Radiation And Transmission Characteristics Of Thz And Infrared Waves By Atmospheric Hydrometeors

With the development of radiation source, detective devices and imaging technique in Terahertz(THz) band, the THz propagation characteristics holds promises in fields of wideband communication, radar detection, electronic warfare, electromagnetic weapons. The radiation and propagation characteristics of THz signal in atmospheric hydrometeors are promising in the research of space environment, such as THz radar detection and communication. And it is the foundation of THz wave/ pulse scattering and transmission simulation models in battlefield environment.Based on the earth and atmospheric transmission and the transmission theory in the discrete random medium, the radiation and transmission properties of infrared/THz waves in atmospheric hydrometeors are studied in this thesis. The variation of complex refractive of water with frequencies in THz band is obtained. With the consideration of the pulse propagation theory in time-varying discrete random medium, the two-frequency mutual coherence function of the infrared/THz signal in different fogs are analyzed. The broadening, distortion and the delay of the THz pulses in the hydrometeors are contrasted. The delays of THz pulses in rain and the infrared Gaussian pulses in the fog/dust aerosols are obtained. Considering the single scattering approximation and multiple scattering theories, the attenuation characteristics in rain and the multiple scattering characteristics in fog are obtained in the infrared and THz bands. With respect to the distribution of droplet particle spectrum, the angle distributions of transmittance and reflectivity of infrared/THz signal, as well as the back-forward scattering intensity is obtained. The main contents of this thesis are shown as follows:1. The spectral characteristics and level transition of gas molecules are analyzed. The typical absorption spectra of atmospheric gas molecules are contrasted. It shows that the absorbing of moisture and oxygen to THz waves is unignorable. Since water(rain, fog, high clouds, and rain clouds) and ice are consisted in hydrometeors, it has great influence on THz waves. The complex refractive indexes of water are obtained from different models, and the variation with THz frequencies is compared with the data obtained from HITRAN libraries.2. The THz radiation characteristics of the atmospheric background are calculated. Theachievement benefits the researches on the control of the location and width of THz atmospheric transmittance windows in different temperature and visibility. Meanwhile it also promotes the application in propagation, communication, radar detection of THz waves.3. Based on the theories of pulse propagation in time-varying discrete random medium, the two-frequency mutual coherence functions of infrared/THz waves in fogs are analyzed. The amplitude and phase changes at different visibility and frequencies are compared. The broadening, distortion and delay of Gaussian pulses are obtained. Additionally, the time delays of Gaussian pulses in fog/dust aerosols and rains are analyzed. The results show that the pulses delay is related to the incident pulse wavelength, propagation distance and particle concentration, which benefits the research on the deformation correction of pulses through atmosphere hydrometeors.4. Based on the physical characteristics of rainfall, the extinction characteristics, scattering properties and phase function of a single spherical particle raindrop are focused on. The THz signal and infrared attenuation from different particle spectrum are compared with the ITU-R data. This has an important theoretical significance for the experimentation of the rain attenuation characteristics in THz band.5. Based on the physical characteristics of fog, the single and multiple scattering characteristics of infrared and THz signals in fog are detailed analyzed. The extinction and scattering characteristics of a single droplet particle are studied. When signals transmit through droplet particles swarm, the angle distribution of transmittance, reflectivity and back-forward scattering intensity is researched. These studies provide preferences for the improvement of existing communication system frequency selection. They also support the analysis of the influence of signal propagation in fog, and have a great influence on the acceleration of communication technology development and application.