Method for measuring the flight altitude of a drone by means of the solar elevation angle
By capturing the solar altitude angle using a drone's camera and calculating the drone's height above the water or ground using proportional geometry, the problem of inaccurate measurement and high cost in existing technologies is solved, enabling fast and accurate altitude measurement without adding hardware.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- NORTH CHINA UNIV OF WATER RESOURCES & ELECTRIC POWER
- Filing Date
- 2024-04-09
- Publication Date
- 2026-05-15
AI Technical Summary
Existing drones cannot accurately measure the actual physical distance from the water surface or the ground during flight, and existing methods require additional hardware, resulting in high costs and short flight time.
By using the principle of capturing the sun's altitude angle through a drone's camera and employing proportional geometry, the drone's height above the water or ground can be calculated, avoiding the need for additional hardware and requiring only an upgrade to the flight control module's program.
This technology enables drones to quickly and accurately measure the altitude above water or land without adding hardware, reducing costs and increasing flight time.
Smart Images
Figure CN118270265B_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of unmanned aerial vehicle (UAV) technology. Background Technology
[0002] Drones are being used more and more widely in various industries. In rivers and lakes, drones can be used to perform tasks such as water quality monitoring, replacing manual water area patrols, hydrological monitoring, inspecting water conservancy projects, and river management.
[0003] During flight, drones need to record their altitude. Currently, the flight control modules of drones record altitudes relative to their takeoff plane. In nature, the altitude of a body of water is lower than that of its banks; for example, the surface of a river is lower than the banks. A drone's takeoff plane is usually not on the water's surface, but rather on the bank. When a drone takes off and flies above a body of water to perform a mission, the recorded altitude is still relative to its takeoff plane, causing a discrepancy between the actual physical distance between the drone and the water surface directly below and the recorded altitude.
[0004] Aside from water bodies, it is normal for the ground to be undulating in areas other than plains. This can cause the actual physical distance between the drone and the ground directly below to differ from the flight altitude recorded by the drone.
[0005] In existing technologies, to obtain the actual physical distance between a drone and the water surface or ground directly below, physical devices such as barometers, GPS, lidar, or ultrasonic sensors need to be installed. This leads to the following problems: 1. Increased drone weight, reducing drone flight time; 2. Increased drone cost, significantly increasing drone price.
[0006] Current drones are equipped with cameras and memory. To reduce drone costs, some researchers are studying a technology that measures the height of objects photographed by drones using the solar altitude angle. Current technologies rely on ground-based markers of known length. A standard marker (of known length) needs to be pre-set at the application site. By calculating the ratio of the marker's length to its shadow, this ratio is used as the ratio of the object being measured to its shadow length, thus calculating the object's height.
[0007] Existing methods are not suitable for situations where there are no standard benchmarks, nor are they suitable for measuring the flight altitude of a drone above the ground or water surface.
[0008] The technical solution of this patent application does not require adding hardware to the drone, and the marginal cost of upgrading the existing drone is zero. It has obvious value for promotion and application, and therefore needs market protection.
[0009] However, the technical solution of this patent application has limitations due to the solar altitude angle and sunlight conditions (no shadow without sunlight). Therefore, the applicant filed a "method for measuring the flight altitude of a drone using tilted laser" at the same time, which eliminates the need to photograph the shadow of the drone itself, thus overcoming the limitations of the technical solution in this patent application. Summary of the Invention
[0010] The purpose of this invention is to provide a method that allows for obtaining the actual physical distance between a drone and the water or ground surface below by processing images using the principle of solar altitude angle, without the need to set up ground or water surface markers.
[0011] To achieve the above objectives, the method for measuring flight altitude of the UAV using the solar altitude angle according to the present invention is performed according to the following steps:
[0012] The first step is takeoff. The staff controls the drone to take off from the takeoff plane. The drone's flight control module continuously records the drone's altitude h1 relative to the takeoff plane during the flight. Before the drone leaves the takeoff plane and enters the plane to be measured, the staff controls the drone's flight control module to take reference photos. The plane to be measured is adjacent to the takeoff plane.
