Method for calculating city space Beidou satellite visibility by using virtual panoramic shielding image

Abstract

The invention discloses a method for calculating urban space Beidou satellite visibility by using a virtual panoramic shielding image, and the method comprises the steps: firstly, generating a high-precision digital surface model based on a point cloud; secondly, calculating an initial region of a high-precision digital surface model of the virtual panoramic shielding image, adaptively selecting a range of the high-precision digital surface model used for constructing the virtual panoramic shielding image at any position of the urban three-dimensional space through continuous iteration, and calculating an azimuth angle and an elevation angle according to the range; converting the obtained cylindrical virtual panoramic shielding image into a semi-spherical virtual panoramic shielding image, and projecting the semi-spherical virtual panoramic shielding image into a plane circumference virtual panoramic shielding image; and finally, judging the visibility of the Beidou satellite according to a plane relation graph of the plane circumference virtual panoramic shielding graph and the sky graph of the Beidou satellite. According to the invention, the precision and resolution of the existing urban digital surface model are improved; the data calculation amount is reduced, the calculation efficiency is improved, and the efficiency and the accuracy of urban three-dimensional space Beidou satellite visibility calculation are improved.

Description

Technical Field

[0001] The invention belongs to the technical field of satellite communications, and in particular relates to a method for calculating the visibility of Beidou satellites in urban space by using a virtual panoramic occlusion map. Background Art

[0002] Low-altitude economy is an emerging economic form. (Ultra) low-altitude flight of unmanned aerial vehicles is an important technology for the development of low-altitude economy. The safe operation of unmanned aerial vehicles in urban three-dimensional space is the basic guarantee for the development of low-altitude economy. However, due to the fact that Beidou satellite signals are easily blocked, reflected and scattered in complex urban environments, the Beidou positioning accuracy and reliability of unmanned aerial vehicles are extremely unstable, resulting in huge challenges in the positioning accuracy and safety of unmanned aerial vehicles taking off and landing and (ultra) low-altitude flight in complex urban environments; at the same time, satellite visibility is an important indicator for evaluating and predicting satellite positioning accuracy and reliability in complex urban environments. Therefore, Beidou satellite visibility is an important guarantee for the safe flight of low-altitude unmanned aerial vehicles in complex urban environments. The existing methods for obtaining Beidou satellite visibility in urban space are mainly divided into actual measurement method and calculation simulation method. The actual measurement method obtains Beidou satellite visibility through field measurement, that is, using Beidou satellite receivers to observe Beidou satellite signals in a static or dynamic manner, and judging Beidou satellite visibility by integrating receiver observation files and Beidou satellite ephemeris files. The computational simulation method uses Beidou satellite ephemeris and other geographic information data to calculate Beidou satellite visibility. Its applicability in complex urban environments is much higher than the actual measurement method.

[0003] Based on the data dimension of the geographic information used, the computational simulation method is divided into the method of calculating satellite visibility using three-dimensional data and the method of calculating Beidou satellite visibility using two-dimensional data. The three-dimensional data includes point clouds (airborne, vehicle-mounted), digital elevation models, digital surface models, and three-dimensional urban models, etc.; the two-dimensional data is the city street view images provided by various map manufacturers. However, there are the following deficiencies when using these geographic information data to calculate Beidou satellite visibility: 1. The method for calculating Beidou satellite visibility in urban space based on digital surface model is to analyze whether the line connecting Beidou satellite and the point to be calculated in urban space is blocked by obstacles such as buildings. It is necessary to traverse all regional digital elevation models in turn, which makes the calculation efficiency of this method extremely low. In addition, the resolution of the public digital surface model is low (usually 30m), which is difficult to meet the precise calculation of Beidou satellite spatiotemporal visibility in urban three-dimensional space; 2. The method for calculating Beidou satellite visibility in urban space based on the urban three-dimensional model is to use the ray tracing method to calculate the occlusion of Beidou satellite signals. However, the detailed urban three-dimensional model cannot be automatically modeled at present, which requires a lot of manpower and time costs. Therefore, there is a disadvantage of too high cost for calculating Beidou satellite visibility in a large-scale three-dimensional space of the city; 3. Point clouds have the advantage of high precision, and point clouds can be used to calculate the satellite visibility of urban three-dimensional space in a refined manner. However, due to the occlusion problem between objects, vehicle-mounted point clouds cannot obtain the surface and object morphology information of the entire city, and are only suitable for calculating the satellite visibility of urban road areas; in addition, both vehicle-mounted point clouds and airborne point clouds have the characteristics of large data volume, which makes it impossible to lightweight calculate the spatial and temporal visibility of Beidou satellites in urban three-dimensional space; 4. Street view images are widely distributed in many urban road areas, and satellite visibility can be calculated quickly and lightly using street view images. However, there are no street view images at any location in the urban three-dimensional space, which makes it impossible to use street view images to calculate Beidou satellite visibility in the urban three-dimensional space. Summary of the invention

