Analysis method of airborne lidar data error source

Airborne lidar system is a complex multi-sensor integrated system, and its accuracy is affected by various components in the system, so the error sources of lidar system are many and complicated. Generally speaking, airborne lidar data, like other spatial data, have three kinds of errors: gross error, random error and systematic error.

I. Measurement error

1, laser ranging error

Laser rangefinder is the most important core equipment of laser radar system. The laser rangefinder is influenced by many factors, mainly including three kinds: ① Observation error caused by rangefinder. Every working process of laser ranging will bring some errors, but the main function is the errors caused by the processing, estimation and time measurement of irregular laser echo signals after ground reflection and space propagation by electronic light path, which can be divided into two categories: time delay estimation errors and time measurement errors. ② Atmospheric refraction error. When laser penetrates the atmosphere, it will be affected by atmospheric (troposphere) refraction error like GPS signal, and its influence degree depends on the wavelength of laser pulse. ③ Error caused by ground objects. When the laser pulse signal is emitted to the ground, different reflections will occur due to the different physical characteristics of the ground. 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 emitted to the surface of a smooth object, it will form a specular reflection, which may lead to the "loss" of the laser ranging signal;

2.DGPS positioning error

DGPS positioning error is the main factor that affects laser step accuracy. GPS dynamic positioning errors mainly include satellite orbit error, satellite clock error, receiver clock error, multipath effect and antenna phase center instability, as well as whether the solution of satellite constellation, observation noise and integer ambiguity is correct or not. Although the GPS positioning error is obvious, it is constantly changing with the change of observation environment, and it is not easy to eliminate or model. In order to weaken the influence of GPS positioning error, the common method is to establish several evenly distributed reference stations in the survey area to ensure that the GPS dynamic positioning calculation will not be too far away from the reference stations.

3. Attitude measurement error

Attitude measurement error is one of the factors that affect the positioning accuracy of airborne lidar system. In the airborne lidar system, the rigid IMU is connected with the laser scanner, and their postures can be said to be completely consistent. The accuracy of IMU attitude measurement will be affected by accelerometer proportional error, constant error of speedometer, random drift, gyro system drift and other factors, and its attitude measurement accuracy will inevitably affect the result of direct positioning. At present, the accuracy level of civil INS system in China is: yaw 0. 1, roll and pitch 0.05, and GPS/INS combined accuracy level 0.03; The advanced accuracy level of GPS/INS combination abroad is: yaw 0.0 1, roll and pitch 0.005.

4. Scanning angle error

Scanning angle error refers to the fact that the rotating shaft direction of the scanning system deviates from the ideal state due to installation, design and other reasons, so that the starting angle of the scanning angle is not zero. This angle is fixed and can be measured when leaving the factory. The uneven rotation of the scanning motor and the vibration of the scanning mirror will also bring errors to the scanning angle; In addition, the existence of torque error also makes the actual scanning angle different from the expected scanning angle. All these will bring errors to the calculation results.

Second, the hardware is misplaced.

1, eccentricity error

Eccentricity error is the translation error between coordinate systems of various instruments. Because the coordinate center of each device is different, it is necessary to accurately determine the position relationship of each device after placement, and the observed value will have certain errors. Generally speaking, this kind of error has been eliminated in data calculation, with little influence. Eccentricity error is mainly the measurement error of the distance from the antenna center of GPS receiver to the laser beam emission point on the scanning mirror.

2. Error of placement angle

The errors produced during the installation of the instrument mainly refer to the systematic error, yaw error, pitch error and rolling error pictures of the laser beam deviating from the low point of the machine due to the installation in the non-scanning state. In the airborne lidar system, the inertial measurement unit is closely and fixedly connected with the laser scanner. During installation, the axis of IMU should be completely parallel to the axis of laser scanning system. But in fact, there is a slight angle difference between the axis of IMU and the axis of laser scanner after installation, that is, eccentricity angle, also known as placement angle. In actual production, the violent vibration of aircraft landing may cause the displacement of instruments and interfere with data. Therefore, it is necessary to study its formation mechanism, influence law and make accurate compensation. In practical application, the eccentricity angle must be calibrated and accurately measured, and this value should be considered in various transformations, so that the attitude data recorded by IMU can be converted into accurate external orientation elements, which can be used in photogrammetry production, especially in airborne lidar, a high-precision application that directly locates the ground.

3. Angle step error

Angle stepping error is the error generated by angle recording device when recording angle change, and it is usually corrected at the factory.

4. Torque error

If the scanning mirror is regarded as a rigid body, the actual rotation angle will inevitably be different from the expected angle (recorded by the recording device) due to the inertia when rotating and swinging, which is the moment error. It is related to the elasticity and mechanical properties of the scanning mirror shaft. At the edge of the scanning band, the actual mirror position of the scanning mirror is slightly different from the calculated position of the encoder under the maximum acceleration. At the center of the belt, there is no torque error, because the acceleration is zero at this time.

Third, data processing errors.

1, time synchronization error

Airborne lidar system is composed of POS and laser scanning system, which are independent system equipment with different time recording devices, and these times are independent of each other. In order to determine the three-dimensional coordinates of the laser point, it is necessary to ensure that the position, attitude and ranging value of the laser emission are simultaneously observed values. If there is a time deviation, or this deviation cannot be accurately determined, it will cause an error at this point. Moreover, this error is variable and will increase with the increase of the change rate of related measurement. For example, when the plane flies smoothly, the time deviation between ranging and attitude measurement has little influence, and the attitude angle generally remains unchanged or changes little; When the flight is not smooth, the deviation of time will have a great influence on the measurement error of laser spot.

2. Interpolation error

Interpolation error is caused by different data recording (sampling) frequencies between laser scanning ranging system and POS system. Generally speaking, the frequency of laser scanning ranging system is the highest, which can reach150 khz; ; IMU is next, about 200Hz; The frequency of DGPS is the lowest, only about 20Hz. Therefore, in order to get the position and attitude of each laser foot point, it is necessary to interpolate the POS data. Obviously, this will bring interpolation error.

3. Coordinate conversion error

The data obtained by airborne lidar system is based on WGS-84 coordinate system. The purpose of measurement is generally to serve the project, and it is necessary to convert the coordinates of laser foot points into local coordinate system. However, due to the influence of height anomaly, there will be errors in this process, which is the coordinate conversion error.