[0013] The baseline photo-taking operation is as follows: the drone's flight control module controls the drone's camera to take a picture of the takeoff plane vertically downwards, and the picture contains the drone's shadow; the picture has a center point, and the flight control module collects the pixel distance L1 information between the shadow and the center point in the picture;
[0014] The second step is to take pictures and measure the plane to be measured; after the staff controls the drone to fly away from the takeoff plane and enter the plane to be measured, the staff controls the drone's flight control module to carry out the picture and measurement operation.
[0015] The photogrammetry operation is as follows: Let h2 be the height of the drone above the plane to be measured. The drone's flight control module controls the drone's camera to take a picture of the plane to be measured vertically downwards. This picture contains the shadow of the drone. This picture has a center point. The flight control module collects the pixel distance L2 between the shadow and the center point in the picture.
[0016] The flight control module calculates the height h2 of the UAV above the plane to be measured using Formula 1. Formula 1 is:
[0017] h2 = h1 × L2 / L1.
[0018] The height difference between the plane to be measured and the takeoff plane is d. After obtaining the value of h2, the flight control module calculates and stores the height difference d according to the following formula: d = h2 - h1.
[0019] During photogrammetry operations, the drone maintains the same altitude and takes multiple pictures by rotating, thereby reducing the requirement for the solar altitude angle.
[0020] When the takeoff plane is the shore of a body of water and the plane to be measured is the surface of the water, the UAV will immediately take pictures and measure after leaving the takeoff plane and entering the plane to be measured.
[0021] This invention also discloses a predictive method for the above-mentioned method of measuring flight altitude by means of solar altitude angle. Before the operator controls the drone to take off from the takeoff plane and fly to the plane to be measured for operation, the operator calculates the minimum solar altitude angle α1 that the drone will definitely be able to capture its own shadow when taking a vertical downward photo according to the formula of certain feasibility. The operator queries or calculates the dates and times in the year in which the solar altitude angle of the area to be operated is greater than or equal to α1 and less than or equal to 90 degrees, based on the latitude information of the area to be operated. The dates and times that meet the conditions are listed as optional dates and times for operation that are certain to be feasible without rotation.
[0022] The definitely feasible formula is: α1 = arctan(100 × H / Rreal); the unit of α1 is degrees;
[0023] H represents the height difference between the plane below the photographed object and the drone, in meters;
[0024] The image captured by the drone camera is a rectangular image, and the intersection of the diagonals of the rectangular image is the center point of the image; the formula for calculating Rreal is: Rreal = r × GSD;
[0025] In the Rreal calculation formula, 'r' means half the pixel length of the shorter side of the rectangular image, in pixels; GSD refers to the actual size represented by a unit pixel in the remote sensing digital image, and the formula for GSD is: GSD = H × a / f; where 'a' is the pixel size, which refers to the actual size represented by one pixel in the length and width directions of the drone camera's photosensitive element, in micrometers; 'f' is the equivalent focal length of the lens used by the drone camera, in millimeters; the values of 'a' and 'f' are derived from the hardware parameters of the drone camera, provided by the drone manufacturer or camera manufacturer; Rreal means the actual length in the physical world corresponding to half the pixel length of the shorter side of the rectangular image.
[0026] After determining the feasible dates and times for operations that do not require rotation, the staff calculates the minimum solar altitude angle α2 required for the drone to capture its own shadow when taking a vertical downward photo by rotating it, based on the rotation feasibility formula. The staff then queries or calculates the dates and times in the year where the solar altitude angle of the area to be operated is greater than or equal to α2 and less than α1, and lists the dates and times that meet the conditions as the feasible dates and times for operations that are guaranteed to be feasible by rotation.
[0027] The feasible rotation formula is: α2 = arctan(100 × H / Dreal); α2 is in degrees; H is the height difference between the plane below the image being photographed and the drone, in meters; Dreal is the actual physical length corresponding to the pixel length between the center point and any vertex of the rectangular image.
[0028] The formula for calculating Dreal is: Dreal = d × GSD; where d is the pixel distance from the center point of the rectangular image captured by the drone camera to any vertex of the image, in pixels.
