Foreign object debris (FOD) on airport runways is an important factor affecting aircraft flight safety, and current FOD detection technologies all have obvious deficiencies. In this paper, an indoor near-infrared (NIR) hyperspectral image data acquisition system with a wavelength range of 900-1700nm was built. The 14 samples of 6 common FODs and airport concrete runways were divided into reference and test sample sets, and the atlas data were collected for two common application scenarios. Preprocessing was performed on the reference sample set of hyperspectral images and reference spectral curves were extracted for 7 types of samples. Six spectral matching algorithms based on spectral angle matching (SAM), spectral information divergence (SID), spectral correlation coefficient (SCC) and their combinations are used to classify pixels one by one. By comparing the classification map, overall accuracy (OA), average accuracy (AA), and Kappa coefficient, a NIR hyperspectral FOD detection method based on SAM-SID (threshold Sc=40 pixel) criterion is obtained. The proposed method obtained ideal classification maps for the test sample set, with OA, AA and Kappa coefficients reaching 92%, 82% and 0.82, respectively, thus achieving good validation.
The propagation characteristics, including beam propagation factor (i.e., M2-factor) and characteristic distances, of truncated Airy (TA) beams is studied in atmospheric turbulence. Based on the extended Huygens-Fresnel principle and the second moments of the Wigner distribution function (WDF), the analytical expressions of the M2-factor of the TA beam in a turbulent atmosphere are derived. By using Andrews spectrum as the atmospheric turbulence model, numerical examples of M2-factor, relative M2-factor, turbulent Rayleigh distance and turbulence distance are given. The results show that the M2-factor, which depends on the truncation factor and the initial first lobe width, decreases with the decrease of the structure constant of turbulence, and with the increase of the inner scale of turbulence. The relative M2-factor decreases with decreasing structure constant of turbulence and truncation factor and with increasing the inner scale. There exists an optimum initial first lobe width which corresponds the minimum of the M2-factor. The turbulent Rayleigh range increases with decreasing truncation factor and inner scale. The turbulence distance increases with decreasing truncation factor and increasing inner scale.
The beam wander properties of electromagnetic Gaussian Schell-model (EGSM) beam propagating in atmospheric turbulence are investigated based on the extended Huygens–Fresnel principle, the second-order moments of the Wigner distribution function (WDF) and the Andrews beam wander theory. The simplified integral formulae for the root-mean-square (rms) beam wander and the relative beam wander of EGSM beams in turbulence have been derived. Our results indicate that in a strong turbulence, the rms beam wander increases obviously with increasing inner scale, and the influence of inner scale of turbulence on the rms beam wander can not be ignored in strong turbulence. The evolution behaviors of the rms beam wander and relative beam wander in atmospheric turbulence are quite different which depend on the initial beam width, the transverse coherence width, the inner and outer scales of turbulence. Both the rms beam wander and relative beam wander can be effectively reduced by increasing the initial beam width and decreasing the transverse coherence width.
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.
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