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# 电子工程代写|通讯系统代写Communication System代考|ELE4606 Scintillation

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## 电子工程代写|通讯系统代写Communication System代考|Ionospheric scintillation

Fluctuations in amplitude, phase, and angle of arrival can occur in transionospheric propagation with changes in frequency, location, time of day, and levels of solar activity. Electron density fluctuations occurring within the auroral ovals at invariant latitudes above $55^{\circ}$ or below $-55^{\circ}$ and the equatorial region between $-20^{\circ}$ and $+20^{\circ}$ invariant latitude (see shaded areas in Figure 1.60) produce measurable scintillation at frequencies between $0.3$ and perhaps $10.0 \mathrm{GHz}$.

The invariant latitudes shown in Figure $1.60$ are for a height of $300 \mathrm{~km}$ (peak of the F2 layer, see Figure 1.29). Annual occurrence statistics for zenith paths are presented for $0.3 \mathrm{GHz}$ (Figure 1.61) and $1.5 \mathrm{GHz}$ (Figure 1.62). The standard deviation statistics summarized in Figure $1.61$ and Figure $1.62$ were collected at frequencies between $0.13$ and $0.4 \mathrm{GHz}$ from 1969 to 1972 during a maximum of the sun spot cycle. The statistics are keyed to the measurement sites, shown as isolated symbols in Figure 1.60. The standard deviation measurements were scaled in frequency by using the $\lambda^{1.5}$ frequency dependence for weak scintillation and the elevation angle adjustment factor model shown in Figure $1.63 .{ }^{29}$

## 电子工程代写|通讯系统代写Communication System代考|Tropospheric scintillation

Tropospheric scintillation. Tropospheric scintillation usually refers to fluctuations in amplitude, phase, or angle of arrival caused by variations in refractive index in the clear atmosphere. Scintillation on paths propagating through the lower atmosphere can also be caused by variations in attenuation or refractive index in clouds or rain (sometimes called wet scintillation), by variations in multipath interference on a moving line-of-sight, or any other process that can produce rapid variations in amplitude or phase. During clear sky conditions, scintillation is caused by turbulent fluctuations in the dry air density and water vapor content.

The time series of standard deviation in attenuation, $\sigma_\chi$ for tropospheric scintillation, and total attenuation for one day of observations at frequencies of $20.2$ and $27.5 \mathrm{GHz}$ are presented in Figure $1.65$ and Figure 1.66, respectively. The standard deviation estimates were calculated from the 60 1-sec average samples that were collected in $1 \mathrm{~min}$. The day included a rain attenuation event that caused a loss of signal at $27.5 \mathrm{GHz}$, attenuation by an earlier shower, attenuation by clouds and periods with clear sky. The clear sky scintillation was higher during the daytime hours (15:00 UT to 21:00 UT or 10:00 a.m. to 6:00 p.m. local time) than at night. The scintillation intensity, $\sigma_{x^{\prime}}$, was higher at $27.5 \mathrm{GHz}$ than at $20.2 \mathrm{GHz}$.

Scintillation can be generated by the diffraction of electromagnetic waves by phase variations produced by refractive index changes or by variations in amplitude caused by changes in specific attenuation along the propagation path. Diffraction by phase variations is a coherent process that affects the phase and amplitude of beacon measurements. Scintillation produced by variations in the specific attenuation affects both beacon measurements (amplitude and phase) and attenuation estimates derived from radiometer observations of changes in received power. Figure $1.67$ and Figure $1.68$ present the standard deviation of attenuation time series for the beacon receiver at $20.2 \mathrm{GHz}$ and $27.5 \mathrm{GHz}$, respectively, and for attenuation estimates derived from radiometer measurements at each frequency. The radiometers used the same antenna as the beacon receiver and an $80-\mathrm{MHz}$ bandwidth centered on the beacon carrier frequency. Scintillation caused by diffraction from phase variations (clear sky) does not cause an increase of scintillation intensity derived from radiometer observations but affects the scintillation on the beacon. The scintillation of attenuation derived from the radiometer shows the component of the fluctuation produced by variations in path attenuation due to water vapor changes, clouds, and rain.

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