e the ionos- group path and phase path, giving rise to time and frequency- phere is less deeply penetrated. Low frequency signals involving dispersed signals. When either the transmitter or receiver is moving, ionospheric propagation are also used for communication with or there are systematic ionospheric movements, the received signal submarines below the surface of the sea, with receivers below the is also Doppler-frequency shifted. Signals propagated simulta- ground and with space vehicles not within line-of-sight of the neously via different ionospheric paths are usually received with transmitter. Other l.f. systems (Stringer, 1969) relying principally differing frequency shifts. Frequency shifts for reflections from the on the ground wave, which are sometimes detrimentally influenced regular layers are usually less than 1Hz, but shifts of up to 20-30Hz by the sky wave at night, include the Decca C.W. navigation system have been reported for scatter-mode signals at low latitudes (Niel- (70-130kHz), the Loran C pulse navigation system (100kHz) and son, 1968). long-wave broadcasting. At medium frequencies (m.f.) daytime absorption is SO high as to 9.3.8 Reflection, scattering and ducting completely suppress the sky wave. Some use is made of the sky wave at night-time for broadcasting, but generally medium frequen- The combined effect of refraction through a number of successive cies are employed for ground-wave services. Despite the advent of slabs of ionisation can lead to ray reflection. This may take place reliable multichannel satellite and cable systems, high frequencies over a narrow height range as at 1.f. or rays may be refracted over continue to be used predominantly for broadcasting, fixed and an appreciable distance in the ionosphere as at h.f. Weak incoherent mobile point-to-point communications, via the ionosphere; there scattering of energy occurs from random thermal fluctuations in are still tens of thousands of such circuits. electron density, and more efficient aspect-sensitive scattering from 