[0029] The present invention has the following advantages:
[0030] The principle of Formula 1 is as follows: The time points for the drone to perform baseline photography and photo-measurement operations are before and after the drone leaves the takeoff plane. If the operation is in water or plains, only two photography operations (baseline photography and photo-measurement) are needed before and after the drone leaves the takeoff plane. At this time, the time difference between the two photography operations is minimal, and there is no measurable change in the solar altitude angle. If the operation is in hilly or mountainous areas, multiple photography and measurement operations are needed to determine the current flight altitude while the drone is performing its own monitoring tasks. Keeping the drone's operation time within half an hour (less than or equal to half an hour allows for multiple operations after battery replacement) is advantageous because the operation time matches the flight time of most drones, and the change in the solar altitude angle within half an hour is very small, approximating as constant, making Formula 1 applicable to measuring the drone's flight altitude.
[0031] In summary, the time difference between the baseline photography operation and the photo-measurement operation is small. Therefore, it can be assumed that the solar altitude angle in the photos taken in both operations is the same. This is the basis of Formula 1 and also the ingenious adaptation that makes Formula 1 faster. Existing research on measuring the height of ground objects using the solar altitude angle (which requires comparison with the shadow of a standard pole of known height set in advance on the ground) not only requires setting a standard pole of known height in advance on the ground, but also requires GPS to obtain the latitude information of the UAV's geographical location in real time, and then obtaining the solar altitude angle information of the current geographical location of the UAV based on the specific time, and incorporating the value of the solar altitude angle into the calculation formula. The whole process is cumbersome and inefficient. This invention, however, utilizes the geometric principle of proportionality. The solar altitude angle does not need to be included in the calculation process. It can quickly calculate the value of h2 using the principle of the shadow formed by the solar altitude angle. The geometric principle of proportionality used in this invention means that since the solar altitude angle is the same, the ratio of the height to the distance of the shadow from the center point of the image is a constant, that is, h1 / L1 = h2 / L2, which leads to Formula 1.
[0032] One innovation of this invention is that it performs baseline photography and photogrammetry during a single drone operation, and makes approximate calculations while ignoring minute changes in the solar altitude angle over a short period. This innovation forms the basis for the second innovation. The second innovation is that h2 is measured using proportional geometric principles between two images. The calculation process does not require obtaining latitude or time information via GPS, resulting in less computation. The efficiency of the program design and actual calculation is significantly improved compared to traditional methods, and it eliminates the need to set up a standard pole of known height on the ground or water surface beforehand as a basis for calculation.
[0033] When the solar altitude angle of the work area is greater than or equal to α1 degrees and less than or equal to 90 degrees, images taken vertically downwards by the drone at any horizontal rotation angle will inevitably contain the drone's shadow. Based on α1 and the latitude information of the work area, a list of possible dates and times throughout the year is compiled that are feasible without rotation (the drone can ensure that its own shadow is captured without rotation). First, a specific work time (such as hydrological monitoring time) is selected from these dates and times. This allows for ensuring that the drone's shadow is included in vertically downward-facing images without rotating the drone during baseline photography and measurement of the target plane, thus minimizing operational complexity and improving efficiency.
[0034] When the solar altitude angle in the work area is greater than or equal to α2 degrees and less than α1 degrees, the drone can take a vertically downward picture from any location. If the picture does not show the drone's own shadow, the drone can rotate its own angle (horizontal rotation) to take multiple pictures vertically downward, ensuring that at least one picture shows the drone's own shadow. Based on α2, α1, and the latitude information of the work area, a list of possible dates and times for the operation is compiled, reducing the requirement for solar altitude angle, expanding the range of possible dates and times, and making it easier for staff to select specific dates and times within a wider range after considering other operational constraints (such as whether it is sunny).
[0035] Rivers or lakes typically have very gentle slopes, so the water surface within the drone's operational area can be considered as a single horizontal plane. Consequently, when operating in water, the drone can perform a single photo measurement immediately upon entering the water, using the drone's height above the water surface as its relative altitude during the operation. This reduces the number of photo measurement operations, lowers the altitude measurement workload, and still meets the need to determine the flight altitude during water operations.
[0036] The technical solution of this patent does not require adding hardware to the drone; it only requires writing a program into the drone's flight control module. It is very suitable for upgrading and modifying existing drones, and the marginal cost of upgrading existing drones is zero, which has obvious value for promotion and application. Attached Figure Description
[0037] Figure 1 This is a schematic diagram illustrating the principle of a drone taking two photos, one on the takeoff plane and the other on the plane to be measured.
[0038] Figure 2 These are images taken during the baseline photography operation.
