Monitor (DIMM), and Tokovinin further improved the technology in 2002 to calculate the Fried parameter, also known as the atmospheric coherence length, from the measured variance of image motion. DIMM is based on a telescope with an aperture of 250-300mm, equipped with two sub-apertures and a prism, providing two images of the same star, which can collect and analyze the motion of star images caused by atmospheric disturbances and calculate the atmospheric coherence length. Miratlas, France, has developed a new device, C-DIMM, which uses two fixed-spaced small-field telescopes to synchronously collect star spots to achieve the measurement of atmospheric coherence length. However, the device only observes the path of a single star and is suitable for the site selection stage of ground stations.

[0003] In the process of satellite-to-ground laser communication, medium and low-orbit satellites will cross different spaces during their movement, and the performance of the satellite-to-ground laser link will be affected by the changes in the intensity of atmospheric optical turbulence along the way. Although the observation results obtained by the single-point observation method of atmospheric optical turbulence are relatively mature, due to the small coverage of the traditional observation of atmospheric coherence length, it is impossible to obtain observations of multi-path atmospheric coherence length. The observation is limited to the position of stars. There is a difference between the single-point data obtained by observing the stars and the turbulence intensity of the surrounding positions. The actual position of the satellite is offset from the observation point, and the correlation between the atmospheric optical turbulence intensity at different positions at the same time is unclear. At present, there is a lack of low-cost, high-resolution visible light atmospheric channel transmission condition perception methods with a large field of view. No researchers have yet provided a multi-path atmospheric coherence length measurement system for satellite-to-ground laser communication. Therefore, it is necessary to measure the multi-path star spots and calculate the atmospheric coherence length to lay the foundation for the prediction of satellite laser link communication windows.

[0004] The Chinese patent application with application number CN202210996124.4 discloses a method and application for visualizing the spatiotemporal distribution of atmospheric optical turbulence parameters. The application discloses the visualization of the atmospheric optical turbulence intensity profile and its integral parameters by computer software to obtain the spatiotemporal distribution diagram of the atmospheric optical turbulence intensity profile and its integral parameters. However, the spatial resolution accuracy is not enough, and the local atmospheric channel transmission parameters are not obtained through actual measurement. The present invention overcomes this problem. The multipath atmospheric coherence length measurement system can be deployed around the ground station to achieve real-time observation.

[0005] The Chinese patent application with application number CN202410061877.5 discloses a method and system for measuring the atmospheric coherence length of the entire layer of the satellite optical transmission path. The application discloses that the DIMM aims at and tracks the target satellite to measure the atmospheric turbulence intensity of the satellite optical transmission path. However, not all transiting low-orbit satellites can provide light sources for tracking. The multi-path atmospheric coherence length measurement system of the present invention obtains the atmospheric coherence length by observing the stars of multiple paths, thus overcoming the limitation that a light source must be provided on the satellite.

[0006] The Chinese patent application with application number CN202411675121.6 discloses a method for measuring whole-layer atmospheric turbulence based on centroid drift and light intensity flicker. The application discloses the measurement of whole-layer atmospheric turbulence intensity, and the obtained atmospheric turbulence intensity parameter is the equivalent mean value of the atmospheric refractive index structure constant. However, the atmospheric refractive index structure constant has a vertical distribution characteristic. In view of this, the present invention provides a measurement of multi-path atmospheric coherence length, which can more accurately characterize the impact of atmospheric turbulence on the transmission quality of satellite-to-ground laser communication links. Summary of the invention

[0007] The purpose of the present invention is to propose a multi-path atmospheric coherence length measurement method and system for satellite-to-ground laser communication, in order to address the problem that the existing atmospheric optical turbulence observation has a small coverage range and cannot obtain the multi-path atmospheric coherence length, and realize the measurement of atmospheric turbulence intensity in satellite-to-ground laser communication. The present invention first uses the photon flux density method to calculate the detection capability of the multi-path atmospheric coherence length measurement system, then selects a dense constellation within a fixed range of zenith angle as the observation area, calculates the angular distance between the stars to be observed, and determines the number of detectable star spots and their probability according to the field of view of the device; then, the optical flow method is used to realize the estimation of pixel motion and the tracking of star point trajectories; finally, the atmospheric coherence length is calculated according to the variance of the center of mass motion, revealing the intensity of atmospheric optical turbulence at different positions. The present invention calculates the detection capability by the photon flux density method, selects a dense constellation within a fixed range of zenith angle as the observation area, calculates the angular distance between stars and determines the number of detectable star spots and their probability. The optical flow method is used to realize pixel motion estimation and star point trajectory tracking, and the atmospheric coherence length is calculated according to the variance of the center of mass motion, revealing the intensity changes of atmospheric optical turbulence at different locations, laying the foundation for satellite laser link communication window prediction, optimizing link planning and improving communication reliability, which is crucial for determining the atmospheric channel quality in satellite-to-ground laser communication links, and also provides a scientific basis for link planning in satellite-to-ground laser communications.

