Method for measuring take-off drift distance of rocket through single-station optical imaging

Abstract

The invention discloses a method for measuring the take-off drift distance of a rocket based on single-station optical imaging, and belongs to the field of rocket pose measurement. The takeoff drift distance of a rocket is calculated through single-station optical equipment, optical equipment parameters and parameters of the rocket serving as a detection target are obtained firstly, and the detection imaging size is obtained through interpretation. Meanwhile, the field angle of the optical equipment during detection imaging is measured, a focal length-field angle function relation is obtained, and a focal length-field angle relation curve is further fitted. According to the focal length-field angle relation curve, the field angle corresponding to any focal length of target imaging is obtained, and the distance information of the target is obtained by combining the imaging size; or calculating the distance information of the target through the imaging results of the plurality of target feature points arranged on the same target. And finally, calculating the spatial position of the rocket relative to the observation point according to the distance information of the target, and further obtaining the take-off drift distance of the rocket. According to the invention, the environmental influence caused by station distribution is avoided, and meanwhile, the measurement precision is improved.

Description

Technical Field

[0001] The invention belongs to the field of rocket posture measurement, and specifically designs a method for measuring rocket takeoff drift by using single-station optical imaging. Background Art

[0002] In a space launch mission, from the time the rocket takes off to the time it leaves the launch tower, the rocket will inevitably swing due to factors such as the uneven thrust of the rocket engine and the overshoot of the rocket control, which manifests as lateral drift. The rocket takeoff drift refers to the lateral offset of the rocket relative to the vertical center of the rocket during the vertical takeoff process, which is mainly used to evaluate the flight control performance of the rocket. If the takeoff drift is large, the rocket may hit the tower, causing a serious accident.

[0003] At present, the launch site mainly uses telemetry and optical measurement methods to measure the attitude of flying targets in the air. Common equipment for attitude measurement by telemetry methods include gyroscopes, inertial navigation instruments, attitude GPS, attitude angle sensors, etc. During the flight, the measurement data of these devices are sent to the ground receiving device to complete the attitude measurement. However, telemetry uses contact attitude angle measurement equipment, which must be installed on the target to be measured. Due to the harsh working environment such as overload and vibration generated during the flight of the target, the measurement equipment will be greatly disturbed. In addition, once a fault occurs or some small targets themselves do not have telemetry data transmitters, reliable attitude data may not be obtained. As for the optical method for attitude parameter measurement, the laser measurement method is currently more mature. Laser measurement uses coaxial emission, divided line of sight, and divided band reception, and then measures the slant distance and relative angular position of the missile head and tail points in real time, and completes the attitude parameter measurement after calculation and processing. Since the laser measurement method requires the setting of laser cooperative targets on the head and tail of the measured object, it cannot be considered as a truly non-contact measurement.

[0004] As a non-contact measurement method, optical measurement can work independently of the measured object and has the advantages of anti-interference and high precision. The current optical measurement equipment is one of the main forces of measurement and control, and the commonly used optical measurement method only treats the flying target as a particle target and only obtains the target's motion trajectory. Obviously, this "particle" treatment of the target discards a lot of useful information in the target image. The post-processing only obtains the speed and acceleration. It can no longer meet the current needs. Therefore, it is of great practical application value to obtain the three-dimensional posture information of the measurement target in the measurement image, or to obtain as much posture information as possible during the flight of the measurement target. If an effective, simple and convenient optical measurement method for posture can be found, it will greatly expand the test field of optical equipment and effectively improve the test capability of the launch site.

[0005] Currently, the measurement of rocket takeoff drift is usually obtained by using the optical intersection method through three high-speed television measuring instruments arranged around the launch tower. However, this model has a large number of equipment for multi-station missions and is also limited by related supporting station conditions such as the site and foundation ring.

[0006] The conventional single-station positioning method is to determine the distance information by adding a laser rangefinder. However, from the ignition and takeoff to the rocket leaving the launcher, the rocket flies at a low altitude, the equipment is too close to the target and is large in size, and the surrounding environment such as the tower is complex, making it impossible to accurately obtain the relevant distance judgment position. Summary of the invention

[0007] In order to solve the problems of increased interference caused by the number of equipment and environment in multi-station measurement in the prior art, and poor accuracy of single-station measurement, the present invention proposes a method for measuring the takeoff drift of a rocket by single-station optical imaging. By taking advantage of the clear imaging of target features of single-station optical equipment and the large longitudinal size of the rocket target, a calibration and ranging algorithm for imaging passive ranging related parameters based on the target feature size is established to obtain high-precision target distance information, and image ranging is combined with optical high-precision angle measurement to achieve high-precision single-station positioning, thereby achieving high-precision measurement of the takeoff drift of the rocket by a single optical device.

[0008] A method for measuring rocket takeoff drift based on single-station optical imaging, the specific steps are as follows:

[0009] Step 1: Establish an optical system for measuring the takeoff drift of a rocket, including a single optical device and a rocket, and obtain the parameters of the optical device and the parameters of the rocket as a detection target;

[0010] When a rocket is used as a detection target, the horizontal marking ring of the rocket or the overall / partial length of the vertical rocket body is selected as the target parameter, as well as the coordinates of the rocket station in a static state, etc.

