Method for Selecting Laser Remote Sensing Landing Safety Zone Considering Obstacle Topography Measurement Error

Abstract

The method for selecting a laser remote sensing landing safety area considering obstacle topography measurement errors includes resampling the three-dimensional point cloud data of the surface of the celestial body in the corresponding area below the lander; determining the side length of the search unit based on the side length of the lander, the navigation, guidance, and control error, and the horizontal direction measurement error of the obstacle; constructing an initial search unit and calculating the parameters of the search unit; judging whether the initial search unit meets the landing conditions through the set safety thresholds of slope, roughness, and maximum elevation difference; the present invention adopts a central double-layer spiral search method, which can quickly select the landing safety area. At the same time, the measurement errors of the obstacle in the horizontal and vertical directions are respectively considered in the calculation of the side length of the search unit and the roughness, and the maximum elevation difference of the laser points in the search unit is restricted, improving the credibility and safety of the landing safety area.

Description

Technical Field

[0001] This invention patent relates to the field of selecting safe landing areas for planetary probes to avoid obstacles on the surface of extraterrestrial celestial bodies, and particularly to a method for selecting laser remote sensing landing safe areas considering obstacle topography measurement errors. Background Art

[0002] The surface topography of extraterrestrial celestial bodies is complex. For example, the surfaces of the moon and Mars are covered with pit and rock obstacles, which pose great risks and challenges to the safe soft landing of probes. Early lunar probes mostly landed in relatively flat lunar maria areas by blind landing. The manned lunar exploration starting from the US "Apollo 11" mainly avoided lunar surface obstacles through visual observation by astronauts and landed in safe areas, which put forward high requirements for the lighting conditions during landing and the observation angle of astronauts. For unmanned system probes of extraterrestrial celestial bodies with limited fuel and long communication delays with the earth, in order to achieve safe soft landing on the surface of extraterrestrial celestial bodies with complex environments and unknown topographies, they must have the ability to obtain the surface topography information of the celestial body and quickly and autonomously select a safe landing area. China's Chang'e-3, Chang'e-4, Chang'e-5 and Tianwen-1 probes hovered at a height of about 100 meters above the surfaces of the moon and Mars and used laser three-dimensional imaging to quickly identify obstacles on the surfaces of the moon and Mars and quickly select landing safe areas, successfully achieving safe soft landings on the surfaces of the moon and Mars.

[0003] However, there are certain errors in measuring the topography of the underlying surface carried on the lander, which may cause misjudgment of the horizontal and vertical dimensions of obstacles, resulting in misselection of the landing safe area and bringing risks to the landing process. To solve this problem, when selecting the landing safe area using during the hovering stage of the lander, it is necessary to consider the topography measurement errors of obstacles on the celestial body surface, so as to further improve the credibility and reliability of the selection of the safe landing area. Summary of the Invention

[0004] The purpose of this part is to outline some aspects of the embodiments of the present invention and briefly introduce some preferred embodiments. Some simplifications or omissions may be made in this part, as well as in the abstract and title of the specification of this application, to avoid obscuring the purpose of this part, the abstract of the specification and the title of the invention, and such simplifications or omissions cannot be used to limit the scope of the present invention.

[0005] In view of the above existing problems, the present invention is proposed. Therefore, the present invention provides a method for selecting a laser remote sensing landing safe area considering obstacle topography measurement errors to solve the problem that errors in measuring the topography of the underlying surface carried on the lander in practical problems lead to misselection of the landing safe area.

[0006] To solve the above technical problems, the present invention provides the following technical solutions:

[0007] The present invention provides a method for selecting a laser remote sensing landing safety area considering obstacle topography measurement errors, including:

[0008] Resample the three-dimensional point cloud data of the surface of the celestial body in the corresponding area below the lander obtained;

[0009] Based on the side length of the lander, the navigation guidance and control error, and the horizontal direction measurement error of the obstacle, determine the side length of the search unit;

[0010] Construct an initial search unit and calculate the parameters of the search unit;

[0011] Judge whether the initial search unit meets the landing conditions through the set slope, roughness, and maximum elevation difference safety thresholds.

[0012] As a preferred solution of the method for selecting a laser remote sensing landing safety area considering obstacle topography measurement errors according to the present invention, wherein: the resampling of the three-dimensional point cloud data of the surface of the celestial body in the corresponding area below the lander obtained includes:

[0013] Calculate the X and Y coordinates of each point to be resampled and the position of each grid through the circumscribed rectangle of the projection points of the laser point cloud data on the XOY plane and the number of points required for resampling in the X and Y directions;

[0014] Based on the position and side length of each grid and the plane coordinates of each laser point output by the lidar, calculate the grid to which it belongs;

[0015] Use the average value of the Z values of the point cloud data in the neighborhood grid of the point to be resampled to calculate the Z value of the point to be resampled.

