ed in the short-term prediction of the ionospheric characteristics used to give the parameters of the Although some experiments on ultra-short-wave techniques were ionospheric models needed for h.f. performance assessment. carried out by Hertz and others more than 90 years ago, it was only Schemes are based either on spatial or temporal extrapolation of after about 1930 that any systematic investigations of tropospheric near real-time data or on correlations with magnetic activity indices. propagation commenced. For a long time it was widely believed that Regression statistics have been produced for the change in the at frequencies above about 30MHz transmission beyond the geome- maximum plasma frequency of the F2-region (foF2) with local tric horizon would be impossible. However, this view was disputed magnetic activity index K, and other work is concerned with pro- by Marconi as early as 1932. He demonstrated that, even with ducing joint correlations with K and with solar flux. relatively low transmitter powers, reception over distances several In principle at h.f. the most reliable, although costly, way of times the optical range was possible. Nevertheless, theoreticians ensuring satisfactory propagation over a given path and of optimis- continued for several years to concentrate on studies of diffraction ing the choice of transmission frequency involves using an oblique- of ultra-short waves around the Earth's surface. However, their incidence sounder over the actual path; in practice, however, results were found to over-estimate the rate of attenuation beyond sounder systems are difficult to deploy operationally, require expert the horizon. To correct for this disparity, the effect of refraction was interpretation of their data, lead to appreciable spectrum pollution, allowed for by assuming a process of diffraction around an Earth 9/12 The lonosphere and the Troposphere with an effective radius of 4/3 times the actual value. In addition, In addition, there has been a recent revival of interest in the effect