CN114399434B - High-precision facula centroid positioning algorithm for establishing space ultra-long distance inter-satellite laser link and identification method thereof - Google Patents
High-precision facula centroid positioning algorithm for establishing space ultra-long distance inter-satellite laser link and identification method thereof Download PDFInfo
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Abstract
According to the high-precision facula centroid positioning algorithm for establishing the space ultra-long-distance inter-satellite laser link and the identification method thereof, according to the characteristics of weak laser facula images captured by a remote satellite, the strongest light intensity point serving as centroid calculation reference and a facula image intercepting area are sequentially optimally designed, and finally, the centroids obtained by solving for multiple times after optimization are averaged, so that the facula centroid calculation error is effectively reduced. The method is suitable for application scenes with extremely weak received light intensity when the space ultra-long-distance inter-satellite laser link is established, solves the problem that the traditional centroid algorithm is difficult to realize high-precision positioning under the condition of low signal to noise ratio, and meets the application requirements of realizing the establishment of the laser link on the ultra-long-distance inter-satellite in the space application fields such as inter-satellite laser communication, space gravitational wave detection and the like.
Description
Technical Field
The invention relates to the technical field of spot centroid positioning, in particular to a high-precision spot centroid positioning algorithm for establishing a space ultra-long-distance inter-satellite laser link and an identification method thereof.
Background
With the rapid development and promotion of space science and technology, space laser application gradually expands to the high-precision application field, and particularly in the space application fields of inter-satellite laser communication, space gravitational wave detection and the like, laser acquisition of two satellites with very far distance is needed for realizing laser communication or precise interferometry, and a laser link is established. In order to achieve the establishment, maintenance, aiming, tracking of the laser link, the optical sensor must be able to determine the centroid position of the laser spot image accurately in real time to calculate its deviation from the standard position, and then correct this deviation by the controller so that the beam is correctly directed towards each other. It can be seen that the accuracy of the centroid position location of the spot image directly determines the laser capture accuracy. In the prior art, various high-precision spot centroid positioning algorithms, such as Gaussian fitting positioning methods, traditional centroid algorithms and the like, are layered in relation to different application fields.
For most laser spots, the ideal spot intensity distribution meets Gaussian distribution, and the Gaussian fitting positioning method has very high positioning precision for perfect Gaussian distribution spots, but captures the light intensity distribution of a camera in real time and meets the Gaussian distribution, so the Gaussian fitting positioning method has the defects of high calculation amount and poor instantaneity although the precision is higher, and cannot meet the requirement of real-time and accurate determination of the centroid of the laser spot during establishment, maintenance and tracking of a laser link.
The traditional centroid algorithm is widely applied to the following characteristics due to simplicity and effectiveness: hartmann-Shack wavefront sensor, laser communication ATP system, etc. The traditional centroid algorithm has the advantages of high operation speed, less influence of uneven light intensity distribution on positioning accuracy and the like, and can meet the application requirement of the far-field beam wave front approximate flat top. However, the traditional centroid algorithm is sensitive to noise, and is difficult to realize high-precision positioning of 0.1pixel under the condition of weak receiving laser and low signal to noise ratio, and the anti-interference capability is too weak to meet the application requirement of establishing a space ultra-long-distance inter-satellite laser link.
But the specific performance of each algorithm in the inter-satellite laser capturing stage is also needed to be analyzed by combining with the actual working condition in the space application. The practical working condition of the space application is that after the laser emitted by the laser emitting satellite is transmitted in a space ultra-long distance, the light intensity is attenuated to nW or even 100pW, the wave front of the far-field beam is expanded to be approximate to a plane, and the laser received by the far-end satellite is Gaussian flat-top beam intercepted by a telescope. The algorithms in the prior art are applied to the inter-satellite laser capturing stage, and have certain defects in detection precision and anti-interference performance.
In addition, the imaging system of the capture CCD/CMOS camera determines that the actual light intensity distribution received by the detector is close to but does not completely accord with the Fraunhofer diffraction distribution, the Fraunhofer diffraction wheel is utilized to deduce the angle distribution of the diffraction light intensity after the parallel light enters the variable-pitch grating, and the centroid of the incident light spot is calculated inaccurately.
