| Spaceborne lidar is an active remote sensing and mapping equipment that can directly obtain three-dimensional coordinates of ground targets with an accuracy of 10 cm at an altitude of hundreds kilometers,and its data products have been widely used in the polar,ocean,and forestry fields.The spaceborne photon counting(PC)Lidar utlizes a single photon detector(SPD)as the detection device,on the basis of the traditional linear Lidar system,which has extremely high sensitivity,enabling the realization of the technical solution of high repetition frequency,micro pulse and multibeams.The ICESat-2(Ice,Cloud and Elevation Satellite-2)boarded with the ATLAS(Advanced Topographic Laser Altimeter System)is currently the only spaceborne PC Lidar and its capacity of obtaining high-precision ground elevations has expanded the implement of satellite laser altimetry technology in monitoring the ice sheet elevation changes,retrieving the tree height,measuring the average sea level,shallow sea topographic mapping and many othermilitary applications.Researches on full-link simulation for spaceborne PC Lidar applications,which includes the single-photon detection theory,ranging accuracy model and the evalution method of the system performance,are of vital importance to the development of domestic spaceborne PC Lidar system.The specific research content of the thesis is as follows:(1)Establishing a complete detection theory of PC Li DAR: the statistical law of spatial and time distribution of the receiving signal photons and the physical responding process of the SPDs are the focus of theoretical research in this paper.Firstly,the suitable statistical reflection paterns of different types of target are derived according to the established target reflection models;then,the response models suitable for nonextended and extended dead time SPDs,as well as their detection theory,were established respectively,according to their different physical detection mechanism.Finally,by taking the multi-channel SPD used on ICESat-2 as the research object,the detection theory was further extended to the scenarios where multi-channel/array detectors were applied.(2)Quantitative evaluation method for PC Lidar point cloud data: Because the point cloud data measured by PC Lidar has strong random characteristics,a set of evaluation methods were proposed,which makes it possible to cross-validate different data products and applies the simulation results to evaluate the top-level design of the system quantitatively.(3)Establishing a full-link simulation model of the spaceborne PC Lidar: Based on surface elevation measuring principle of spaceborne photon counting laser radar,geometric simulation model and radiation simulation model are set up,the former focuses on making the simulation have the ability to analyze the measurement accuracy of the system,which is based on a complete set of coordinate system,set up from the foot to the laser ranging results of three-dimensional coordinates calculating theory model of the whole process.Meanwhile,it can analyze and simulate the errors caused by hardware system and atmospheric refraction in the measurement process,and realize the quantitative evaluation and analysis of the horizontal positioning accuracy and elevation accuracy of the measurement results of the input system design scheme;the latter focuses on the simulation of the reflected signal energy of different ground objects.Based on the geometry and reflection characteristics of the target,the model simulates the echo waveform obtained after the response function of the target system,and simulates the photon events output by the single photon detector with the response model of the detector.The two theoretical models are coupled by the distance measurement value,and the simulated photon events and their three-dimensional coordinates are output finally,which realizes the modeling of the spaceborne photon counting laser radar full link simulation.Finally,according to the actual requirements of the researching insititution for scheme demonstration and load performance evaluation,the simulation software is programmed and realized based on the full-link simulation model.(4)Full link simulation validation and analysis of the influence of detector on the detection performance of the system: Based on the evaluation indexes of photon point cloud data noise rate,average signal photon event number,point cloud SNR and elevation accuracy,the full-link simulation model established in this paper is verified from two aspects of radiation characteristics and accuracy through the measured data of ATLAS system.On this basis,the detection results of two single photon detectors,GM-APD and PMT,in different terrain environments are used,meanwhile,the recognition and accuracy of the system detection results are quantitatively analyzed by the SNR and the elevation accuracy of point cloud,which provides a reference for the design and optimization of the spaceborne photon counting lidar detection system in China in the future.(5)Signal processing with low signal-to-noise ratio: The minimum signal-to-noise ratio(SNR)required to extract effective signals from the measurement data of spaceborne photon-counting lidar system is further studied.Taking ICESat-2 point cloud data(measured by weak beam in mountain area)as test data,combined with the detection theory of photon counting lidar,the signal extraction algorithm under the condition of extremely low SNR is studied.When the SNR is lower than 4d B,the precision and recall ratio of signal photons are both better than 90%.Furthermore,in the case of no signal extraction,only use the difference of noise rate of point cloud data to achieve ground object classification.Without relying on optical image,the overall accuracy of point cloud data classification for snow and bare ground is better than 90%. |
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