a communication path ground-reflection losses and polarisation-coupling losses. Pre- link. Here attention is concentrated on the weak signals which can dictions may be further extended by including estimates of the day be received for a large percentage of the time, say 99.99%. Sec- to day variability in signal intensity. Calculations are prohibitively ondly, it is necessary to consider the problem of interference caused lengthy without computing aids and a number of computerised by abnormal propagation and unusually strong, unwanted signals of prediction schemes have been produced (CCIR, 1990f; Teters, et. the same frequency as the wanted transmission. al., 1983). Nowadays such procedures are available for microcom- In both these aspects of propagation, the radio refractive index of puter evaluation. By means of estimates of background noise intens- the troposphere plays a dominant role. This parameter depends on ity, and from the known required signal/noise ratio, the mode the pressure, temperature and humidity of the atmosphere. Its verti- reliability may also be determined. This is the fraction of the days cal gradient and local fluctuations about the mean value determine that the signals are received with adequate strength. For some the mode of propagation in many important practical situations. systems involving fast data transmission, predictions of the prob- Hence the interest in the subject of radio meteorology, which seeks ability of multipath, with two or more modes of specified com- to relate tropospheric structure and radio-wave propagation. In most parable amplitude with propagation delays differing by less than ground-to-ground systems the height range 0-2km above the Earth's some defined limit, are also useful and can be made. surface is the important region, but in some aspects of Earth-space transmission, the meteorological structure at greater heights is also 9.6.2 Short-term predictions and real-time significant. channel sounding 9.7.1 Historical background Some limited success has been achiev