 random thermal fluctuations in electron density which exist in a few degrees to minimise the ground area illuminated. Long linear a plasma in quasi-equilibrium. Several important parameters of the antenna arrays with beam slewing, and circularly-disposed banks of plasma affect the scattering such that each of these can be deter- log-periodic antennas with monopulsing are used. Oblique-in- mined separately. The power, frequency spectrum and polarisation cidence sounders are adaptions of vertical-incidence ionosondes of the scattered signals are measured and used to give the height with the transmitter and receiver controlled from stable-syn- distributions of electron density, electron temperature, ionic compo- chronised sources. Atlases of characteristic records obtained from sition, ion-neutral atmosphere and ion-ion collision frequencies, the two types of sounder under different ionospheric conditions and the mean plasma-drift velocity. Tristatic receiving systems have been produced. Mean models of the ionosphere over the enable the vertical and horizontal components of the mean plasma sounding paths may be deduced by matching measured data with drift to be determined. Radars operate at frequencies of 40- ray-tracing results (Croft, 1968). 1300MHz using either pulse or C.W. transmissions. Transmitter peak Large upwards-pointing antenna arrays fed from very high power powers of the order of 1MW, complex antenna arrays (Figure 9.3), transmitters operating at v.l.f. through to u.h.f. are capable of lead- and sophisticated data processing procedures are needed. Ground ing to ionospheric modification at F-region heights with the excita- clutter limits the lowest heights that can be investigated to around tion of plasma instabilities by non-linear wave-interaction 100km. processes. Measurements mainly at h.f. are the subject of ongoing Electron densities, ion temperatures and ionic composition out to research involving a variety of phenomena including the generation several Earth radii may be studied using naturally 