[0039] Figure 3 These are pictures taken during a photogrammetry operation.
[0040] Figure 4 This is the schematic diagram corresponding to a formula that is guaranteed to be feasible. Figure 4 The circular outline in the image represents the set of possible locations where the drone's shadow might appear when the solar altitude angle is equal to α1. Figure 4 The set of possible locations where the shadow of a drone may appear when the solar altitude angle is greater than or equal to α1 and less than or equal to 90 degrees, along with the outline of the medium circle and its internal region.
[0041] Figure 5 This is the schematic diagram corresponding to the rotational feasibility formula; Figure 5 There are two concentric circles. The outline of the larger circle represents the set of possible positions where the UAV's shadow may appear when the solar altitude angle is equal to α2 (if the shadow exceeds...). Figure 5 The large circle in the image represents the location where the drone's shadow can be captured when the solar altitude angle is less than α2. The outline of the small circle represents the set of possible locations where the drone's shadow can appear when the solar altitude angle is equal to α1. Figure 5 The annular area between the small and large circles is the region where the drone's shadow can be captured by horizontal rotation.
[0042] Figure 5 There are solid rectangles (horizontally and vertically straight rectangles) and dashed rectangles (slanted rectangles). The solid rectangles represent the area covered by the image captured by the inorganic camera when the drone's shadow is not included; the dashed rectangles represent the area covered by the image captured by the drone when it is rotated and includes the drone's shadow. Figure 5 The black dot in the upper right corner represents the shadow of the drone.
[0043] Figure 4 and Figure 5 In the diagram, point A represents the intersection of the perpendicular line from the center of the image to the long side of the image taken by the drone and the long side of the image itself, while point B is a vertex of the rectangular image taken by the drone. Detailed Implementation
[0044] like Figures 1 to 5 As shown, this invention provides a method for measuring the flight altitude of a drone using the solar altitude angle, which is carried out according to the following steps:
[0045] The first step is takeoff. The operator controls the drone to take off from takeoff plane 1 (such as a riverbank). During flight, the drone's flight control module continuously records the drone's altitude h1 (in meters) relative to takeoff plane 1; this is existing technology. Before the drone leaves takeoff plane 1 and enters the target plane 2 (such as the surface of a river or lake, or ground at a different level from the takeoff plane), the operator uses the drone's remote controller to control the drone's flight control module to perform baseline photography. The target plane 2 is adjacent to takeoff plane 1. Figure 1 The direction indicated by the middle arrow is the flight direction of UAV 3 from takeoff plane 1 to the plane to be measured 2.
[0046] The baseline photo-taking operation is as follows: The flight control module of UAV 3 controls the camera of UAV 3 to take a picture of the takeoff plane 1 vertically downward. The picture contains the shadow 4 of UAV 3 (that is, the shadow 4 of UAV 3 projected on the takeoff plane 1 under the sunlight). The picture has a center point 5 (that is, the point located directly below the camera, which is also the intersection of the diagonals of the rectangular picture). The flight control module collects the pixel distance L1 (pixels) between the shadow 4 and the center point 5 in the picture.
[0047] The second step is to take pictures and measure the plane to be measured. The staff controls the drone 3 to fly away from the takeoff plane 1 (at this time, there is a height difference between the water surface or ground below the drone 3 and the takeoff plane 1) and enter the plane to be measured 2 (such as flying from the bank of the takeoff plane 1 to the river). The staff controls the flight control module of the drone 3 to take pictures and measure. Alternatively, the flight control program can be set to take pictures and measure periodically and automatically, such as taking pictures and measuring once every 10 seconds.
[0048] The photo measurement operation is as follows: Let the height of the drone 3 above the plane 2 to be measured be h2 (meters). The flight control module of the drone 3 controls the camera of the drone 3 to take a picture of the plane 2 to be measured vertically downward. The picture contains the shadow 4 of the drone 3 (that is, the shadow 4 of the drone 3 projected on the plane 2 to be measured under sunlight). The picture has a center point 5 (that is, the point located directly below the camera). The flight control module collects the pixel distance L2 (pixels) between the shadow 4 and the center point 5 in the picture.