[0008] The above purpose is achieved through the following technical solutions:

[0009] The present invention first provides a multi-path atmospheric coherence length measurement system for satellite-to-ground laser communication, the system comprising: a binocular image acquisition module, a control module, a data processing module and a storage module, wherein:

[0010] The binocular image acquisition module is used to collect paired star spot images at different positions;

[0011] The control module is used to control the binocular image acquisition module to effectively acquire star spot images; through the preset exposure time, acquisition frequency and signal gain, the binocular image acquisition module is controlled to synchronously acquire star spots according to the selected observation area;

[0012] The data processing module is used to extract the centroid of the star spot from the two frames of synchronously collected images, and calculate the centroid of the star spot using the differential image motion method to obtain the atmospheric coherence length and seeing parameter results on the path where the star spot is located;

[0013] The storage module is used to store the collected star spot images and calculated data results;

[0014] The binocular image acquisition module, control module, data processing module and storage module work together; according to the set exposure time, acquisition frame rate and signal gain, a set of images including the centroid of the star spot, atmospheric coherence length and seeing are output at each interval.

[0015] Furthermore, the binocular image acquisition module includes two visible light industrial cameras, a lens, a box and a high-speed data interface; the visible light industrial camera is installed inside the box, and the distance between the two cameras must be greater than or equal to twice the lens aperture, and the camera has an independent image sensor; the box meets IP67 waterproof requirements and is installed with a high-speed data interface; the binocular image acquisition module registers a callback function for the data stream of each camera, which is used for data processing immediately after the image data is captured.

[0016] The present invention also provides a multi-path atmospheric coherence length measurement method for satellite-to-ground laser communication. The method runs the multi-path atmospheric coherence length measurement system for satellite-to-ground laser communication. The specific steps include:

[0017] S1. The binocular image acquisition module ensures that the star spots can be observed in the field of view of the measurement system under short exposure time, and records the required short exposure time and the signal gain of the visible light industrial camera in the binocular image acquisition module;

[0018] S2. Select a dense constellation within a fixed zenith angle range as the observation area of ​​the binocular image acquisition module, calculate the angular distance between the stars to be observed, and determine the number and probability of detectable star spots according to the field of view of the binocular image acquisition module;

[0019] S3. The control module sets the parameter values ​​of the exposure time and signal gain of the measurement system according to the short exposure time required to ensure that the star spots can be observed in the field of view of the measurement system and the signal gain of the visible light industrial camera in step S1, and sets the parameter values ​​of the acquisition frame rate of the binocular image acquisition module and the output time of the data processing module. In the observation area described in step S2, the star spots are collected and the pixel motion of the star spots is estimated and the star point trajectory is tracked;

[0020] S4. The data processing module calculates the variance of the center of mass motion based on the star spots collected in step S3, and calculates the atmospheric coherence length and seeing from the variance of the center of mass motion, revealing the intensity of atmospheric optical turbulence at different locations.

[0021] Furthermore, the specific process of step S1 is as follows:

[0022] Adjust the exposure time and signal gain until the binocular image acquisition module can successfully detect the star spot, freeze the atmospheric turbulence under short exposure time, and realize the measurement of multi-path atmospheric coherence length, so that the signal-to-noise ratio of the optical signal Ability to exceed detectable signal-to-noise threshold , target signal photoelectron number and background noise photoelectron number for: (1) In the formula, is the receiving area of ​​the multipath atmospheric coherence length measurement system, and are the transmittance coefficients of the multipath atmospheric coherence length measurement system for the target signal spectrum and the background noise spectrum, and are the average quantum efficiencies of the reference signal and background noise, is the transmittance of the optical system, is the spatial solid angle corresponding to one pixel, and the spatial solid angle of the whole pixel and zenith angle , Field of view The relationship between , is the exposure time, is the number of pixels occupied by the star imaging, is the photon flux density of the background light per unit solid angle, and the photon flux density of the star spot : (2) In the formula, is the magnitude, is the wavelength, is Planck's constant, is the speed of light, in equation (2), the photon flux density of the background light per unit solid angle at night for: (3) Where K is the Boltzmann constant, is the thermodynamic temperature;

[0023] Photon flux density of background light for: (4) In the formula, The background light intensity of the night sky;

[0024] Substituting equations (2), (3) and (4) into equation (1), we can obtain the target signal photoelectron number: and background noise photoelectrons : (5)

[0025] When the signal-to-noise ratio When , the star spots can be detected: (6).

[0026] Furthermore, the specific process of step S2 is as follows:

[0027] S2.1 First, it is necessary to clarify the field of view of the measurement system. The field of view of the multipath atmospheric coherence length measurement system and focal length The relationship with the detector's photosensitive area is: ; (7) In the formula, is the horizontal width of the visible light camera target surface, is the vertical width of the visible light camera target surface; after the detectable field size is determined, the angular distance between the stars to be measured is calculated to determine the corresponding field of view requirements. Assume that the right ascension and declination coordinates of stars A and B in the celestial coordinate system are , , then their corresponding unit direction vectors in the celestial coordinate system are , They are: (8).

[0028] S2.2 If the angular distance between the observed stars is smaller than the field of view of the observation device, then the two stars can be observed simultaneously; based on the unit direction vector, the angular distance between stars A and B in the celestial coordinate system can be calculated for: (9).

[0029] The multi-path atmospheric coherence length measurement system is used to conduct synchronous observations at space points to reflect the influence of atmospheric turbulence on the satellite-to-ground laser link; the average number of stars provided by the star catalog With magnitude The relationship function is: (10).

[0030] The number of detectable stars in the equivalent field of view is: (11) In the formula, is the equivalent field of view, Indicates field of view The average number of stars in the corresponding area of ​​the sky, The magnitude of the star in the whole sky is less than or equal to The total number of stars with brightness exceeding the third magnitude can be detected when setting the resolution, pixel size, focal length and field of view of the multi-path atmospheric coherence length measurement system. .

