The Study On Modeling And Simulation, Information Processing Algorithm Of Differential Absorption Lidar

With the development of modern industry and science and technology, atmosphere environmental protection has already aroused people's broad attention. At the same time, faced to the risk of chemical toxicant used in battlefield never stopped despite numerous bans, it is important both in civil and military area to establish a long-range real-time monitoring system to .measure the kind, concentration, distribution and so on of polluted gas and chemical toxicant in atmosphere. Differential absorption lidar (DIAL) has proved to be a well-established technology for remote sensing of trace gases in the atmosphere. It not only can be used in civil field such as environmental monitoring, measuring the concentration of all kinds of atmospheric gas and establishing relevant database to analyze and predict the weather, but also can used in military field such as measuring battlefield chemical agents in real time, quantitatively researching the effect of chemical weapon blast and so on.Through the research on modeling and simulation of DIAL and establishing the system model, it is convenient and direct to evaluate and predict the performance of DIAL under various conditions of environment and system. It can simulate and estimate the main characteristic information of DIAL realistically too. This research can help to design the scheme, evaluate the system and improve performance in DIAL system. It is an economical and effective science means. In this dissertation, we model and simulate the far-infrared direct detection long-range DIAL. Based on the simulated return signal of DIAL, the information processing algorithms of discriminating and estimating gas and extracting the weak return pulse signal are researched. The results of research show that the simulation model is more accorded with actual instance, the information processing algorithms can estimate the gas concentration and extract the weak return pulse signal buried in noises more accurately.The main contribution in this dissertation is given below:1. The return signal statistical model of DIAL is the basis of modeling and simulation of DIAL. However, the known statistical models do not consider the effect of laser scintillation resulting from atmospheric turbulence on return signal. Considered the effects of laser scintillation resulting from atmospheric turbulence, target reflective speckle, receiving and detecting noise and the fluctuation of transmitted power, a modified return signal statistical model is presented and its statistical characteristics are analyzed. Moreover, based on the modified return signal statistical model, two factors which have effects on measurement sensitivity: laser scintillation resulting from atmospheric turbulence and laser speckle are analyzed.2. For the traditional two-wavelength DIAL, in order to estimate the concentration of target gas accurately and in real time, multi-resolution Kalman filtering algorithm is firstly applied in DIAL to estimate the gas concentration. According to this algorithm, an improved multi-resolution Kalman filtering algorithm for gas concentration estimating is presented, which uses the stationary wavelet transform (SWT) as a means for mapping data between different resolution levels. Under the same environment and system condition, this algorithm can estimate the gas concentration more accurately and in real time.3. Multiwavelength DIAL offers the potential ability for simultaneously detect and estimate multiple gas materials. An algorithm is presented for simultaneously detecting and estimating multiple gas materials with multiwavelength DIAL, which based on the maximum likelihood estimation methodology. Only known the return signal of each wavelength and the absorption spectra of the target vapors, this algorithm can discriminate various gas materials from mixtures and estimate their concentrations. Its estimated performance is quite well.4. An effective noise reduction algorithm based on the wavelet domain spatial filtration is presented, which is specially suited for processing sharp edge signal and pulse signal. This algorithm is applied in the differential absorption lidar to extract the weak return pulse signal. It can not only extract the weak return pulse signal buried in noises effectively, but also identify the edge of the return pulse signal. Therefore, this algorithm can measure the range of target accurately, and extract return signal buried in noise better.5. Based on the model of DIAL system established, applied the modified return signal statistical model and some information processing algorithms of discriminating target gases and estimating their concentration, a DIAL simulated system is established. It can simulate both traditional two-wavelength DIAL and multiwavelength DIAL and generate the return signals of various simulated environment and system conditions visually and calculate their SNR value. Carry out the information processing on generated return signals, one can obtain information such as gas kind, concentration and distribution and verify the information processing algorithms visually. This simulated system has provided the advanced science implement for scheme design, system evaluation and improved performance of DIAL, provided the laboratory environment for algorithmic research of information processing. By now, the DIAL simulated system combined with information processing algorithms has not been reported.