[0016] As a preferred solution of the method for selecting a laser remote sensing landing safety area considering obstacle topography measurement errors according to the present invention, wherein: determining the side length of the search unit based on the side length of the lander, the navigation guidance and control error, and the horizontal direction measurement error of the obstacle includes:

[0017] Set the side length of the search unit as L, the side length of the lander as LanderSize, the navigation guidance and control error of the lander as ε_GNC, and the measurement error in the horizontal direction of the kth obstacle as ε_horizontal_meansure k , and obtain the formula:

[0018]

[0019] As a preferred solution of the method for selecting a laser remote sensing landing safety area considering obstacle topography measurement errors according to the present invention, wherein: constructing an initial search unit and calculating the parameters of the constructed initial search unit includes:

[0020] With the center position of the resampled point cloud as the center and the determined side length of the search unit as the side length, the resampled point cloud within the region is overall constructed into an initial search unit;

[0021] Calculate the average slope of the initial search unit and the maximum elevation difference of the resampled laser points within the unit, and further calculate the roughness, safety radius, and safety factor of the initial search unit in combination with the measurement error in the vertical direction of the obstacle.

[0022] As a preferred solution of the method for selecting a laser remote sensing landing safety zone considering the obstacle topography measurement error according to the present invention, wherein: calculating the average slope of the initial search unit and the maximum elevation difference of the resampled laser points within the unit, and further calculating the roughness of the initial search unit in combination with the measurement error in the vertical direction of the obstacle, includes:

[0023] For the resampled laser points within the initial search unit, use least squares fitting to construct the average slope of the initial search unit and calculate the average slope;

[0024] Calculate the maximum elevation difference of the resampled laser points within the initial search unit;

[0025] Calculate the distance from each resampled laser point within the initial search unit to the constructed average slope and the roughness in a central spiral manner.

[0026] As a preferred solution of the method for selecting a laser remote sensing landing safety zone considering the obstacle topography measurement error according to the present invention, wherein: calculating the distance from each resampled laser point within the initial search unit to the constructed average slope and the roughness in a central spiral manner, includes:

[0027] Using the point-to-plane distance formula, obtain the distance d from the i-th laser point within the initial search unit to the average slope i ;

[0028] After considering the measurement error in the vertical direction of the obstacle, obtain the distance d' from the laser point to the average slope i as:

[0029]

[0030] wherein, is the measurement error in the vertical direction of the k-th obstacle;

[0031] Obtain the roughness of the search unit as:

[0032] roughness = max(d′ i ).

[0033] As a preferred embodiment of the method for selecting a laser remote sensing landing safety zone considering obstacle topography measurement errors according to the present invention, it includes: calculating the safety radius of the initial search unit, including:

[0034] Mark the positions of the laser points within the initial search unit whose distances to the average slope are greater than the roughness safety threshold as dangerous pixels, and take the minimum distance from the first dangerous pixel encountered during the search process to the center of the initial search unit as the safety radius of the initial search unit.

[0035] As a preferred embodiment of the method for selecting a laser remote sensing landing safety zone considering obstacle topography measurement errors according to the present invention, it includes: calculating the safety factor of the initial search unit, including:

[0036] Based on the calculated average slope of the initial search unit and the safety radius of the search unit, calculate the safety factor S of the initial search unit by weighted summation. The calculation formula is as follows:

[0037]

[0038] where k s and k r respectively represent the weights of the slope and the safety radius, T s and T r respectively represent the safety thresholds of the slope and the safety radius, and θ and r respectively represent the average slope and the safety radius of the initial search unit.

[0039] As a preferred embodiment of the method for selecting a laser remote sensing landing safety zone considering obstacle topography measurement errors according to the present invention, it includes: judging whether the initial search unit meets the landing conditions through the set safety thresholds of the slope, roughness, and maximum elevation difference, including:

[0040] If it meets the landing conditions, select the initial search unit as the safe landing zone; if it does not meet the landing conditions, move the center of the initial search unit to the next position in a spiral manner with a set fixed search step length to construct a new initial search unit and continue the safety threshold judgment.

[0041] As a preferred embodiment of the method for selecting a laser remote sensing landing safety zone considering obstacle topography measurement errors according to the present invention, it includes: the step of moving the center of the initial search unit to the next position in a spiral manner with a set fixed search step length, including:

[0042] When all the initial search units have been traversed, sort the safety factors of all the initial search units, and select the center point of the initial search unit with the largest safety factor as the safe landing zone.