[0049] The flight control module calculates the height h2 of the UAV 3 above the plane 2 to be measured using Formula 1. Formula 1 is:
[0050] h2 = h1 × L2 / L1; The principle of Formula 1 is as follows: The time points for the drone to perform baseline photography and photo-measurement operations are before and after the drone leaves the takeoff plane. If the operation is in water or plains, only two photography operations (baseline photography and photo-measurement) are needed before and after the drone leaves the takeoff plane. At this time, the time difference between the two photography operations is minimal, and there is no measurable change in the solar altitude angle. If the operation is in hilly or mountainous areas, multiple photo-measurement operations are needed to determine the current flight altitude while the drone is performing its own monitoring task. Keeping the drone's operation time within half an hour (less than or equal to half an hour allows for multiple operations after battery replacement) is advantageous because the operation time matches the endurance of most drones, and the change in the solar altitude angle within half an hour is very small, approximating as constant, making Formula 1 applicable to measuring the drone's flight altitude.
[0051] In summary, the time difference between the baseline photography operation and the photo-measurement operation is small. Therefore, it can be assumed that the solar altitude angle is the same in both operations. This is the basis of Formula 1 and a clever adaptation that reduces the amount of calculation and speeds up the calculation. Existing research on measuring the height of ground objects using the solar altitude angle (which requires comparison with the shadow 4 of a standard pole with a known height set in advance on the ground) not only requires setting a standard pole with a known height in advance on the ground, but also requires GPS to obtain the latitude information of the location of UAV 3 in real time, and then, based on the specific time, obtain the solar altitude angle information of the current location of UAV 3, and incorporate the value of the solar altitude angle into the calculation formula. The whole process is cumbersome and inefficient. This invention, however, utilizes the geometric principle of proportionality. The solar altitude angle does not need to be included in the calculation process. The value of h2 can be quickly calculated using the principle of the shadow 4 formed by the solar altitude angle. The geometric principle of proportionality used in this invention means that since the solar altitude angle is the same, the ratio of the height to the distance of the shadow 4 from the center point 5 of the image is a constant, that is, h1 / L1=h2 / L2, which leads to Formula 1.
[0052] One innovation of this invention is that it performs baseline photography and photogrammetry during a single drone operation, and makes approximate calculations while ignoring minute changes in the solar altitude angle over a short period. This innovation forms the basis for the second innovation. The second innovation is that h2 is measured using proportional geometric principles between two images. The calculation process does not require obtaining latitude or time information via GPS, resulting in less computation. The efficiency of the program design and actual calculation is significantly improved compared to traditional methods, and it eliminates the need to set up a standard pole of known height on the ground or water surface beforehand as a basis for calculation.
[0053] The height difference between the plane to be measured 2 and the takeoff plane 1 is d. After obtaining the value of h2, the flight control module calculates and stores the height difference d according to the following formula: d = h2 - h1.
[0054] During the photogrammetry operation, UAV 3 maintains the same altitude and takes multiple pictures by rotating to reduce the requirement for the solar altitude angle.
[0055] When the takeoff plane is the shore of a body of water (i.e., a river or lake) and the plane to be measured is the surface of the water, the UAV will immediately take pictures and measure after leaving the takeoff plane and entering the plane to be measured.
[0056] Rivers and lakes typically have very gentle slopes, so the water surface within the drone's operational area can be considered a level plane. Therefore, when operating in water, the drone can perform a single photographic measurement immediately upon entering the water, using the drone's height above the water surface as its relative altitude during the operation. This reduces the number of photographic measurements, lowers the altitude measurement workload, and still meets the need to determine flight altitude during water operations. Of course, in other locations, especially in hilly areas, multiple photographic measurements are required to determine the drone's current flight altitude.
[0057] Throughout the year, the solar altitude angle in the same region constantly changes. When the solar altitude angle is 90 degrees (there are instances of a 90-degree solar altitude angle between the Tropic of Cancer and the Tropic of Capricorn), any drone with a camera function can capture its own shadow (4) vertically downwards (the shadow is directly below the drone). When the solar altitude angle is lower (greater tilt), for a specific drone model, it is not possible to capture its own shadow (4) vertically downwards on all dates and at all times. Therefore, it is necessary to predict in advance the suitable dates and times for the current drone model to operate and plan accordingly to avoid delays.
