HF propagation - What we know and what we need to know

Although recent technological developments have permitted frequencies above the HF band to be used in telecommunications, the HF spectrum has become saturated with users while performance requirements for HF systems have become more stringent. After assessing the state of knowledge concerning the ionosphere from the viewpoint of ionosphere-dependent telecommunications performance optimization, suggestions are made concerning areas where further research is required. An attempt is made throughout this discussion to closely relate the ionosphere and its structure to the pertinent aspects of ionospheric propagation systems.