A study of HF communication link and network availability has been carried out using frequency modulated continuous wave (FMCW) swept-frequency sounders operating in an oblique-incidence configuration. The sounder deployment enabled 28 middle- and high-latitude paths to be evaluated; the time period of the study was from December 1994 until the summer of 1996. Propagation conditions including ionospheric mode information, maximum observable frequencies, signal-to-noise ratios, and channel availabilities for digital data communication were derived and archived. The objective of the study was to ascertain the efficacy of HF data link communication for a proposed aeronautical-mobile service. Data were used to simulate the propagation environment which would be experienced in actual operation and to evaluate the value of path and frequency diversity in overcoming various propagation effects. It has been concluded that path and frequency diversity will lead to channel availabilities approaching nearly 100% and that a practical engineering solution was possible. These positive results are achievable if and only if dynamic frequency management methods are invoked. The paper outlines the nature of a real-time system, which is based upon a terrestrial FMCW swept-frequency sounder constellation and by which communication nowcasts and short-term forecasts are developed to drive a dynamic frequency management system.
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