Fixed star map simulation method based on in-orbit measured data of low-orbit moonlet

Abstract

The invention provides a fixed star map simulation method based on in-orbit measured data of a low-orbit moonlet, which comprises the following steps of: calculating orbit and attitude data of four corners of a star map CCD (Charge Coupled Device) according to row and column number interpolation of a to-be-simulated satellite star map image on the basis of satellite orbit, attitude and row time data measured in-orbit by the low-orbit moonlet in combination with a fixed star catalog; calculating right ascension and declination of angular points by combining internal and external orientation element information of a satellite, and selecting fixed star points falling into a star map in a star catalogue; the row and column numbers of the fixed star points in the star map are obtained by adopting a grid point bilinear interpolation method, and the gray scale information of the fixed star points is obtained by adopting a two-dimensional Gaussian distribution method. The star sky environment is further simulated by adding noise, and a simulation star map is output according to detailed information of result star points. According to the method, the simulation star map can be obtained quickly and efficiently, and the method has positive significance in improving the geometric calibration precision of the visible light load of the low-orbit moonlet through fixed star calibration.

Description

Technical Field

[0001] The present invention relates to a method for simulating a star map based on on-orbit measured data of a low-earth orbit small satellite, belonging to the field of satellite remote sensing image processing. Background Technique

[0002] During on-orbit operation, the interior and exterior orientation elements of an optical remote sensing satellite are affected by various factors, and their accuracy will be affected. Traditional geometric calibration is carried out by photographing a ground calibration field, which has some disadvantages: (1) Affected by weather, advance mission planning is required; (2) For long-term and high-frequency calibration, the human and material costs are high.

[0003] Currently, for multiple remote sensing satellites launched in China, in addition to photographing the ground, through satellite attitude adjustment, the ability to image the starry sky by photographing deep space has enabled the ability to photograph stars. Geometric correction of the satellite can also be carried out through star calibration, and it has many advantages: (1) Star photography is not affected by the earth's atmosphere; (2) As a distant reference object, the position of a star can be considered stationary, and the distribution of a large number of stars has the characteristics of ground control points; (3) Only satellite attitude adjustment is required to complete star photography, and no calibration field is needed, etc. However, there are also some problems in current star calibration photography, such as the actual photography frequency is not high, and the actual imaging quality is relatively poor. Summary of the Invention

[0004] The technical problem to be solved by the present invention is: overcoming the deficiencies of the prior art, providing a method for simulating a star map based on on-orbit measured data of a low-earth orbit small satellite, overcoming the problems of current photography frequency and quality, and providing a basis for further research on star calibration.

[0005] The technical solution of the present invention is: In the first aspect, a method for simulating a star map based on on-orbit measured data of a low-earth orbit small satellite is disclosed, including:

[0006] Reading satellite attitude, orbit and time-of-flight data, as well as the right ascension, declination and magnitude information of stars in the SAO star catalog;

[0007] Establishing the conversion relationship between the camera CCD and the celestial coordinate system; converting the four corner points of the camera CCD into the celestial coordinate system to determine the range of the four corner points of the camera CCD in the celestial coordinate system;

[0008] Selecting stars falling within the range of the four corner points of the camera CCD according to the right ascension and declination of the stars in the star catalog; dividing the camera CCD into grid points, obtaining the camera coordinate system vectors corresponding to the CCD pixels of each grid point, and converting them into the celestial coordinate system;

[0009] For each star within the range of the four corner points of the camera CCD, search for the four grid points closest to the star among the grid points in the celestial coordinate system, and calculate the row and column numbers of the star point in the camera CCD and the corresponding star point gray value;

[0010] Based on the obtained star points of the stars, generate a new image with the same size as the original CCD image according to their row and column numbers in the camera CCD and the corresponding star point gray values, and output it as a simulated star map of the stars after adding background noise.

[0011] Preferably, the orbital data are the XYZ three-dimensional coordinates and velocities of the satellite in the Earth-Centered Earth-Fixed coordinate system;

[0012] The attitude data are the roll angle, pitch angle, and yaw angle of the satellite relative to the orbital coordinate system;

[0013] The line time data are the time information corresponding to each line during the camera shooting.

