ng C.W. bistatic systems in which the transmitter perature, retarding potential analysers and sampling mass spec- and receiver are separated by several hundred kilometres, Since the trometers for ion density, quadrupole and monopole mass irregularities are known to be elongated and aligned along the spectrometers for ion and neutral-gas analysis and retarding poten- direction of the Earth's magnetic field and since at the higher tial analysers for ion temperature measurements. frequencies efficient scattering can also occur under restricted con- Ground measurements of satellite beacon signals permit studies ditions, the scattering centres may readily be located. Using low- of total-electron content, either from the differential Doppler fre- power v.h.f. beacon transmitters, this technique has proved very quency between two harmonically related h.f./v.h.f. signals (Gar- popular with radio amateurs. Pulsed meteor radars incorporating riott and Nichol, 1965) or from the Faraday rotation of a single v.h.f. Doppler measurements indicate the properties and movements of transmission (Ross, 1965). Beacons on geostationary satellites are meteor trains (Sugar, 1964). valuable for investigations of temporal variations. The scintillation Two other oblique-path techniques, giving information on the of satellite signals at v.h.f. and u.h.f. gives information on the regular ionospheric regions, are high-frequency ground backscatter incidence of ionospheric irregularities, their heights and sizes sounding and variable-frequency oblique sounding. The former (Aarons, 1973). uses a nearby transmitter and receiver, and record interpretation A powerful tool for ionospheric investigations up to heights of generally involves identifying the skip distance (see Figure 9.1) several thousand kilometres is the vertical-incidence incoherent- where the signal returns are enhanced because of ray convergence. scatter radar. The technique makes use of the very weak scattering It is important to use antennas with azimuthal beamwidths of only from