| Monitoring of vegetation growth condition plays an important role for the research in ecosystem, and is closely related to environmental monitoring, biodiversity conservation, agriculture and forestry, etc. In the aspect of monitoring crops, in particular, effective use of nitrogen and to reduce the economic and environmental damage caused by excess of nitrogen application are of profound significance. With the advantages of being all-time, fast, real-time and the ability to monitor with high precision, remote sensing techniques have been well used in motoring of nitrogen stress of vegetation, especially crops.As an active remote sensing measure, LiDAR has unique edges in obtaining vertical distribution of vegetation compared with other existing remote sensing means. However, subject to the limit of a single wavelength, its application in monitoring vegetation growth condition has certain restrictions. Laser-induced fluorescence remote sensing, with a wealth of fluorescence spectral information, can be used to observe the vegetation growth condition, especially nitrogen stress. In addition, it has the ability of achieving early detection of nitrogen stress, thus having great application potentials.However, fluorescent signal is extremely weak and is seriously affected by external factors such as stray light, complex environment, fluorescent detection, etc. In comparison with steady-state fluorescence for vegetation detection, time-resolved fluorescence which has been widely used in the field of biochemistry can provide more spectral information, and can well reflect the change of fluorescence over time. Especially, as the intrinsic parameters of the fluorescence signal, fluorescence lifetime signals are less affected by external factors and have better stability and higher accuracy.Combining the advantages of time-resolved fluorescence spectroscopy and LiDAR, this paper proposes the establishment of a novel time-resolved fluorescence LiDAR and developed a prototype system. This system is designed to preserve the capability of LiDAR in detecting three dimensional-distribution characteristics of vegetation and the ability of time-resolved fluorescence spectroscopy detection in order to achieve precise and stable remote sensing detection of vegetation growth condition, especially nitrogen stress. For time-resolved fluorescence LiDAR, the novel active remote sensing technique, this paper carried out the following theoretical methods and technology researches:(1) For different nitrogen stress application of crops, paddy was chosen as typical crop in central China to be the experimental subject. Analysis of laser-induced fluorescence characteristics has been carried out. Through processing and spectrally analyzing the fluorescence data of different nitrogen stress levels, the optimal sensitive bands towards nitrogen stress were selected and used for reception for monitoring the condition of vegetation nitrogen stress with fluorescence spectral data. The above analysis can provide vital theoretical basis and guidance for the development of time-resolved fluorescence LiDAR.(2) The technological research of the key unit in time-resolved fluorescence LiDAR was conducted, in order to lay the technical foundation for the development of the prototype system. The emitting system employs the coaxial design of fluorescence excitation laser and the ranging laser beams, and implementing simultaneous three-dimensional detection of vertical distribution and fluorescence spectra by two-dimensional scanning. The receiving system uses grating and ICCD to receive weak fluorescence signals with high sensitivity. In addition, triggered by a third party and sampled by nonlinear multi-delay pulses, optimal timing control of time-resolved fluorescence LiDAR is realized.(3) Upon completion of the researches of the key cell technology of time-resolved fluorescence LiDAR, the integration and control researches on the prototype of time-resolved fluorescence LiDAR system were carried out. Additionally, based on the fluorescence signals detected by ICCD, the studies of processing time-resolved fluorescence spectral signals and intensity correction were conducted, mainly focusing on analyzing the influential factors of fluorescence spectrum signals during detection. Further study was carried out concerning removal of relative noises and wavelength calibration.(4) Finally, basing on the developed time-resolved fluorescence LiDAR system, vegetation remote sensing detection experiments were conducted to demonstrate the detecting potentials of time-resolved LiDAR in the vertical distribution of vegetation canopy and time-resolved fluorescence spectrum. Experimental results show that the time-resolved fluorescence LiDAR system not only displays the traditional advantage of LiDAR in three-dimensional detecting, but also the remote sensing detecting capability towards vegetation time-resolved fluorescence spectra. What's more, applied research has been carried out based on time-resolved fluorescence spectral data to obtain fluorescence lifetime, thus characterizing the state of vegetation stress more intuitively. Also, by remote sensing observation of paddy under different conditions of nitrogen stress, the relevance between fluorescence lifetime and paddy's state of nitrogen stress is proved, showing the huge application potential of time-resolved fluorescence LiDAR in vegetation stress conditions especially nitrogen stress. |
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