Research On The Single-wavelength Airborne LiDAR Bathymetry Full-waveform Data Processing Algorithm And And Its Application

The acquisition of submarine topography is one of the key areas of hydrographic basic surveying and charting.It plays an important role in economic construction,maintenance of marine rights and interests,national defense and scientific construction.Airborne LiDAR bathymetry technology is an advanced submarine topography detection system and is one of the most important marine surveying and charting technologies.By applying the ALB technique,the data can be obtained from waveform measurements of the received LiDAR pulse.This method can provide rapid operation with an acceptable accuracy and a high-resolution.The airborne LiDAR bathymetry system includes GPS,IMU,laser scanners and digital cameras.It is an active remote sensing technology.The airborne LiDAR bathymetry technique uses a pulsed blue-green laser as a bathymetric sensor that can penetrate the water to obtain the bathymetric data according to the differential arrival times of the laser echo signals reflected by the water surface and the sea bottom.At present,the typical airborne LiDAR bathymetry system adopts a green/infrared dual-band laser configuration,in which the green wavelength laser is mainly used for obtaining bottom terrain information,and the infrared laser basically cannot penetrate the water but only for extracting the water surface elevation.The use of dual-band,high-power laser transmitters and multi-channel,high-sensitivity signal receivers has resulted in high hardware costs,which has limited the large-scale deployment of airborne LiDAR bathymetry technology to a large extent.At present,with the development of low cost and miniaturization of airborne LiDAR bathymetry systems,a single-band airborne LiDAR bathymetry system with low power and low signal-to-noise ratio has emerged.The energy of the laser pulses emitted by this type of equipment is usually low and can only penetrate shallow water of 10 to 15 meters,but the compromise in the maximum detectable depth results in a higher frequency of pulse emission.Under the identical flight conditions,the underwater laser spot density obtained by the single-segment airborne LiDAR bathymetry system can reach several times or even more than the dual-band system.On the other hand,the technical disadvantages of single-band hardware devices are also very obvious.Due to the lack of infrared wavelengths,single-segment airborne LiDAR bathymetry systems can only rely on green-wave lasers to obtain surface elevations.When the water depth is very shallow,it is difficult to detect the water surface and bottom reflection signals from the hybrid waveform,which seriously affects the accuracy of the water depth measurement.In addition,because the energy of the laser pulse emitted by the single-band hardware system is usually relatively low,the amplitude of the bottom reflection signal is weak and difficult to distinguish from the noise,resulting in a low echo detection rate,and the underwater laser point cloud is missing.Comparing with the discrete laser point cloud obtained by real-time detection by on-board signal processing hardware,post-processing of airborne LiDAR waveform data can significantly improve the ranging resolution,measurement accuracy,and echo detection rate of laser signal,which has become a hot area in LiDAR research in recent years.At present,all commercial and experimental single-segment airborne LiDAR bathymetry systems that have been successfully developed,they all have the capability of full-waveform digital recording,but the data processing algorithms of these systems are all confidential.However,most existing literatures still directly uses laser point cloud data from hardware systems to construct underwater digital elevation models.The research on LiDAR bathymetry full-waveform data processing is lacking.Based on the data characteristics and underwater laser propagation characteristics of the single-segment airborne LiDAR bathymetry system,this paper thoroughly studies on how to improve the near-shore bathymetry accuracy and data integrity of the single-band airborne LiDAR bathymetry system.A series of effective airborne LiDAR bathymetry full waveform data processing methods are proposed:(1)For the maximum detectable water depth of airborne LiDAR bathymetry system is mainly affected by the turbidity of water,taking the Northern South China sea as an example,we study the relationship between turbidity of water and the bathymetry performance of an airborne LiDAR bathymetry system,and presented an algorithm that can estimate the spatial distribution of maximum detectable water depth in the Northern South China sea by using the values of diffuse attenuation coefficient of sea water.Firstly,we studied the K_d(490)inversion algorithm in the experimental water region.Secondly,the relationship between the diffuse attenuation coefficient K_d(490)and K_d(532)was established by using the measured optical profile data in this region.Then,the relationship between the diffuse attenuation coefficient K_d(532)and the maximum detectable depth was summarized.Finally,the spatial distribution of K_d(532)and maximum detectable depth in the Northern South China sea were retrieved by using MODIS data.The results provide a reference for the time selection and flight scheme of LiDAR bathymetric operation in the Northern South China sea.(2)Under the conditions of large noise pollution,weak bottom return and large diffuse attenuation coefficient,it is very difficult to extract bottom signal from airborne LiDAR bathymetry full waveform data.Therefore,in order to retrieve high-precision bathymetry,it is a key method that denoising from the bathymetric full waveform data.In order to solve this question,we presented a new denoising method based on bathymetric full waveform data.First all,we used the Richardson-Lucy deconvolution method to process the full waveform LiDAR data.Then,excluding abnormal bottom return based on four times filter.At last,we retrieved the exact location of water surface and bottom.The experimental demonstrates that the proposed denoising algorithm can effectively remove the noise signal and retain the echo signal reflected from the bottom,so as to obtain a more accurate bathymetry value.(3)For the hardware performance of the single-segment airborne Lidar bathymetry system,the existing full-waveform data processing methods of the airborne LiDAR bathymetry system have been summarized and evaluated,the evaluation index and evalution procedure of the full waveform data processing algorithm of the LiDAR bathymetry systems have been constructed.The proposed nine methods were tested on a simulated dataset and a real dataset,with the focus being mainly on the performance of retrieving bottom response and water depths.We also investigated the influence of the parameter settings on the accuracy of the bathymetry estimates by using the Monte Carlo method.The influenced factors were analyzed that hardware parameters and observation conditions of the system to the bathymetry accuracy and data integrity.The applicable conditions of various types of full waveform processing algorithms have been summarized,and some waveform processing algorithms have been further improved and optimized.(4)An improved algorithm based on a mixture of Gaussian and quadrilateral functions is presented to process airborne bathymetric LiDAR waveforms.In the presented method,the LiDAR waveform is fitted to a combination of three functions:one Gaussian function for the water surface contribution,another Gaussian function for the water bottom contribution,and a new quadrilateral function to fit the water column contribution.The proposed method was tested on a simulated dataset and a real dataset,with the focus being mainly on the performance of retrieving bottom response and water depths.We also investigated the influence of the parameter settings on the accuracy of the bathymetry estimates.The results demonstrate that the improved quadrilateral fitting algorithm shows a superior performance in terms of low RMSE and a high detection rate in the water depth and magnitude retrieval,especially performed better performance in very shallow water(0–3 m)and deep water(>11 m).What's more,compared with the use of a triangular function or the existing quadrilateral function to fit the water column contribution,the presented method retrieved the least noise and the least number of unidentified waveforms,showed the best performance in fitting the return waveforms,and had consistent fitting goodness for all different water depths.