Research On Automatic Correction Technique Of Geometric Overlap Factor Of Lidar System

Lidar is an active optical remote sensing technology,which has been widely used in the fields of atmospheric remote sensing,climate and environmental monitoring in recent years.Mie scattering lidar,polarization lidar,Raman scattering lidar,differential absorption lidar,high spectral resolution lidar,Rayleigh scattering lidar,resonance fluorescence lidar,and Doppler lidar are the main types of atmospheric detection lidar forms,which play an increasingly important role in climate and environmental monitoring fields such as atmospheric temperature,humidity,aerosols,clouds,sand and haze.The geometric overlap factor is an important parameter index of atmospheric detection lidar,which seriously affects the detection performance of the lidar system.It can characterize the field of view matching between the laser transmitting subsystem and the telescope receiving subsystem.When the optical axis between the transmitting and receiving subsystems is parallel,the lidar can obtain high-performance and high-precision detection results,when the optical axis between the transmitting and receiving subsystems is not parallel,the lidar’s detection accuracy and detection capabilities are limited.Therefore,carrying out research on the correction technology of lidar geometric overlap factor is an important part of improving the detection performance of lidar.This paper is based on the non-coaxial lidar system to carry out the research on the automatic correction technology of the lidar geometric overlap factor to improve the lidar system performance.The paper first analyzes the influencing factors of the geometric overlap factor of the non-coaxial lidar,analyzes the influence of the four system parameters on the geometric overlap factor,which include the laser radar beam divergence angle,the telescope receiving field of view,the optical axis separation distance of the transmitting system and the receiving system,and the inclination of the optical axis of the transmitting system and the receiving system.Especially the distribution of the geometric overlap factor of the system when the optical axes of the transmitting system and the receiving system are parallel and non-parallel respectively.The influence of the change of the geometric overlap factor on the detection performance of the lidar system is simulated.On the basis of simulation analysis,the paper builds an automatic correction hardware system for the geometric overlap factor of the lidar,and uses the lidar signal intensity method to automatically determine the performance of the lidar system,and uses a combination of coarse scanning and fine scanning to adjust the inclination of the optical axis of the system.The lidar geometric overlap factor automatic correction hardware system not only includes the lidar transmitting and receiving subsystem,photoelectric monitoring and data acquisition subsystem,but also includes a two-dimensional tilt angle adjustment platform and a laser intensity correction subsystem.The laser intensity correction subsystem mainly eliminates the error source of the optical axis correction criterion caused by the laser pulse jitter,and the two-dimensional tilt angle adjustment platform uses a worm gear for transmission and torque conversion,and is driven by two two-phase stepping motors.The minimum inclination angle of 0.0025 mrad/step can be adjusted under the signal drive control of the controller SR2.The two-dimensional tilt angle adjustment platform allows the laser emission axis to be adjusted in two timings:coarse scanning and fine scanning.The coarse scanning adopts the circular scanning sequence,the step angle is adjusted to 0.4 mrad,and the fine scanning adopts S-shaped scanning and half-folding.The method scans two time sequences to increase the adjustment speed,and the step angle is adjusted to 0.02 mrad.The lidar geometric overlap factor self-correction software system is designed using the Lab VIEW platform and the Quartus Ⅱ platform respectively.The Lab VIEW platform mainly designs the atmospheric echo signal processing software and the optical axis tilt adjustment platform control software to realize the real-time acquisition,display and storage of lidar data.Manual/automatic control of two-dimensional tilt adjustment platform,Quartus Ⅱ platform realizes the logic control design of coarse scanning and fine scanning of laser emission optical axis.Based on the atmospheric echo signal intensity method,the software designed three best position criterion methods,including atmospheric echo signal integration,limit detection range,and signal-to-noise ratio,to realize the self-correction function of the lidar geometric overlap factor.After completing the construction of the lidar geometric overlap factor self-correction software and hardware system,the experimental verification of the lidar geometric overlap factor self-correction system was carried out.The experimental results show that the coarse scan of the two-dimensional inclination platform can be completed in 16 minutes and 1 seconds,and the best correction range is locked within the inclination range of 0.4 mrad×0.4 mrad;the fine scan can be completed in 7 minutes and 21 seconds and realized 0.0025 mrad inclination accuracy adjustment.The worm gear of the two-dimensional inclination platform can realize the self-locking of the system,maintain the stability of the laser radar emission axis for a long time,and provide a reliable system guarantee for the accurate retrieval of atmospheric state information.