Interference and SWF effects on HF skywave communications reliability

HF propagation prediction programs for skywave and, to a lesser extent, groundwave communication systems are widely used. Some means of including an allowance for the interference background in the procedures for estimating communications reliability is required. In the paper a simple procedure is proposed which combines the results from an HF skywave prediction program with those from a spectrum occupancy model. The reliability of an example HF skywave communication system is computed with and without the interference model extension. A second influence on the reliability of HF skywave systems is solar flares which give rise to shortwave fadeouts (SWFs) lasting typically 30 minutes. When HF radio systems are being designed to achieve very high reliabilities the sporadic outages due to SWFs may be a significant factor. In the paper, reported statistics of solar flare and SWF occurrence are evaluated. A rule-of-thumb is developed for estimating the proportion of time for which a system will be affected by SWFs at different phases of the sunspot cycle.