Therefore, the method for positioning the centroid of the laser spot suitable for the laser capturing stage in the inter-satellite optical communication system solves the problems of insufficient detection precision and interference resistance of the centroid of the laser spot with non-Gaussian light intensity distribution characteristic under the weak received light intensity of 100pW magnitude in the prior art, and is a technical problem to be solved urgently by those skilled in the art.
Disclosure of Invention
The invention provides a high-precision spot centroid positioning algorithm with high measurement precision and high anti-interference performance, which is applied to the establishment of a space ultra-long inter-satellite laser link, aiming at the problems of low detection precision and low anti-interference performance in the spot centroid positioning of a laser capturing stage in an inter-satellite optical communication system in the prior art.
For this purpose, the above object of the present invention is achieved by the following technical solutions: the high-precision facula centroid positioning algorithm for establishing the space ultra-long inter-satellite laser link is characterized by comprising the following steps of:
in step S-1, image data with a spot is acquired, the image data having to contain one spot, and only one brightest spot,
Step S-2: calculating the gray maximum coordinate point of the image, which is marked as (Xmax, ymax), the gray value of the light intensity of the point is M, taking the point as the calculated centroid reference point,
Step S-3: calculating a coordinate point with the gray value of M/2 of the image, which is marked as (Xhalf, yhalf),
Step S-4: the distance between the strongest light intensity point (Xmax, ymax) and the M/2 light intensity point coordinate point (Xhalf, yhalf) is calculated and is marked as R,
Step S-5: taking the maximum point (Xmax, ymax) of the light intensity as a center point, selecting a square with a side length of 4R to intercept the light spot image, calculating the centroid coordinates of the intercepted square image by adopting a traditional gray centroid algorithm, marking as (Xmeasure, ymeasure),
Step S-6: the distance between the centroid (Xmeasure, ymeasure) of the square image and the M/2 light intensity point (Xhalf, yhalf), denoted as R',
Step S-7: taking the centroid (Xmeasure, ymeasure) of the square image as the center, making a new square with the side length of 4R', continuously solving the centroid of the new square image to be Xcn, ycn by using the traditional gray centroid algorithm in S-5,
Step S-8: repeating steps S-1 to S-7N times, recording the image centroid (Xc 1,Yc1)、(Xc2,Yc2)、Xc3,Yc3) … … (Xcn, ycn) in each step S-7, wherein N is an integer, and 0 < N < 500,
Step S-9: when the number of times of N in the recorded centroids (Xcn, ycn) in the step S-8 is equal to N, the loop is exited, the step S-9 is automatically entered, the image centroids (Xcn, ycn) in each time of S-7 are recorded, the average value of N light spot centroids (Xcn, ycn) recorded in the step S-8 is solved and recorded as (Xcenter, ycenter), and the average value (Xcenter, ycenter) is the calculated image centroid;
In step S-5, the conventional gray centroid algorithm is calculated by the following formula, where x ij represents the x coordinate of the calculation pixel, y ij represents the y coordinate of the calculation pixel, and I ij represents the light intensity value of the calculation pixel:
the second object of the invention is to provide a spot centroid identification method for establishing a space ultra-long inter-satellite laser link.
In order to achieve the above object, the present invention is realized by the following scheme:
a spot centroid identification method established by a space ultra-long distance inter-satellite laser link comprises the following steps:
Step one, dividing a rough calculation region: identifying a point (Xmax, ymax) with the maximum gray value of the whole image as the center of an initial calculation area, determining the gray value of the point, and marking the gray value as M; searching the pixel point (Xhalf, yhalf) with the gray value closest to M/2, and recording the distance between (Xmax, ymax) and (Xhalf, yhalf) as R, wherein the side length of the calculated area is 4R, namely the rough calculated area is a square area taking (Xmax, ymax) as the center and 4R as the side length;
Step two, calculating the center position of the coarse light spot: obtaining a coarse light spot center position (Xmeasure, ymeasure) in the calculation region divided in the first step by using a centroid algorithm;
Step three, obtaining the accurate light spot center position: since the center of the calculated area should be an integer, the integer pixel (Xm, ym) nearest to (Xmeasure, ym) is selected as the new area center, the distance between (Xm, ym) and (Xhalf, yhalf) is calculated as R ', a new square area with (Xm, ym) as the center and 4R' as the side length is segmented again, and the centroid algorithm is used again in the new area to identify the accurate centroid position of the light spot.
