I. Measurement Errors
1. Laser Ranging Errors
The laser rangefinder is the most important core equipment of the LiDAR system, and laser ranging is affected by a variety of factors, mainly three categories: ① Observation errors caused by the rangefinder. Each working process of laser ranging will bring a certain amount of error, but the main role is played by the electronic optical circuitry after ground reflection and space propagation of irregular laser echo signal processing, estimation and time measurement errors, respectively, when the time-delayed estimation error and time measurement error of two types. ② Atmospheric refraction error. When the laser penetrates the atmosphere, like the GPS signal, it will be affected by the refraction error of the atmosphere (troposphere), and the degree of its influence depends on the wavelength of the laser pulse. (iii) Errors caused by ground targets. When the laser pulse signal is transmitted to the ground, different reflections are generated due to different physical characteristics of the ground surface. When the signal diffuse reflection occurs when a large number of reflected signals are received, will form a large reception noise; when the signal is launched to the smooth surface of the object will form a specular reflection, which may cause the laser ranging signal "lost";
2, DGPS positioning error
DGPS positioning error is the main factor that affects the accuracy of the laser footpoints. Accuracy of the main factors, GPS dynamic positioning error mainly includes satellite orbit error, satellite clock difference, receiver clock difference, multipath effect, antenna phase center instability, there are satellite constellations, observation noise, the whole week fuzzy degree of the solution is correct or not, etc., despite the GPS positioning error is more obvious, but it is with the observation of the environment changes and constantly changing, not easy to eliminate or model. In order to weaken the impact of GPS positioning errors, the method usually used is to establish a number of evenly distributed reference stations in the measurement area to ensure that the GPS dynamic positioning calculation is not too far from the reference 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 with the laser scanner, the attitudes of the two can be said to be identical. the accuracy of the IMU attitude measurement will be affected by the accelerometer scale error, the velocity meter constant error, random drift, gyro various system drift and other factors, and the accuracy of the attitude measurement is bound to affect the results of direct positioning. At present, in the domestic civil INS system accuracy level is: heading 0.1, side roll and pitch 0.05 °, using GPS/INS combination of the accuracy level of 0.03 °; foreign advanced GPS/INS combination of the accuracy level of 0.01 °, side roll and pitch 0.005 °.
4, scanning angle error
Scanning angle error refers to the installation, design and other reasons that make the scanning system axis direction deviates from the ideal state, so that the scanning angle of the starting angle of the angle is not zero, which is fixed, can be measured in the factory; the scanning motor of the non-uniform speed of rotation, as well as the vibration of the scanning mirror and so on, will also bring errors to the scanning angle; in addition, the existence of torque error also makes the actual scanning angle and the expected scanning angle is not the same as that of the scanning angle, so that the scanning angle will be different from the expected scanning angle. In addition, the existence of torque error also makes the actual scanning angle different from the expected scanning angle. These will bring errors to the calculation results.
Second, the hardware placement error
1, eccentricity error
Eccentricity error is the translation error between the coordinate system of each instrument. Since each device has a different coordinate system center, it is necessary to accurately determine the interrelationship of the position of each device after placement, and there will be a certain amount of error in the observed value. Generally, this error is eliminated during data solving and brings little effect. 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
Instrument placement error, mainly refers to the non-scanning state, due to the installation of the laser beam deviated from the underwater point of the system error flight error, pitch error, side roll error picture. In the airborne LiDAR system, the IMU and the laser scanner are tightly fixed, and the installation tries to ensure that each axis of the IMU and the laser scanner system are pointing accurately and parallel, but in fact, after the installation, there is a small angular difference between each axis of the IMU pointing and the laser scanner pointing, i.e., the eccentric angle, also known as the placement angle, and in the actual production, the violent vibration of the airplane when it is landing may result in the instrument's shift and interfere with the data. Therefore, it is necessary to study its formation mechanism, influence law, and make accurate compensation. The eccentricity angle must be checked and accurately determined in practical applications, and the value must be taken into account in various conversions in order to convert the attitude data recorded by the IMU into accurate external orientation elements that can be used in photogrammetric production, especially important in the high-precision application of LiDAR, which is a kind of direct positioning to the ground on board the aircraft.
3. Angle stepping error
Angle stepping error is the error generated by the angle recording device when recording the angle change, which is usually corrected at the factory.
4, torque error
If the scanning mirror as a rigid body, the rotation and swing due to inertia of the actual angle of rotation will inevitably be different from the expected (recording device recording value) angle, this is the torque error. With the elasticity of the scanning mirror axis of rotation and mechanical properties related to the edge of the scanning tape, the scanning mirror in the maximum acceleration, the actual mirror position and the encoder calculated the position of the slight difference; and in the center of the tape, there is no torque error, because at this time the acceleration is zero.
Third, data processing errors
1, time synchronization errors
Airborne LiDAR system consists of POS as well as laser scanning system, which are their own 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 ranging values of the laser emission are observed at the same moment, and if there is a time bias, or if this bias can not be precisely determined, it will result in an error in the point position. Moreover, this error is variable and increases with the rate of change of the relevant measurements. For example, when the airplane is flying smoothly, the effect of time deviation between ranging and attitude measurement is small, when the attitude angle generally remains constant or changes very little; while when the flight is not smooth, the time deviation will have a great impact on the measurement error of the laser point.
2, interpolation error
Interpolation error is due to the laser scanning ranging system and the POS system has a different data recording (sampling) frequency caused by. Generally speaking, the frequency of laser scanning ranging system is the highest, up to 150kHz; IMU is the second highest, about 200Hz; DGPS has the lowest frequency, only about 20Hz. Therefore, to get the position and attitude of each laser footpoint, the POS data must be interpolated. Obviously, this will bring interpolation error.
3. Coordinate conversion error
The data obtained by the airborne LiDAR system is based on the WGS-84 coordinate system. The purpose of the measurement is generally for engineering services, the need to convert the coordinates of the laser footpoints to the local coordinate system, and due to the influence of elevation anomalies, this process will also appear error, which is the coordinate conversion error.