[0014] Preferably, establish the conversion relationship between the camera CCD and the celestial coordinate system, specifically:

[0015] Construct the camera observation vector ox = [tan(psi_x), -tan(psi_y), 1], where point o is the focus, psi_x is the angle between the vector ox and the CCD pixel along the flight direction, psi_y is the angle between the vector ox perpendicular to the flight direction and the CCD pixel, and normalize the vector ox to obtain ox';

[0016] ox' is rotated according to the installation matrix and converted from the camera coordinate system to the body coordinate system to obtain the observation vector ox” in the body coordinate system;

[0017] ox” is further represented as OX' in the celestial coordinate system after coordinate rotation:

[0018] OX' = ox”·R a ·R b ·R c

[0019] where, R a 、R b 、R c are respectively the rotation matrix of the exterior orientation elements, the rotation matrix of the satellite attitude angle, and the rotation matrix of the satellite orbital coordinate system:

[0020]

[0021]

[0022] In the formula: omega, phi, and kappa are respectively the cross-track tilt angle, along-track tilt angle, and rotation angle around the main axis in the exterior orientation elements of the camera, and is the rotation matrix about the three exterior orientation elements angles; yaw, pitch, and roll are the satellite yaw angle, pitch angle, and roll angle respectively, and is the rotation matrix about the three attitude angles; Ω is the right ascension of the ascending node of the satellite, i is the inclination of the satellite orbit, and u is the argument of latitude of the satellite, and is the rotation matrix about the three orbital angles.

[0023] Preferably, when dividing the camera CCD into grid points, every 100 - 200 pixel points are used as grid points in the row and column directions of the CCD respectively.

[0024] Preferably, when calculating the row and column numbers of the corresponding star points and the corresponding star point brightness:

[0025] The row and column numbers of each star point in the image are calculated by the method of bilinear interpolation, and the brightness of the current star point is calculated using two-dimensional Gaussian blur.

[0026] Preferably, the row and column numbers of each star point in the image are calculated by the method of bilinear interpolation, specifically:

[0027] If the latitudes and longitudes of the four nearest grid points of star point i are denoted as: (lat A , lon A ), (lat B , lon B ), (lat C , lon C ), (lat D , lon D ), and the corresponding row and column numbers are denoted as (r A , c A ), (r B , c B ), (r C , c C ), (r D , c D ), and the right ascension and declination of the star point are (lat O , lon O ), then the row and column numbers (r iO , c iO ) of the center point O of star point i in the camera CCD are:

[0028]

[0029] Preferably, the gray value within the region where the star point is located is calculated using two-dimensional Gaussian blur, and the size of the region where the star point is located is taken as 5×5 pixels, specifically:

[0030] Then the gray value g of the star point i i The expression is:

[0031]

[0032] Where: Let the maximum quantization bit number of the star map be a, and m i is the magnitude of the star point i, and m max is the magnitude of the brightest star among the star points, and g max is the gray value, and g max <2 a -1, and the magnitude size is negatively correlated with the brightness of the star;

[0033] The energy distribution of the star point follows a two-dimensional Gaussian distribution, and the gray value of any pixel point (x, y) in the region is:

[0034]

[0035] (r iO , c iO ) is the coordinate of the center point O of the star point i, and σ is the diffusion radius.

[0036] In a second aspect, the present invention discloses a terminal device, which is characterized by comprising:

[0037] A memory for storing instructions executed by at least one processor;

[0038] A processor for executing the instructions stored in the memory to implement the method for simulating a star map of a constant star based on the on-orbit measured data of a low-earth orbit small satellite as described above.

[0039] In a third aspect, the present invention discloses a computer-readable storage medium, which is characterized in that the computer-readable storage medium stores computer instructions, and when the computer instructions are run on a computer, the computer is made to execute the method for simulating a star map of a constant star based on the on-orbit measured data of a low-earth orbit small satellite as described above.