[0043] Compared with the prior art, the beneficial effects of the invention are as follows: The method provided by the present invention can support the rapid selection of the landing safety area; by adopting the search method of the central double-layer helix (the search unit moves in a spiral manner, and each resampled laser point in the search unit traverses in a spiral manner), and spreading outwards circle by circle around the center of the point cloud field of view for search, the landing safety area closest to the hovering position of the lander can be quickly selected, providing guarantee for the fast and safe soft landing of a deep space exploration lander with fuel constraints; at the same time, it can support the reliable selection of the landing safety area; by respectively considering the size measurement errors of the obstacles obtained by the three-dimensional laser scanning during the hovering section of the lander in the horizontal and vertical directions in the calculation of the side length and roughness of the search unit, the selected landing safety area is more credible and safer; in addition, by considering the maximum elevation difference constraint of the laser points in the search unit, those search units with part on flat ground, part in a low-lying area (such as a pit) or on a slope can be excluded, further improving the safety of the selected landing area. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings. Among them:

[0045] Figure 1 It is the overall flowchart of the method for selecting the landing safety area by laser remote sensing considering the obstacle morphology measurement error according to an embodiment of the present invention;

[0046] Figure 2 It is the comparison diagram of the selection results (position and quantity) of the safety area under different morphology measurement errors of the method for selecting the landing safety area by laser remote sensing considering the obstacle morphology measurement error according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0047] In order to make the above objects, features, and advantages of the present invention more obvious and understandable, the following will describe the specific embodiments of the present invention in detail with reference to the drawings of the specification. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0048] In the following description, many specific details are set forth in order to fully understand the present invention. However, the present invention can also be implemented in other ways different from those described herein. Those skilled in the art can make similar extensions without departing from the connotation of the present invention. Therefore, the present invention is not limited by the specific embodiments disclosed below.

[0049] Second, the "one embodiment" or "embodiment" referred to herein means a specific feature, structure, or characteristic that may be included in at least one implementation manner of the present invention. The appearances of "in one embodiment" in different places in this specification do not all refer to the same embodiment, nor are they separate or alternative embodiments that are mutually exclusive of other embodiments.

[0050] The present invention is described in detail in conjunction with schematic diagrams. When describing the embodiments of the present invention in detail, for the convenience of explanation, the cross-sectional views showing the device structure will be enlarged locally in a non-general proportion, and the schematic diagrams are only examples and should not limit the scope of protection of the present invention herein. In addition, in actual production, three-dimensional spatial dimensions including length, width, and depth should be included.

[0051] At the same time, in the description of the present invention, it should be noted that the orientation or positional relationship indicated by terms such as "upper, lower, inner, and outer" is based on the orientation or positional relationship shown in the drawings, and is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation to the present invention. In addition, the terms "first, second, or third" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance.

[0052] Unless otherwise clearly defined and limited in the present invention, the terms "installed, connected, and coupled" should be understood in a broad sense. For example: it can be a fixed connection, a detachable connection, or an integral connection; it can also be a mechanical connection, an electrical connection, or a direct connection, and can also be indirectly connected through an intermediate medium, or can be the communication inside two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0053] Embodiment 1

[0054] Referring to Figure 1 , this is the first embodiment of the present invention. This embodiment provides a method for selecting a laser remote sensing landing safety zone considering obstacle topography measurement errors, including:

[0055] S1. Resample the three-dimensional point cloud data of the surface of the celestial body in the corresponding area below the lander;

[0056] Further, the resampling includes:

[0057] Calculate the X and Y coordinates of each point to be resampled and the position of each grid by the circumscribed rectangle of the projection points of the laser point cloud data on the XOY plane and the number of points required for resampling in the X and Y directions;

[0058] Calculate the grid to which it belongs based on the position and side length of each grid and the plane coordinates of each laser point output by the lidar;

[0059] Calculate the Z value of the resampling point to be processed using the mean of the Z values of the point cloud data in the neighborhood grid of the resampling point to be processed;

[0060] S2. Determine the side length of the search cell based on the side length of the lander, the navigation guidance and control error, and the horizontal measurement error of the obstacle;

[0061] Further, determining the side length of the search cell includes:

[0062] Set the side length of the search cell as L, the side length of the lander as LanderSize, the navigation guidance and control error of the lander as ε_GNC, and the horizontal measurement error of the k-th obstacle as ε_horizontal_meansure k , and obtain the formula:

[0063]

