Cross-sections (hatched areas) of ellipsoids of constant autocorrelation of electron density fluctuations with the plane associated either with the slab of irregularities (a) or with the wave propagation direction (b). Here the local coordinate system x,y,z (North, East, Down) originates at height z above the observation plane XY. The coordinate system x′, y′, z′ is chosen in such a way that the z′ axis is pointed along the wave vector k while the x′ and z′ axes lie in the plane formed by k and the down axis z. In turn, the direction of the wave vector is defined in terms of the zenith angle θ and the azimuth angle ϕ. The models (a) and (b) also assume the flat and spherical geometries, respectively. This results in different definitions of the slant ranges and transformation rules for θ, ϕ into the observer’s zenith, θ_o, and azimuth ϕ_o, angles (see the text for more details).