[0011] The optical device parameters include: the total number of pixels and target size of the detector target surface in the imaging direction, the image pixels of the target on the target surface, the focal length of the optical detector at the time of imaging, the coordinates of the optical device, etc.

[0012] Step 2: The optical device detects and images the stationary rocket or launch pad, and obtains the image size (number of pixels) of the feature points through interpretation.

[0013] The image recorded by the optical device is processed by the interpretation software to determine the image size (number of pixels) in the corresponding direction of the target, specifically:

[0014] First, coordinates are established in the X and Y directions, with the center point of the target surface (the number of pixels in the X-axis direction is F / 2, and the number of pixels in the Y-axis direction is G / 2) as the origin of the coordinates, and all pixels (F×G) on the target surface are evenly distributed;

[0015] Then, for any point in the image, its coordinate position on the X and Y axes of the target surface is directly obtained through the interpretation software. According to the selected target features and shapes, the pixel positions of the two polar edges of the target in the corresponding directions are directly obtained in the form of manual interpretation. After calculating the difference, the imaging size of the target in the corresponding direction is determined.

[0016] Finally, the error in the interpretation of the image size was reduced by taking the average value through multiple interpretations.

[0017] According to the characteristics of the selected target object, in the corresponding imaging direction of the target surface, the direction of the rocket's vertical takeoff is taken as the vertical direction, and the direction of the rocket's cross section is taken as the horizontal direction. The imaging size of the target in the horizontal and vertical directions is represented by the number of pixels in the corresponding direction, m is the number of pixels for the horizontal target imaging, and n is the number of pixels for the vertical target imaging.

[0018] Step 3, based on the optical device parameters, the field of view angle of the optical device during detection imaging is measured to obtain a functional relationship between focal length and field of view angle;

[0019] The field of view angle is measured with the camera lens of the optical device as the vertex, and the maximum angle range of the image of the object to be measured that can be formed through the lens. The specific measurement process is:

[0020] First, by continuously zooming from small to large, the tracking system of the optical device directly reads the focal length value fed back by the zoom system, and the field of view changes from large to small accordingly;

[0021] Then, select certain zoom positions at appropriate intervals during the entire zoom process to calibrate the field of view angle. The field of view angle corresponding to each group of focal lengths is measured more than 3 times, and the corresponding field of view angle is determined after average calculation;

[0022] Finally, after recording and arranging the data of the field of view angles corresponding to different focal length values, MATLAB was used to fit the functional relationship between focal length and field of view angle.

[0023] Step 4: According to the focal length-field of view function, select the appropriate Fourier function fitting order, and fit the focal length-field of view relationship curve in MATLAB;

[0024] Step 5, according to the focal length-field angle relationship curve, the field angle corresponding to any focal length selected by the optical device after changing the target imaging ratio by zooming is obtained, and combined with the imaging size, the distance information between the target center point and the center of the optical device is further obtained;

[0025] Taking the rocket axis as the target measurement object, the distance R from the target center to the center of the optical device is calculated as follows:

[0026]

[0027] Among them, θ H is the field of view angle, h is the target feature size, and h << R; B is the total number of pixels of the camera target surface in the target imaging direction, and n is the number of pixels occupied by the target imaging on the target surface.

[0028] Step six, when the direction of the device target surface is tilted or flipped, by setting two or more target feature points on the same target, calculate the distance between the centroid of the feature points and the camera based on the imaging results of each feature point, which is the distance information of the target;

[0029] Select a target feature points on the same target respectively, a ≥ 2. In the same frame of optical image recorded by the optical device, calculate the distance from the center point of each target feature to the device respectively. Taking a = 2 as an example, the specific process is as follows:

[0030] L1 is the body size between the two marker rings at the upper end of the rocket, and the corresponding imaging pixel number is n1. L2 is the body size between the two marker rings at the lower end of the rocket, and the corresponding imaging pixel number is n2. ψ is the tilt angle of the rocket in the vertical image plane direction.

[0031] Based on the imaging pixels n1 and n2 of the two known body lengths, and the measured distance information r1 and r2 from the center points of the two body segments to the optical system respectively, according to the target density centroid measurement principle, the distance R from the shape centroid position of the entire target composed of the two body segments to the camera is:

[0032]

[0033] Step seven, calculate the spatial position of the rocket relative to the observation point according to the distance information of the target.

[0034] Define the station coordinate system O-XYZ, where the origin O is the observation point of the optical device, the X-axis points to the due north direction, the Y-axis points to the zenith, and the Z-axis points to the due east direction. Assume that the spatial position of the target at a certain moment is represented by point M(X c ,Y c ,Z c ). Therefore, OM is the distance R from the observation point to the flying target. At this time, project the target position M onto the horizontal plane XOZ of the coordinate system, and the projection point is denoted as M'.

