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Abstruct-A simple method is described that accounts for tropospheric scatter using the parabolic equation portion of the radio physical optics (WO) model. RPO is a hybrid propagation model that combines ray-optics and parabolic-equation methods to assess realistic range-dependent tropospheric effects at frequencies from 100 MHz to 20 GHz. A semiempirical scatter model adds a random refractive-index fluctuation to the mean refractive-index value at each height considered by the parabolic equation method. The results of this scatter model are compared with another scatter model and with a few sample radio measurements.

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A comparison of 3, 9.6, and 18 GHz low-altitude overthe-horizon propagation, as influenced by the evaporation duct, is presented. Both theoretical and experimental results are given for a 35 km over-water path, where the transmitters were located about 5 m above mean sea level and the receivers were located either 4-5 m or 18-19 m above sea level. Results are presented in terms of the one-way propagation factor in decibels at each frequency. Long-term cumulative frequency distributions of calculated and observed propagation factors are presented. Particular attention is given to the relative performance of each radio frequency to investigate frequency-diversity improvements that may be available on such paths. In most cases presented, there is a close agreement of the theory and observations. It is concluded that substantial improvements in received signal levels are Likely to be achieved in most areas of the world by a suitable choice of two frequencies in the 10-20 GHz range.