OnAirWeather & science · Experimental
Decode Ionospheric sounders with an SDR
OnAir receives Ionospheric sounders. HF chirp sounders (ionosondes and oblique sounders, 0 - 30 MHz at 100 - 500 kHz/s): sweeps found by dechirping, their rate, period and start offset, an ionogram of group range against frequency, a timeline, PNG / CSV export; no decoder.
What Ionospheric sounders is
Ionosondes are HF chirp transmitters that probe the ionosphere. Their sweeps are visible all over the shortwave bands.
Frequencies and where it is used
No decoder: HF chirp ionosondes and oblique sounders sweeping 0 - 30 MHz (100, 125, 150, 200, 500 kHz/s).
In OnAir the tuning shortcut is Chirp sweeps, 0 - 30 MHz; you can enter any frequency your radio reaches.
What OnAir shows for Ionospheric sounders
Sweeps found by dechirping, their rate, period and start offset; an ionogram of group range against frequency built from every sweep through the captured span (a HackRF at 20 Msps sees 18 MHz of it), the band coverage of a narrow radio, a timeline of sweeps, PNG / CSV export.
This receiver is built from the published specification and checked end to end on simulated signals. It has had little or no testing on real transmissions yet, so reports and recordings on Discord are welcome.
Which radio you need
This is a shortwave mode, so the radio has to tune HF. An SDRplay, Airspy HF+ or an RTL-SDR in direct-sampling mode do that directly; with a HackRF an HF upconverter or a direct-sampling receiver is the usual route. A long wire or loop antenna matters more than the radio.
Questions about Ionospheric sounders
Can I decode Ionospheric sounders with an RTL-SDR?
An RTL-SDR can receive HF only in direct-sampling mode, which OnAir offers in its radio settings; results depend on the antenna. SDRplay and Airspy HF+ radios are better suited.
Is it legal to receive Ionospheric sounders?
Receiving these signals is legal in most countries. OnAir is a receiver.
Can I try Ionospheric sounders in OnAir without a radio?
Yes. Most modes include a built-in synthetic test signal, so you can open the screen and try the controls before you own a radio.
Try it without a radio
OnAir includes built-in synthetic test signals for most modes, so you can open this screen, see the layout and try the controls before you own a radio. The Tour button walks you through the app.