[0031] S2.3 After the detectable limit magnitude, number of stars and field angle are determined, further clarify the detected The probability of a star for: (12).

[0032] Each star spot represents the spatial location of an observation, and its spatial distribution reflects the estimated distribution of turbulence on that plane.

[0033] Furthermore, the specific process of step S3 is as follows:

[0034] S3.1 setting the parameter values ​​of the exposure time and signal gain of the measurement system according to the short exposure time required to ensure that the star spots can be observed in the field of view of the measurement system and the signal gain of the visible light industrial camera in step S1, and setting the parameter values ​​of the acquisition frame rate of the measurement system and the output time of the data processing module;

[0035] S3.2 Use the equatorial mount to accurately point to the selected star group, and then use the optical flow method to track the position changes of the star points in the continuously collected multi-frame star map: (13) In the formula, optical flow Represents the moving speed in the x direction, optical flow Represents the moving speed in the y direction. In the Cartesian coordinate system, the x-axis and y-axis represent the horizontal and vertical directions on the image plane, and the z-axis represents the change of the gray value of the spot in the depth direction; Represents the gradient of the light spot brightness in the x-axis direction in the image plane, Represents the gradient of the light spot brightness in the y-axis direction in the image plane, is the partial derivative on the z-axis;

[0036] S3.3 Calculate the optical flow field between adjacent frames, that is, the speed change of pixels; observation data composition Matrix, the optical flow velocity vector is , the temporal gradient of the image is : Q= , ,b= (14) In the formula, It's a pixel The brightness gradient of the light spot in the x and y directions, because it is necessary to obtain the star light spot within 6×6 pixels, , It is in pixels To track and analyze the light spot, we only need to find a set of velocity parameters , that is, the system of equations The required spot movement speed to minimize the following expression: (15);

[0037] S3.4 The optical flow problem of spot tracking involves local nonlinear optimization. The least squares solution is obtained by pseudo-inverse method. The value of the point where the derivative is 0 is the optimal value. The symbol T represents the transpose of the matrix. Find the partial derivative: (16)

[0038] Use the optical flow method to track the position changes of multiple star spots in multiple frames of images. Let the size of the star map be m×n. is the gray value of the CCD pixel, and For each pixel, the horizontal and vertical coordinates are The coordinates of the center of mass of the light spot are , change the coordinates of the center of mass of the light spot from Updated to for: (17)

[0039] Save the Multipath Atmospheric Coherence Length Measurement System Camera Multi-location data , and camera Multiple location data at the same time The measurement system synchronously collects data at M different times for the calculation of atmospheric coherence length and seeing.

[0040] Furthermore, the specific process of step S4 is as follows:

[0041] S4.1 At the ground receiving end, the influence of the arrival angle fluctuation on the optical system is mainly considered, and the influence of the amplitude is ignored; the covariance of the phase can be obtained from the covariance of the arrival angle fluctuation and the phase structure function, and then the motion variance of the two spots in the horizontal and vertical directions can be further calculated. Wavefront phase error Proportional, , The arrival angle fluctuation component in the direction for: (18)

[0042] Covariance of arrival angle fluctuations , the phase structure function is introduced as: D ϕ (ξ,η)=2[ B ϕ (0,0)- B ϕ (ξ,η)] (19) In the formula, Represents the phase structure function, which is used to describe the phase fluctuation between two points in space , The difference between represents the value of the covariance function of the bit fluctuation at the origin, The covariance function representing the phase fluctuation is at the point The value at represents the horizontal distance, Indicates vertical distance;

[0043] S4.2 Approximate Kolmogorov turbulence, the phase structure function is given by It means that the covariance of phase fluctuation is for: (20) in, represents the atmospheric coherence length, represents the Euclidean distance from the origin. According to the centroid of the star spot observed by the multipath atmospheric coherence length measurement system, let , and get the longitudinal covariance; let , and obtain the transverse covariance to further calculate the atmospheric coherence length;

[0044] S4.3 The aperture spacing between the two cameras in the multipath atmospheric coherence length measurement system is , the diameter of the aperture is , the ratio of aperture spacing to aperture diameter ,when When the value is 6.5~7, the consistency is the best. When the value is , the consistency will decrease; is the variance of the differential image motion. The horizontal and vertical values ​​can be used to obtain the Atmospheric coherence length , in centimeters: (20) in, is the wavelength, the seeing pass The value is calculated using the following formula: (twenty one),

[0045] The storage module saves the calculation results output by the data processing module at fixed time intervals, with a typical time interval of 1 minute. The calculation results include the coordinates of the center of mass of the light spot, the atmospheric coherence length and the seeing.

[0046] Compared with the prior art, the present invention has the following beneficial effects:

[0047] This method uses starlight imaging technology to monitor the transmission characteristics of the atmospheric channel in real time, so as to accurately determine the communication window of the satellite laser communication link and enhance the stability and reliability of the satellite-to-ground laser communication link. It breaks through the limitation of the traditional "differential image motion monitor" that can only observe the atmospheric optical turbulence intensity of a single path. The design is more flexible. It can simultaneously observe multiple stars without the need for multiple devices to achieve differential imaging, ensuring measurement accuracy and improving the convenience of application. The multi-path atmospheric coherence length measurement system for satellite-to-ground laser communication adopts a box-type structure design, which has better sealing performance, simple structure, high cost-effectiveness, and meets the IP67 level of waterproof and dustproof requirements, which is particularly suitable for outdoor environment deployment. The system is not only crucial for obtaining astronomical observation data, but also provides a solid foundation for studying the spatial distribution characteristics of the atmospheric coherence length. In addition, the system can effectively deal with the deviation problem between the actual position of the satellite and the observation point, and perform spatial interpolation to obtain the intensity of atmospheric optical turbulence on different paths, which is helpful for link selection of satellite-to-ground laser communication and promotes the realization of efficient and reliable satellite-to-ground laser communication networking. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 This is a flow chart of a multi-path atmospheric coherence length measurement method based on satellite-to-ground laser communication of the present invention;

