ediate wideband ditions signals are enhanced at high latitudes, but are little affected electromagnetic pulse is produced which can be monitored throug- at low latitudes. Directional transmitting and receiving antennas hout the world at v.l.f. and h.f. Also, enhanced D-region ionisation, with intersecting beams are required. Particular attention has to be lasting for several days over a wide geographical area, produces an paid to avoiding interference from signal components scattered identifiable change in the received phase of long-distance v.l.f. from sporadic-E ionisation or irregularities in the F-region. Special ionospheric signals. Other more localised effects which can be frequency-modulation techniques involving time division multiplex detected include the generation of irregularities in the F-region. are used to combat Doppler effects. Systems with 16 channels, automatic error correction and operating at 100 words per minute, Atmospherics may be monitored at v.l.f. out to distances of several now exist. thousand kilometres and by recording simultaneously the arrival azimuths at spaced receivers the locations of thunderstorms may be determined and their movements tracked as an aid to meteorological 9.4.2 Monitoring systems warning services. High frequency (h.f.) signals propagated obliquely via the ionos- phere and scattered at the ground back along the reverse path may lonospheric probing techniques be exploited to give information on the characteristics of the scat- tering region (Croft, 1972). Increased scatter results from moun- tains, from cities and from certain sea waves. Signals backscattered There are a wide variety of methods of sounding the state of the from the sea are enhanced when the signal wavelength is twice the ionosphere involving single- and multiple-station ground-based component of the sea wavelength along the direction of incidence equipments, rocket-borne and satellite probes (see Table 9.2). A of the signal, since round-trip signals reflected from successive comprehensive survey o