Introduces information on time variations of HF congestion, and proposes a short-term forecasting method. The work is based on continuous records of the HF spectrum, within chosen frequency bands, over 15-day periods. Using this data, the authors have studied the probability that a frequency band will be unoccupied for J consecutive days, at a given time, and at a specified threshold level. Such a function is the complement of the congestion, and is referred to as 'clearness'. It gives a measure of the availability of a channel.
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