[0049] Figure 2 It is a flow chart of the dual-camera SDK synchronous acquisition to calculate the measurement parameters of the present invention;

[0050] Figure 3 A schematic diagram of a method for measuring multi-path atmospheric coherence length in satellite-to-ground laser communications according to the present invention;

[0051] Figure 4 A grayscale schematic diagram of the optical flow method of the present invention for tracking star spots;

[0052] Figure 5 Schematic diagram of realizing star point tracking between multiple frames using the optical flow method of the present invention: Figure 5 (a) is a schematic diagram of the first group of star point tracking; Figure 5 (b) Schematic diagram of the second set of star point tracking;

[0053] Figure 6 This is a schematic diagram of multiple star acquisition of the multi-path atmospheric coherence length measurement system of the present invention. Figure 6 (a) is a schematic diagram of two stars. Figure 6 (b) is a schematic diagram of three stars. DETAILED DESCRIPTION

[0054] By combining the drawings in the embodiments of the present invention, the technical scheme is clearly and completely described. It should be pointed out that the embodiments only represent a part of the present invention, not all of it. According to these embodiments, other embodiments obtained by ordinary technicians without innovative work are all within the scope of protection of the present invention.

[0055] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0056] Figure 1 A flow chart of a multi-path atmospheric coherence length measurement method for satellite-to-ground laser communication provided by an embodiment of the present invention, first, the detection capability of the measurement system is calculated by a binocular image acquisition module using a photon flux density method to determine whether star spots can be observed in the field of view of the measurement system; then, a dense constellation within a fixed zenith angle range is selected by a control module as an observation area, and the number of detectable star spots and their probability are determined according to the field of view of the device; then, on the basis of the sky area determined by the equatorial mount pointing, the optical flow method is used to realize the estimation of pixel motion and the tracking of star point trajectories, and the synchronous acquisition of a multi-path atmospheric coherence length measurement system is realized; a data processing module calculates the centroid motion variance based on the collected star spots, and the atmospheric coherence length and seeing are calculated based on the motion variance, revealing the intensity of atmospheric optical turbulence on different paths; finally, a storage module saves the collected star spot images and the calculated data results;

[0057] A multi-path atmospheric coherence length measurement system for satellite-to-ground laser communication of the present invention comprises: a binocular image acquisition module, a control module, a data processing module and a storage module, wherein:

[0058] The binocular image acquisition module is used to collect paired star spot images at different positions;

[0059] The control module is used to control the binocular image acquisition module to effectively acquire star spot images; through the preset exposure time, acquisition frequency and signal gain, the binocular image acquisition module is controlled to synchronously acquire star spots according to the selected observation area;

[0060] The data processing module is used to extract the centroid of the star spot from the two frames of synchronously collected images, and calculate the centroid of the star spot using the differential image motion method to obtain the atmospheric coherence length and seeing parameter results on the path where the star spot is located;

[0061] The storage module is used to store the collected star spot images and calculated data results;

[0062] The binocular image acquisition module, control module, data processing module and storage module work together; according to the set exposure time, acquisition frame rate and signal gain, a set of images including the centroid of the star spot, atmospheric coherence length and seeing are output at each interval.

[0063] The binocular image acquisition module in this embodiment includes two visible light industrial cameras, a lens, a box and a high-speed data interface; the visible light industrial camera is installed inside the box, and the distance between the two cameras must be greater than or equal to twice the lens aperture, and the camera has an independent image sensor; the box meets the IP67 waterproof requirement and is installed with a high-speed data interface; the binocular image acquisition module registers a callback function for the data stream of each camera, which is used to process the data immediately after the image data is captured.

[0064] The present invention provides a multi-path atmospheric coherence length measurement method for satellite-to-ground laser communication, which runs on the multi-path atmospheric coherence length measurement system for satellite-to-ground laser communication, and specifically comprises the following steps:

[0065] S1. The binocular image acquisition module ensures that the star spots can be observed in the field of view of the measurement system under short exposure time, and records the required short exposure time and the signal gain of the visible light industrial camera in the binocular image acquisition module;

[0066] S2. Select a dense constellation within a fixed zenith angle range as the observation area of ​​the binocular image acquisition module, calculate the angular distance between the stars to be observed, and determine the number and probability of detectable star spots according to the field of view of the binocular image acquisition module;

[0067] S3. The control module sets the parameter values ​​of the exposure time and signal gain of the measurement system according to the short exposure time required to ensure that the star spots can be observed in the field of view of the measurement system and the signal gain of the visible light industrial camera in step S1, and sets the parameter values ​​of the acquisition frame rate of the binocular image acquisition module and the output time of the data processing module. In the observation area described in step S2, the star spots are collected and the pixel motion of the star spots is estimated and the star point trajectory is tracked;

[0068] S4. The data processing module calculates the variance of the center of mass motion based on the star spots collected in step S3, and calculates the atmospheric coherence length and seeing from the variance of the center of mass motion, revealing the intensity of atmospheric optical turbulence at different locations.