[0058] This invention also discloses a predictive method for the above-mentioned method of measuring flight altitude by means of solar altitude angle of the UAV. Before the operator controls the UAV 3 to take off from the take-off plane 1 and fly to the plane to be measured 2 for operation, the operator calculates the minimum solar altitude angle α1 that the UAV 3 will definitely be able to capture its own shadow 4 when taking pictures vertically downwards according to the formula of certain feasibility. The operator queries or calculates the dates and times in the year when the solar altitude angle of the area to be operated is greater than or equal to α1 and less than or equal to 90 degrees (such as from 11:00 am to 1:00 pm on a certain day of a certain month) based on the latitude information of the area to be operated. The date and time that meet the conditions are listed as the optional dates and times of operation that are certain to be feasible without rotation.
[0059] Querying or calculating the solar altitude angle for specific dates and times in the coming year based on latitude information is a standard technique and will not be elaborated upon.
[0060] The definitely feasible formula is: α1 = arctan(100 × H / Rreal); the unit of α1 is degrees;
[0061] H is the height difference between the plane below the photographed object and the drone 3 (in this invention, "height difference" and "height difference" are synonyms), and the unit is meters;
[0062] The image captured by the drone's three cameras is a rectangular image. The intersection of the diagonals of the rectangle is the center point 5 of the image. A circle is formed with center point 5 as the center and a radius equal to half the length of the shorter side of the rectangle. Figure 4 The possible range of the shadow 4 of the drone 3 in the image when the solar altitude angle is not less than α1 in the middle circular region.
[0063] The formula for calculating Rreal is: Rreal = r × GSD;
[0064] In the Rreal calculation formula, r represents half the pixel length of the shorter side of the rectangular image, measured in pixels; GSD refers to the actual size represented by a unit pixel in remote sensing digital imagery, which is related to the flight altitude of the aircraft, the focal length of the lens of the imaging system, and the size of the pixel. The formula for calculating GSD is: GSD = H × a / f; where a is the pixel size, which refers to the actual size represented by one pixel in the length and width directions of the UAV 3 camera's photosensitive element, measured in micrometers (μm); f is the equivalent focal length of the lens used by the UAV 3 camera, measured in millimeters (mm); the values of a and f are derived from the hardware parameters of the UAV 3 camera, provided by the UAV 3 manufacturer or the camera manufacturer; Rreal means the actual length (horizontal length) in the physical world corresponding to half the pixel length of the shorter side of the rectangular image; when H is constant, Rreal is determined by the specific camera hardware parameters.
[0065] When the solar altitude angle of the work area is greater than or equal to α1 degrees and less than or equal to 90 degrees, any image taken vertically downwards by UAV 3 at any horizontal rotation angle will inevitably contain the shadow 4 of UAV 3. Based on α1 and the latitude information of the work area, a list of possible dates and times throughout the year that are feasible without rotation (UAV 3 can ensure that its own shadow 4 is captured without rotation) is compiled. First, a specific work time (such as hydrological monitoring time) is selected from these dates and times. This allows for ensuring that the image taken vertically downwards contains the shadow 4 of UAV 3 without rotating UAV 3 in place during benchmark photography and measurement of the target plane, thus minimizing operational complexity and improving operational efficiency.
[0066] After determining the feasible dates and times for operations that do not require rotation, the staff calculates the minimum solar altitude angle α2 required for the drone 3 to capture its own shadow 4 when taking a vertical downward photo by rotating it, based on the rotation feasibility formula. The staff then queries or calculates the dates and times in the year where the solar altitude angle of the area to be operated is greater than or equal to α2 and less than α1, and lists the dates and times that meet the conditions as the feasible dates and times for operations that are guaranteed to be feasible by rotation.
[0067] The feasible rotation formula is: α2 = arctan(100 × H / Dreal); α2 is in degrees; H is the height difference between the plane below the image being photographed and the drone 3, in meters; Dreal is the actual physical length (horizontal length) corresponding to the pixel length between the center point 5 of the rectangular image and any vertex; when H is constant, Dreal is determined by the specific camera hardware parameters. Dreal is also the farthest real distance between the drone 3's own shadow 4 and the center point 5 of the image, determined by the specific camera hardware parameters.
[0068] The formula for calculating Dreal is: Dreal = d × GSD; the meaning and calculation formula of GSD are as described above. Where d is the pixel distance from the center point 5 of the rectangular image captured by the drone's three cameras to any vertex of the image (the rectangle has four vertices), in pixels.
