Very high frequency (v.h.f.) communication relying on ionos- ionospheric irregularities gives rise to direct backscattered and pheric scatter propagation between ground-based terminals is forward-scatter signals. Ducting of signals to great distances can possible. Two-way error-correcting systems with scattering from lonospheric probing techniques 9/7 intermittent meteor trains can be used at frequencies of 30-40MHz filtering can also resolve signals reflected or scattered back along over ranges of 500-1500km (Sugar, 1964). Bursts of high-speed ionosphere paths from aircraft, rockets, rocket trails or ships. This data of about one second duration with duty cycles of the order of technique is exploited in military over-the-horizon (OHD) radar 5% can be achieved, using transmitter powers of about 1kW. Me- systems which involve huge steered antenna arrays for fan-beam teor-burst systems find favour in certain military applications be- surveillance (Anderson, 1986; Headrick and Skolnik, 1974). cause they are difficult to intercept, since the scattering is usually Studies of the Doppler shift of stable frequency, h.f., C.W. signals confined to 5-10 degrees from the great-circle path. Forward-scatter propagated via the ionosphere between ground-based terminals communication systems at frequencies of 30-60MHz, also opera- provide important information about infrasonic waves in the F-re- ting over ranges of about 1000km, rely on coherent scattering from gion originating from nuclear explosions (Baker and Davies, 1968), field-aligned irregularities in the D-region at a height of about 85km earthquakes (Davies and Baker, 1965), severe thunderstorms (Baker (Bailey et. al., 1955). They are used principally at low and high and Davies, 1969), and air currents in mountainous regions. latitudes. Signal intensities are somewhat variable, depending on High-altitude nuclear explosions lead to other effects which may the incidence of irregularities. During magnetically disturbed con- be detected by radio means (Pierce, 1965). An imm