Abstract. 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 sweptfrequency sounder constellation and by which communication nowcasts and short-term forecasts are developed to drive a dynamic frequency management system. IntroductionThere is a common perception that long-haul HF communications are intrinsically unreliable owing to the fact that the sky wave channel is a dispersive, birefringent, and dissipative medium. This perception has largely been based upon the experience obtained over the years prior to the advent of digital data communications and the development of modern digital signal processing (DSP) technologies and adaptive HF schemes. While the reality is distinctly different from perception in the modern era, designers of communication systems must still be mindful of the variabilities of the HF channel. The HF channel has a rich personality which is far from featureless. Even the benign channel exhibits a diurnal texture, and frequency management strategies must account for variations in the instantaneous propagating band-

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Since 1993, TCL/BR investigators have been examining the potential of frequency and path diversity in solving many of the problems which have longed plagued HF communication systems and have hindered its effectiveness. While our results have application to voice communication, our primary motivation is directed toward the improvement in reliability for conveyance of digital data. Paths include polar cap, auroral zone, high-latitude trough, as well as midlatitude channel environments. These paths have been selected to ascertain the relative correlation properties under both benign and pathological conditions, and the resultant data are being used to validate emerging real-time ionospheric and HF performance prediction models. These results have application for design and operation of highly reliable HFDL service for DoD applications in stressed environments and in various aeronautical-mobile and maritime applications. In this paper, we will outline the importance of path and frequency diversity toward the improvement in system availability. Emphasis will be given to results which have been obtained from an analysis of radio paths within the high latitude and arctic regions. These have been selected since the pathological behavior exhibited by the relevant skywave channels provides us with an abundant opportunity to test our several design hypotheses based upon application of diversity. We have developed estimates of performanFLfor an HF data link service based upon Chirpsounder records and HFDL modem specifications [ 11.

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