[0069] Step S1, specific implementation steps include:

[0070] Adjust the exposure time and signal gain until the binocular image acquisition module can successfully detect the star spot, freeze the atmospheric turbulence under short exposure time, and realize the measurement of multi-path atmospheric coherence length, so that the signal-to-noise ratio of the optical signal Ability to exceed detectable signal-to-noise threshold , target signal photoelectron number and background noise photoelectron number for: (1) In the formula, is the receiving area of ​​the multipath atmospheric coherence length measurement system, and are the transmittance coefficients of the multipath atmospheric coherence length measurement system for the target signal spectrum and the background noise spectrum, and are the average quantum efficiencies of the reference signal and background noise, is the transmittance of the optical system, is the spatial solid angle corresponding to one pixel, and the spatial solid angle of the whole pixel and zenith angle , Field of view The relationship between , is the exposure time, is the number of pixels occupied by the star imaging, is the photon flux density of the background light per unit solid angle, and the photon flux density of the star spot : (2) In the formula, is the magnitude, is the wavelength, is Planck's constant, is the speed of light, in equation (2), the photon flux density of the background light per unit solid angle at night for: (3) Where K is the Boltzmann constant, is the thermodynamic temperature;

[0071] Photon flux density of background light for: (4) In the formula, The background light intensity of the night sky;

[0072] Substituting equations (2), (3) and (4) into equation (1), we can obtain the target signal photoelectron number: and background noise photoelectrons : (5)

[0073] When the signal-to-noise ratio When , the star spots can be detected: (6).

[0074] The multi-path atmospheric coherence length measurement system proposed in the present invention takes the spatial position into consideration. On the basis of detecting the star spots, it is necessary to further clarify the number of observable star spots in the equivalent field of view to ensure the accuracy and reliability of the measurement results. Table 1 is a table of meteorological observation simulation parameters;

[0075] Table 1 Meteorological observation simulation parameters

[0076] Step S2, specific implementation steps include:

[0077] S2.1 First, it is necessary to clarify the field of view of the measurement system. The field of view of the multipath atmospheric coherence length measurement system and focal length The relationship with the detector's photosensitive area is: ; (7) In the formula, is the horizontal width of the visible light camera target surface, is the vertical width of the visible light camera target surface; after the detectable field size is determined, the angular distance between the stars to be measured is calculated to determine the corresponding field of view requirements. Assume that the right ascension and declination coordinates of stars A and B in the celestial coordinate system are , , then their corresponding unit direction vectors in the celestial coordinate system are , They are: (8).

[0078] S2.2 If the angular distance between the observed stars is smaller than the field of view of the observation device, then the two stars can be observed simultaneously; based on the unit direction vector, the angular distance between stars A and B in the celestial coordinate system can be calculated for: (9)

[0079] The multi-path atmospheric coherence length measurement system is used to conduct synchronous observations at space points to reflect the influence of atmospheric turbulence on the satellite-to-ground laser link; the average number of stars provided by the star catalog With magnitude The relationship function is: (10)

[0080] The number of detectable stars in the equivalent field of view is: (11) In the formula, is the equivalent field of view, Indicates field of view The average number of stars in the corresponding area of ​​the sky, The magnitude of the star in the whole sky is less than or equal to The total number of stars with brightness exceeding the third magnitude can be detected when setting the resolution, pixel size, focal length and field of view of the multi-path atmospheric coherence length measurement system. ;

[0081] S2.3 After the detectable limit magnitude, number of stars and field angle are determined, further clarify the detected The probability of a star for: (12)

[0082] Each star spot represents the spatial location of an observation, and its spatial distribution reflects the estimated distribution of turbulence on that plane.

[0083] Figure 2 The flowchart of the dual-camera SDK synchronous acquisition to calculate the measurement parameters of the present invention, first obtain the list of currently connected devices, check and open the two camera devices; synchronously control the parameters including the camera exposure time and trigger mode, and obtain the camera data stream; initialize an empty list for each camera to store the centroid data; register a callback function for the data stream of each camera to process the captured image data; start the camera acquisition process, triggering the acquisition every 10ms. Save the centroid data to a file, calculate the atmospheric coherence length and seeing, and write the calculated value to the file.

[0084] Each star spot represents the spatial position of an observation value, and its spatial distribution reflects the estimated distribution of turbulence on the plane. By using meteorological observation equipment to perform spatially distributed synchronous observations of the entire atmospheric optical turbulence, multiple star observations are performed to reflect the impact of atmospheric turbulence on the satellite-to-ground laser link. It is necessary to calculate the angular resolution of Capella (magnitude 0.24) and Epsilon Aurigae (magnitude 3.14), as well as the angular resolution of Betelgeux (magnitude 1.68), Bellatrix (magnitude 1.62), and Rigel (magnitude 2.13). The typical star parameter table is shown in Table 2, and the interstellar angular distance can be further calculated.