According to the high-precision light spot centroid positioning algorithm for establishing the space ultra-long inter-satellite laser link and the identification method thereof, the centroid calculation is carried out by adopting different reference points in the twice calculation area intercepted in the algorithm, the noise influence of other pixel points is filtered out by screening out the light spot points of interest, and the anti-interference performance in light spot centroid positioning is improved; the distance R is automatically calculated by judging the position of the maximum light intensity point of 1/2, so that the intercepting range can be changed along with the size of the light spot, and the calculated centroid position of the light spot is more accurate; the method calculates the average value by intercepting the image for N times, reduces the error of the barycenter coordinates, wherein the larger the value of N is, the more accurate the calculated coordinate position is, and has important practical value and wide application prospect in the field of spot barycenter positioning of the space ultra-long inter-satellite laser link.
Drawings
FIG. 1 is a flow chart of a high-precision spot centroid positioning algorithm for spatial ultra-long-range inter-satellite laser link establishment of the present invention;
FIG. 2 is a diagram of an experimental apparatus for a high-precision spot centroid positioning algorithm established based on a spatially ultra-long inter-satellite laser link;
FIG. 3 is an image acquired by a camera in an experimental setup of a high-precision spot centroid positioning algorithm based on a spatially ultra-long inter-satellite laser link;
In the accompanying drawings: a laser 1; a light intensity attenuation system 2; an imaging system 3; the camera 4 is captured.
Detailed Description
The invention will be described in further detail with reference to the drawings and specific embodiments.
Example 1
Fig. 1 is a flowchart of a high-precision facula centroid positioning algorithm for establishing a space ultra-long inter-satellite laser link, and as shown in fig. 1, the high-precision facula centroid positioning algorithm for establishing a space ultra-long inter-satellite laser link is further improved on the traditional centroid positioning algorithm, and the accuracy of the algorithm is improved by adopting methods of changing a reference point, calculating for a plurality of times and the like, so that the method is more suitable for detecting facula centroids in extremely weak received light intensity application scenes when the space ultra-long inter-satellite laser link is established than the traditional algorithm.
The invention discloses a high-precision facula centroid positioning algorithm for establishing a space ultra-long inter-satellite laser link, which comprises the following steps:
Step S-1: acquiring an image with a light spot by a camera, wherein the image must contain one light spot and only one brightest light spot;
Step S-2: converting the image into a gray image, calculating a gray maximum coordinate point of the gray image, marking the gray maximum coordinate point as (Xmax, ymax), wherein the light intensity gray value of the point is M, and taking the point as a calculated centroid reference point;
step S-3: a coordinate point with an image gray value of M/2 is calculated and is marked (Xhalf, yhalf).
Step S-4: calculating the distance between the strongest light intensity point (Xmax, ymax) and the M/2 light intensity point coordinate point (Xhalf, yhalf), and marking as R;
Step S-5: taking a light intensity maximum point (Xmax, ymax) as a center point, selecting a square with a side length of 4R to intercept the light spot image, calculating the centroid coordinates of the intercepted square image by adopting a traditional gray centroid algorithm, and marking the centroid coordinates as (Xmeasure, ymeasure);
the formula of the traditional gray level centroid algorithm is as follows, wherein x ij represents the x coordinate of the calculation pixel point, y ij represents the y coordinate of the calculation pixel point, and I ij represents the light intensity value of the calculation pixel point;
step S-6: calculating the distance between the centroid (Xmeasure, ymeasure) of the square image and the M/2 light intensity point (Xhalf, yhalf), denoted as R';
step S-7: taking the centroid (Xmeasure, ymeasure) of the square image as the center, making a new square with the side length of 4R', continuously solving the centroid of the new square image to be Xcn, ycn by using the traditional gray centroid algorithm in S-5,
Step S-8: repeating steps S-1 to S-7 for N times, recording the image centroid (Xc 1,Yc1)、(Xc2,Yc2)、Xc3,Yc3) … … (Xcn, ycn) in each step S-7, and when the number of times of N in the recorded centroid (Xcn, ycn) is equal to N, exiting the loop, automatically entering step S-9,N to be an integer, wherein 0 is less than N is less than 500. In the invention, N is adjusted according to the actual situation, and if N is too large, the accuracy is not obviously improved. The N adjustment needs to select a lower N value according to the camera frame frequency being too low or the processor performance being poor, otherwise, a higher N value is selected.
