ng and a range the available bandwidth, but since ionospheric attenuation is very of fading rates. The slowest fades are usually those due to absorp- low, near world-wide coverage can be achieved. Unfortunately the tion changes which have a period of about 10 minutes. The deepest radiation of energy is difficult at such frequencies and complex and most rapid fading occurs from the beating between two signal transmitting antenna systems, coupled with large transmitter power- components of comparable amplitude propagated along different S, are needed to overcome the high received background noise from paths. A regularly reflected signal together with a signal scattered atmospherics, the electromagnetic radiation produced by lightning from spread-F irregularities can give rise to so-called flutter fading, discharges. Despite the expanding introduction of alternate satellite with fading rates of about 10Hz. A good general survey of fading links such as NAVSTAR and GPS, because of the stability of effects, including a discussion of fading statistics, has been pro- propagation, v.l.f. systems continue to be used for the transmission duced (CCIR, 1990b). On operational communication circuits fad- of standard time signals and for C.W. navigation systems which rely ing may be combated by space diversity or polarisation-diversity on direction-finding techniques, or on phase comparisons between receiving systems and by the simultaneous use of multiple-fre- spaced transmissions as in the Omega system (10-14kHz) (Pierce, quency transmissions (frequency diversity). 1965). At low frequencies (1.f.) increased propagation losses limit area coverage, but simpler antenna systems are adequate and lower 9.3.7 Frequency deviations transmitter powers can be employed because of the reduced atmos- pheric noise. Low frequency systems are used for communication by on-off keying and frequency-shift keying. Propagation condi- Amplitude fading is accompanied by associated fluctuations in tions are more stable than at higher frequencies becaus