[0064] S3. With the center position of the resampled point cloud as the center and combining the determined side length of the search cell as the side length, construct the resampled point cloud in the area as an initial search cell as a whole; calculate the average slope of the initial search cell and the maximum elevation difference of the resampled laser points in the cell, and calculate the roughness, safety radius, and safety factor of the initial search cell in combination with the vertical measurement error of the obstacle, including the following steps:

[0065] Further, for the resampled laser points in the search cell, use the least squares fitting to construct the average slope of the search cell and calculate the average slope;

[0066] Further, calculate the maximum elevation difference of the resampled laser points in the search cell;

[0067] Further, in combination with the vertical measurement error of the obstacle, calculate the distance from each resampled laser point in the search cell to the constructed average slope and the roughness of the search cell in a central spiral manner. The specific calculation method is as follows:

[0068] Using the distance formula from a point to a plane, obtain the distance d from the i-th laser point in the initial search cell to the average slope i ;

[0069] After considering the vertical measurement error of the obstacle, obtain the distance d' from the laser point to the average slope i as:

[0070]

[0071] where, is the vertical measurement error of the k-th obstacle;

[0072] Obtain the roughness of the search cell as:

[0073] roughness = max(d i );

[0074] It should be noted that the central spiral method refers to the inner spiral method in the central double - layer spiral;

[0075] Furthermore, mark the positions of the laser points in the initial search unit whose distance to the average slope surface is greater than the roughness safety threshold as dangerous pixels, and take the minimum distance from the first dangerous pixel encountered during the search process to the center of the initial search unit as the safety radius of the initial search unit;

[0076] Furthermore, based on the calculated average slope gradient and safety radius of the initial search unit, calculate the safety factor S of the initial search unit by the method of weighted summation. The calculation formula is as follows:

[0077]

[0078] where k s and k r respectively represent the weights of the slope gradient and the safety radius, T s and T r respectively represent the safety thresholds of the slope gradient and the safety radius, and θ and r respectively represent the average slope gradient and the safety radius of the initial search unit;

[0079] S4. Determine whether the initial search unit meets the landing conditions through the set safety thresholds of the slope gradient, roughness, and maximum elevation difference;

[0080] Furthermore, if it meets the landing conditions, select the initial search unit as the safe landing area; if it does not meet the landing conditions, move the center of the initial search unit to the next position in a spiral manner with a set fixed search step length to construct a new initial search unit and continue to judge the safety threshold;

[0081] It should be noted that the next position refers to the position after moving the "fixed search step length" (for example, 5 pixels) from the current position (for example, the coordinate is (0, 12)) (such as (0, 17));

[0082] Furthermore, when all the initial search units have been traversed, sort the safety factors of all the initial search units, and select the center point of the initial search unit with the largest safety factor as the safe landing area.

[0083] Embodiment 2

[0084] Refer to Figure 2, which is the second embodiment of the present invention. This embodiment provides a method for selecting a laser remote sensing landing safety zone considering obstacle topography measurement errors, including:

[0085] Experimental verification of the safety zone selection results of the method proposed in the present invention under different topography measurement errors is carried out;

[0086] As Figure 2 shown, half of the lander size plus the lander GNC error is set to 6m, and the topography measurement errors in the horizontal and vertical directions vary between 0 and 0.11m; the slope safety threshold is set to 8°, the roughness safety threshold is set to 0.2m, and the maximum elevation difference safety threshold is set to 0.2m;

[0087] The set of positions of the selected safe landing zones under different topography measurement error settings is as Figure 2 (a) to Figure 2 (g) shown (the black boxes in each subfigure); Figure 2 (a) is the set of safe zones selected without considering the obstacle topography measurement error, Figure 2 (b) to Figure 2 (g) are the sets of safe zones selected when considering the obstacle topography measurement error by gradually increasing the topography measurement errors in the horizontal and vertical directions;

[0088] It can be seen from this that all the selected safe landing zones are located in flat areas and avoid obstacles such as pits, rocks, and slopes with large gradients in the site, verifying the feasibility of the method for finding a safe landing area; as the horizontal and vertical detection errors increase (from 0m to 0.11m), the number of searched safe landing zones gradually decreases, but they are farther away from the surrounding obstacles, proving that the present invention can select a safer and more reliable landing area than when not considering the obstacle topography measurement error after considering the obstacle topography measurement error.

[0089] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the spirit and scope of the technical solutions of the present invention, and they should all be covered within the scope of the claims of the present invention.