Step S-9: when the number of times N in the recorded centroids (Xcn, ycn) in the step S-8 is equal to N, the loop is exited, the step S-9 is automatically entered, the image centroids (Xcn, ycn) in each time S-7 are recorded, the average value of the N light spot centroids (Xcn, ycn) recorded in the step S-8 is solved and is recorded as (Xcenter, ycenter), and the average value (Xcenter, ycenter) is the calculated image centroid.
The high-precision facula centroid positioning algorithm for the establishment of the space ultra-long-distance inter-satellite laser link provided by the invention has the advantages that the acquired image is an image of a facula type, and the size of the image is not limited. If a plurality of light spots exist in one picture, one light spot is required to be selected for calculation.
According to the high-precision light spot centroid positioning algorithm for establishing the space ultra-long-distance inter-satellite laser link, the two calculation areas are intercepted in the algorithm process, centroid calculation is carried out by adopting different reference points, the interested light spot points are selected through screening to filter out noise influence of other pixel points, and the distance R is automatically calculated by judging the position of the 1/2 maximum light intensity point, so that the intercepted range can be changed along with the size of the light spot, and the centroid precision of the image light spot obtained through the gray centroid algorithm is higher. According to the algorithm, the centroid coordinates of each image light spot are calculated through intercepting the image for N times, and the error of the centroid coordinates is reduced through calculating the average value of the centroid coordinates of each light spot, wherein the larger the value of N is, the more accurate the calculated coordinate position is.
In order to verify the accuracy of the algorithm, the invention builds an experimental system as shown in fig. 2. The experimental objective of the system is that the accuracy of calculating the centroid of the light spot is better than 0.1pixel under the condition that the input light is 100 pW. The experimental system comprises a 1064nm Nd YAG laser 1, a light intensity attenuation system 2, an imaging system 3 and a TEKWIN SH640 capturing camera 4, the resolution is 640 x 512pixel, the pixel size is 15 μm, and the photoelectric conversion efficiency (eta) is 0.7A/W@1064nm. The output power of the laser is 8mW, the attenuation system 3 consists of 3 medium-density attenuation sheets, and the maximum attenuation rate of each sheet is 1000 times, so that the optical power can be attenuated to be below 100pW, and the attenuation system can be used for simulating a scene with extremely weak received light intensity when a space ultra-long-distance inter-satellite laser link is established. The imaging system can reduce the effect of laser dithering while reducing the spot size on the photosurface.
Fig. 3 (a) and 3 (b) are images taken by a camera with and without an imaging system, respectively, with an input light of 100 pW. It can be seen that the imaging system can reduce the spot size on the one hand and improve the contrast of the spot image on the other hand. The spot size shown in fig. 3 (b) is 7pixel, the fig. 3 (a) is obtained through the experimental device diagram of the invention, and the high-precision spot centroid calculation is performed on the spot by adopting the high-precision spot centroid positioning algorithm for the establishment of the space ultra-long inter-satellite laser link.
The experimental device of the high-precision facula centroid positioning algorithm established based on the space ultra-long distance inter-satellite laser link adopts the facula centroid recognition method in the experimental process as follows:
Step one, dividing a rough calculation region: first, a point (Xmax, ymax) with the maximum gray value of the whole image is found as the center of the initial calculation area, and the gray value of the point is denoted as M. The pixel with the gray value closest to M/2 is then retrieved (Xhalf, yhalf), and the distance between (Xmax, ymax) and (Xhalf, yhalf) is denoted as R. To ensure that the segmented regions contain almost all photoelectrons, the calculated region is made to have a side length of 4R, i.e., the rough calculated region is a square region centered on (Xmax, ymax), 4R being the side length.