[0040] The present invention has the following advantages compared with the prior art:

[0041] The star map simulation carried out by the present invention can be calculated using satellite orbits, attitudes and travel time data under different conditions, improving the imaging quality while increasing the shooting frequency, fully overcoming the problems of few and poor-quality actual on-orbit star maps of visible light satellites. Compared with traditional geometric calibration research, it has the characteristics of being fast, efficient and low-cost, and has good significance for further research on using stars for geometric calibration and saving calibration labor and material costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 It is a flowchart of the method of the present invention;

[0043] Figure 2 Schematic diagram of the conversion from the camera coordinate system to the celestial sphere coordinate system of the present invention;

[0044] Figure 3 Schematic diagram of the camera coordinate system of the present invention;

[0045] Figure 4 Schematic diagram of the row and column number interpolation of the stellar points of the present invention;

[0046] Figure 5 Simulation result of the star map of the present invention and its partial enlarged view. Detailed implementation manner

[0047] Based on the satellite orbit, attitude and time-of-flight data of a low-earth orbit small satellite in orbit, the present invention converts the coordinates of the four corner points of the CCD to the celestial sphere coordinate system, calculates the stars falling within the range of the four corner points, calculates the row number and column number of the star points of each star on the image by the method of bilinear interpolation, calculates the star point brightness by two-dimensional Gaussian blur and outputs a star map of stars. Through the real data (orbit, attitude, time-of-flight, etc.) of a low-earth orbit small satellite, combined with a star catalog, the star map of stars can be simulated. The simulated star map can simulate the star map under different environments, overcome the current problems of shooting frequency and quality, and provide a basis for further research on star calibration.

[0048] As Figure 1 shown, a method for simulating a star map of stars based on the measured data of a low-earth orbit small satellite in orbit includes the following steps:

[0049] (1) Read the input data, including satellite attitude, orbit and time-of-flight data; read the SAO (Smithsonian Astrophysical Observatory) star catalog data;

[0050] According to the row and column numbers of the four corner points of the simulated star map image (referring to the row and column sizes of the star map to be simulated, refer to the detailed step (5). For example, if a 4096×4096 star map is simulated, then the coordinates of the four corner points are (0,0), (0,4096), (4096,0), (4096,4096)), calculate the time-of-flight of the corresponding row and interpolate to calculate the satellite attitude and orbit at the corresponding moment. The satellite orbit is interpolated by the Lagrange interpolation method, and the attitude data is interpolated by the linear interpolation method. The SAO star catalog reads the right ascension, declination and stellar magnitude of each star respectively.

[0051] (2) Perform coordinate transformation to convert the coordinates of the four corner points from the camera coordinate system to the celestial sphere coordinate system;

[0052] Convert the coordinates of the four corner points from the camera coordinate system to the celestial sphere coordinate system. First, calculate the three-dimensional vector of each pixel in the camera coordinate system and convert it from the camera coordinate system to the body coordinate system. After performing the transformation of the exterior orientation elements, satellite attitude, and orbital angle on this vector and rotating it to the celestial sphere coordinate system, the right ascension and declination of the four corner points in the celestial sphere coordinate system are obtained.

[0053] Traverse the longitude and latitude of the stars in the star catalog, and select the star points within the range of the four corner points as the points finally output on the star map.

[0054] (3) Solve the specific row and column numbers of the star points in the star map;

[0055] First, perform grid point interpolation of the CCD. In the CCD, every 100 - 200 pixel points in the row and column directions are used as grid points. Solve the row and column numbers of each grid point respectively, and use the row and column numbers to solve information such as the row time, orbit, and attitude at this moment. Construct the camera coordinate system vector corresponding to the detection element, and convert the vector of each grid point to the celestial sphere coordinate system through coordinate transformation. The specific transformation steps can be seen in the transformation of the four corner points in (2).

[0056] (4) Calculation of the star points of the stars;

[0057] The calculation of the row and column numbers of the star points first calculates the grid where the point is located according to the right ascension and declination of the star point, and uses the right ascension, declination, and row and column numbers of 4 grid points to perform bilinear interpolation to calculate the row number and column number of the star point on the image.

[0058] The gray level of the star point is set according to the relationship of the stellar magnitude and the maximum quantization bit number of the image. For every difference of one stellar magnitude between star points, the brightness value approximately differs by a factor of 2.512. The specific energy distribution of each star point is set in the manner of a two-dimensional Gaussian distribution.

[0059] (5) Output of the star map of the stars. The final output star map has the same number of rows as the row time file and the same number of columns as the number of pixels of the CCD sensor. All the star points are sorted according to the row and column numbers in step (4). To simulate the actual starry sky, background noise can be added to the image.