[0069] When the solar altitude angle of the work area is greater than or equal to α2 degrees and less than α1 degrees, UAV 3 takes a vertical downward picture from any location. If the picture does not contain its own shadow 4, UAV 3 rotates its own angle (horizontal rotation) to take multiple pictures vertically downward, ensuring that at least one picture contains the shadow 4 of UAV 3. Based on α2, α1, and the latitude information of the work area, a list of possible dates and times for the operation is compiled, reducing the requirement for solar altitude angle, expanding the range of possible dates and times, and making it easier for staff to select specific dates and times within a wider range of dates and times after considering other operational constraints.
[0070] Determining the pixel distance between two specific pixels in a given image is a common technique in image processing and will not be elaborated upon. Querying or calculating the solar altitude angle of the region throughout the year based on its latitude information is also a common technique. The principle is briefly explained below (this can be used when predicting suitable dates and times for implementing drone-based altitude measurement methods using solar altitude angle): The solar altitude angle varies with local time and solar declination.
[0071] Solar declination (also known as the declination angle, is the angle between the Earth's equatorial plane and the line connecting the Sun and the Earth's center. The declination angle moves annually within the range of +23°26' and -23°26') is denoted by δ. The geographical latitude of the observation location is denoted by φ (both solar declination and geographical latitude are positive for north latitude and negative for south latitude). Local time (hour angle, defined as the spherical angle formed by the celestial meridian and the right ascension circle of a celestial body at the North Pole, or the arc measured along the celestial equator. The Earth rotates 360 degrees, corresponding to 24 hours; therefore, the corresponding hour angle is 15 degrees per hour. This means that as the Earth rotates, the hour angle of a celestial body increases by 15° per hour) is denoted by t. The formula for calculating the solar altitude angle is:
[0072] sin(α)=sin(φ)sin(δ)+cos(φ)cos(δ)cos(t)(9).
[0073] Where α represents the solar altitude angle (the value to be calculated), in degrees (°); φ represents the geographic latitude, in degrees (°); δ represents the solar declination, in degrees (°); and t represents the hour angle, in degrees (°). The formula for calculating solar declination is:
[0074] cos (δ) = sin (φ) cos (∈) + cos (φ) sin (∈) cos (t) (10).
[0075] ∈ represents the obliquity of the ecliptic (the obliquity of the ecliptic is the angle between the Earth's orbital plane, i.e., the ecliptic plane, and the equatorial plane, i.e., the celestial equator plane; it is also known as the solar declination angle or the greatest elongation of the ecliptic. The obliquity of the ecliptic is not a completely fixed value; it has varied throughout history. Its range is 22°00'-24°30', with a period of approximately 4.1 × 10^4 years. However, because this variation is very small, it can be ignored in the short term. Therefore, we generally consider the obliquity of the ecliptic to be relatively stable, with a value of approximately 23°26').
[0076] By combining the formulas for calculating the solar altitude angle and solar declination, and inputting the geographical latitude φ and hour angle t of the observation location, the local solar declination angle δ and solar altitude angle α can be obtained. By inputting all hour angles of the observation location throughout the day, the range of solar altitude angle variation at that location can be obtained. Comparing this with the solar altitude angle at which a specific model of drone 3 can capture its own shadow 4, it can be determined whether the specific model of drone 3 has time to capture its own shadow 4 at that location during the day, and the time range within which it can capture its own shadow 4, thus providing accurate guidance for the operator. In summary, under the guidance of this invention, a drone 3 can independently complete altitude measurement operations by vertically shooting its own shadow 4 downwards.
[0077] The above embodiments are only used to illustrate and not limit the technical solutions of the present invention. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the present invention without departing from the spirit and scope of the present invention. Any modifications or partial substitutions should be covered within the scope of the claims of the present invention.
