KEYWORDS: Signal to noise ratio, Iterative methods, Hyperspectral imaging, Computer simulations, Chemical elements, Vegetation, Minerals, Hematite, Chemical species, Algorithms
Sparse unmixing has been proven to be an effective hyperspectral unmixing method. The row-sparsity model (using l2,0 norm to control the sparsity) has outperformed single-sparsity unmixing methods in many scenarios. However, to avoid the NP-hard problem, most algorithms adopt a convex relaxation strategy to solve the l2,0 norm at the expense of unmixing accuracy and sparsity. In addition, the row-sparsity model might cause aliasing artifacts on the boundaries. To solve these problems, a novel algorithm called two-step iterative row-sparsity hyperspectral unmixing via a low-rank constraint (TRSUnLR) is proposed. TRSUnLR introduces a row-hard-threshold function to solve the l2,0 norm directly. The low-rank constraint, which can make full use of the global structure of data, is imposed to alleviate the aliasing artifacts. The reweighting strategy is used to further enhance the sparsity. Then we adopt the two-step iterative method under the alternating direction method of multipliers framework to solve the proposed algorithm. Specifically, the current solution is computed by a linear combination of the solutions of two previous iterations. Simulated and real data experiments have proven that the proposed algorithm can obtain better unmixing results.
Repetition frequency laser used in precision strike system can transmit a series of laser pulse trains. The repetition frequency pulse transmitted by common repetition frequency laser device does not meet the high precision measurement and target indication requirements due to the existing of “relaxation oscillation” effect. Thus, a Laser Q modulation technology must be used for the laser pulse shaping. Q switch is formed by applying high voltage to the electro-optical crystal cut at a certain angle through electro-optical Q modulation technology. At present, the Q switch of the high repetition frequency laser system is easy to be damaged under the state of high repetition frequency. “Piezoelectric bell response” exists with the over-high voltage and that will affect the repetition frequency accuracy. Aiming at LGS crystal applied in the repetition frequency laser equipment, we carry out an experimental research in order to solve the technical problems, such as optical rotation effect, frequency stabilization driver, and develop LGS Q switch, driver module and repetition frequency laser system. The experiment results show that the Q switch applied in the high repetition frequency laser system is capable to produce the anti-damage and high repetition frequency effect.
Ranging distance was one of the most important index of laser rangefinders. However, it could be affected by the atmospheric turbulence. Effect on maximum distance critical value of laser rangefinders with atmospheric turbulence was discussed. Firstly, the atmospheric structure constant of refractive index (C2n) of specific area was monitoring and the variable regularity was analyzed. With the assumption that the target was the small target, the power of small target was calculated. Then, the receiving power of detector was obtained. Lastly, the maximum distance critical value with the same visibility and different turbulence intensity was got. When the refractive index (C2n) was 5×10-15m-2/3, 5×10-14 m-2/3, 5×10-13 m-2/3 and 5×10-12 m-2/3, the maximum distance critical value was 1900m, 1850m, 1850-1810m and 1810m. It could be generalized that the maximum distance critical value of laser rangefinders could be affected by the atmospheric turbulence.
The working processes of various APD (Avalanche photodiode)laser active detection equipment includes: laser emission, laser transmission in the atmosphere, and APD laser detection three parts. Among them, due to the influence of atmospheric turbulence on the laser, the intensity of laser on the APD detection surface randomly fluctuates over time, after the laser is transmitted through the turbulence atmosphere. The process of APD detecting the laser light reaching its receiving end face is accompanied by noise generation. Combined with the central limit theorem, the APD output current conforms to a Gaussian distribution. The CLAP software was used to simulate the propagation of the laser in the turbulent atmosphere, and the probability distribution of the intensity fluctuation over a limited area under various atmosphere fluctuation conditions was obtained. Using several distribution functions to fit the probability distribution of light intensity fluctuations, it was found that the Exponential Weibull distribution has a good fitting effect on simulation data. Based on the above research results, a detection model of the Exponential Weibull-Gaussian double stochastic process of APD is established. Using the obtained Exponential Weibull distribution parameters and the dual stochastic process model to simulate the APD detection process, the probability density curve of the ADP output current is obtained and compared with the fluctuation of the light intensity at the receiving end surface, thereby verifying the influence of the double stochastic process on the detection process.
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