Browsing by Author "Smith, William Travis"

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    Statistical modeling for land mobile satellite communications
    Smith, William Travis (Virginia Polytechnic Institute and State University, 1986)
    The Land Mobile Satellite System (LMSS) to be available in the 1990 time frame will provide connection between mobile vehicles and the conventional terrestrial communication network. The design is dependent on the propagation characteristics of the land mobile satellite signal. Unlike fixed satellite links, there is blockage in the line of sight path, mainly in the form of vegetative shadowing. The focus of this study is to develop models for the fading of the received satellite signal. A brief review of the physics and statistics associated with mobile propagation is presented. This is followed by a review of current literature and experiments. The modeling of the cumulative distribution function for a totally vegetatively shadowed mobile (VS distribution) is presented. The VS distribution is then used in a model for the cumulative distribution function of a partially shadowed mobile. The complete model for partially shadowed routes permits calculations for arbitrary combinations of open and forested terrain. Comparisons are made to data reported for partially shadowed routes. The deterministic path model (DPM) developed in an earlier effort is a geometrically based tool for determining the signal path length through a stand of trees. It is expanded to give approximate expressions for the statistical parameters describing the fading of the line-of-sight component of the received signal. New expressions for the secondary statistics of a totally vegetatively shadowed mobile are derived. These new expressions are then used in models for the level crossing rate and average fade duration of a partially shadowed mobile. Comparisons are made to data reported for partially shadowed routes.
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    A synthesis procedure for array feeds to improve radiation performance of large distorted reflector antennas
    Smith, William Travis (Virginia Tech, 1990)
    Surface errors on parabolic reflector antennas degrade the overall performance of the antenna. They cause amplitude and phase errors in the aperture field which lower the gain, raise the side lobes, and fill in the nulls. These are major problems in large ->space reflector antenna systems. F or example, future multiple beam antenna systems requiring spatial isolation to allow frequency reuse could be rendered useless if high side lobes are present. Space antenna structures are difficult to build. They must maintain a nearly perfect parabolic shape in a harsh environment while remaining lightweight. The restrictions on the structure become more severe as science and technology requirements demand electrically large antennas. Mechanically, there are technologies [4)r building antennas with adaptive surfaces that can compensate for many of the larger distortions caused by thermal and gravitational forces. However, as the frequency and size of the reflectors increase, the subtle surface errors become significant and degrade the overall radiation pattern. It is for this reason that another method must be used to further improve the radiation pattern. Electromagnetic compensation for surface errors in large apace reflector antennas has been the topic of several research studies. Most of these studies try to correct the focal plane fields of the reflector near the radiation pattern. The compensation is implemented by weighting the elements of an array feed. In most of the studies, a precise knowledge of the reflector surface is required. An alternative approach to electromagnetic compensation is presented in this study. The proposed technique uses pattern synthesis to compensate for the surface errors. It differs from previous methods in two major respects. The previous studies used global algorithms that try to correct the entire focal plane field near the focal point or the aperture plane field and, hence, modify the entire radiation pattern. The pattern synthesis approach uses a localized algorithm in which pattern corrections are directed specifically towards portions of the pattern requiring improvement. The second major difference is that the pattern synthesis technique does not require knowledge of the reflector surface, but instead uses radiation pattern data to perform the compensation.