[0004] In view of the problems existing in the above-mentioned background technology, the purpose of the present invention is to provide a method for calculating the Beidou satellite visibility in urban space by using a virtual panoramic occlusion map, so as to achieve high-precision, lightweight and fast calculation of the spatiotemporal visibility of Beidou satellite in urban three-dimensional space.

[0005] In order to achieve the above object, the present invention adopts the following technical solution: The method for calculating the visibility of Beidou satellites in urban space using a virtual panoramic occlusion map comprises the following steps: S1. Generate high-precision digital surface model based on point cloud: Extract ground points from point cloud data as the basis for generating digital surface models, divide the extracted ground points into a certain grid structure, and assign elevation values ​​to each grid unit to obtain a high-precision digital surface model; solve the problems of low resolution and poor accuracy of existing urban digital surface models, as well as low computational efficiency caused by large amount of point cloud data; S2. Constructing a virtual panoramic occlusion map based on a high-precision digital surface model: S201. Calculate the maximum radius of the initial selection range of the high-precision digital surface model using formula (1) ,by The circular area formed as the radius of the plane is the initial area of ​​the high-precision digital surface model of the virtual panoramic occlusion map; (1), In the formula, is the maximum cut-off elevation angle of Beidou satellite, is the maximum height of the obstruction in the area to be calculated; S202. Adaptive selection method for high-precision digital surface model area: Calculate the height of the highest occluder in the initial area of ​​the high-precision digital surface model of the obtained virtual panoramic occlusion map The height of the point to be calculated from the ground , using formula (2) to calculate the radius of the new area : (2), In the new area of ​​the obtained high-precision digital surface model, equation (2) is used to iterate continuously. As the height of the point to be calculated is As the radius increases, the radius of the selected high-precision digital surface model area will also decrease, and a continuously shrinking range of high-precision digital surface models will be obtained, until the range of high-precision digital surface models used for adaptively selecting any position of the urban three-dimensional space to construct a virtual panoramic occlusion map is achieved; S203, calculating the azimuth and altitude angles of all grids within the range of the high-precision digital surface model from the center of the circle based on the adaptively selected high-precision digital surface model range; S204, sequentially arranging the calculated altitude angles corresponding to the azimuth angles from 0° to 360° to obtain a cylindrical virtual panoramic occlusion map; S205, converting the cylindrical virtual panoramic occlusion map into a hemispherical virtual panoramic occlusion map by projection, and then projecting the hemispherical virtual panoramic occlusion map into a planar circular virtual panoramic occlusion map; By using high-precision digital surface models to construct virtual panoramic occlusion maps suitable for urban three-dimensional space, the existing problems of low algorithm efficiency and long calculation time when using digital surface models to calculate altitude angles and azimuth angles to determine Beidou satellite visibility are solved; S3. Calculate Beidou satellite visibility based on virtual panoramic occlusion map: S301, calculating the real-time position and predicted position of the Beidou satellite in the geocentric rectangular coordinate system; using a seven-parameter conversion model to convert the Beidou satellite coordinates calculated by different ephemeris from the geocentric rectangular coordinate system to the station-centered polar coordinates with the point to be calculated as the origin; S302, calculating the altitude angle and azimuth angle of the Beidou satellite, and converting the Beidou satellite coordinates from the station-centered rectangular coordinate system to the station-centered polar coordinate system with the point to be calculated as the origin; S303, respectively using the broadcast ephemeris of the Beidou satellite to calculate the real-time Beidou satellite coordinates and using the two-line element orbit data to predict the Beidou satellite position, and draw the real-time and predicted Beidou navigation satellite sky map; S304, projecting the intersection of the Beidou satellite signal and the hemispherical virtual panoramic occlusion map onto the planar circular virtual panoramic occlusion map, calculating the projection position of the projection point in the planar circular virtual panoramic occlusion map, and obtaining a planar relationship diagram between the planar circular virtual panoramic occlusion map and the Beidou satellite sky map; S305. Determine the visibility of Beidou satellites based on the planar relationship diagram between the planar circular virtual panoramic occlusion map and the Beidou satellite sky map: when the projection position is blocked, the Beidou satellite is not visible, otherwise it is visible; calculate the visibility of all Beidou satellites at that moment and count the number of visible Beidou satellites; use the virtual panoramic occlusion map of any Beidou satellite in the city to calculate the number of visible Beidou satellites at any position in the city and draw a three-dimensional space-time visibility map of Beidou satellites in the city.