Claims
1. A method for measuring flight altitude of an unmanned aerial vehicle (UAV) using the solar altitude angle, characterized in that: The method for measuring flight altitude using the solar altitude angle by a drone is performed as follows: The first step is takeoff. The staff controls the drone to take off from the takeoff plane. The drone's flight control module continuously records the drone's altitude h1 relative to the takeoff plane during the flight. Before the drone leaves the takeoff plane and enters the plane to be measured, the staff controls the drone's flight control module to take reference photos. The plane to be measured is adjacent to the takeoff plane. The baseline photo-taking operation is as follows: the drone's flight control module controls the drone's camera to take a picture of the takeoff plane vertically downwards, and the picture contains the drone's shadow; the picture has a center point, and the flight control module collects the pixel distance L1 information between the shadow and the center point in the picture; The second step is to take pictures and measure the plane to be measured; after the staff controls the drone to fly away from the takeoff plane and enter the plane to be measured, the staff controls the drone's flight control module to carry out the picture and measurement operation. The photogrammetry operation is as follows: Let h2 be the height of the drone above the plane to be measured. The drone's flight control module controls the drone's camera to take a picture of the plane to be measured vertically downwards. This picture contains the shadow of the drone. This picture has a center point. The flight control module collects the pixel distance L2 between the shadow and the center point in the picture. The flight control module calculates the height h2 of the UAV above the plane to be measured using Formula 1. Formula 1 is: h2 = h1 × L2 / L1; Before the staff controls the drone to take off from the takeoff plane and fly to the plane to be measured to carry out the operation, they calculate the minimum solar altitude angle α1 that the drone will definitely be able to capture its own shadow when taking pictures vertically downwards according to the formula of certain feasibility. The staff then queries or calculates the dates and times in the year in which the solar altitude angle of the area to be operated is greater than or equal to α1 and less than or equal to 90 degrees. The dates and times that meet the conditions are listed as the optional dates and times for operation that are certain to be feasible without rotation. The definitely feasible formula is: α1 = arctan(100 × H / Rreal); the unit of α1 is degrees; H represents the height difference between the plane below the photographed object and the drone, in meters; The image captured by the drone camera is a rectangular image, and the intersection of the diagonals of the rectangular image is the center point of the image; the formula for calculating Rreal is: Rreal = r × GSD; In the Rreal calculation formula, 'r' means half the pixel length of the shorter side of the rectangular image, in pixels; GSD refers to the actual size represented by a unit pixel in the remote sensing digital image, and the formula for GSD is: GSD = H × a / f; where 'a' is the pixel size, which refers to the actual size represented by one pixel in the length and width directions of the drone camera's photosensitive element, in micrometers; 'f' is the equivalent focal length of the lens used by the drone camera, in millimeters; the values of 'a' and 'f' are derived from the hardware parameters of the drone camera, provided by the drone manufacturer or camera manufacturer; Rreal means the actual length in the physical world corresponding to half the pixel length of the shorter side of the rectangular image.
2. The method for measuring flight altitude of a UAV using the solar altitude angle according to claim 1, characterized in that: After determining the feasible dates and times for operations that do not require rotation, the staff calculated the minimum solar altitude angle α2 that would ensure the drone could capture its own shadow when taking photos while rotating it, based on the rotation feasibility formula. Staff members use the latitude information of the area to be worked on to query or calculate the dates and times in the year when the solar altitude angle of the area is greater than or equal to α2 and less than α1, and list the dates and times that meet the conditions as the optional dates and times for the operation that are guaranteed to be feasible. The feasible rotation formula is: α2 = arctan(100 × H / Dreal); α2 is in degrees; H is the height difference between the plane below the photographed object and the drone, in meters; Dreal is the actual physical length of a rectangular image, corresponding to the pixel length between the center point and any vertex. The formula for calculating Dreal is: Dreal = d × GSD; where d is the pixel distance from the center point of the rectangular image captured by the drone camera to any vertex of the image, in pixels.
3. The method for measuring flight altitude of a UAV using the solar altitude angle according to claim 1, characterized in that: The height difference between the plane to be measured and the takeoff plane is d. After obtaining the value of h2, the flight control module calculates and stores the height difference d according to the following formula: d = h2 - h1.
4. The method for measuring flight altitude of a UAV using the solar altitude angle according to claim 1, characterized in that: During photogrammetry operations, the drone maintains the same altitude and takes multiple pictures by rotating, thereby reducing the requirement for the solar altitude angle.
5. The method for measuring flight altitude of a UAV using the solar altitude angle according to claim 2, characterized in that: When the takeoff plane is the shore of a body of water and the plane to be measured is the surface of the water, the UAV will immediately take pictures and measure after leaving the takeoff plane and entering the plane to be measured.