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10"

Fig. 7. Diurnal plots of F-region peak electron densities for two stations near the magnetic equator, Kodaikamal and Jicamarca.

The day-to-day variability in F-region parameters, apparently unrelated to any specific solar or magnetic event is perhaps the biggest challenge to ionospheric forecasters and the problem is particularly severe in low latitudes [Lakshmi, 1994], There have been some attempts to generate statistical pictures of ionospheric variability using a large volume of data [Aggarwalet al., 1979], but the problem persists. One consequence is that HF operators looking for higher reliability are forced to operate at frequencies much lower than the optimum working frequencies, thus crowding the lower part of the HF spectrum and also using higher powers due to higher ionospheric absorption and higher Atmospheric Radio Noise at lower frequencies. This variability, also known as ionospheric weather, is obviously a consequence of variable electric fields, thermospheric winds etc. and will be impossible to predict; dynamic adaptive systems may be the only solution with proper caution on spectrum management. In summary, HF has several advantages at low latitudes, in addition to being very simple and inexpensive. It is also particularly suitable to vast countries dominated by rural and scattered communities which abound in the tropical region.

5.3 Restrictions imposed by Ionosphere on Trans-Ionospheric propagation

The presence of the ionosphere in the propagation path between a satellite and the ground receiver gives rise to various effects that degrade the performance of the

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