[0035] For a single optical device, its shaft angle encoder system obtains the azimuth angle A and elevation angle E of the target relative to the optical axis of the device. Combining with the calculated distance information R of the target, obtain the position of the target in space through the following conversion relationship:

[0036]

[0037] Step eight, calculate the rocket takeoff drift. The coordinates of the rocket axis in the station system during the waiting stage are (X c,Y c ,Z c ), the coordinates of the center of mass of the rocket shape are (X, Y, Z), then the rocket takeoff drift p is:

[0038]

[0039] The drift azimuth angle θ is the angular amount by which the drift deviates from the OX axis:

[0040] θ=tg -1 [(ZZ c ) / (XX c )]-θ0

[0041] Where θ0 is the launch azimuth angle.

[0042] The advantages of the present invention are:

[0043] (1) Compared with the existing multi-station positioning method, the single-station method for measuring rocket takeoff drift based on optical imaging passive ranging in the present invention reduces the number of rocket takeoff drift measurement devices.

[0044] (2) The present invention uses a large-sized longitudinal rocket as the distance measurement object, which can greatly reduce the distance error. Combined with the high-precision angle measurement of optical images, the measurement accuracy of the rocket takeoff drift can be greatly improved.

[0045] (3) The present invention makes full use of rocket imaging information and effectively avoids the influence of the rocket's inclination during flight on the ranging accuracy by selecting multiple target characteristic sizes of the rocket to obtain the center of mass distance of the rocket image. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 It is a schematic diagram of the rocket takeoff drift during the rocket launch;

[0047] Figure 2 This is a schematic diagram of a rocket prototype;

[0048] Figure 3 It is a schematic diagram of imaging principle of the detector target surface end of the optical device;

[0049] Figure 4 It is the principle diagram of target imaging;

[0050] Figure 5 It is a flow chart of a method for measuring rocket takeoff drift at a single station based on optical imaging passive ranging according to the present invention;

[0051] Figure 6 It is a schematic diagram of the single-station positioning principle of the optical device in the present invention;

[0052] Figure 7 It is a schematic diagram of the characteristic parts of different rockets;

[0053] Figure 8 It is the working interface of the interpretation software;

[0054] Fig. 9 This is the principle diagram of the relationship between focal length and field of view;

[0055] Fig.10 It is a program interface of MATLAB fitting the relationship between focal length and field angle in an embodiment of the present invention;

[0056] Fig.11 is a fitting curve of horizontal focal length-field angle obtained in an embodiment of the present invention;

[0057] Fig.12 is a fitting curve of vertical focal length-field angle obtained in an embodiment of the present invention;

[0058] Fig.13 The corresponding viewing angle is calculated by the MATLAB fitting curve obtained in the embodiment of the present invention;

[0059] Fig.14 It is a schematic diagram of calculating target distance through multiple target feature points in an embodiment of the present invention. DETAILED DESCRIPTION

[0060] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0061] For the rocket launch process, in the launch coordinate system, if Figure 1 As shown in the figure, the rocket should fly vertically upward along the Y axis of the plumb line from the launch point during takeoff. Ideally, there is no displacement on the X and Z axes. However, due to the unevenness of the rocket thrust, vibration during launch, and external environmental influences such as weather, the rocket's vertical flight has a rise and fall on the Y axis compared to the actual trajectory, and there is a certain displacement on the X and Z axes. The axis position of the rocket's vertical flight at a certain moment ( Figure 1 The displacement of the projection point (marked as point a) on the XOZ plane of the launch coordinate system relative to the launch origin O is called the rocket takeoff drift, and the angle between the rocket drift direction and the launch direction is called the drift azimuth.

[0062] Describe the rocket as a line segment on the axis, so that different positions on the rocket will have different takeoff drift. Figure 2 As shown, b position and size relative marking rings are arranged on its outer shell. Each marking ring can be used as a target point for judging and measuring the rocket takeoff drift. Generally, the rocket takeoff drift is calculated with reference to the intersection position of a certain marking ring plane and the axis line.

[0063] As optoelectronic passive ranging technology becomes more mature, passive ranging methods based on image information are a new direction for realizing ranging functions.

[0064] As known from geometric optics, the pixel size of the optical detector at the image plane end is d, and the focal length of the detector is f. The detector is equivalent to an array composed of regular pixels. Taking the horizontal direction as an example, O is the center of the lens, and OO' is the center line of the optical axis. The imaging geometric relationship is as Figure 3 shown.

[0065] Among them:

[0066]

[0067] By analogy, it can be obtained that:

[0068]

[0069] In the linear interval within the detector plane, then

[0070] θ1 = θ N , N ≥ 2 (5)

[0071] The target is imaged on the target surface, and the angular spread relative to the center point of the lens is θ h :

[0072]

[0073] Generally, when 10Nd < f, there is

[0074]

[0075] When the central axis of the target coincides with the optical axis of the imaging system, as Figure 3 shown, then there is:

[0076]

[0077] In the formula, h is the target feature size, and R is the distance from the target to the center of the optical device. It can be obtained that

[0078]

[0079] According to the target imaging principle diagram as Figure 4 shown, when the size and focal length of the camera sensor's target surface are certain, there corresponds a unique camera field of view angle θ H , generally h << R. According to the size (number of pixels) of the target imaged on the target surface, θ h is converted into the camera field of view angle θ H , and by simplifying Equation (9), it can be obtained that

[0080]

[0081] Among them, B is the total number of pixels in the target imaging direction on the camera target surface, and n is the number of pixels occupied by the target imaged on the target surface.