[0006] Furthermore, in S1, the point cloud data is first processed to remove noise, outliers and duplicate points to improve the accuracy and efficiency of subsequent processing of the point cloud data; then, the ground points are extracted from the processed point cloud data using a cloth simulation filtering algorithm.

[0007] Furthermore, in S2, the maximum cutoff elevation angle of the Beidou satellite The value is set to 15°.

[0008] Further, in S203, in the calculation of the azimuth angle and the altitude angle, when there are multiple altitude angles for the same azimuth angle, the maximum altitude angle is retained.

[0009] Furthermore, in S205, the hemispherical virtual panoramic occlusion map is projected into a planar circular virtual panoramic occlusion map using an isometric projection method.

[0010] Further, in S301, the seven-parameter conversion model is as follows: (3), In the formula, and They are the coordinates of the Beidou satellite and the station in the geocentric rectangular coordinate system. is the coordinate of the BeiDou satellite in the station-centered polar coordinate system, and are the geodetic latitude and longitude of the station, respectively.

[0011] Further, in S302, the altitude angle The calculation formula is as follows: (4), The azimuth The calculation formula is as follows: (5).

[0012] Furthermore, in S304, the projection position is calculated as follows: First, according to the altitude angle Calculate the distance from the projection point to the center of the plane circular virtual panoramic occlusion map , and then according to the center distance and azimuth Calculate the projection position , the calculation formula is as follows: (6), (7), In the formula, is the radius of the hemispherical virtual panorama occlusion map.