Step two, calculating the center position of the coarse light spot: and (3) obtaining a coarse light spot center position (Xmeasure, ymeasure) in the calculation region separated in the step one by using a centroid algorithm.
Step three, obtaining the accurate light spot center position: since the center of the calculation region should be an integer, the integer pixel (Xm, ym) nearest to (Xmeasure, ymeasure) is defined as the new region center. Calculating the distance R 'between (Xm, ym) and (Xhalf, yhalf), dividing a new square area with (Xm, ym) as the center and 4R' as the side length again, and using a centroid algorithm again in the new area to obtain the accurate spot center position.
As can be seen from fig. 3, the spot is very energy weak on the one hand and very small in size, with only a few pixels, due to the very large spatial distance. If denoising is carried out on the whole picture, the operation amount is large, and the real-time performance is poor. The algorithm of the invention selects the spot position to calculate, so that the algorithm complexity can be greatly reduced, and the pixel number introduced into calculation can be reduced, so that the position fluctuation error can be reduced.
Taking the spot diameter 7 pixels, the dividing area 15 pixels, the camera dark current electron number standard deviation mu=1.5×10 5e- and the dark current electron number expected sigma=4.5×10 3e- as examples, the high-precision spot centroid identification method based on the space ultra-long inter-satellite laser link establishment is used, in the first step, the pixel point (Xmax, ymax) with the maximum gray value is preset as the center of an initial calculation area, and the (Xmax, ymax) and the actual spot center position have certain offset in consideration of the influence of pixel quantization, noise electron number fluctuation and diffraction. Under the action of the imaging system, the intensity distribution of the light spot is similar to the fraunhofer diffraction distribution, i.e. the light intensity changes rapidly around (Xmax, ymax). On the other hand, the pixels located near the spot center have a far greater photoelectron number than the noise electron number even in the case of weak light, and therefore the actual position of the spot center will be located between two pixels adjacent to (Xmax, ymax), i.e., the maximum value Δx 0 of the deviation between the actual spot center position and the calculated area center in step one is 2 pixels. At this time, for a weak received light intensity of 100pW, a fixed position deviation delta less than or equal to 0.164pixel is obtained through theoretical analysis, and a position fluctuation deviation is obtainedThe centering accuracy can be defined by/>And estimating, wherein in the second and third steps, the deviation between x measure and the actual position of the light spot is smaller than 0.17pixel. Since the center of the calculation region is an integer, the maximum value of Δx 0 in the third step is reduced to 0.67pixel, and the maximum value of Δwill be reduced to 0.055pixel, so that the positioning accuracy of the improved centroid algorithm can be estimated as/>
Therefore, by using the high-precision light spot centroid positioning algorithm established based on the space ultra-long inter-satellite laser link and the identification method thereof, the fluctuation of the light spot center position can be obtained through the root mean square value of 500 image measurement results:
The traditional centroid algorithm position fluctuation is as follows: Δx rms,tr = 0.0437pixel
The improved centroid algorithm position fluctuation is: Δx rms,im =0.0032 pixel
It can be seen that the position fluctuation of the improved centroid algorithm is much smaller than that of the conventional centroid algorithm. The maximum value of the measured fixed position deviation is as follows through noise analysis: Δ 100pW,max =0.055 pixel.
Thus, the maximum error for the improved centroid algorithm can be calculated as:
Δ100pW,max+Δxrms,im＝0.0582pixel
From the analysis, the maximum error of the improved centroid algorithm is 0.0582pixel, the obtained positioning precision is better than the experimental target precision of 0.1pixel, and the proposed improved centroid algorithm can meet the requirement of the experiment under the condition of weak light receiving of 100 pW.
Compared with the traditional centroid positioning algorithm, the high-precision facula centroid positioning algorithm for establishing the space ultra-long-distance inter-satellite laser link and the identification method thereof are more suitable for application scenes with extremely weak received light intensity when the space ultra-long-distance inter-satellite laser link is established, can effectively reduce facula centroid calculation errors, solve the problem that the traditional centroid algorithm is difficult to realize high-precision positioning under the condition of low signal to noise ratio, and meet the application requirements of realizing the establishment of the laser link on the ultra-long-distance inter-satellite in the space application fields such as inter-satellite laser communication, space gravitational wave detection and the like.
