HF Propagation: Professional, free, and Real-time

IRTAM shows the actual state of the ionosphere – see text.

Without proper propagation, world-wide HF communications simply doesn’t exist. We, hams and SWLs, depend on the supporting power of the ionosphere and sometimes struggling with its capricious behavior. Many forecast models had been developed, VOACAP the most prominent among them. Like some far-looking weather models, they deliver broad probabilities – more the climate of the quarter than the weather in the afternoon. Even smart and processor-hungry 3D-raytracing software, taking into account more factors than just the average sunspot number of the month, do face challenges.

Here, IRTAM comes into play. The acronym means “IRI Real-Time Assimilative Mapping”, where IRI stands for the International Reference Ionosphere. This model is the base which is updated by the data of many of so-called digisondes. They are regularily probing the ionosphere at many locations of the world in time increments up to as short as five minutes.

The processed data reveal the actual space weather at this location. Experience, models and clever algorithms are used to spread (assimilate) these results over a world map, and, even more, to produce an animation of the last 24 hours – see the screenshop on the top. Click here to see the last 24 hours.

They show the frequencies, just reflected by the F2 layer under an angle of 90° (vertical sounding). You have to multiply these frequencies with a factor of about 3 to get the highest frequency, being reflected (indeed: refracted) for usual HF communications, or oblique sounding.

Additionally to this, the map will also show e.g. “deviation from climate”. By this map you can compare your VOACAP results (“climate”) to get an impression of the deviations – plus or minus, location, time.

It is a free service of a team around Prof. Bodo Reinisch, supported by world-wide data of their Lowell Digital Ionosondes, the gold standard in this field.


  • Thanks, Nils, most impressive. But I’m ashamed to admit that (even at my advanced age) these maps mean little to me. Would I have to get serious with a good propagation primer? Or is there a gentle “walk-through”, map examples translated into likely path scenarios?

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