[0013] Compared with the shortcomings and deficiencies of the prior art, the present invention has the following beneficial effects: (1) Using airborne point clouds to build high-precision and high-resolution urban digital surface models, the high-precision advantages of airborne point clouds are retained, and the problem of large data volume when directly using point clouds to calculate Beidou satellite visibility is solved, thus improving the accuracy and resolution of existing urban digital surface models; (2) By adaptively selecting the range of the digital surface model, the global data is avoided from being involved in the calculation when the digital surface model is used to construct a virtual panoramic occlusion map, which reduces the amount of data calculation and improves the calculation efficiency; (3) The method of constructing a virtual panoramic occlusion map using a high-precision digital surface model avoids the problem of digital surface models directly participating in Beidou satellite visibility calculations and solves the problem of urban three-dimensional space without panoramic image coverage; (4) By combining the virtual panoramic occlusion map and the Beidou satellite sky map, the refined calculation of Beidou satellites in the urban three-dimensional space is realized, which improves the efficiency and accuracy of satellite visibility calculation in the urban three-dimensional space. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 It is a flow chart of a method for calculating Beidou satellite visibility in urban space using a virtual panoramic occlusion map provided by an embodiment of the present invention; Figure 2 is a schematic diagram of a method for adaptively selecting a range of a high-precision digital surface model provided by an embodiment of the present invention; Figure 3 is a schematic diagram of calculating azimuth and altitude angles based on a high-precision digital surface model provided by an embodiment of the present invention; Figure 4 is a cylindrical virtual panoramic occlusion map provided by an embodiment of the present invention; Figure 5 is a planar circular virtual panoramic occlusion map provided by an embodiment of the present invention; Figure 6 is a schematic diagram of a hemispherical equidistant projection of a virtual panoramic occlusion map provided by an embodiment of the present invention; Figure 7 It is a planar relationship diagram between a planar circular virtual panoramic occlusion map and a Beidou satellite sky map provided by an embodiment of the present invention; Figure 8It is a virtual panoramic occlusion map of different heights and different positions of the experimental area provided by an embodiment of the present invention; Fig. 9 is a Beidou satellite sky map used in the experiment provided in the embodiment of the present invention; Fig.10 It is a single-point Beidou satellite visible map calculated by using a virtual panoramic occlusion map in the experimental area provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0015] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0016] The present invention proposes a method for calculating the visibility of Beidou satellites in urban space using a virtual panoramic occlusion map, the flow chart of which is as follows: Figure 1 As shown, it includes three parts: generating a high-precision digital surface model based on point cloud, constructing a virtual panoramic occlusion map based on the high-precision digital surface model, and calculating Beidou satellite visibility based on the virtual panoramic occlusion map. Each part is explained in detail below.

[0017] S1. Generate high-precision digital surface model based on point cloud: In view of the problems of low resolution and poor precision of existing urban digital surface models, as well as low computational efficiency caused by large amount of point cloud data, the present invention proposes the following method for generating high-precision urban digital surface models using point clouds: First, airborne LiDAR equipment is used to collect point cloud data of urban areas; Secondly, a median filter is used to remove noise, outliers and duplicate points in the point cloud data to improve the accuracy and efficiency of subsequent processing of the point cloud; Thirdly, the cloth simulation filtering algorithm is used to extract ground points from the point cloud data as the basis for generating a high-precision digital surface model; Finally, the extracted ground points are divided according to a certain grid structure (grid and triangulated network), and an elevation value is assigned to each grid cell to obtain a high-precision digital surface model.

[0018] In view of the problems of low algorithm efficiency and long calculation time in the existing method of using digital surface model to calculate altitude angle and azimuth angle to judge Beidou satellite visibility, the present invention proposes the following method for calculating Beidou satellite visibility in urban three-dimensional space based on virtual panoramic occlusion map.