[0085] Table 2 Typical stellar parameters

[0086] Step S3, specific implementation steps include:

[0087] S3.1 setting the parameter values ​​of the exposure time and signal gain of the measurement system according to the short exposure time required to ensure that the star spots can be observed in the field of view of the measurement system and the signal gain of the visible light industrial camera in step S1, and setting the parameter values ​​of the acquisition frame rate of the measurement system and the output time of the data processing module;

[0088] S3.2 Use the equatorial mount to accurately point to the selected star group, and then use the optical flow method to track the position changes of the star points in the continuously collected multi-frame star map: (13) In the formula, optical flow Represents the moving speed in the x direction, optical flow Represents the moving speed in the y direction. In the Cartesian coordinate system, the x-axis and y-axis represent the horizontal and vertical directions on the image plane, and the z-axis represents the change of the gray value of the spot in the depth direction; Represents the gradient of the light spot brightness in the x-axis direction in the image plane, Represents the gradient of the light spot brightness in the y-axis direction in the image plane, is the partial derivative on the z-axis;

[0089] S3.3 Calculate the optical flow field between adjacent frames, that is, the speed change of pixels; observation data composition Matrix, the optical flow velocity vector is , the temporal gradient of the image is : Q= , ,b= (14) In the formula, It's a pixel The brightness gradient of the light spot in the x and y directions, because it is necessary to obtain the star light spot within 6×6 pixels, , It is in pixels To track and analyze the light spot, we only need to find a set of velocity parameters , that is, the system of equations The required spot movement speed to minimize the following expression: (15).

[0090] S3.4 The optical flow problem of spot tracking involves local nonlinear optimization. The least squares solution is obtained by pseudo-inverse method. The value of the point where the derivative is 0 is the optimal value. The symbol T represents the transpose of the matrix. Find the partial derivative: (16)

[0091] Use the optical flow method to track the position changes of multiple star spots in multiple frames of images. Let the size of the star map be m×n. is the gray value of the CCD pixel, and For each pixel, the horizontal and vertical coordinates are The coordinates of the center of mass of the light spot are , change the coordinates of the center of mass of the light spot from Updated to for: (17)

[0092] Save the Multipath Atmospheric Coherence Length Measurement System Camera Multi-location data , and camera Multiple location data at the same time The measurement system synchronously collects data at M different times for the calculation of atmospheric coherence length and seeing.

[0093] Figure 3 The present invention proposes a method for measuring the multi-path atmospheric coherence length in satellite-to-ground laser communication. A multi-path atmospheric coherence length measurement system is deployed near the ground station. The system tracks star spots based on the optical flow method. Under the conditions of short exposure time and high frame rate, the system collects star spot images, analyzes the collected star spots, and calculates the motion variance of the spot mass center in the horizontal and vertical directions. Finally, the atmospheric coherence length is calculated based on the calculated motion variance.

[0094] Figure 4The grayscale schematic diagram of the optical flow method of the present invention to achieve star spot tracking is very important to ensure the continuous identification of the same star point. The optical flow method accurately tracks the position of the star point by analyzing the pixel movement in consecutive frames. Even if some star points are invisible, it does not affect the identification of other star points, which helps to reduce the centroid drift error and improve the accuracy of star point tracking;

[0095] Figure 5 This is a schematic diagram of the results of implementing the Lucas-Kanade optical flow method of the present invention to track star points between multiple frames. When tracking star points in multiple frames, it is very important to ensure that the same star point is continuously identified. Figure 5 (a) and (b) are schematic diagrams of the movement trends of the positions of different star spots in the same star map. By analyzing the pixel movement in continuous frames, accurate tracking of the star point positions can be achieved.

[0096] Step S4, specific implementation steps include:

[0097] S4.1 At the ground receiving end, the influence of the arrival angle fluctuation on the optical system is mainly considered, and the influence of the amplitude is ignored; the covariance of the phase can be obtained from the covariance of the arrival angle fluctuation and the phase structure function, and then the motion variance of the two spots in the horizontal and vertical directions can be further calculated. Wavefront phase error Proportional, , The arrival angle fluctuation component in the direction for: (18)

[0098] Covariance of arrival angle fluctuations , the phase structure function is introduced as: D ϕ (ξ,η)=2[ B ϕ (0,0)- B ϕ (ξ,η)] (19) In the formula, Represents the phase structure function, which is used to describe the phase fluctuation between two points in space , The difference between represents the value of the covariance function of the bit fluctuation at the origin, The covariance function representing the phase fluctuation is at the point The value at represents the horizontal distance, Indicates vertical distance;

[0099] S4.2 Approximate Kolmogorov turbulence, the phase structure function is given by It means that the covariance of phase fluctuation is for: (20) in, represents the atmospheric coherence length, represents the Euclidean distance from the origin. According to the centroid of the star spot observed by the multipath atmospheric coherence length measurement system, let , and get the longitudinal covariance; let , and obtain the transverse covariance to further calculate the atmospheric coherence length;

[0100] S4.3 The aperture spacing between the two cameras in the multipath atmospheric coherence length measurement system is , the diameter of the aperture is , the ratio of aperture spacing to aperture diameter ,when When the value is 6.5~7, the consistency is the best. When the value is , the consistency will decrease; is the variance of the differential image motion. The horizontal and vertical values ​​can be used to obtain the Atmospheric coherence length , in centimeters: (20) in, is the wavelength, the seeing pass The value is calculated using the following formula: (twenty one),

[0101] The storage module saves the calculation results output by the data processing module at fixed time intervals, with a typical time interval of 1 minute. The calculation results include the coordinates of the center of mass of the light spot, the atmospheric coherence length and the seeing.

