A FOT Prediction Procedure for HF Communications Frequency Management

Use of the ionospheric sounder as an aid in HF frequency management has proved to be more successful than the use of ionospheric models, but the disadvantages of cost, airwave pollution, and cumbersomeness of operation prevent its direct, routine use on most communications paths. Instead, we believe that results nearly as good can be obtained with models which are updated by "recent" sounder measurements made on a "nearby" path. The exact meanings of "recent" and "nearby" depend, respectively, on the temporal and spatial correlation of ionospheric properties over long periods of time and over large geographical regions. We describe here the measurement of correlation of Maximum Observable Frequencies, and show how the results may be used to predict the Frequency of Optimum Transmission (FOT) on a path distant from the sounded path.