[0060] The following further describes the specific implementation manner of the present invention in detail with reference to the accompanying drawings. The main steps are as follows:

[0061] (1) Read the input data required for star map simulation

[0062] Read the orbital data, attitude data, and line time data at the star map simulation moment. The orbital data are the XYZ three-dimensional coordinates and velocities of the satellite in the ECEF (Earth-Centered, Earth-Fixed coordinate system); the attitude data are the roll, pitch, and yaw angles of the satellite relative to the orbital coordinate system; the line time data are the time information corresponding to each line during camera shooting. Assume that the CCD of the star map shooting satellite has L rows and R columns. The satellite line time data should include the shooting moments of each of the L rows, and the orbital and attitude data include the satellite orbit and attitude data during this shooting time period. The star catalog of the satellite uses the SAO star catalog, and read the right ascension, declination, and magnitude information of the stars in this star catalog.

[0063] (2) Calculate the right ascension and declination of the four corner points of the star map to be simulated

[0064] The right ascension and declination coordinates of the stars are described based on the celestial coordinate system. To calculate which stars can fall within the shooting range of the star camera, it is necessary to convert the four corner points of the CCD shooting star map to the celestial coordinate system, as shown in the figure. The row and column numbers of the four corner points correspond to N1 = 0, M1 = 0; N2 = 0, M2 = R; N3 = L, M3 = 0; N4 = L, M4 = R. N i and M i are the row and column numbers. Calculate the corresponding line time according to the row number of each point. According to the line time, the orbit and attitude at this moment can be interpolated. The orbit is interpolated using the Lagrange interpolation method, and the attitude can be interpolated using the linear interpolation method.

[0065] As Figure 3 shown, construct the camera observation vector ox = [tan(psi_x), -tan(psi_y), 1]. The o point is the focus. The vector ox' after normalizing the vector ox. psi_x is the angle between the vector along the flight direction and the CCD detector element, and psi_y is the angle between the direction perpendicular to the flight direction and the CCD detector element. In the ideal state, it can be considered that psi_x = 0.

[0066] The normalized vector ox' first needs to be rotated according to the installation matrix of the satellite to complete the rotation from the camera coordinate system to the body coordinate system to obtain the rotated vector ox”. The installation matrix is a 3×3 matrix.

[0067] The vector ox” after being rotated by the installation matrix is then rotated through the coordinate system to obtain its vector OX' in the celestial coordinate system, satisfying:

[0068] OX' = ox”·R a ·R b ·R c

[0069] R a 、R b 、R cThey are respectively the rotation matrix of exterior orientation elements, the rotation matrix of satellite attitude angles, and the rotation matrix of the satellite orbit coordinate system:

[0070]

[0071] In the formula: omega, phi, and kappa are respectively the cross-track tilt angle, the along-track tilt angle, and the rotation angle around the principal axis in the exterior orientation elements; yaw, pitch, and roll are the yaw (the selection order can be chosen according to the actual rotation order of the satellite, such as the order of the PRSS-1 satellite is pitch - roll - yaw), pitch, and roll angles of the satellite; Ω is the right ascension of the ascending node of the satellite, i is the inclination angle of the satellite orbit, and u is the argument of latitude of the satellite. Further, in the formula: and are respectively the rotation matrices around the three exterior orientation element angles; and are respectively the rotation matrices around the three attitude angles; and are respectively the rotation matrices around the three orbit angles.

[0072] Under the celestial coordinate system, OX' satisfies:

[0073]

[0074] In the formula: as Figure 2 shown, α and δ are the right ascension and declination of the celestial body.

[0075] (3) Selection of stars within the star map range

[0076] According to the right ascension and declination of the stars in the star catalog, select the stars that fall within the range of the star Figure 4 corner points as the stars shown on the star map.

[0077] In order to accurately calculate the row and column numbers of each star point, it is necessary to divide the CCD into grid points. Every 100 - 200 pixel points are used as grid points in the row and column directions of the CCD respectively. Solve the row and column numbers of each grid point respectively, use the row and column numbers to solve the information such as the row time, orbit, and attitude at this moment, construct the camera coordinate system vectors of the corresponding pixels of each grid point, and convert the vectors of each grid point to the celestial coordinate system through coordinate transformation. The specific transformation steps can be seen in the transformation of the four corner points in (2).

