Damage to the skin coating of the Cessna 172R aircraft is an unavoidable and significant issue due to long flight training. Traditional detection methods are easy to missed and false detections, and hyperspectral technology can significantly improve. The spectral curves of the damaged and undamaged skin coating pixels in the near-infrared band (900-1700 nm) of the Cessna 172R aircraft skin samples were used to establish three spectral indices: ASCI-I, ASCI-II, and ASCI-III. Then the decision tree is used to perform recognition experiments on the two types of skin samples. The experimental results show that the decision tree model based on ASCI-I has the best recognition performance. Its global image recognition results are closely related to the spatial distribution of the actual targets. The Producer accuracy, User accuracy, and overall classification accuracy are all over 90%, and the Kappa coefficient is greater than 0.88.
At present, there are few studies on nondestructive testing of aircraft surface based on hyperspectral imaging at home and abroad. Therefore, an indoor near infrared (NIR) hyperspectral damage detection system with a spectral resolution of 5nm was established, and the paint damage on the sample surface was identified. The reflectance calibration, average reflectance calculation and principal component analysis (PCA) dimensionality reduction were performed on the collected hyperspectral data. On this basis, the unsupervised classification iterative self-organizing Data analysis algorithm (ISODATA) is used to identify the damaged samples. The results show that the spectral curves of the damaged and undamaged pixels of the sample are significantly different at about 910nm. The first 10 principal components selected can contain 97% of the sample data information, which can realize the effective identification of damage samples based on ISODATA. In this study, paint damage was taken as an experimental sample to verify the feasibility of using near-infrared hyperspectral imaging technology for damage identification. In addition, preliminary outfield experiment results also show that it is feasible to apply this technology to aircraft surface damage detection.
For the two-point spherical wave structure function, we propose a modified quadratic approximation, which can be used to investigate the second-order coherence properties (such as the beam spreading, average intensity, cross-spectral density function) of partially coherent beams through the turbulent atmosphere. We prove that the modified quadratic approximation, different from usual one, can be used to study the effects of inner and outer scales of turbulence, and is better than the usual one for a less coherent beam or for a very strong turbulence. A more accurately analytical expression for average intensity of Gaussian-Schell model beams is derived based on the modified quadratic approximation. These results are also illustrated by investigating the average intensity for Gaussian-Schell model beams in turbulence.
Based on the carbon dioxide mass mixing ratio concentration profiles of Hefei Science Island from 2013 to 2016, the distribution characteristics of carbon dioxide in the subtropical monsoon climate and the characteristics of carbon dioxide source and sink in Hefei Science Island were analyzed from the night, season and year respectively. At lower altitudes, CO2 builds up during nighttime hours as respiration is trapped within the nocturnal boundary layer. Throughout this nighttime the CO2 concentration at 390m shows that only little variation. After sunrise, convection begins to spread the CO2 trapped at low levels to higher altitudes. When the measurement height was more than 100m, the obvious seasonal change was shown, with the minimum in summer and the maximum in winter, which the difference in concentration of about 19.32mg/m3 . The annual distribution of the concentration of carbon dioxide from 2013 to 2016 had no significant difference in the gradient change of the height above 100m, the correlation coefficient was above 0.9, and the CO2 vertical concentration goes up with a rate of about 4.35mg/m3 per year. Through data analysis from three different timescales, it could be concluded that the concentration of carbon dioxide near the ground was greatly influenced by the atmospheric environment. The seasonal distribution of carbon dioxide concentration was the result of a combination of atmospheric movement and the activities of plants and animals. In the process of long-term carbon dioxide cycling, there was a current that the near-surface carbon dioxide transported to a higher altitude.
The aerosols near the ground are closely related to human health and climate change, the study on which has important significance. As we all know, the aerosol is inhomogeneous at different altitudes, of which the phase function is also different. In order to simplify the retrieval algorithm, it is usually assumed that the aerosol is uniform at different altitudes, which will bring measurement error. In this work, an experimental approach is demonstrated to measure the scattering phase function of atmospheric aerosol particles at different heights by CCD lidar system, which could solve the problem of the traditional CCD lidar system in assumption of phase function. The phase functions obtained by the new experimental approach are used to retrieve the aerosol extinction coefficient profiles. By comparison of the aerosol extinction coefficient retrieved by Mie-scattering aerosol lidar and CCD lidar at night, the reliability of new experimental approach is verified.
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