Browsing by Author "Haidara, Fatim M."

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    Characterization of tropospheric scintillations on Earth-space paths in the Ku and Ka frequency bands using the results from the Virginia Tech OLYMPUS experiment
    Haidara, Fatim M. (Virginia Tech, 1993-05-15)
    There has been increasing interest in the use of Very Small Aperture Terminals (VSATs) in satellite communication links operating in the Ku and Ka bands. Such systems offers the advantage of low signal margins and therefore, low cost. For the same reason, however, VSATs links can be strongly impaired by tropospheric scintillations. The VA Tech OLYMPUS propagation experiment which includes 12, 20, 30 GHz beacon receivers at an elevation angle of 14° provides us with valuable multi-frequency scintillation data. This dissertation focuses on the characterization of tropospheric scintillations on earth-space paths. Tropospheric scintillations are rapid fluctuations of signal caused by multiple scattering from the sma11 scale turbulent refractive index inhomogeneities in the troposphere. Analysis of the scintillation results from the VA Tech OLYMPUS experiment for both short term and long term studies is presented. The results include spectral characteristics, frequency scaling and statistics of the scintillation intensity, statistics of scintillation fading, diurnal and meteorological trends as well as a comparison with the current CCIR predictive model. Sma11 scale spatial diversity is a potential means to counteract tropospheric scintillation fading; it can be cost effective because of the low cost of VSATs. The VA Tech experiment includes 12, 20, 30 GHz beacon receivers and colocated 20 and 30 GHz diversity receivers that allows us to study this restoration technique. In this dissertation the effects of spatial diversity are studied through a cross-correlation analysis. Experimental results are compared to a theoretical model in which the atmosphere is characterized by an "anisotropic" Kolmogorov spectrum. A method to determine minimum vertical spacing between terminals to achieve good diversity performance is introduced.
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    Rain scatter interference in satellite links
    Haidara, Fatim M. (Virginia Tech, 1988-12-15)
    Scattering by rain in addition to inducing attenuation and depolarization in satellite links can create intersystem interference. This type of interference was investigated extensively in the 1970's for terrestrial links, but little has been done to study its effects on modern and future satellite links. This thesis reports on studies of the potential interference created by the scattering of an uplink signal into an adjacent satellite and the inverse case where a downlink signal is scattered into the earth station of an adjacent satellite system. The thesis describes the rain medium in a way which emphasizes the computation of the needed single-drop scattering coefficients using the extended boundary condition method (EBCM). The development of the experimental and theoretical study of rain scattering interference is then reviewed. Three computation methods for the interfering power are presented and compared. The first one, based on the radiative transfer equation, includes multiple scattering considerations, while the two other techniques take only the first order multiple scattering into account. The impact of such interference is then studied and an experiment involving the OLYMPUS and ACTS spacecraft is proposed.