[0078] (4) Calculation of star points of stars

[0079] The calculation of star points of stars includes the calculation of the row and column numbers of the stars on the star map and the calculation of the gray values.

[0080] For the calculation of the row and column numbers, according to the right ascension and declination of the star point, search for the 4 grid points with the closest distance in the grid points, and calculate the row and column numbers of the star point through bilinear interpolation, such as Figure 4As shown, the latitudes and longitudes of the four neighboring grid points of the center point o of the star point i are respectively (lat A , lon A ), (lat B , lon B ), (lat C , lon C ), (lat D , lon D ), and the row and column numbers are (r A , c A ), (r B , c B ), (r C , c C ), (r D , c D ). The right ascension and declination of the star point center are (lat O , lon O ), and the row and column numbers of the center point O of the star point i are (r iO , c iO ) as follows:

[0081]

[0082] Calculation of the gray value of the current star point of a star (a star for which the row and column numbers of the star in the star chart have been obtained). Assume that the maximum quantization bit number of the star chart is a, the magnitude of the star point i is m i , the magnitude of the brightest star in the star point is m max , and the gray value is g max (g max < 2 a - 1). The magnitude and the brightness of the star are negatively correlated. For every magnitude difference, the gray value approximately differs by a factor of 2.512. Then the gray value g i of the star point i is expressed as:

[0083]

[0084] Use two-dimensional Gaussian blur to calculate the gray value of the area where the star point is located (the size of the area where the star point is located is generally taken as 5×5). The gray value of any pixel point (x, y) in the grid area is:

[0085]

[0086] (r iO , c iO ) is the coordinate of the center point O of the star point i, σ is the dispersion radius. If σ is taken as 0.671, it can ensure that 90% of the energy is concentrated within 5×5 pixels.

[0087] (5) Output of the star chart of stars

[0088] The number of rows in the final output star map is the same as that in the row time file, and the number of columns is the same as the number of detection elements of the CCD sensor. All the star points are arranged according to the row and column numbers in step (4). To simulate the actual starry sky, certain background noise can be added to the image.

[0089] As Figure 5 shown, it is a set of simulated star maps, and the size of this star map is 4096*4096. During simulation, it is assumed that there are 4096 columns of CCD detection elements, the change of interior orientation elements is from -20° to 20°, the row time file has 4096 rows, the change of row time is 40 seconds, the attitude pitch angle changes from 0° to 40° within this time interval, covering a sky area of 40°×40°. Stars with a magnitude less than 6 in the SAO star catalog are selected, and there are 74 stars in this area.

[0090] In a second aspect, the present invention discloses a terminal device, which is characterized by including:

[0091] A memory for storing instructions executed by at least one processor;

[0092] A processor for executing the instructions stored in the memory to implement the method as described above.

[0093] In a third aspect, the present invention discloses a computer-readable storage medium, which is characterized in that the computer-readable storage medium stores computer instructions, and when the computer instructions are run on a computer, the computer is enabled to execute the method as described above.

[0094] The content not described in detail in the specification of the present invention belongs to the prior art well-known to those skilled in the art.

Claims

1. A star map simulation method based on the on-orbit measured data of a low-orbit small satellite, characterized in that include: Read satellite attitude, orbit and time data, as well as right ascension, declination and magnitude information of stars in the SAO star catalog; Establish the conversion relationship between the camera CCD and the celestial coordinate system; convert the four corner points of the camera CCD to the celestial coordinate system, and determine the range of the four corner points of the camera CCD in the celestial coordinate system; According to the right ascension and declination of stars in the star catalog, stars falling within the range of the four corner points of the camera CCD are selected; the camera CCD is divided into grid points, the camera coordinate system vector corresponding to the camera CCD pixel of each grid point is obtained, and it is converted to the celestial coordinate system; For each star within the four corner points of the camera's CCD, search for the four grid points closest to the star in the grid points converted to the celestial coordinate system, and calculate the row and column numbers of the star point in the camera's CCD and the corresponding star point grayscale value; Based on the obtained star points, according to their row and column numbers in the camera CCD and the corresponding star point grayscale values, a new image with the same size as the original camera CCD image is generated, and the background noise is superimposed as a star simulation star map output.

