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The Dutton-Dougherty (DD) model for predicting microwave attenuation distribution on an annual basis on earth-space telecommunications links was first developed at the Institute for Telecommunication Sciences in 1973. The version for making predictions throughout the United States, with year-to-year variability allowance, was developed in 1977 and had been essentially unmodified until the present improvement efforts were undertaken. The refinements of the Dutton-Dougherty model discussed in this paper include an extension of the attenuation distribution prediction range to 0.001% of a year, whereas it had previously extended only to 0.01% of a year. This extension is accomplished in two alternative ways: empirically and analytically. Both extension procedures give nearly identical prediction results. An analysis is conducted of the pertinency of ‘effective path length’ models for evaluating rain attenuation at microwave frequencies to the DD model. It is determined that whereas effective path length methodologies may be useful in their own right, it would be difficult and unwieldy to incorporate this concept into the DD model, at present.