OnAirWeather & science · Experimental
Decode Radio astronomy with an SDR
OnAir receives Radio astronomy. Radio astronomy: the 21 cm hydrogen line (1420.4058 MHz) and the OH lines; long integration, on - off reference, baseline fit, radial velocity, drift-scan waterfall, CSV export; no decoder.
What Radio astronomy is
The hydrogen line at 1420.4058 MHz lets amateurs map the Milky Way with a small dish and an SDR.
Frequencies and where it is used
No decoder: the 21 cm hydrogen line (1420.4058 MHz) and the OH lines.
In OnAir the tuning shortcut is Hydrogen line, 1420 MHz; you can enter any frequency your radio reaches.
What OnAir shows for Radio astronomy
Long integration of the spectrum (1 s to 1 h, FFT 1024 - 16384), on - off reference or frequency switching, polynomial baseline outside a line window, radial velocity axis (km/s), peak marker, drift-scan waterfall, CSV export, a short how-to (LNA, filter, horn or dish).
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 mode sits at a high frequency, so check that your radio tunes it: a HackRF covers 1 MHz to 6 GHz, and RTL-SDR stops near 1.7 GHz. Some modes at these frequencies also need a fast radio (10 to 20 Msps) or a dish and an LNB.
Questions about Radio astronomy
Can I decode Radio astronomy with an RTL-SDR?
Probably not: an RTL-SDR tunes up to about 1.7 GHz. A HackRF (1 MHz to 6 GHz) or another wideband radio is the usual choice for this frequency range.
Is it legal to receive Radio astronomy?
Receiving these signals is legal in most countries. OnAir is a receiver.
Can I try Radio astronomy 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.