2. The method for simulating a star map based on the on-orbit measured data of a low-orbit small satellite according to claim 1, characterized in that: The orbital data are the XYZ three-dimensional coordinates and velocity of the satellite in the Earth-centered Earth-fixed coordinate system; The attitude data are the roll, pitch and yaw angles of the satellite relative to the orbital coordinate system; The row time data is the time information corresponding to each row when the camera takes pictures.

3. The method for simulating a star map based on the on-orbit measured data of a low-orbit small satellite according to claim 1, characterized in that: Establish the conversion relationship between the camera CCD and the celestial coordinate system, specifically: Construct the camera observation vector ox = [tan(psi_x), -tan(psi_y), 1], where point o is the focus, psi_x is the angle between the vector ox and the CCD pixel along the flight direction, and psi_y is the angle between the vector ox and the CCD pixel perpendicular to the flight direction. Normalize the vector ox to get ox'; ox' is rotated according to the installation matrix and converted from the camera coordinate system to the body coordinate system to obtain the observation vector ox" in the body coordinate system; ox" is further rotated and expressed as OX' in the celestial coordinate system: OX'=ox”·R a ·R b ·R c Among them, R a , R b , R c They are the rotation matrix of the exterior orientation element, the satellite attitude angle rotation matrix and the rotation matrix of the satellite orbit coordinate system: Where: omega, phi and kappa are the side tilt angle, heading tilt angle and rotation angle around the main axis in the camera's exterior orientation elements, respectively. and is the rotation matrix around the three exterior orientation element angles; yaw, pitch and roll are the satellite yaw, pitch and roll angles respectively. and is the rotation matrix around the three attitude angles; Ω is the satellite ascending node right ascension, i is the satellite orbit inclination, u is the satellite argument, and is the rotation matrix about the three orbital angles.

4. The method for simulating a star map based on the on-orbit measured data of a low-orbit small satellite according to claim 1, characterized in that: When dividing the camera CCD into grid points, every 100 to 200 pixels in the row and column directions of the CCD are used as grid points.

5. The method for simulating a star map based on the on-orbit measured data of a low-orbit small satellite according to claim 1, characterized in that: When calculating the row and column numbers of the corresponding star points and the corresponding star point brightness: The row and column numbers of each star point on the image are calculated by bilinear interpolation, and the brightness of the current star point is calculated using two-dimensional Gaussian blur.

6. The method for simulating a star map based on the on-orbit measured data of a low-orbit small satellite according to claim 5, characterized in that: The row and column numbers of each star point on the image are calculated by bilinear interpolation, specifically: If the latitude and longitude of the four nearest grid points of star point i are recorded as: (lat A ,lon A )、(lat B ,lon B )、(lat C ,lon C )、(lat D ,lon D ), the corresponding row and column numbers are recorded as (r A ,c A )、(r B ,c B )、(r C ,c C )、(r D ,c D ), the right ascension and declination of the star point are (lat O ,lon O ), then the coordinates of the center point O of the star point i in the camera CCD are in the row and column numbers (r iO ,c iO )for:

7. The method for simulating a star map based on the on-orbit measured data of a low-orbit small satellite according to claim 5, characterized in that: The grayscale value of the area where the star point is located is calculated using two-dimensional Gaussian blur. The size of the area where the star point is located is 5×5 pixels, specifically: Then the gray level g of star point i i The expression is: Among them: Assume that the maximum quantization bit of the star map is a, m i is the magnitude of star point i, m max is the magnitude of the brightest star among the stars, g max is grayscale, g max <2 a -1, the magnitude is negatively correlated with the brightness of the star; The energy distribution of the star point follows a two-dimensional Gaussian distribution, and the grayscale value of any pixel point (x, y) in the region is: (r iO ,c iO ) are the coordinates of the center point O of star point i, and σ is the diffusion radius.

8. A terminal device, characterized in that: include: a memory for storing instructions executed by at least one processor; A processor, configured to execute instructions stored in a memory to implement a method according to any one of claims 1 to 7.

9. A computer-readable storage medium, characterized in that: The computer-readable storage medium stores computer instructions, and when the computer instructions are executed on a computer, the computer is enabled to execute the method according to any one of claims 1 to 7.

Images