[0082] In the embodiment of the present invention, one of the three substation measuring instruments of the optical system responsible for measuring the drift of the rocket takeoff is selected as the verification platform of the ranging method. The equipment is mainly composed of a high-speed television subsystem, a small precision tracking frame, an image tracking and processing subsystem, a tracking control subsystem, an image recording subsystem, an image interpretation system, a data communication subsystem, etc., which fully meets the hardware conditions for verifying the imaging ranging method. In addition, the equipment is close to the launch station (about 300 meters), the target imaging field of view accounts for a large proportion (more than 60%), and has obvious advantages in extracting and identifying feature points such as marker rings.

[0083] A method for measuring rocket takeoff drift based on single-station optical imaging, such as Figure 5 As shown, the specific steps are as follows:

[0084] Step 1: Establish an optical system for measuring the takeoff drift of a rocket, including a single optical device and a rocket, obtain the parameters of the optical device and the parameters of the rocket as a detection target, and simultaneously obtain the coordinates of the stationary state;

[0085] The selection of target features requires two considerations: first, whether the target's actual size parameters can be accurately obtained; second, whether the target size will be deformed during the measurement process. Try to select feature points whose feature size is fixed during the target's motion and whose image size on the target surface is consistent with the target's actual size during the interpretation process as the measurement target. Figure 7 As shown in , you can select the various horizontal marker rings of the rocket as the target, whose size parameter is the diameter width of the rocket; you can also select the entire rocket body, that is, the full length of the rocket, as the target size, or select the rocket body section between two marker rings as the target.

[0086] The optical device parameters are shown in Table 1, including: the total number of pixels and target size of the detector target surface in the imaging direction, the image pixels of the target on the target surface, and the focal length of the optical detector at the time of imaging.

[0087] Table 1 Optical detector related parameters

[0088]

[0089] Step 2: The optical device detects and images the stationary rocket or launch pad, and obtains the image size (number of pixels) of the feature points through interpretation.

[0090] According to the characteristics of the selected target object, in the corresponding imaging direction of the target surface, the direction of the rocket's vertical takeoff is taken as the vertical direction, and the direction of the rocket's cross section is taken as the horizontal direction. That is, the horizontal direction generally targets a certain rocket mark ring, and the vertical direction generally targets rocket segments of different lengths. In the image recorded in real time by the optical system, the imaging size of the horizontal and vertical targets is represented by the number of pixels in the corresponding direction, m is the number of pixels for the horizontal target imaging, and n is the number of pixels for the vertical target imaging.

[0091] The image of the rocket's real-time takeoff process recorded by the optical device is checked, data is verified, and image interpretation is performed through the interpretation software to determine the image size in the corresponding direction of the target. The software working interface is as follows: Figure 8 As shown, specifically:

[0092] First, coordinates are established in the X and Y directions, with the center point of the target surface (the number of pixels in the X-axis direction is F / 2, and the number of pixels in the Y-axis direction is G / 2) as the origin of the coordinates, and all pixels (F×G) on the target surface are evenly distributed;

[0093] Then, for any point in the image, its coordinate position on the X and Y axes of the target surface is directly obtained through the interpretation software. According to the selected target features and shapes, the pixel positions of the two polar edges of the target in the corresponding directions are directly obtained in the form of manual interpretation. After calculating the difference, the imaging size of the target in the corresponding direction is determined.

[0094] Finally, the error in the interpretation of the image size was reduced by taking the average value through multiple interpretations.

[0095] Step 3, based on the optical device parameters, the field of view angle of the optical device during detection imaging is measured to obtain a functional relationship between focal length and field of view angle;

[0096] The detector's field of view angle is an important parameter for measuring the distance to the target when imaging the target. The field of view angle is also called the field of view. It determines the field of view of the optical instrument. The field of view angle is related to the target size and focal length of the camera sensor. For the optical system selected in the present invention (whose target size is 25.6mmx16mm), because it has a continuous zoom function, the field of view angle corresponding to each focal length design of the lens zoom system is unique and different. The field of view angle is measured by taking the optical device camera lens as the vertex and the maximum angle range in which the image of the measured target can be imaged through the lens, such as Fig. 9 shown.

[0097] In order to accurately achieve the target distance measurement requirements, it is key to obtain accurate calibration of the focal length-field of view function relationship. The present invention uses the maximum angle of azimuth and pitch angle corresponding to the critical position of imaging in the field of view to measure a fixed object at different focal lengths, that is, the maximum field of view angle in the vertical or horizontal direction corresponding to the focal length value f. The specific measurement process is:

[0098] First, by continuously zooming from small to large, the tracking system of the optical device directly reads the focal length value fed back by the zoom system, and the field of view changes from large to small accordingly;

[0099] Then, select certain zoom positions at appropriate intervals during the entire zoom process to calibrate the field of view angle. The field of view angle corresponding to each group of focal lengths is measured more than 3 times, and the corresponding field of view angle is determined after average calculation;

[0100] Finally, after recording and arranging the data of the field of view angles corresponding to different focal length values, MATLAB was used to fit the functional relationship between focal length and field of view angle.