[0019] S2. Constructing a virtual panoramic occlusion map based on a high-precision digital surface model: S201. Calculate the maximum radius of the initial selection range of the high-precision digital surface model using formula (1) ,by The circular area formed as the radius of the plane is the initial area of ​​the high-precision digital surface model of the virtual panoramic occlusion map; (1), In the formula, is the maximum height of the occluder in the area to be calculated, is the maximum cut-off elevation angle of Beidou satellite. According to the measurement experience and specifications of Beidou satellite system in cities, the maximum cut-off elevation angle of Beidou satellite is The value of is set to 15°; S202, propose an adaptive selection method for high-precision digital surface model areas, such as Figure 2 As shown, the height of the highest occluder is calculated in the initial area of ​​the high-precision digital surface model of the obtained virtual panoramic occlusion map. The height of the point to be calculated from the ground , the radius of the new area of ​​the high-precision digital surface model is calculated using formula (2) : (2), because , which significantly reduces the scope of digital surface models involved in constructing virtual panoramic occlusion maps; In the new area of ​​the obtained high-precision digital surface model, equation (2) is used to iterate continuously. As the height of the point to be calculated is As the radius increases, the radius of the selected high-precision digital surface model area will also decrease, and a continuously shrinking range of high-precision digital surface models will be obtained, until the range of high-precision digital surface models used for adaptively selecting any position of the urban three-dimensional space to construct a virtual panoramic occlusion map is achieved; S203, based on the adaptively selected high-precision digital surface model range, calculate the azimuth and altitude angles of all grids within the range from the center of the circle to the high-precision digital surface model, such as Figure 3 As shown, when there are multiple altitude angles at the same azimuth, the maximum altitude angle is retained; S204, arranging the calculated altitude angles corresponding to the azimuth angles from 0° to 360° in sequence, and obtaining Figure 4 The cylindrical virtual panoramic occlusion map shown; S205, converting the cylindrical virtual panoramic occlusion map into a hemispherical virtual panoramic occlusion map by projection, and then projecting the hemispherical virtual panoramic occlusion map into a planar circular virtual panoramic occlusion map by using an equidistant projection method, such as Figure 5 shown.

[0020] S3. Calculate Beidou satellite visibility based on virtual panoramic occlusion map: S301, using the broadcast ephemeris and two-line element orbital data of the Beidou satellite system to respectively calculate the real-time position and predicted position of the Beidou satellite in the geocentric rectangular coordinate system; using the seven-parameter conversion model to convert the Beidou satellite coordinates calculated by different ephemeris from the geocentric rectangular coordinate system to the station-centered rectangular coordinate system with the point to be calculated as the origin. The seven-parameter conversion model is as follows: (3), In the formula, and They are the coordinates of the Beidou satellite and the station in the geocentric rectangular coordinate system. is the coordinate of the BeiDou satellite in the station-centered polar coordinate system, and are the geodetic latitude and longitude of the station, respectively; S302, use equations (4) and (5) to calculate the altitude angle of the Beidou satellite and azimuth , convert the Beidou satellite coordinates from the geocentric rectangular coordinate system to the station-centered polar coordinate system with the point to be calculated as the origin; (4), (5); S303, respectively using the broadcast ephemeris of the Beidou satellite to calculate the real-time Beidou satellite coordinates and using the two-line element orbit data to predict the Beidou satellite position, and draw the real-time and predicted Beidou satellite sky map; S304, projecting the intersection of the Beidou satellite signal and the hemispherical virtual panoramic occlusion map onto the plane circular virtual panoramic occlusion map, such as Figure 6 As shown, P The point is the location of the Beidou satellite receiver. O Point is the center of the virtual panoramic occlusion map of the plane circle, and is the radius of the virtual panoramic occlusion map of the hemispherical surface. Q Point is the intersection of Beidou satellite signal and hemispherical virtual panoramic occlusion map. T Point is Q The point is projected onto the virtual panoramic occlusion map on the plane circle. When the Beidou satellite altitude angle is When the intersection point Q is projected, the distance from point T to the center point O of the plane circular virtual panoramic occlusion map is calculated. : (6), In the formula, is the radius of the hemispherical virtual panoramic occlusion map; According to the center distance and azimuth Calculate the projection position of the projection point T in the plane circular virtual panoramic occlusion map : (7), Figure 7 It is a planar relationship diagram between the plane circular virtual panoramic occlusion map and the Beidou satellite sky map, which more intuitively shows the positional relationship of the Beidou satellite position projected onto the plane circular virtual panoramic map, providing an important reference for Beidou satellite visibility judgment; S305, judging the visibility of Beidou satellites according to the plane relationship diagram of the plane circumference virtual panoramic occlusion map and the Beidou satellite sky map: when the projection position When there is occlusion, Beidou satellites are not visible, otherwise they are visible; by calculating the visibility of all Beidou satellites at that moment, the number of visible Beidou satellites is counted; using the virtual panoramic occlusion map of any Beidou satellite in the city, the number of visible Beidou satellites at any location in the city is calculated, and the spatial-temporal visibility map of Beidou satellites in the three-dimensional space of the city is drawn.

