High-precision tree height measuring device and method based on Beidou multi-sensor fusion

Abstract

The invention relates to the field of tree height measurement, in particular to a high-precision tree height measurement device and method based on Beidou multi-sensor fusion, and the device is composed of an unmanned aerial vehicle, a Beidou differential positioning module, a laser ranging module, a camera, a communication module, a GNSS antenna, a WIFI module, a digital level, a display screen, a storage module, a power module and the like. According to the device, three measurement sensors including a meter ruler, a Beidou differential positioning module and a laser range finder are combined for measurement, laser ranging, Beidou differential positioning height values and multiple measurement values of the meter ruler are fused to obtain an average value, Kalman filtering estimation processing is carried out on the measurement values, and an optimal measurement result is estimated. The device realizes miniaturization of equipment volume, has the advantages of easy installation, low maintenance cost, low equipment power consumption, high multi-sensor fusion measurement precision and the like, solves the problems of large volume, inconvenience in installation, low measurement efficiency and the like of the existing tree height measurement device, and particularly plays a certain role in nondestructive measurement of famous and ancient trees.

Description

Technical Field

[0001] The present invention relates to the field of high-precision height measurement technology, and specifically to a high-precision tree height measurement device and method based on Beidou multi-sensor fusion. Background Art

[0002] Tree height is an important parameter for tree measurement in forestry. In the past two years, the height of trees in my country has been constantly updated. The scientific expedition team has made great gains in the field species survey in Tongmai Town, Bomi County, Nyingchi City. In addition to discovering the tallest Tibetan cypress tree in Asia, a large number of giant trees with a height of more than 85 meters were discovered, which exceeded the 83.4-meter-high Yunnan yellow fruit fir in Zayu County, Tibet. There are as many as 25 trees with a height of more than 90 meters, which greatly enriched my country's tall tree information resource library. At the same time, it also made Tongmai Town, Bomi County, Nyingchi City, Tibet, the area with the highest distribution density of tall trees in my country and even in Asia. In addition, in May 2023, my country's scientific expedition team discovered a Tibetan cypress that was 102.3 meters tall.

[0003] In the forest resource survey, in order to more accurately measure parameters such as tree height, a set of precise and convenient measuring instruments is needed. At present, the measurement of tree height mainly includes mechanical instruments, theodolites, total stations, and laser instruments. However, these instruments have different problems. Mechanical instruments have low accuracy in measuring tree height, low efficiency in operation and measurement, single functions, and troublesome processing of collected data. Although electronic theodolites and total stations save time, improve efficiency and accuracy, and realize automatic data collection, transmission and calculation, these instruments are not only difficult to carry, but also cumbersome to measure, and require high professional knowledge. Although laser instruments are easy to carry and have high accuracy, they are often expensive.

[0004] The Global Positioning System (GPS) has been widely used in surveying and mapping and geographic information related fields. It has developed from a single-star GPS system to dual-star and multi-star systems. In particular, after China's independently developed Beidou Satellite Navigation System (BDS) was officially put into global operation on July 31, 2020, many domestic companies are currently investing in the research and development of Beidou series products and have launched a number of multi-star (GPS+BDS, GPS+GLONASS, GPS+BDS+GLONASS) satellite receivers.

[0005] Beidou positioning equipment is generally divided into low-precision and high-precision: the accuracy ranges from ten meters to sub-meter level, and then to RTK centimeter level. Beidou RTK equipment with different accuracy can be provided according to the actual measurement requirements of customers. Beidou RTK equipment can not only provide comprehensive Beidou RTK equipment application software, but also be compatible with the secondary development of various mainstream GIS software platforms, and can achieve seamless data docking. The current Beidou RTK equipment adopts an integrated design, high-precision satellite positioning technology, high-end system configuration, and mature CORS (continuously operating reference station). After searching for satellites such as BDS and GPS, the current position information and differential correction information are obtained, and then the point coordinates are collected to achieve the purpose of high-precision measurement.