[0102] Figure 6 This is a schematic diagram of multiple star acquisition by the multi-path atmospheric coherence length measurement system of the present invention. Multiple stars are observed simultaneously with an exposure time of 10ms. Figure 6 The distribution and angular distance of stars are marked in Figure 6 In (a), two stars can be detected. Figure 6 In (b), three stars can be detected. The collected light spot occupies more than 80% of the energy in the 3*3 pixel, which is enough to meet the needs of atmospheric turbulence measurement. Based on the differential image motion of the same light spot in the two cameras, the atmospheric coherence length at different positions is obtained. and seeing.

Claims

1. A multi-path atmospheric coherence length measurement system for satellite-to-ground laser communications, characterized in that: The system includes: a binocular image acquisition module, a control module, a data processing module and a storage module, wherein: The binocular image acquisition module is used to collect paired star spot images at different positions; The control module is used to control the binocular image acquisition module to effectively acquire star spot images; through the preset exposure time, acquisition frequency and signal gain, the binocular image acquisition module is controlled to synchronously acquire star spots according to the selected observation area; The data processing module is used to extract the centroid of the star spot from the two frames of synchronously collected images, and calculate the centroid of the star spot using the differential image motion method to obtain the atmospheric coherence length and seeing parameter results on the path where the star spot is located; The storage module is used to store the collected star spot images and calculated data results; The binocular image acquisition module, control module, data processing module and storage module work together; according to the set exposure time, acquisition frame rate and signal gain, a set of images including the centroid of the star spot, atmospheric coherence length and seeing are output at each interval.

2. The multi-path atmospheric coherence length measurement system for satellite-to-ground laser communication according to claim 1 is characterized in that: The binocular image acquisition module includes two visible light industrial cameras, a lens, a box and a high-speed data interface; the visible light industrial camera is installed inside the box, and the distance between the two cameras must be greater than or equal to twice the lens aperture, and the camera has an independent image sensor; the box meets IP67 waterproof requirements and is installed with a high-speed data interface; the binocular image acquisition module registers a callback function for the data stream of each camera, which is used to process the data immediately after the image data is captured.

3. A method for measuring multi-path atmospheric coherence length for satellite-to-ground laser communication, the method being operated on the multi-path atmospheric coherence length measurement system for satellite-to-ground laser communication as claimed in any one of claims 1-2, characterized in that: The specific steps include: S1. The binocular image acquisition module ensures that the star spots can be observed in the field of view of the measurement system under short exposure time, and records the required short exposure time and the signal gain of the visible light industrial camera in the binocular image acquisition module; S2. Select a dense constellation within a fixed zenith angle range as the observation area of ​​the binocular image acquisition module, calculate the angular distance between the stars to be observed, and determine the number and probability of detectable star spots according to the field of view of the binocular image acquisition module; S3. The control module sets the parameter values ​​of the exposure time and signal gain of the measurement system according to the short exposure time required to ensure that the star spots can be observed in the field of view of the measurement system and the signal gain of the visible light industrial camera in step S1, and sets the parameter values ​​of the acquisition frame rate of the binocular image acquisition module and the output time of the data processing module. In the observation area described in step S2, the star spots are collected and the pixel motion of the star spots is estimated and the star point trajectory is tracked; S4. The data processing module calculates the variance of the center of mass motion based on the star spots collected in step S3, and calculates the atmospheric coherence length and seeing from the center of mass motion variance, revealing the intensity of atmospheric optical turbulence at different locations.

4. The multi-path atmospheric coherence length measurement method for satellite-to-ground laser communication according to claim 3 is characterized in that: The specific process of step S1 is as follows: Adjust the exposure time and signal gain until the binocular image acquisition module can successfully detect the star spot, freeze the atmospheric turbulence under short exposure time, and realize the measurement of multi-path atmospheric coherence length, so that the signal-to-noise ratio of the optical signal Ability to exceed detectable signal-to-noise threshold , target signal photoelectron number and background noise photoelectron number for: (1)， In the formula, is the receiving area of ​​the multipath atmospheric coherence length measurement system, and are the transmittance coefficients of the multipath atmospheric coherence length measurement system for the target signal spectrum and the background noise spectrum, and are the average quantum efficiencies of the reference signal and background noise, is the transmittance of the optical system, is the spatial solid angle corresponding to one pixel, and the spatial solid angle of the whole pixel and zenith angle , Field of view The relationship between , is the exposure time, is the number of pixels occupied by the star imaging, is the photon flux density of the background light per unit solid angle, and the photon flux density of the star spot : (2)， In the formula, is the magnitude, is the wavelength, is Planck's constant, is the speed of light, in equation (2), the photon flux density of the background light per unit solid angle at night for: (3)， Where K is the Boltzmann constant, is the thermodynamic temperature; Photon flux density of background light for: (4)， In the formula, The background light intensity of the night sky; Substituting equations (2), (3) and (4) into equation (1), we can obtain the target signal photoelectron number: and background noise photoelectrons : (5)， When the signal-to-noise ratio When , the star spots can be detected: (6)。 5. The multi-path atmospheric coherence length measurement method for satellite-to-ground laser communication according to claim 3 is characterized in that: The specific process of step S2 is as follows: S2.1 First, it is necessary to clarify the field of view of the measurement system. The field of view of the multipath atmospheric coherence length measurement system and focal length The relationship with the detector's photosensitive area is: ； (7)， In the formula, is the horizontal width of the visible light camera target surface, is the vertical width of the visible light camera target surface; after the detectable field size is determined, the angular distance between the stars to be measured is calculated to determine the corresponding field of view requirements. Assume that the right ascension and declination coordinates of stars A and B in the celestial coordinate system are , , then their corresponding unit direction vectors in the celestial coordinate system are , They are: (8)， S2.2 If the angular distance between the observed stars is smaller than the field of view of the observation device, then the two stars can be observed simultaneously; based on the unit direction vector, the angular distance between stars A and B in the celestial coordinate system can be calculated for: (9)， The multi-path atmospheric coherence length measurement system is used to conduct synchronous observations at space points to reflect the influence of atmospheric turbulence on the satellite-to-ground laser link; the average number of stars provided by the star catalog With magnitude The relationship function is: (10)， The number of detectable stars in the equivalent field of view is: (11)， In the formula, is the equivalent field of view, Indicates field of view The average number of stars in the corresponding area of ​​the sky, The magnitude of the star in the whole sky is less than or equal to The total number of stars with brightness exceeding the third magnitude can be detected when setting the resolution, pixel size, focal length and field of view of the multi-path atmospheric coherence length measurement system. ; S2.3 After the detectable limit magnitude, number of stars and field angle are determined, further clarify the detected The probability of a star for: (12)， Each star spot represents the spatial location of an observation, and its spatial distribution reflects the estimated distribution of turbulence on that plane.

