IMPROVED SHORT-TERM PREDICTIONS OF F(0)F(2) USING GPS TIME-DELAY MEASUREMENTS

Reliable HF communications along short-, medium- and long-range paths require propagation assessment. Such assessment could be facilitated w ith the monitoring of ionospheric characteristics by continuously avai lable passive means, i.e., measurement of the total electron content ( TEC) using satellite-emitted signals without a need for burdening the electromagnetic spectrum. With ubiquitous Global Positioning System (G PS) providing instantaneous time delay, or equivalently, TEC, values w hen needed, an assessment of HF propagation conditions may be availabl e on a near-real-time basis. Both TEC and the peak electron density of the ionosphere, which determines the ordinary upper frequency limit ( f(o)F(2)) for HF sky wave vertical propagation, vary strongly with sol ar and geomagnetic parameters. Their ratio, the equivalent slab thickn ess, may vary to a lesser degree and hence be modeled with greater acc uracy. A slab thickness model combined with real-time TEC measurement anywhere on the globe may possibly yield an improved HF parameter pred iction algorithm. To test the efficacy of the hypothesis, one has to a scertain the correlation, as exhibited by the correlation coefficient, between the TEC daily variability about the monthly mean and the f(o) F(2) variability. To determine such correlation, a study compared Fara day TEC data as well as GPS-generated TEC data collected in Israel and with corresponding f(o)F(2) values obtained from vertical sounder mea surements near the appropriate subionospheric location in Cyprus. The analysis shows that for large percentages of the time, very good corre lation exists between TEC and f(o)F(2) short-term variations. The corr elation coefficient varies between 0.7 or better during winter and sum mer months to about 0.5-0.6 during equinox months. A study of the diur nal dependence of the correlation indicates that a better correlation exists during daytime than nighttime. There was no indication that the coefficient is dependent on geomagnetic activity or on protonospheric electron content during the period of this study.