【作者】 冀航；

【导师】 王宏远； 马泳；

【作者基本信息】 华中科技大学 ， 信息与通信工程， 2007， 博士

【摘要】 利用激光雷达实现水下目标探测,抑制海水的后向散射是核心问题。本文通过研究海水的光传输特性,解决了后向散射率的求取以及对后向散射体分布随机性的表述等问题,构建了以后向散射模型和目标反射模型为主的激光雷达系统的海水信道模型。该模型不论从时域还是从频域的角度分析,都与海水实际的光学特性相似,成为本文理论研究的工具。分析信道特性可知,目标反射信号可以保持探测信号的频率特性,而海水后向散射来自随机分布的散射体的反射,大量随机反射的叠加导致后向散射信号呈现出低频特性。基于该频域特性,本文提出了引入频域滤波法抑制后向散射,即在发射端将探测信号调制到高频、在接收端进行频域检测、利用带通滤波器滤除低频信号的方法。根据这一思想,设计了调制脉冲激光雷达系统的实现方案,并利用理论推导和仿真计算,详细分析了海水信道的信号传输过程和系统的信号处理方法,通过比较频域滤波前后的目标对比度,论证了方案的有效性。为使调制脉冲激光雷达系统达到最优设计,研究了系统参数与性能之间的关系,提出了发射脉冲能量、调制频率范围、调制指数、滤波带宽、飞行高度等系统参数的设计原则,设计过程中既考虑到参数对系统性能的影响,又要结合当前的技术水平或技术发展趋势,提出切合实际的设计参数,为实物研制或技术发展方向提供了参考性意见。另外研究了环境参数与系统性能之间的关系,归纳出调制脉冲激光雷达系统适用于在常见海水水质条件下进行浅水域探测。本文针对调制脉冲激光雷达系统区别于以往机载激光雷达的器件和技术,如对光脉冲的调制方法、具有快速响应时间和高灵敏度的光探测器、微波信号滤波和处理模块等,提供了一些可用于系统实现的器件、技术和设计方案,论证了方案的可实现性。经仿真论证,该系统对后向散射信号有18dB的抑制能力,在浅水域可以提高目标对比度10dB以上,验证了系统在常见水质的浅水域具有优于一般方法的探测效果。因此,该系统除了应用于水下探测外,还可以用于水下成像。其他各种后向散射和多次散射严重的信道,也可利用频域滤波法进行系统设计,实现目标识别。更多还原

【Abstract】 The principal task of the lidar system design for ocean exploration is to restrain the backscattering. By investigating the light transmission character in seawater, the backscattering rate and the expression of random distribution sources of backscattering are acquired, and the seawater channel model of the lidar system based on backscattering model and target reflection model is formed. This model is similar to the actual optic characters no matter on time domain or frequency domain. It is an important tool of theoretical research in this paper.As known from the analysis of the channel characters, reflection signal from the target can retain the frequency character of the detection signal. Due to the fact that the seawater backscattering comes from the reflection of the random distributed scattering sources, a large amount of random reflection signals overlap to each other so that the backscattering signal shows a low frequency character. Based on such frequency character, a method of using frequency filtering to restrain the backscattering signal is introduced in this paper. This is a method to modulate the detection signal to a high frequency at the transmitter, do frequency detection at the receiver, and use bandpass filter to cease the low frequency signal. Based on this algorithm, the fulfilling scheme of the modulated-pulse lidar system is proposed. Furthermore, by using theoretical deduction and simulating calculation, the signal transmission process in seawater channel and the signal processing ways of the system are fully analyzed. In the end, the validity of this method is illustrated by comparing the target contrast before and after the frequency filtering.In order to optimize the design of the modulated-pulse lidar system, the relation between system parameters and the performance are investigated, and the principle of system parameter design is introduced. System parameters such as transmitting pulse power, range of modulating frequency, modulation index, bandwidth of filter and height of fight are included. In the designing process, the influence of parameters to the system performance must be taken into consideration, and also practical designing parameters will be raised according to nowadays technique level or the developing trend, so that it can give reference opinions to the entity research or the technique developing direction. Besides, a conclusion that the modulated-pulse lidar system is suitable for shallow water exploration in ordinary seawater condition is made based on study of the relation between environment and system performance.Instruments and techniques of modulated-pulse lidar system such as modulation methods of laser pulse, laser detector with fast response time and high sensitivity, filtering and processing module of microwave signal are different compared with previous lidar system. In this paper several suitable instruments, techniques and design scheme for modulated-pulse lidar system are proposed according to these differences and the feasibility of this scheme is validated.The modulated-pulse lidar system has reduced backscattering signals by 18dB and enhanced the target contrast over 10dB in the shallow water in computer simulation, which shows that this system has better detection effect than ordinary algorithms in shallow water with common water quality parameters. The simulation result shows that this system can be applied in underwater imaging other than underwater detection. The frequency filtering method can also be applied to the system design for target identify in other channels with serious backscattering and multi-scattering.更多还原