[0006] As cities develop rapidly, the application of smart city GIS (geographic information system) information collector equipment is becoming more and more widespread. In the face of tall trees, my country's methods of measuring trees are mostly drone laser radar technology measurement and manual tree climbing and hanging ruler measurement. In the case of large trees in the middle of the road, railings, green belts, etc., it is difficult for personnel to reach the location information of the target point. The Beidou RTK equipment on the market is particularly insufficient, which brings great difficulty to tree height measurement. Therefore, a high-precision tree height measurement device based on Beidou and drone multi-sensor fusion is very necessary. Summary of the invention

[0007] The technical problem to be solved by the present invention is to overcome the deficiencies in the prior art and provide a high-precision tree height measurement device and method based on Beidou multi-sensor fusion.

[0008] The measuring device consists of a drone, Beidou differential positioning module, laser ranging module, camera, communication module, GNSS antenna, WIFI module, digital level, display screen, storage module, power module, etc. Optional accessories include reflectors, rulers, etc. Figure 1 The functions of each part are shown below.

[0009] Beidou differential positioning module: Using the differential positioning module to receive Beidou satellite navigation signals and Beidou CORS network differential correction information can obtain centimeter-level elevation, longitude and latitude coordinates and other information.

[0010] Beidou differential positioning module (Beidou RTK module) is a carrier phase differential positioning module of three systems and eight frequencies GPS: L1, L2, L5, BDS: B1, B2, B3, GLONASS: G1, G2. Beidou RTK module can obtain centimeter-level positioning by receiving GNSS satellite navigation signals and CORS network differential correction information. Beidou RTK module communicates with microprocessor through serial port. Beidou RTK module obtains local CORS network differential correction information (differential information includes satellite ephemeris error, satellite ephemeris clock error, satellite orbit error, ionosphere, troposphere, etc.) by receiving GNSS satellite navigation signals and communication module, and wireless communication, and obtains centimeter-level positioning.

[0011] The GNSS carrier phase differential positioning module goes through four different positioning accuracy modes: first, standard single-point positioning is obtained; then, pseudo-range differential RTD positioning is quickly obtained after differential correction information is obtained; then, RTK positioning with carrier phase integer ambiguity as a floating point solution is obtained; finally, RTK positioning with carrier phase integer ambiguity as a fixed solution is obtained.

[0012] The four different positioning accuracy modes are: 10 meters, meters, decimeters, and centimeters. The GNSS carrier phase differential positioning module uses the Chinese CGCS2000 geodetic coordinate system and follows the NMEA-0183 format to output information such as longitude, latitude, and elevation.

[0013] The laser ranging module is used to obtain the distance to the ground: the laser ranging module includes a laser transmitter and a laser receiver, and a button is provided on the laser transmitter;

[0014] Among them: the laser ranging module uses 635nm (nanometer) visible light laser, laser grade II safety laser, and has a ranging range of up to 100 meters. It is used to obtain the distance between the reference point and the target point. The laser ranging module calculates the time difference of the emitted light beam after passing through the air and the reflector and returning, and then obtains the time difference. The reflector is used for measurement, which is accurate, fast and reliable.

[0015] The camera is used to take photos of the target and transmit videos and pictures in real time through the WIFI module, so that the three points of the camera crosshairs, the aiming fulcrum and the treetop are in line.

[0016] The communication module is used to obtain the local area CORS network differential correction information.

[0017] The GNSS antenna (differential positioning antenna) is a high-precision measurement three-system eight-frequency built-in spiral antenna, which reduces the size of the equipment and includes circuits such as low-noise amplifier LNA and SAW surface acoustic filter.

[0018] The antenna is fed and transmitted through a standard SMA connector. The antenna connector can be connected to the signal connector of the terminal device to fix the antenna. The antenna adopts a four-arm spiral structure, has a stable phase center, and adopts anti-interference technology, so it can still work normally in a complex environment with certain interference.

[0019] Digital level: The angle sensor senses the horizontal tilt angle and is connected to the internal microprocessor. It has a high resolution, high precision, large range, and low power consumption. It can detect the slight tilt changes of the X-axis and Y-axis, and communicate with the microprocessor through the serial port to determine whether the device is placed horizontally. The digital level on the drone ensures that the drone is in a horizontal state, so that the laser beam of the laser ranging module is perpendicular to the reflector, ensuring that the beam propagation distance is the shortest, and the incident angle and refraction angle are approximately equal to 0°.