[0101] Tables 2 and 3 record the focal length-field angle measurement results of the optical system in the horizontal direction and the vertical direction in this embodiment.

[0102] Table 2 Horizontal focal length and field of view angle measurement results

[0103]

[0104] Table 3 Measurement results of vertical focal length and field of view angle

[0105]

[0106] The above two sets of focal length values ​​in the horizontal and vertical directions and the corresponding field of view angle measurement data can be used to obtain the functional relationship between focal length and detector field of view angle through Fourier fitting method in MATLAB. The specific program is written as follows: Fig.10 shown.

[0107] Step 4: According to the focal length-field of view function, select the appropriate Fourier function fitting order, and fit the focal length-field of view relationship curve in MATLAB;

[0108] The focal length-field angle relationship curve is fitted in MATLAB. According to the matching with the recorded data, the appropriate Fourier function fitting order is selected. In the embodiment of the present invention, the 7th order Fourier function expression is selected for the horizontal direction. The fitting curve is as follows: Fig.11 As shown; the 6th-order Fourier function expression is used in the vertical direction, and the fitting curve is as follows Fig.12 shown.

[0109] Step 5, according to the focal length-field angle relationship curve, the field angle corresponding to any focal length selected by the optical device after changing the target imaging ratio by zooming is obtained, and the distance information of the target is further obtained in combination with the imaging size;

[0110] according to Fig.11 , Fig.12 The MATLAB fitting curves in the horizontal and vertical directions are shown in Fig.13 After inputting the current focal length value, the field of view angle parameter corresponding to the focal length can be directly obtained, and directly brought into formula (10) to calculate the distance information R of the target.

[0111] Considering that the target may be tilted or flipped in the direction of the target surface of the device during flight, this will affect the size of the target image to a certain extent and cause a large error in ranging. Fig.14 The single-camera ranging method shown is to set two or more target feature points on the same target. This embodiment takes two target feature points as an example. In the same frame of optical image captured and recorded by the device, the distances from the center points of the two target features to the device are calculated respectively.

[0112] L1 is the size of the rocket body between the two marking rings at the upper end of the rocket, and the corresponding number of imaging pixels is n1. L2 is the size of the rocket body between the two marking rings at the lower end of the rocket, and the corresponding number of imaging pixels is n2. ψ is the inclination angle of the rocket in the direction vertical to the image plane.

[0113] according to Fig.14 As shown, based on the known imaging pixels n1 and n2 of the two segments of the arrow body, and the corresponding measurable distance information r1 and r2 from the center point of the two segments to the optical system, the distance R from the center of mass of the entire target composed of the two segments of the arrow body to the camera can be obtained based on the target density centroid measurement principle:

[0114]

[0115] By using this method, the distances from two target points to the device can be obtained, thereby determining the positions of two characteristic parts of the flying target in space. In formula (11), R combines the ranging results of r1 and r2. According to the characteristic parts of the selected target, not only can the distance information from the target centroid or the center of the two characteristic points to the device be obtained by comprehensive calculation, but also, compared with the ranging of a single point of the target, the ranging accuracy is further improved, thereby more accurately determining the position information of the target in space.

[0116] Step six, calculate the spatial position of the rocket relative to the observation point based on the target's distance information.

[0117] like Figure 6 As shown in the figure, in order to better describe the position change relationship of the flying target relative to the observation equipment, the station coordinate system O-XYZ is defined, the origin O is the observation point of the optical equipment, the X axis points to the north, the Y axis points to the zenith, and the Z axis points to the east. Assume that the spatial position of the target at a certain moment is represented by point M (X c ,Y c ,Z c ), so OM is the distance R from the observation point to the center point of the flying target. At this time, the target position M is projected onto the horizontal plane XOZ of the coordinate system, and the projection point is recorded as M'.

[0118] For a single optical device, its axis encoder system obtains the azimuth angle A and elevation angle E of the target relative to the optical axis of the device, and combines the calculated distance information R of the target to obtain the position of the target in space through the following conversion relationship:

[0119]

[0120] Step seven, calculate the rocket takeoff drift.

[0121] The coordinates of the rocket axis in the station coordinate system are (X c ,Y c ,Z c ), the coordinates of the center of mass of the rocket shape are (X, Y, Z), and the rocket takeoff drift p is:

[0122]

[0123] The drift azimuth angle θ is the angular amount by which the drift deviates from the OX axis:

[0124] θ=tg -1 [(ZZ c ) / (XX c )]-θ0 (14)

[0125] Where θ0 is the launch azimuth angle.