The above detailed description is intended to illustrate the present invention by way of example only and not to limit the invention to the particular embodiments disclosed, but to limit the invention to the precise embodiments disclosed, and any modifications, equivalents, improvements, etc. that fall within the spirit and scope of the invention as defined by the appended claims.
Claims (2)
1. The high-precision light spot centroid positioning method for establishing the space ultra-long inter-satellite laser link is characterized by comprising the following steps of:
in step S-1, image data with a spot is acquired, the image data having to contain one spot, and only one brightest spot,
Step S-2: calculating the gray maximum coordinate point of the image, which is marked as (Xmax, ymax), the gray value of the light intensity of the point is M, taking the point as the calculated centroid reference point,
Step S-3: calculating a coordinate point with the gray value of M/2 of the image, which is marked as (Xhalf, yhalf),
Step S-4: the distance between the strongest light intensity point (Xmax, ymax) and the M/2 light intensity point coordinate point (Xhalf, yhalf) is calculated and is marked as R,
Step S-5: taking the maximum point (Xmax, ymax) of the light intensity as a center point, selecting a square with a side length of 4R to intercept the light spot image, calculating the centroid coordinates of the intercepted square image by adopting a traditional gray centroid algorithm, marking as (Xmeasure, ymeasure),
Step S-6: the distance between the centroid (Xmeasure, ymeasure) of the square image and the M/2 light intensity point (Xhalf, yhalf), denoted as R',
Step S-7: taking the centroid (Xmeasure, ymeasure) of the square image as the center, making a new square with the side length of 4R', continuously solving the centroid of the new square image to be Xcn, ycn by using the traditional gray centroid algorithm in S-5,
Step S-8: selecting integer pixel (Xn, yn) nearest to (Xmeasure, ymeasure) as new region center, repeating steps S-1-S-7 for N times, recording image centroid (Xc 1, yc 1), (Xc 2,Yc2)、Xc3,Yc3) … … (Xcn, ycn) in each step S-7, when the number of times N in the recorded centroid (Xcn, ycn) is equal to N, exiting the loop, automatically entering step S-9,N as integer, and 0 < N < 500,
Step S-9: when the number of times of N in the recorded centroids (Xcn, ycn) in the step S-8 is equal to N, the loop is exited, the step S-9 is automatically entered, the image centroids (Xcn, ycn) in each time of S-7 are recorded, the average value of N light spot centroids (Xcn, ycn) recorded in the step S-8 is solved and recorded as (Xcenter, ycenter), and the average value (Xcenter, ycenter) is the calculated image centroid;
In step S-5, the conventional gray centroid algorithm is calculated by the following formula, where x ij represents the x coordinate of the calculation pixel, y ij represents the y coordinate of the calculation pixel, and I ij represents the light intensity value of the calculation pixel:
2. the method for identifying the light spot centroid for establishing the space ultra-long distance inter-satellite laser link adopts the high-precision light spot centroid positioning method for establishing the space ultra-long distance inter-satellite laser link according to claim 1, and comprises the following steps:
Step one, dividing a rough calculation region: identifying a point (Xmax, ymax) with the maximum gray value of the whole image as the center of an initial calculation area, determining the gray value of the point, and marking the gray value as M; searching the pixel point (Xhalf, yhalf) with the gray value closest to M/2, and recording the distance between (Xmax, ymax) and (Xhalf, yhalf) as R, wherein the side length of the calculated area is 4R, namely the rough calculated area is a square area taking (Xmax, ymax) as the center and 4R as the side length;
step two, calculating the center position of the coarse light spot: obtaining a coarse light spot center position (Xmeasure, ymeasure) in the calculation region divided in the first step by using a centroid algorithm;
Step three, obtaining the accurate light spot center position: since the center of the calculated area should be an integer, the integer pixel (Xn, yn) nearest to (Xmeasure, ymeasure) is selected as the new area center, the distance between (Xn, yn) and (Xnhalf, ynhalf) is calculated as R ', a new square area with (Xn, yn) as the center and 4R' as the side length is segmented again, and the centroid algorithm is used again in the new area to identify the accurate centroid position of the light spot.
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