Claims

1. Method for selecting laser remote sensing landing safety area considering obstacle topography measurement error, characterized in that, comprising: Resampling the three-dimensional point cloud data of the star surface in the corresponding area below the lander; Determining the side length of the search unit based on the side length of the lander, navigation guidance and control error, and horizontal direction measurement error of the obstacle; Constructing an initial search unit and calculating the parameters of the search unit, including: Taking the center position of the resampled point cloud as the center and combining the determined side length of the search unit as the side length, and constructing the resampled point cloud in the area as a whole into an initial search unit; Calculating the average slope of the initial search unit and the maximum elevation difference of the resampled laser points in the unit, and further calculating the roughness, safety radius and safety factor of the initial search unit in combination with the vertical direction measurement error of the obstacle; Among them, calculating the distance and roughness from each resampled laser point in the initial search unit to the constructed average slope in a central spiral manner, including: Using the distance formula from a point to a plane, the distance d from the i-th laser point in the initial search unit to the average slope surface is obtained i ; After considering the vertical measurement error of the obstacle, the distance d' from the laser point to the average slope surface is obtained i as follows: Among them, is the measurement error of the k-th obstacle in the vertical direction; Obtaining the roughness of the search unit as: roughness = max(d′ i ); Judging whether the initial search unit meets the landing condition through the set slope, roughness and maximum elevation difference safety thresholds.

2. The method for selecting laser remote sensing landing safety area considering obstacle topography measurement error according to claim 1, characterized in that, The resampling of the three-dimensional point cloud data of the star surface in the corresponding area below the lander includes: Calculating the X and Y coordinates of each point to be resampled and the position of each grid according to the circumscribed rectangle of the projection points of the laser point cloud data on the XOY plane and the number of points required for resampling in the X and Y directions; Calculating the grid to which each laser point belongs based on the position and side length of each grid and the plane coordinates of each laser point output by the lidar; Calculating the Z value of the point to be resampled by using the average value of the Z values of the point cloud data in the neighborhood grid of the point to be resampled.

3. The method for selecting laser remote sensing landing safety area considering obstacle topography measurement error according to claim 2, characterized in that, Determining the side length of the search unit based on the side length of the lander, navigation guidance and control error, and horizontal direction measurement error of the obstacle, including: Set the side length of the search unit as L, the side length of the lander as LanderSize, the guidance, navigation and control error of the lander as ε_GNC, and the measurement error of the k-th obstacle in the horizontal direction as ε_horizontal_meansure k , and the formula is obtained as follows:

4. The method for selecting laser remote sensing landing safety area considering obstacle topography measurement error according to claim 3, characterized in that, Calculating the average slope of the initial search unit and the maximum elevation difference of the resampled laser points in the unit, and further calculating the roughness of the initial search unit in combination with the vertical direction measurement error of the obstacle, including: For the resampled laser points in the initial search unit, using the least squares fitting to construct the average slope of the initial search unit and calculating the average slope; Calculating the maximum elevation difference of the resampled laser points in the initial search unit; Calculating the distance and roughness from each resampled laser point in the initial search unit to the constructed average slope in a central spiral manner.

5. The method for selecting laser remote sensing landing safety area considering obstacle topography measurement error according to claim 4, characterized in that, Calculating the safety radius of the initial search unit, including: Mark the positions of the laser points within the initial search unit whose distances to the average slope surface are greater than the roughness safety threshold as dangerous pixels, and take the minimum distance from the first dangerous pixel encountered during the search process to the center of the initial search unit as the safety radius of the initial search unit.

6. The method for selecting a laser remote sensing landing safety area considering obstacle topography measurement errors as claimed in claim 5, characterized in that calculating the safety factor of the initial search unit, including: Based on the calculated average slope of the initial search unit and the safety radius of the search unit, calculate the safety factor S of the initial search unit by means of weighted summation. The calculation formula is as follows: Among them, k s and k r represent the weights of the slope and the safety radius respectively, T s and T r represent the safety thresholds of the slope and the safety radius respectively, and θ and r represent the average slope surface gradient and the safety radius of the initial search unit respectively.

7. The method for selecting a laser remote sensing landing safety area considering obstacle topography measurement errors as claimed in claim 6, characterized in that judging whether the initial search unit meets the landing conditions through the set safety thresholds of slope, roughness and maximum elevation difference, including: If the landing conditions are met, select the initial search unit as the safe landing area; if the landing conditions are not met, move the center of the initial search unit to the next position in a spiral manner with a set fixed search step length to construct a new initial search unit and continue to judge the safety threshold.

8. The method for selecting a laser remote sensing landing safety area considering obstacle topography measurement errors as claimed in claim 7, characterized in that the moving the center of the initial search unit to the next position in a spiral manner with a set fixed search step length includes: When all the initial search units have been traversed, sort the safety factors of all the initial search units, and select the center point of the initial search unit with the largest safety factor as the safe landing area.

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