This paper compares several methods for predicting the frequency of optimum transmission (FOT) during periods of time when the geomagnetic field and the ionosphere are disturbed. The predictions will be made with a relatively simple ionospheric model (PROPHET) updated in near-real time with information from an oblique-incidence sounder operating on a different (control) path. Current and previous methods used to predict the FOT are examined along with a new technique which employs additional sounder information from the control path. The use of multipath reduction factors and CCIR-recommended FOT factors, derived for use with monthly predictions during undisturbed periods, are also examined for the near-real-time application with the simple model in the presence of a disturbed ionosphere. Predictions made using each method are compared to measured FOT data scaled from oblique-incidence ionograms gathered during a 29-day test in the continental United States during the geomagnetically active summer of 1982. Finally, a comparison is made with data taken during a 1-week test in the continental United States during the fall of 1984 when the ionosphere and geomagnetic field were not disturbed (Ap ≤ 24).
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