[0126] In practical applications, after obtaining r1 and r2 using the above method, the feature point L can be obtained using formula (12): l The three-dimensional position information of L1 and L2 in the spatial coordinate system, that is, the coordinates of the two feature points are (X1, Y1, Z1) and (X2, Y2, Z2) respectively. At the same time, their imaging positions on the target surface are determined, and the axial direction of the rocket can be obtained by the above formula with the line connecting these two points as the target. Since the center of mass of the targets L1 and L2 is a straight line, that is, the central axis of the target, its unit vector is equal to the unit vector of the central axis. Then the yaw angle and pitch angle of the axis in space are:

[0127]

[0128] As for the rolling angle, another attitude angle parameter of the flying target, it is necessary to measure the rolling angle mark on the rocket body (a group of small red square patterns arranged in a straight line parallel to the direction of the rocket body). At present, when the optical equipment is used for close-up shooting at a single station, high-definition imaging is used, and advanced digital image extraction and processing technology and details are used to interpret the details, or a group of rolling angle marks have been obtained. The position changes within a certain period of time, after coordinate conversion calculation, can be obtained. The rotation angle of the target around the central axis (flight direction), i.e., the y-axis, in the carrier coordinates can be obtained.

[0129] Therefore, if there are two or more target feature points in the field of view in the optical image, the distance from the center of the two targets to the device can be accurately obtained by the above-mentioned ranging method. According to the measurement principle of the optical attitude angle, the spatial position and attitude angle of the target in the measuring station system can be determined, and the tilt swing of the target relative to the device on the launch axis at that moment can be offset, including in the image plane direction (the left and right swing of the target in the field of view relative to the vertical axis) and the vertical image plane direction (the forward and backward swing of the target along the vertical axis relative to the vertical image plane direction), thereby eliminating the influence of the rocket attitude change on the ranging in the actual measurement.

[0130] L1 is the size of the rocket body with two marking rings on the upper end, and the corresponding number of imaging pixels is n1. L2 is the size of the rocket body with two marking rings on the lower end, and the corresponding number of imaging pixels is n2. ψ is the inclination angle of the rocket in the vertical image plane direction. Then:

[0131]

[0132] In the formula, r1 is the distance from the center of the L1 segment to the camera, r2 is the distance from the center of the L2 segment to the camera, f is the focal length of the current device, d is the pixel size, and ψ is the tilt angle of the rocket toward the camera. Eliminating the common terms (cosψ, d, f), the equation can be transformed into:

[0133]

[0134] According to the figure, Δr=r2-r1, then:

[0135]

[0136] From this, we can get the angle between the rocket and the camera image plane, that is, the inclination angle of the rocket in the direction perpendicular to the image plane:

[0137]

[0138] The inclination angle of the rocket in the camera image plane can be directly calculated by the following formula β, that is, the inclination angle of the rocket in the camera image plane:

[0139]

[0140] Therefore, as long as at least three marker rings or endpoints are in the field of view at the same time in each frame of the image, and the three feature points divide the rocket into two sections, the distance between the two target center points can be obtained. Fig.14 As long as the spatial positions of the two center points are determined, according to the principle of two points determining a line, the line connecting the two center points can determine the target, thereby obtaining the swing angle of the target relative to the transmitting vertical line in the direction of the device image plane and the tilt angle perpendicular to the image plane. At the same time, the error of target positioning can be further corrected.

[0141] Through the above method, the position of the target center or centroid after comprehensive calculation is continuously tracked and measured. According to the equipment site information, after coordinate conversion, the spatial position of the target centroid (or center) in the geodetic coordinate system can be continuously obtained, thereby obtaining the flight trajectory of the target in space.

[0142] Furthermore, when selecting two target feature points, the following requirements must be met:

[0143] Taking the horizontal target as an example, the total differentiation of R in formula (12) is:

[0144]

[0145] Dividing equation (22) by equation (10) yields the relative ranging accuracy:

[0146]

[0147] Since the field of view angle of the optical system can be effectively eliminated through a large amount of experimental data, the error rate is not large; and the target feature size error does not take into account the deformation of the target during flight, so the size error is also small. Therefore, the main factor affecting the ranging error is the target imaging pixel judgment error. The judgment error is directly related to the target imaging size.

[0148] According to equation (23), ignoring the influence of target size error, that is, only considering the interpretation error of imaging pixels, we can obtain:

[0149]

[0150] Taking the measurement of rocket takeoff drift as an example, its accuracy is usually required to be less than 0.2m. Considering the angle measurement error of the current equipment for measuring rocket takeoff drift (the design value is less than 1′), the ranging accuracy of the optical equipment (station distance is 300m, and the detector resolution of the optical system is 2560×1600) at the initial position must meet 0.18m. Assuming that the current digital image interpretation error is 0.4 pixels, the calculation according to formula (15) under the current equipment conditions meets the minimum condition of the corresponding target imaging size when the measurement accuracy is met. When two segments of the target are connected in series (corresponding to imaging sizes n1 and n2), the maximum length of the target n′=n1+n2 combination is the total length of the rocket, and the target imaging size is:

[0151]

[0152] It can be seen that when there is a tilt angle or a swing angle in the image plane direction or the vertical image plane direction, and the takeoff drift measurement accuracy conditions need to be met, when the two segments of the rocket are selected as the targets of the rocket's takeoff drift, the imaging size of the two-segment combined target must satisfy n1+n2>1333 pixels. At this time, the imaging sizes are: rocket as a whole>n1+n2>1333 pixels. At this time, the imaging ratio of the combined target in the field of view satisfies 1333 / 1600=83.3%, that is, the two selected target segments must occupy more than 83.3% of the field of view in total to meet the accuracy requirements and realize the single-station optical measurement task of the rocket's takeoff drift through the ranging method in this paper.