[0020] The reflector comes with a bubble level for visually checking the level of the reflector.

[0021] The advantages of the present invention are: combining three sensors, namely a ruler, Beidou differential positioning and a laser rangefinder, for measurement, with high measurement accuracy, miniaturization of the equipment, easy installation, low maintenance cost and low equipment power consumption, etc., solving the problems of large size and inconvenient measurement of existing tree height measuring devices. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is the principle block diagram of the high-precision tree height measurement device based on Beidou multi-sensor fusion;

[0023] Figure 2 This is a schematic diagram showing the three points of the camera crosshairs center point, the aiming fulcrum, and the treetop being collinear;

[0024] Figure 3 This is a schematic diagram of laser ranging measurement. DETAILED DESCRIPTION

[0025] Implementation method:

[0026] The measuring device has three methods for measuring tree height: suitable for windless or light wind conditions, the drone is placed in a relatively open area, the area is as horizontal as possible with the tree roots (buildings) at the same height, and there are no obstructions such as branches above the area to avoid affecting the drone's rise.

[0027] ①Measurement method 1--Measurement with a ruler

[0028] Starting position measurement: Before the drone rises, measure the horizontal position with the tree root, that is, the height corresponding to the three points of the camera crosshairs center point, the alignment support point, and the tree root when they are in line, to ensure that the starting measurement point of the ruler is consistent with the horizontal position of the tree root.

[0029] Height measurement: A ruler is hung on the drone. As the drone rises, the drone is positioned in the center of the reflector as much as possible, and the ruler is continuously pulled up. The drone is integrated with a camera, and the camera has a built-in aiming icon. The drone takes off and rises in an open field. The camera transmits video images to the remote control terminal in real time. When the drone is about to reach the treetop, adjust the direction and position of the drone. After hovering, the drone is in a horizontal state (determined by a digital level). After reaching the target point, when the center point of the camera crosshairs, the aiming fulcrum, and the treetop are in a straight line, the length of the ruler pulled up is the height of the tree.

[0030] The process of camera aiming is somewhat similar to the process of drone strike. The drone strikes at an angle and aims at the target on the ground. When measuring the height of a tree, the drone is in a horizontal position with an angle of 0 degrees. Figure 2 shown.

[0031] ②Measurement method 2--Beidou RTK high-precision positioning

[0032] Starting position measurement: Before the drone ascends, measure the horizontal position with the tree root, that is, the height h1 corresponding to the collinear center point of the camera crosshairs, the fulcrum, and the tree root.

[0033] Height measurement: After arriving at the destination, when the center point of the camera crosshairs, the aiming fulcrum, and the treetop are in a straight line, start Beidou RTK high-precision positioning and measure the height h2. The difference between the two heights (h2-h1) is the measured tree height.

[0034] ③Measurement method 3--laser rangefinder

[0035] Starting position measurement: The reflector is integrated with a bubble level. The reflector is placed horizontally and as level as possible with the tree roots (at the same height as the tree roots, that is, the center point of the camera crosshairs, the alignment fulcrum, and the tree roots are in a straight line). This can enhance the laser reflection signal strength and increase the ranging range.

[0036] Height measurement: Then the drone gradually rises. When the drone rises to the vicinity of the treetops, the drone is in a horizontal position, and the center point of the camera crosshairs, the aiming fulcrum, and the treetops are in a straight line, the laser ground distance measurement is started, and there is a reflector on the ground to cooperate with the laser distance measurement. Since the geometric centers of the laser distance measurement module transmitter and receiver are 2cm apart, when the reflector is stationary, the reflector has its own bubble level, which is used to visually check the horizontal placement of the reflector; the laser emits a parallel beam vertically to the ground, which will be reflected at the reflector surface, and the reflected beam is finally sent to the receiver. Therefore, the laser distance measurement module needs to focus on processing the round-trip time difference of the received reflected beam signal to obtain the distance measurement value.

[0037] The propagation speed of laser in vacuum is C, the speed of air as medium is C / n, the refractive index is n, the round-trip transmission time is t, and H is the height value. The specific derivation process is shown in the following formula.