6. The multi-path atmospheric coherence length measurement method for satellite-to-ground laser communication according to claim 3 is characterized in that: The specific process of step S3 is as follows: S3.1 setting the parameter values ​​of the exposure time and signal gain of the measurement system according to the short exposure time required to ensure that the star spots can be observed in the field of view of the measurement system and the signal gain of the visible light industrial camera in step S1, and setting the parameter values ​​of the acquisition frame rate of the measurement system and the output time of the data processing module; S3.2 Use the equatorial mount to accurately point to the selected star group, and then use the optical flow method to track the position changes of the star points in the continuously collected multi-frame star map: (13)， In the formula, optical flow Represents the moving speed in the x direction, optical flow Represents the moving speed in the y direction. In the Cartesian coordinate system, the x-axis and y-axis represent the horizontal and vertical directions on the image plane, and the z-axis represents the change of the gray value of the spot in the depth direction; Represents the gradient of the light spot brightness in the x-axis direction in the image plane, Represents the gradient of the light spot brightness in the y-axis direction in the image plane, is the partial derivative on the z-axis; S3.3 Calculate the optical flow field between adjacent frames, that is, the speed change of pixels; observation data composition Matrix, the optical flow velocity vector is , the temporal gradient of the image is : Q= , ,b= (14), In the formula, It's a pixel The brightness gradient of the light spot in the x and y directions, because it is necessary to obtain the star light spot within 6×6 pixels, , It is in pixels To track and analyze the light spot, we only need to find a set of velocity parameters , that is, the system of equations The required spot movement speed to minimize the following expression: (15)， S3.4 The optical flow problem of spot tracking involves local nonlinear optimization. The least squares solution is obtained by pseudo-inverse method. The value of the point where the derivative is 0 is the optimal value. The symbol T represents the transpose of the matrix. Find the partial derivative: (16)， Use the optical flow method to track the position changes of multiple star spots in multiple frames of images. Let the size of the star map be m×n. is the gray value of the CCD pixel, and For each pixel, the horizontal and vertical coordinates are The coordinates of the center of mass of the light spot are , change the coordinates of the center of mass of the light spot from Updated to for: (17)， Save the Multipath Atmospheric Coherence Length Measurement System Camera Multi-location data , and camera Multiple location data at the same time The measurement system synchronously collects data at M different times for the calculation of atmospheric coherence length and seeing.

7. The multi-path atmospheric coherence length measurement method for satellite-to-ground laser communication according to claim 3 is characterized in that: The specific process of step S4 is as follows: S4.1 At the ground receiving end, the influence of the arrival angle fluctuation on the optical system is mainly considered, and the influence of the amplitude is ignored; the covariance of the phase is obtained from the covariance of the arrival angle fluctuation and the phase structure function, and then the motion variance of the two spots in the horizontal and vertical directions is further calculated, and the wavefront Wavefront phase error Proportional, , The arrival angle fluctuation component in the direction for: (18)， Covariance of arrival angle fluctuations , the phase structure function is introduced as: (19)， In the formula, Represents the phase structure function, which is used to describe the phase fluctuation between two points in space , The difference between represents the value of the covariance function of the bit fluctuation at the origin, The covariance function representing the phase fluctuation is at the point The value at represents the horizontal distance, Indicates vertical distance; S4.2 Approximate Kolmogorov turbulence, the phase structure function is given by It means that the covariance of phase fluctuation is for: (20)， in, represents the atmospheric coherence length, represents the Euclidean distance from the origin. According to the centroid of the star spot observed by the multipath atmospheric coherence length measurement system, let , and get the longitudinal covariance; let , and obtain the transverse covariance to further calculate the atmospheric coherence length; S4.3 The aperture spacing between the two cameras in the multipath atmospheric coherence length measurement system is , the diameter of the aperture is , the ratio of aperture spacing to aperture diameter ,when When the value is 6.5~7, the consistency is the best. When the value is , the consistency will decrease; is the variance of the differential image motion, obtained by the values ​​in the horizontal and vertical directions at any position Atmospheric coherence length , in centimeters: (20)， in, is the wavelength, the seeing pass The value is calculated using the following formula: (21)， The storage module saves the calculation results output by the data processing module at fixed time intervals, with a typical time interval of 1 minute. The calculation results include the coordinates of the center of mass of the light spot, the atmospheric coherence length and the seeing.

Images