Improving foF2 Prediction for the Sunrise Transition Period

: During the 2- to 3-hr sunrise transition period, the relative change in foF2 is 20-60 percent per hour and is independent of solar activity. The change is largest in winter and smallest in summer. To prevent the skip distance from changing by more than 250 km, that is, to maintain continuous illumination of at least 50 percent of the barrier (nominal width 500 km), requires that the operating frequency be changed for every 6 percent change in foF2. Thus, in the case of an OTH system, the operating frequency has to be adjusted during the sunrise period every 6 to 20 min (seasonal dependence) for satisfactory performance. To accommodate such rapid changes, an empirical algorithm has been tested for improved foF2 predictions during sunrise. The prediction scheme is based on the availability of relevant (under the oblique raypath) real time ionospheric data.