[0153] Considering that formula (15) does not require that n1 and n2 cannot overlap, if the overlap is considered, the target n1+n2<2n', that is, the maximum length of the two-segment rocket body combination is less than twice the total length of the rocket, then

[0154]

[0155] This shows that if the selected targets can overlap, that is, the lengths of the two target rocket bodies are increased accordingly, that is, the imageable size of each target segment on the target surface must meet the condition of >667 pixels. At this time, the imaging ratio of each target length (also the total target length) in the vertical direction of the rocket accounts for more than 41.7% of the field of view (667 / 1600=41.7%). Therefore, the method of overlapping target selection to measure the takeoff drift can further reduce the requirements for the rocket imaging size during the mission, effectively broaden the selectable conditions for measuring targets, and provide more implementation paths and measurement solutions for the measurement task of the rocket takeoff drift that meets the accuracy requirements of a single optical device according to the ranging method in this article.

[0156] It can be seen from the above that through the aforementioned imaging passive ranging method, using the existing high-speed television system, not only can the rocket's attitude angle measurement and flight trajectory measurement be realized, but also when the rocket's axis meets the imaging size range, the task of single-station measurement of the rocket's takeoff drift can be successfully achieved.

[0157] Technical Effects

[0158] The present invention uses the maximum angle of the azimuth and pitch angles corresponding to the critical position where an image can be formed in the field of view to measure a fixed object at different focal lengths, which is the maximum field of view angle in the vertical or horizontal direction corresponding to the focal length value f. Through continuous zooming from small to large, the tracking system of the optical system can directly read the focal length value fed back by the zoom system. At this time, the field of view changes from large to small. Some zoom positions approximately spaced apart during the whole process of distributed zooming are selected for field of view angle calibration. The field of view angle corresponding to each group of focal lengths is measured more than 3 times, and the corresponding field of view angle is determined after average calculation. After recording and arranging the data of the field of view angles corresponding to different focal length values, MATLAB is used to fit the functional relationship between the focal length and the field of view angle. Tables 2 and 3 record the focal length-field of view angle measurement results of the optical system in the horizontal direction and the vertical direction, respectively.

[0159] The above two sets of focal length values ​​in the horizontal and vertical directions and the corresponding field of view angle measurement data can be used to obtain the functional relationship between focal length and detector field of view angle through Fourier fitting method in MATLAB. The specific program is written as follows: Fig.10 shown.

[0160] The focal length-field of view relationship curve is fitted in MATLAB. According to the matching with the recorded data, the appropriate Fourier function fitting order is selected. In this paper, the 7th-order Fourier function expression is selected for the horizontal direction. The fitting curve is as follows: Fig.11 As shown; the 6th-order Fourier function expression is used in the vertical direction, and the fitting curve is as follows Fig.12 According to the fitting curve, the field of view angle corresponding to any focal length selected by the optical system after changing the target imaging ratio by zooming can be deduced.

[0161] according to Fig.11 , Fig.12 The MATLAB fitting curves in the horizontal and vertical directions are shown in Fig.13 After inputting the current focal length value, the field of view angle parameter corresponding to the focal length can be directly obtained, and directly brought into formula (10) to calculate the distance information of the target.

[0162] In the process of optical measurement, the azimuth angle A, pitch angle E, and distance information R are obtained according to the angle measurement system of the equipment. At the same time, according to the position offset of the target feature point relative to the center of the optical axis in the field of view, the linear position of the target point relative to the vertical optical axis is determined. The distance value from the relevant part of the rocket to the center of the equipment is obtained experimentally by using the visual imaging passive ranging method. According to the working principle of optical equipment measurement, in the equipment station system, the spatial position of the rocket feature point is uniquely determined by the three measurement elements A, E, and R, so that single-station positioning can be achieved. The launch origin and the station address information of the equipment in the geodetic measurement can also be used to determine the distance of the rocket axis center from the original vertical state in the launch coordinate system. In this way, the mission goal of measuring the takeoff drift of the rocket by a single optical device can be achieved.

