Athens Digisonde

Short electromagnetic pulses in the frequency range between 1 and 20 MHz are vertically transmitted into the ionosphere and are received locally after their ionospheric reflection. From the ionospheric echoes received in the four DPS4D receiving antennas, their amplitude, travelling time (virtual reflection height), Doppler shift, polarization, angle of arrival and the electron density profile are derived. Vertical incidence ionograms display the virtual height (the range) of the HF waves. The ordinary waves echoes are marked in red, the extraordinary echoes in green. Other colors indicate off vertical echoes. The calculated electron density profile shows the real reflection heights of the ordinary wave. The topside profile is modeled assuming an α-Chapman profile with constant effective scale height.

The transmitted signal illuminates a large area in the ionosphere, typically a few hundred kilometers in diameter. At the point where the local index of refraction becomes equal to zero, the sounding pulse is “reflected” from the ionosphere. If the normal to the surface of equal electron density contour points exactly towards the sounder, then the reflected signal returns to the sounder. Each such reflection point can be considered as a “source” of reflected signal. A map showing locations of all such “sources” is called a “skymap”. From the collection of the sources in the skymap the orientation of the normal vector characterizing the ionospheric tilt of an imaginary reflecting plane can be derived. In essence, this approach assumes a mirror type reflection of HF waves, a very simple model that neglects refraction from underlying layers and consider a flat Earth/ionosphere reflection geometry, which nonetheless can be efficiently used to characterize HF wave reflections from the ionosphere as long as magnetic field effects are not very strong.