[0021] In order to verify the effectiveness of the method of calculating BeiDou satellite visibility in urban space using virtual panoramic occlusion map, airborne point cloud data of an experimental area with a total area of ​​about 1 square kilometer were collected, and the airborne point cloud was preprocessed to remove noise points. The cloth simulation filtering method was used to separate ground points and non-ground points, and a high-precision digital surface model of the experimental area was established. Three different scenes were randomly selected to construct a virtual panoramic occlusion map using the high-precision digital surface model. Figure 8 These are virtual panoramic occlusion images of three random scenes at 7m, 20m, and 30m above the ground. Figure 8 The first and second row of views are scenes where buildings and trees jointly block the view. Figure 8 The third row of views shows the scene with only buildings blocking the sky. It can be found that in the blocking environment of buildings and buildings and trees, the virtual panoramic blocking map can fully describe the sky blocking situation, and the sky becomes wider as the altitude increases. In order to draw the visible Beidou satellite map of the city in three-dimensional space and better serve the low-altitude reliable navigation of various devices, the Beidou satellite sky map drawn using the broadcast ephemeris at 01:00 on May 1, 2024, Coordinated Universal Time, is shown in the figure below: Fig. 9 shown. Fig.10 It is a single-point Beidou satellite visible map calculated by the experimental area using a virtual panoramic occlusion map. The single-point Beidou satellite visible map can be used to draw a Beidou satellite visible map in the urban three-dimensional space. Through experimental verification, the present invention can calculate the number of Beidou satellite visible in the urban three-dimensional space with light weight and high precision. The Beidou satellite visible map drawn in the urban three-dimensional space can accurately reflect the visibility changes of Beidou satellites in the urban space, and also provide reliable navigation for the safe flight of unmanned aerial vehicles in the urban three-dimensional space, and provide security for the development of low-altitude industries.

[0022] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for calculating Beidou satellite visibility in urban space using a virtual panoramic occlusion map, characterized in that: The following steps are involved: S1. Generate high-precision digital surface model based on point cloud: Extract ground points from point cloud data as the basis for generating digital surface models, divide the extracted ground points into a certain grid structure, and assign elevation values ​​to each grid unit to obtain a high-precision digital surface model; S2. Constructing a virtual panoramic occlusion map based on a high-precision digital surface model: S201. Calculate the maximum radius of the initial selection range of the high-precision digital surface model using formula (1) ,by The circular area formed as the radius of the plane is the initial area of ​​the high-precision digital surface model of the virtual panoramic occlusion map; （1）， In the formula, is the maximum cut-off elevation angle of Beidou satellite, is the maximum height of the obstruction in the area to be calculated; S202. Adaptive selection method for high-precision digital surface model area: Calculate the height of the highest occluder in the initial area of ​​the high-precision digital surface model of the obtained virtual panoramic occlusion map The height of the point to be calculated from the ground , using formula (2) to calculate the radius of the new area : （2）， Formula (2) is used to continuously iterate in the obtained new area of ​​the high-precision digital surface model to obtain a continuously reduced range of the high-precision digital surface model until the range of the high-precision digital surface model used to construct a virtual panoramic occlusion map at any position in the urban three-dimensional space is adaptively selected; S203, calculating the azimuth and altitude angles of all grids within the range of the high-precision digital surface model from the center of the circle based on the adaptively selected high-precision digital surface model range; S204, sequentially arranging the calculated altitude angles corresponding to the azimuth angles from 0° to 360° to obtain a cylindrical virtual panoramic occlusion map; S205, converting the cylindrical virtual panoramic occlusion map into a hemispherical virtual panoramic occlusion map by projection, and then projecting the hemispherical virtual panoramic occlusion map into a planar circular virtual panoramic occlusion map; S3. Calculate Beidou satellite visibility based on virtual panoramic occlusion map: S301, calculating the real-time position and predicted position of the Beidou satellite in the geocentric rectangular coordinate system; using a seven-parameter conversion model to convert the Beidou satellite coordinates calculated by different ephemeris from the geocentric rectangular coordinate system to the station-centered polar coordinates with the point to be calculated as the origin; S302, calculating the altitude angle and azimuth angle of the Beidou satellite, and converting the Beidou satellite coordinates from the station-centered rectangular coordinate system to the station-centered polar coordinate system with the point to be calculated as the origin; S303, respectively using the broadcast ephemeris of the Beidou satellite to calculate the real-time Beidou satellite coordinates and using the two-line element orbit data to predict the Beidou satellite position, and draw the real-time and predicted Beidou satellite sky map; S304, projecting the intersection of the Beidou satellite signal and the hemispherical virtual panoramic occlusion map onto the planar circular virtual panoramic occlusion map, calculating the projection position of the projection point in the planar circular virtual panoramic occlusion map, and obtaining a planar relationship diagram between the planar circular virtual panoramic occlusion map and the Beidou satellite sky map; S305. Determine the visibility of Beidou satellites based on the planar relationship diagram between the planar circular virtual panoramic occlusion map and the Beidou satellite sky map: when the projection position is blocked, the Beidou satellite is not visible, otherwise it is visible; calculate the visibility of all Beidou satellites at that moment and count the number of visible Beidou satellites; use the virtual panoramic occlusion map of any Beidou satellite in the city to calculate the number of visible Beidou satellites at any position in the city and draw a three-dimensional space-time visibility map of Beidou satellites in the city.

