troposphere 9/11 and give much redundant information. Some schemes involve low- March, 1958 Longitude UT 00 West East power channel monitoring of the phase-path stability on each auth- 15° 0° 15° 45° 60' orised frequency, to ensure that at all times the best available is used. Real-time sounding on one path can aid performance predictions for another. Examples include ray tracing through mean ionospheric models simulated from measured backscatter or oblique-incidence soundings. Many engineers operating established radio circuits prefer, for frequency management, to rely on past experience, rather than to use predictions. This is not SO readily possible for mobile applications. Real-time sounding schemes involving ground trans- missions on a range of frequencies to an aircraft, but only single-fre- quency transmission in the reverse direction, have proved successful (Stevens, 1968). 9.7 The troposphere 45°60' The influence of the lower atmosphere, or troposphere, on the propagation of radio waves is important, in several respects. At all frequencies above about 30MHz refraction and scattering caused by local changes in atmospheric structure become significant, espe- cially in propagation beyond the normal horizon. In addition, at frequencies above about 5GHz, absorption in oxygen and water Figure 9.4 Predicted median, foF2, MHz for 00h, UT in vapour in the atmosphere is important at certain frequencies corre- March 1958 (Reproduced by permission of the Institute for sponding to molecular absorption lines. An understanding of the Telecommunication Sciences, Boulder, USA) basic characteristics of these effects is thus essential in the planning of very high frequency communication systems. The main features of tropospheric propagation are summarised from a practical point factors. These include transmitting and receiving antenna gain, of view as follows. There are two general problems: firstly, the spatial attenuation, ray convergence gain, absorption, intermediate- influence of the troposphere on the reliability of 