On this page you can view ionograms as received by the WebSDR receiver located at the University of Twente in Enschede, the Netherlands.
These ionograms are made by receiving so-called chirp transmitters: transmitters that are not on a fixed frequency but run from e.g. 2 to 30 MHz in de course of e.g. 5 minutes. Thus, one can see on what frequencies propagation to/from the transmitter sites is possible. Furthermore, one can discern between multiple reflecting layers in the ionosphere; hence the name "ionogram".
In each ionogram, the horizontal axis shows the frequency in MHz. The vertical axis shows the reception time of the signal, in seconds, and normalized to the corresponding starting time of the sweep at 0 MHz. Practically speaking, this means for (most) GPS-locked chirps that the digits after the decimal point give the propagation time in milliseconds. For non-GPS-locked chirps, the vertical axis still indicates different propagation times, but the absolute value is meaningless (unless you want to compare reception with another chirp receiver).
Note that not all these transmitters are on continuously, so if no signal appears, that may simply mean the transmitter is off, rather than that no propagation is possible. Also, note that not all transmitters start as low as 2 MHz, nor do they all go all the way up to 30 MHz.
For more information, see G3PLX's article about chirping (note though that the list of chirp transmitters on that site is outdated), and the article I wrote recently about ionograms, their interpretation, and how this receiver works. Comments can be sent to firstname.lastname@example.org.
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|Others in and near Europe:|
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|Australian radars: (about 0.5 MW each)|
|Australian ionosounders: (very much less powerful)|
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