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The behaviour of the ionosphere and HF [High Frequency] wave propagation parameters and their possible changes can be described using ionospheric oblique sounding (IOS) data. With the help of this method the maximum observed frequency (MOF), the maximum useable frequency (MUF), the lowest observed frequency (LOF), the number of propagation modes and the signal delays can be determined.  2, fiche 1, Anglais, - oblique%20ionospheric%20sounding

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The changing state of the ionosphere is generally monitored by networks of vertical ionosondes. These radars transmit and receive HF (1- 30MHz) radio waves to and from the ionosphere directly above the monitoring station. The result is real-time information on the state of the ionosphere needed by communications users allowing them to adapt their operational systems accordingly. The oblique ionospheric sounder extends this idea. The transmitter and receiver of the oblique sounder are not co-located like the vertical but are generally hundreds thousands of kilometres apart. So the instrument is able to study how the radio signals of real communications (below 30MHz) propagate via the ionosphere under a variety of conditions. However the interpretation of the oblique propagation is also significantly harder than the case of the vertical.  3, fiche 1, Anglais, - oblique%20ionospheric%20sounding

Fiche 1,  Justifications,  Français

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Station de réception par sondage oblique passif.  3, fiche 1, Français, - sondage%20oblique

Fiche 1,  Justifications,  Espagnol