[0038] H=C×t / 2 / n

[0039] The refractive index of air is known to be n = 1.0003. From the above formula, we can get the round trip time difference and calculate the "distance" value H. When measuring laser distance, it is necessary to place a 1m×1m reflector on the bottom of the drone to enhance the reflected signal and increase the measurement distance. The schematic diagram of laser distance measurement is as follows: Figure 3 shown.

[0040] In order to obtain high-precision distance measurement, data can be measured multiple times; at the same time, Beidou RTK is used to measure altitude information, and the laser ground distance measurement and Beidou altitude measurement values ​​are merged to obtain the average value, and the average value is filtered and estimated. Finally, the accurate distance is sent to the microprocessor, and the microprocessor obtains a more accurate distance through Kalman filtering.

[0041] ④Data filtering processing

[0042] Since wireless ranging uses laser ranging sensors, it is affected by local temperature, humidity, wind speed, air composition (density), terrain (such as valleys), atmospheric circulation, weather conditions, measurement errors and other factors. Therefore, it is necessary to perform filtering and estimation processing on the collected data.

[0043] Kalman filtering is a time domain filtering method that is described using the state space method. The algorithm uses a recursive form and has a small data storage volume. It can handle stationary random processes, multidimensional and non-stationary random processes. It is one of the most important optimal estimation theories and is widely used in various fields.

[0044] The dynamic system described by the Kalman filter state space model is shown in equations (1) and (2):

[0045] X(n+1)=ΦX(n)+ΓW(n) (1)

[0046] Z(n)=HX(n)+V(n) (2)

[0047] Formula (1) is the state equation, Formula (2) is the observation equation, n is the discrete time, and the pressure state of the system at time n is X(n)∈R n , Z(n)∈R m is the corresponding state observation signal, W(n)∈R r is process noise (white noise), V(n)∈R m Observation (measurement) noise, Φ is the state transfer matrix, Γ is the noise driving matrix, and H is the observation matrix.

[0048] Assumptions: W(n) and V(n) are uncorrelated white noises with mean 0 and variances Q and R respectively; the initial state X(0) is uncorrelated with W(n) and V(n), E[X(0)] = μ0, E[(X(0)-μ0)(X(0)-μ0) T ]=P0.

[0049] According to equation (1), equation (2) and the above assumptions, the recursive Kalman filter is obtained as follows:

[0050] The state is further predicted: X(n+1|n)=ΦX(n|n) (3)

[0051] One-step prediction covariance: P(n+1|n)=ΦP(n|n)Φ T +ΓQΓ T (4)

[0052] Filter gain matrix: K(n+1)=P(n+1|n)H T [HP(n+1|n)H T +R] -1 (5)

[0053] State update: ε(n+1)=Z(n+1)-HX(n+1|n) (6)

[0054] X(n+1|n+1)=X(n+1|n)+K(n+1)ε(n+1) (7)

[0055] Covariance matrix update: P(n+1|n+1)=[I n -K(n+1)H]P(n+1|n) (8)

[0056] Where X(0|0)=μ0, P(0|0)=P0.

[0057] In this algorithm, it is assumed that when the laser ranging sensor is working, it is affected by many external factors, and the variance of the process noise W(n) introduced is Q, and the size is Q=0.04; the noise V(n) measured by the laser ranging sensor for the nth time has a variance of R. The state X(n) is the distance value at time n. Since X(n) is a one-dimensional variable distance value, that is, the state transfer matrix Φ=1, the noise driving matrix Γ=1, and the observation matrix H=1, the state equation corresponding to the established Kalman filter state space model is shown in formula (9):

[0058] X(n+1)=ΦX(n)+ΓW(n)=X(n)+W(n) (9)

[0059] Assuming that the factory measurement error of the laser ranging sensor is ±0.6cm, it can be known that the variance R of the laser ranging sensor is 0.36, that is, the measurement data is not completely correct and there is a measurement error V(n). Then the observation equation corresponding to the Kalman filter state space model is shown in formula (10):

[0060] Z(n)=HX(n)+V(n)=X(n)+V(n) (10)

[0061] Assuming that we want to estimate the actual distance value at time n, we must first predict the distance value at time n based on the distance value at time n-1.

