f the different techniques has been pro- sea-wave crests then arrive in phase to give coherent addition. The duced by a Working Group of the International Union of Radio Doppler shift of backscatter returns from sea waves due to the sea Science (URSI) (Smith, 1970). Some of the techniques involve motion usually exceeds that imposed on the received signals by the complex analysis procedures and require elaborate and expensive ionosphere, SO that Doppler filtering enables the land- and sea-scat- equipment and antenna systems (Figure 9.3), others need only a tered signals to be examined separately (Blair et. al., 1969). Doppler single radio receiver. Figure 9.3 The EISCAT radar antenna (Photograph courtesy of Rutherford Appleton Laboratory) 9/8 The lonosphere and the Troposphere Table 9.2 Principal ionospheric probing techniques Height range Technique Parameters monitored Above 100km Vertical-incidence Up to height of maximum ionisation-electron density Ground sounding Topside sounding From height of satellite to height of maximum ionisation-electron Satellite density; electron and ion temperatures; ionic composition Incoherent scatter Up to few thousand km - electron density; electron temperature; ion Ground temperature; ionic composition; collision frequencies; drifts of ions and electrons Faraday rotation Total electron content Satellite-ground and differential Doppler In-situ probes Wide range of parameters Satellite C.W. oblique Solar flare effects; irregularities; travelling disturbances; radio Ground incidence aurorae Pulse oblique Oblique modes by ground backscatter and oblique sounding; Ground incidence meteors; radio aurorae; irregularities and their drifts Whistlers Out to few Earth radii-electron density; ion temperature; ionic Ground.or satellite composition Below 100km Vertical-incidence Absorption Ground sounding Riometer Absorption Ground C.W. and pulse Electron density and collision frequency Ground oblique incidence Wave fields Electron density and collision frequency Rocket-ground In situ probes E