es 400Hz and 1200Hz respectively. It is worth commenting that noise reduction techniques used in analogue audio recording, such as the Dolby TM system, make use noise masking. 8/16 Sound, speech and hearing Threshold Shift, dB Figure 8.18 Noise masking by 1200Hz tone 8.7 References Richards (1973) Telecommunication by Speech, Butterworths, Lon- Smith (1970) Acoustics, Longmans, London. Gilford (1972) Acoustics for Radio and Television Studios, Peter Taylor C.A. (1965) The Physics of Musical Sounds, English Univer- Peregrinus. sities Press. The Ionosphere and the Troposphere PA Bradley BSc MSc CEng MIEE Rutherford Appleton Laboratory (Sections 9.1 - 9.6) JA Lane DSc CEng FIEE FInstP Radio Communications Agency (Sections 9.7 - 9.11) Contents The ionosphere 9/3 Formation of the ionosphere and its morphology 9/3 Ionospheric effects on radio signals 9/5 Communication and monitoring systems relying on ionospheric propagation 9/6 lonospheric probing techniques 9/7 Propagation prediction procedures 9/10 The troposphere 9/11 Survey of propagation modes in the troposphere 9/12 Tropospheric effects in space communications 9/15 Propagation and co-channel interference 9/16 Techniques for studying tropospheric structures 9/17 References 9/17 This page intentionally left blank The ionosphere 9/3 The ionosphere 2. It requires specific engineering technologies to derive ex- perimental probing facilities to assess its characteristics, both The ionosphere is an electrified region of the Earth's atmosphere for communication-systems planning and management, and for situated at heights of from about fifty kilometres to several thousand scientific investigations. kilometres. It consists of ions and free electrons produced by the It permits the remote monitoring by sophisticated techniques of ionising influences of solar radiation and of incident energetic solar certain distant natural and man-made phenomena occurring on and cosmic particles. The ionosphere is subject to marked geo- the ground, in the air and in space. graphic and tempo