Electromagnetic waves impinging upon a plasma at frequencies larger than the plasma frequency, suffer weak scattering. The scattering arises from the existence of electron density fluctuations. The so-called incoherent scattering theory basically deals with fluctuations of random thermal origin; however, for practical purposes, it must also take account of these fluctuations caused by streaming photo-electrons. As is well known, in any scattering experiment, the received signal corresponds to a particular spatial Fourier component of the fluctuations, the wavevector of which is a function of the wavelength of the radiowave. Wavelengths short with respect to the Debye length of the medium relate to fluctuations due to non-interacting Maxwellian electrons, while larger wavelengths relate to fluctuations due to collective Coulomb interactions. In the latter case, the scattered signal exhibits a spectral distribution which is characteristic of the main properties of the electron and ion gases and, therefore, provides a powerful diagnosis of the state of the plasma, in our case, the ionosphere.