Claims

1. A method for measuring rocket takeoff drift based on single-station optical imaging, characterized in that: The specific steps are as follows: Step 1: Establish an optical system for measuring the takeoff drift of a rocket, including a single optical device and a rocket, and obtain the parameters of the optical device and the parameters of the rocket as a detection target; When the rocket is used as the detection target, the horizontal marker ring of the rocket or the overall / partial length of the vertical rocket body is selected as the target parameter, and the coordinates of the rocket station in the static state are obtained at the same time; Step 2: The optical device detects and images the stationary rocket or launcher, and obtains the number of pixels of the characteristic point imaging by interpretation; According to the characteristics of the selected target object, in the corresponding imaging direction of the target surface, the direction of the vertical take-off of the rocket is taken as the vertical direction, and the direction of the rocket cross section is taken as the horizontal direction; the imaging size of the target in the horizontal and vertical directions is represented by the number of pixels in the corresponding direction, m is the number of pixels of the horizontal target imaging, and n is the number of pixels of the vertical target imaging; Step 3, based on the optical device parameters, the field of view angle of the optical device during detection imaging is measured to obtain a functional relationship between focal length and field of view angle; The field of view angle is measured with the camera lens of the optical device as the vertex, and the maximum angle range of the image of the object to be measured that can be formed through the lens. The specific measurement process is: First, by continuously zooming from small to large, the tracking system of the optical device directly reads the focal length value fed back by the zoom system, and the field of view changes from large to small accordingly; Then, select certain zoom positions at appropriate intervals during the entire zoom process to calibrate the field of view angle. The field of view angle corresponding to each group of focal lengths is measured more than 3 times, and the corresponding field of view angle is determined after average calculation; Finally, after recording and collating the data of the field of view angles corresponding to different focal length values, MATLAB was used to fit the functional relationship between focal length and field of view angle; Step 4: According to the focal length-field of view function, select the appropriate Fourier function fitting order, and fit the focal length-field of view relationship curve in MATLAB; Step 5, according to the focal length-field angle relationship curve, the field angle corresponding to any focal length selected by the optical device after changing the target imaging ratio by zooming is obtained, and the distance information of the target is further obtained in combination with the imaging size; Taking the rocket axis as the target measurement object, the distance R from the target to the center of the optical device is calculated as: where θ H is the field of view angle, h is the target feature size, and h << R; B is the total number of pixels of the camera target surface in the target imaging direction, and n is the number of pixels occupied by the target imaging on the target surface; Step 6: When the target surface of the device is tilted or flipped, two or more target feature points are set on the same target, and the distance between the centroid of the feature point and the camera is calculated based on the imaging results of each feature point, which is the distance information of the target. Select a target feature points on the same target, a≥2, and calculate the distance from the center point of each target feature to the device in the same frame of optical image recorded by the optical device. Taking a=2 as an example, the specific process is as follows: L1 is the size of the rocket body between the two marking rings at the upper end of the rocket, and the corresponding number of imaging pixels is n1. L2 is the size of the rocket body between the two marking rings at the lower end of the rocket, and the corresponding number of imaging pixels is n2. ψ is the inclination angle of the rocket in the vertical image plane direction. Based on the known imaging pixels n1 and n2 of the two segments of the arrow body, and the corresponding distance information r1 and r2 from the center point of the two segments of the arrow body to the optical system, the distance R from the shape center position of the entire target composed of the two segments of the arrow body to the camera is obtained according to the target density centroid measurement principle: Step 7: Calculate the space position of the rocket relative to the observation point based on the distance information of the target, and calculate the rocket takeoff drift; The coordinates of the rocket axis in the station system during the launch phase are (X c ,Y c ,Z c ), the coordinates of the center of mass of the rocket shape are (X, Y, Z), then the rocket takeoff drift p is: The drift azimuth angle θ is the angular amount by which the drift deviates from the OX axis: θ=tg -1 [(Z-Z c ) / (X-X c )]-θ0 Where θ0 is the transmitting azimuth angle.

2. The method for measuring rocket takeoff drift based on single-station optical imaging according to claim 1, characterized in that: The optical device parameters include: the total number of pixels and target size of the detector target surface in the imaging direction, the image pixels of the target on the target surface, the focal length of the optical detector at the time of imaging, and the coordinates of the optical device.

3. The method for measuring rocket takeoff drift based on single-station optical imaging according to claim 1, characterized in that: The image recorded by the optical device is processed by the interpretation software to determine the image size in the corresponding direction of the target, specifically: First, coordinates are established in the X and Y directions, with the center point of the target surface as the origin of the coordinates. The number of pixels in the X-axis direction is F / 2, the number of pixels in the Y-axis direction is G / 2, and all pixels on the target surface are evenly distributed F×G. Then, for any point in the image, its coordinate position on the X and Y axes of the target surface is directly obtained through the interpretation software. According to the selected target features and shapes, the pixel positions of the two polar edges of the target in the corresponding directions are directly obtained through manual interpretation. After calculating the difference, the imaging size of the target in the corresponding direction is determined. Finally, the error in the interpretation of the image size was reduced by taking the average value through multiple interpretations.

4. The method for measuring rocket takeoff drift based on single-station optical imaging according to claim 1, characterized in that: Step 6 The calculation process of the rocket's spatial position relative to the observation point is: Define the geodetic coordinate system O-XYZ, where the origin O is the observation point of the optical device, the X axis points to the north, the Y axis points to the zenith, and the Z axis points to the east. Assume that the spatial position of the target at a certain moment is represented by point M (X c ,Y c ,Z c ), so OM is the distance R from the observation point to the flying target. At this time, the target position M is projected onto the horizontal plane XOZ of the coordinate system, and the projection point is recorded as M'; For a single optical device, its axis encoder system obtains the azimuth angle A and elevation angle E of the target relative to the optical axis of the device, and combines the calculated distance information R of the target to obtain the position of the target in space through the following conversion relationship:

Images