Wavelet parameters (e.g., wavelet type, level of decomposition) affect the performance of the wavelet denoising algorithm in hyperspectral applications. Current studies select the best wavelet parameters for a single spectral curve by comparing similarity criteria such as spectral angle (SA). However, the method to find the best parameters for a spectral library that contains multiple spectra has not been studied. In this paper, a criterion named normalized spectral angle (NSA) is proposed. By comparing NSA, the best combination of parameters for a spectral library can be selected. Moreover, a fast algorithm based on threshold constraint and machine learning is developed to reduce the time of a full search. After several iterations of learning, the combination of parameters that constantly surpasses a threshold is selected. The experiments proved that by using the NSA criterion, the SA values decreased significantly, and the fast algorithm could save 80% time consumption, while the denoising performance was not obviously impaired.
The phase shift error (PSE) for DS detection is analyzed using frequency domain measure theory of DS. Two factors
lead to the difference among different pulse waves, and introduce PSE into DS. Firstly, the period, amplitude and the
base line of pulse wave are unstable. Secondly, the phases of pulse waves under different wavelengths are different. The
PSE of transmitted and reflected pulse waves are both discussed quantitatively. The results showed that the PSE is
correlated with the position of rectangular intercept window that intercepts pulses from the waves. It can be minimized
into about 10% by selecting the start points of the windows. Because of the sensor contact pressure, the shapes of
transmitted and reflected pulse waves are different. Thus the right intercept locations are at the start of ascending limb
and the dicroticpulse respectively according to minimum PSE rule.
Nowadays, the edge detecting technology based on Brillouin scattering signal has became the most advanced research of
lidar. This technology is widely applied to the areas of detecting atmosphere wind, space environment, and space objects
and so on. In this lidar system based on edge detecting technology, it is important to analyze measuring error. In order to
research measuring error, the space atmosphere channels are analyzed and the measuring error formula of detecting
Brillouin scattering frequency shift is educed. At the same time, Brillouin scattering echo signal received by the edge
detecting technology is estimated. A program is designed to simulate measuring error of detecting Brillouin scattering
frequency shift at different altitudes and with different Brillouin scattering frequency shift. From the simulation results,
we can find that in the ideal atmosphere conditions, the measuring error of Brillouin scattering frequency shift is less
than 20MHz at 100km height and less than 5MHz at 30km height in the atmosphere. Based on the results above, we can
draw a final conclusion: the edge detecting technology holds high signal-to-noise ratio and small measuring error.
To eliminate the communications blackout phenomenon of reentry vehicle during reentry, characters of the
communications channel of plasma sheath is studied. And based on the electron density and temperature of the sheath,
the transmission model of optical signal in plasma sheath is formed. Making use of the light transmission model in
plasma sheath to do simulation and calculation, the refraction and attenuation state can be got. It illustrates that the
influence of sheath to the transmission of optical signal is limited, and demonstrates the theoretical feasibility of light
communications in reentry communications.
In this paper, an improved semi-analytic Monte Carlo method is used to simulate the lidar received backscattering signals. The H-G function is used to approximate the scattering phase function of seawater, from which we can derive the scattering angle directly, and a modified H-G function is used to calculate the probability of the photons received by the receiver at each scattering point, which greatly improves the accuracy of the simulation. The simulation result shows that the different parameters of air-sea system of lidar, such as lidar’s field of view, attenuation coefficient and single scattering albedo of seawater, greatly influence the lidar received backscattering signal waveform. Multiple scattering is studied to explain these phenomena.
There have been many mature results about reflection and refraction characteristic on medium surface of Gaussian beam, which are verified by practical applications. But, those are limited to regular medium surface such as plane and sphere. When the medium surface is wavy, the reflection and refraction characteristic is greatly different comparing with regular one. In the paper, according to the statistical description of direction distribution on wavy surface by Cox, we have set up a physical model of reflection of laser beam on wavy surface, derived that a beam reflected by wavy surface is also a Gaussian beam when the incident beam is a Gaussian beam, and set up the relationship between Gaussian beam’s light spot size and wind speed over sea surface. According to the wave model on the water surface, the returned laser power expression for the airborne laser bathymetry is derived from. The influence of the wavy water surface, the field of view for the IR received system on returned laser power is discussed.