2. The method for calculating BeiDou satellite visibility in urban space using a virtual panoramic occlusion map as claimed in claim 1, characterized in that: In S1, the point cloud data is first processed to remove noise, outliers and duplicate points, and then the ground points are extracted from the processed point cloud data using a cloth simulation filtering algorithm.

3. The method for calculating BeiDou satellite visibility in urban space using a virtual panoramic occlusion map as claimed in claim 1, characterized in that: In S2, the maximum cut-off elevation angle of the Beidou satellite The value is set to 15°.

4. The method for calculating BeiDou satellite visibility in urban space using a virtual panoramic occlusion map as claimed in claim 1, characterized in that: In S203, in the calculation of the azimuth angle and the altitude angle, when there are multiple altitude angles for the same azimuth angle, the maximum altitude angle is retained.

5. The method for calculating BeiDou satellite visibility in urban space using a virtual panoramic occlusion map as claimed in claim 1, characterized in that: In S205, the hemispherical virtual panoramic occlusion map is projected into a planar circular virtual panoramic occlusion map by using an isometric projection method.

6. The method for calculating BeiDou satellite visibility in urban space using a virtual panoramic occlusion map according to claim 1, characterized in that: In S301, the seven-parameter conversion model is as follows: （3）， In the formula, and They are the coordinates of the Beidou satellite and the station in the geocentric rectangular coordinate system. is the coordinate of the BeiDou satellite in the station-centered polar coordinate system, and are the geodetic latitude and longitude of the station, respectively.

7. The method for calculating BeiDou satellite visibility in urban space using a virtual panoramic occlusion map as claimed in claim 6, characterized in that: In S302, the altitude angle The calculation formula is as follows: （4）， The azimuth The calculation formula is as follows: （5）。 8. The method for calculating BeiDou satellite visibility in urban space using a virtual panoramic occlusion map as claimed in claim 7, characterized in that: In S304, the projection position is calculated as follows: First, according to the altitude angle Calculate the distance from the projection point to the center of the plane circular virtual panoramic occlusion map , and then according to the center distance and azimuth Calculate the projection position , the calculation formula is as follows: （6）， （7）， In the formula, is the radius of the hemispherical virtual panorama occlusion map.

Images