Propagation models for HF radio service planning

Control of usage of the radio spectrum is vested in the International Telecommunication Union and the Radiocommunication Sector of that organisation (ITU-R) is responsible for developing and maintaining propagation models to be used in service planning. For many years the ITU-R, and the former CCIR from which it has evolved, have provided a forum for HF propagation modelling, both in the development of new models and in the testing of existing models against measurement databases. Procedures have now been produced which augment monthly median estimates with day-to-day ionosphere, signal and background noise variability statistics to yield defined reliability and compatibility probabilities. Modern HF radio systems require additional information. Over-the-horizon radars, HF target location complexes and other remote-sensing facilities need a means of relating measured arrival angles and group-path delays to specific signal source positions. Knowledge of multi-mode time spread and Doppler dispersion is important to the design and operation of communications modems. Besides the range of likely statistics, methods are needed of determining the conditions on individual days, so that available operational parameters may be optimised. So, near real-time estimates of ionospheric conditions must be derived. This paper provides an overview of the current state of HF propagation predictions, highlights the principal problem areas, and offers suggestions for the ways ahead.