[0062] This design combines multiple sensors for measurement, fuses the laser ranging sensor, Beidou differential positioning and meter ruler measurement values ​​to take the average value, and performs Kalman filter estimation on the measurement values ​​to estimate the optimal measurement result. The multi-sensor fusion measurement has high accuracy, realizes miniaturization of equipment, is easy to install, has low maintenance cost and low power consumption.

[0063] Advantages of this design:

[0064] (1) Compared with the UAV laser radar technology and the manual tree climbing hanging ruler measurement method, the low-cost solution of using Beidou + laser rangefinder has a strong cost advantage, reduces the size of the equipment, and reduces the load on users;

[0065] (2) Using a three-system eight-frequency high-performance and high-precision carrier phase differential positioning module to achieve centimeter-level positioning accuracy, compared with the ordinary GPS meter-level positioning accuracy, the positioning accuracy can be improved by 2 orders of magnitude;

[0066] (3) Users can obtain accurate target height information without direct contact when measuring height in the field, helping users obtain height information of difficult-to-reach target points;

[0067] (4) The use of drones improves safety and reduces the difficulty of field work. It is particularly suitable for situations where it is difficult for humans to reach the target point, thus improving work efficiency;

[0068] (5) The laser distance measurement module uses a safe laser, which is generally considered safe for human eyes. Normal exposure to the beam of this laser will not cause permanent damage to the retina of the eye. The 100m / 200m laser distance measurement is accurate, fast and reliable. The measurement distance is 120m without a reflector and can reach 200m with a reflector.

[0069] (6) Low material cost, small size, low power consumption, and compatible with data acquisition.

[0070] Laser distance measurement is now widely used, from laser measurement of the moon and satellites to ordinary distance measurement for home use. Laser distance measurement has entered the homes of ordinary people. As a major design of mankind, laser is called "the most accurate ruler", "the fastest knife" and "the brightest light". The divergence of the laser beam is extremely small, only about 0.001 radians, and it diverges in one direction in a nearly parallel manner with almost no attenuation. We generally regard laser as parallel light. The use of laser distance measurement takes distance measurement technology a step further. Laser rangefinder uses laser to accurately measure the distance to the target (also known as laser rangefinder).

[0071] The total weight of the helical antenna is light, which greatly reduces the load burden. The antenna can be fixed by connecting the antenna connector to the signal connector of the terminal device, without the need for additional mounting holes. The antenna adopts a four-arm helical structure. Compared with traditional microstrip antennas, it has better low elevation gain and wide-angle circular polarization characteristics, and can receive more satellite signals at low elevation angles.

[0072] The beneficial effects of the present invention are as follows: the device combines three measurement sensors, namely a ruler, a Beidou differential positioning module and a laser rangefinder, to perform measurement, fuses the laser rangefinder, the Beidou RTK height and the ruler measurement values ​​to obtain an average value, performs Kalman filter estimation processing on the measurement values, estimates the optimal measurement result, and realizes the miniaturization of the equipment. It also has the advantages of easy installation, low maintenance cost, low equipment power consumption, high multi-sensor fusion measurement accuracy, etc. It solves the problems of large size, inconvenient installation and low measurement efficiency of existing tree height measurement devices, and realizes non-destructive measurement of famous and ancient trees.

[0073] The preferred specific embodiments of the present invention are described in detail above. It should be understood that a person skilled in the art can make many modifications and changes based on the concept of the present invention without creative work. Therefore, any technical solution that can be obtained by a person skilled in the art through logical analysis, reasoning or limited experiments based on the concept of the present invention on the basis of the prior art should be within the scope of protection determined by the claims.

Claims

1. A high-precision tree height measurement device based on Beidou multi-sensor fusion, characterized by: The system comprises a microprocessor, which is connected to a power module, a WIFI module, a differential positioning module, a communication module, a storage module, a digital level, a laser ranging module, a display screen and a drone. The drone is connected to a camera, the differential positioning module is also connected to a GNSS antenna, and the communication module is connected to a communication antenna and a SIM card.

