What Causes Drone LiDAR Data Errors?
FDG-130X drone lidar system
UAV LiDAR is an integrated system of multiple sensors, so the mapping accuracy is jointly affected by various components in the system, so there are many reasons for the error of the LiDAR system. Generally speaking, UAV LiDAR data has three types of errors like other spatial data: measurement error, hardware placement error, and data processing error.
1. Laser ranging error
The laser rangefinder is the most important core equipment of the LiDAR system. The laser rangefinder is affected by many factors. There are three main categories: ① The observation error caused by the rangefinder. Each working process of laser ranging will bring certain errors, but the main role is the error caused by the electronic optical circuit's processing, estimation and time measurement of the irregular laser echo signal after ground reflection and space propagation , respectively, the time delay estimation error and the time measurement error. ② Atmospheric refraction error. As the laser penetrates the atmosphere, it is affected by atmospheric (tropospheric) refraction errors like GPS signals, the extent of which depends on the wavelength of the laser pulse. ③ Errors caused by object targets. When the laser pulse signal is transmitted to the ground, different reflections are produced due to the different physical characteristics of the surface. When the signal is diffusely reflected, a large number of reflected signals are received, which will form a large receiving noise; when the signal is transmitted to the surface of a smooth object, it will form a specular reflection, which may cause the "loss" of the laser ranging signal;
2. DGPS positioning error
The positioning error of DGPS is the main factor affecting the accuracy of the laser foothold. The dynamic positioning error of GPS mainly includes satellite orbit error, satellite clock error, receiver clock error, multipath effect, and unstable antenna phase center, as well as satellite constellation, observation Whether the solution of noise and whole-cycle ambiguity is correct or not, although the GPS positioning error is obvious, it changes constantly with the change of the observation environment, and it is not easy to eliminate or model. In order to weaken the influence of GPS positioning error, the usual method is to establish multiple base stations with relatively uniform distribution in the survey area to ensure that the GPS dynamic positioning calculation is not too far from the base station.
3. Attitude measurement error
Attitude measurement error is one of the factors affecting the positioning accuracy of airborne LiDAR systems. In the airborne LiDAR system, by connecting the rigid body IMU and the laser scanner, the attitudes of the two can be said to be exactly the same. The accuracy of IMU attitude measurement will be affected by factors such as accelerometer proportional error, speedometer constant error, random drift, and various system drifts of gyroscope. The accuracy of its attitude measurement will inevitably affect the result of direct positioning. At present, the accuracy level of domestic civil INS system is: yaw 0.1, roll and pitch 0.05°, the accuracy level of GPS/INS combination is 0.03°; the accuracy level of foreign advanced GPS/INS combination is: yaw 0.01 °, roll and pitch 0.005°.
4. Scanning angle error
The scanning angle error refers to the deviation of the rotation axis direction of the scanning system from the ideal state due to installation, design and other reasons, so that the starting angle of the scanning angle is not zero, which is fixed and can be measured at the factory; the non-uniform rotation of the scanning motor and the The vibration of the scanning mirror will also bring errors to the scanning angle; in addition, the existence of the torque error also makes the actual scanning angle different from the expected scanning angle. These will bring errors to the calculation results.
The hardware placement error
1. Eccentricity error
Eccentricity error is the translation error between each instrument coordinate system. Since each device has a different coordinate system center, it is necessary to accurately measure the relationship between the positions of each device after placement, and there will be certain errors in the observed values. Generally speaking, this kind of error is eliminated in the data calculation, and the impact is not large. The eccentricity error is mainly the measurement error of the distance from the center of the GPS receiver antenna to the point where the laser beam is emitted on the scanning mirror.
2. Placement angle error
The error generated when the instrument is installed mainly refers to the system error of the laser beam deviating from the lower point of the machine due to the installation in the non-scanning state. Yaw error, pitch error, roll error picture. In the airborne LiDAR system, the IMU and the laser scanner are tightly connected. During installation, try to ensure that the axes of the IMU and the laser scanner system point precisely and parallel to each other. However, after installation, the axes of the IMU point to the axes of the laser scanner. There is a slight angle difference between the points, namely the eccentric angle, also known as the placement angle. In actual production, the violent vibration of the aircraft when it lands may cause the displacement of the instrument and interfere with the data. Therefore, it is necessary to study its formation mechanism, influence law, and make accurate compensation. In practical applications, the eccentric angle must be calibrated, accurately measured, and taken into account in various conversions, so that the attitude data recorded by the IMU can be converted into accurate external orientation elements that can be used for photogrammetry production, especially in machine tools. It is especially important in high-precision applications such as direct-to-ground positioning such as LiDAR.
3. Angle step error
The angle step error is the error generated when the angle recording device records the angle change, and is generally corrected at the factory.
4. Torque error
If the scanning mirror is regarded as a rigid body, the actual angle of its rotation due to inertia during rotation and swing will inevitably be different from the expected (recorded value recorded by the recording device) angle, which is the torque error. It is related to the elasticity and mechanical properties of the rotation axis of the scanning mirror. At the edge of the scanning belt, when the scanning mirror is at the maximum acceleration, there is a slight difference between the actual mirror position and the calculated position of the encoder; and at the center of the belt, there is no torque error. , because the acceleration is zero at this time.
Data processing errors
1. Time synchronization error
The airborne LiDAR system consists of a POS and a laser scanning system, which are independent system equipment with different time recording devices, which are independent of each other. In order to determine the three-dimensional coordinates of a laser point, it is necessary to ensure that the position, attitude and distance measurement value of the laser emission are the observed values at the same time. If there is a time deviation, or the deviation cannot be accurately determined, it will cause point errors. And this error is variable and increases with the rate of change of the associated measurement. For example, when the aircraft is flying smoothly, the time deviation between ranging and attitude measurement has little effect. At this time, the attitude angle generally remains unchanged or changes very little; and when the flight is not stable, the time deviation will affect the laser point. The measurement error has a great impact.
2. Interpolation error
The interpolation error is caused by the different data recording (sampling) frequencies of the laser scanning ranging system and the POS system. Generally speaking, the frequency of the laser scanning ranging system is the highest, up to 150kHz; the IMU is next, about 200Hz; the frequency of DGPS is the lowest, only about 20Hz. Therefore, in order to obtain the position and attitude of each laser foot point, the POS data must be interpolated. Obviously, this will introduce interpolation errors.
3. Coordinate conversion error
The data obtained by the airborne LiDAR system is based on the WGS-84 coordinate system. The purpose of measurement is generally for engineering services, and it is necessary to convert the coordinates of the laser foot point into the local coordinate system. Due to the influence of elevation anomalies, errors will also occur in this process, which is the coordinate conversion error.
Above is the analysis of the reasons that may cause the drone lidar data error, hope it can be helpful to you.