2. The high-precision tree height measurement device based on Beidou multi-sensor fusion according to claim 1 is characterized in that: The differential positioning module is a carrier phase differential positioning module of three-system eight-frequency GPS: L1, L2, L5, BDS: B1, B2, B3, GLONASS: G1, G2, and the GNSS antenna is a three-system eight-frequency built-in helical antenna, including a low-noise amplifier LNA and a SAW surface acoustic filter circuit; The laser ranging module is used to obtain the ground distance measurement, and includes a laser transmitter and a laser receiver; The digital level is provided with two groups, including one set on the measuring device and the drone respectively, and the horizontal tilt monitoring device and the drone are sensed by the angle sensor. The drone is also connected to a reflector set to cooperate with the laser ranging module, and the reflector is connected to the bubble level.

3. The high-precision tree height measurement device based on Beidou multi-sensor fusion according to claim 1 is characterized in that: The camera is used to take pictures of the target, and the WIFI module transmits videos and pictures in real time, so that the three points of the camera crosshair center, the aiming fulcrum and the treetop are in line.

4. The high-precision tree height measurement device based on Beidou multi-sensor fusion according to claim 1 is characterized in that: The communication module is used to obtain local area CORS network differential correction information; The differential positioning module receives Beidou satellite navigation signals and Beidou CORS network differential correction information, and obtains local area CORS network differential correction information by receiving GNSS satellite navigation signals and communication module information; The differential positioning module first obtains standard single-point positioning, then obtains pseudo-range differential RTD positioning after obtaining differential correction information, then obtains RTK positioning with carrier phase integer ambiguity as a floating point solution, and finally obtains RTK positioning with carrier phase integer ambiguity as a fixed solution; The positioning accuracy is 10 meters, meters, decimeters and centimeters respectively.

5. The high-precision tree height measurement device based on Beidou multi-sensor fusion according to claim 4 is characterized in that: The differential correction information includes satellite ephemeris error, satellite ephemeris clock error, satellite orbit error, ionosphere, and troposphere.

6. A high-precision tree height measurement method, using the high-precision tree height measurement device based on Beidou multi-sensor fusion according to any one of claims 1 to 5, characterized in that: It includes using a drone to determine the horizontal position of tree roots through a camera, and hanging a meter ruler on the drone; The drone rises so that it is in the center of the reflector. The camera transmits video images to the device in real time. The direction and orientation of the drone are adjusted to a horizontal state at the treetop position, and the ruler is read.

7. A high-precision tree height measurement method, using the high-precision tree height measurement device based on Beidou multi-sensor fusion according to any one of claims 1 to 5, characterized in that: It includes measuring the horizontal position of the tree root once before the drone ascends, that is, the height h1 corresponding to the three points of the camera crosshair center, the aiming fulcrum, and the tree root are in a straight line. After arriving at the destination, when the three points of the camera crosshair center, the aiming fulcrum, and the treetop are in a straight line, the Beidou RTK high-precision positioning is started to measure the height h2. The difference between the two heights h2-h1 is the measured tree height.

8. A high-precision tree height measurement method, using the high-precision tree height measurement device based on Beidou multi-sensor fusion according to any one of claims 1 to 5, characterized in that: The reflector is used in conjunction with a bubble level. The reflector is placed horizontally and its position is as level as possible with the tree roots. The drone gradually rises. When the drone rises near the treetops, the drone is in a horizontal position. When the center point of the camera crosshairs, the alignment fulcrum, and the treetops are in a straight line, the laser is started to measure the distance to the ground. There is a reflector on the ground to cooperate with the laser distance measurement. When the reflector is stationary, the level of the reflector is measured by the bubble level. The laser emits a parallel light beam vertically to the ground, and the reflective surface is reflected. The reflected light beam is finally sent to the receiver, and the distance measurement value is obtained based on the time difference.

9. A high-precision tree height measurement method according to any one of claims 6 to 8, characterized in that: It includes using Beidou RTK to measure altitude information, and fusing laser ground ranging with Beidou altitude values. The microprocessor uses Kalman filtering to obtain a more accurate distance.

10. A high-precision tree height measurement method according to claim 9, characterized in that: The observation equation of the dynamic system space model of the Kalman filter is: Z(n)=HX(n)+V(n)=X(n)+V(n).

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