OnAirAviation · Experimental
Decode FLARM with an SDR
OnAir receives FLARM. FLARM: glider and light-aircraft collision-avoidance broadcasts on 868.2 and 868.4 MHz (Europe), received like the Open Glider Network does. Old-protocol positions are decoded (device ID, type, altitude, speed, track, climb) and shown in a table and on the live map with distance and bearing from your receiver; the newer protocol is only counted. Receive only; aircraft that set the no-track or stealth flag are only counted, never shown or saved.
What FLARM is
FLARM is the collision-avoidance broadcast of gliders and light aircraft on 868 MHz.
Frequencies and where it is used
Glider and light-aircraft collision-avoidance broadcasts on 868.2 and 868.4 MHz (both from one radio on 868.3 MHz, at least 1 Msps, so an RTL-SDR works), 2-FSK 100 kchip/s Manchester.
In OnAir the tuning shortcut is Gliders, 868.2 / 868.4 MHz; you can enter any frequency your radio reaches.
What OnAir shows for FLARM
The old protocol is decoded (device ID, aircraft type, altitude, speed, track, climb, level, distance and bearing) in a table and on the live map; the newer protocol is only counted ("FLARM (new protocol)"), without positions. Receive only, as the Open Glider Network does: positions are what the aircraft broadcast openly. The no-track and stealth flags are respected: such aircraft (and anonymous addresses) are never shown, logged, saved or exported, only counted as "privacy-flagged aircraft". Needs the receiver position (shared with ADS-B) and the computer clock set right (the packet key changes every 64 s).
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
Any radio that tunes this range works. An RTL-SDR covers roughly 24 MHz to 1.7 GHz, and every other radio in the radio guide reaches it too. Antenna height and a clear view of the transmitter matter most.
Questions about FLARM
Can I decode FLARM with an RTL-SDR?
Usually yes for narrowband signals, as an RTL-SDR tunes roughly 24 MHz to 1.7 GHz at up to about 2.4 Msps. Wide signals such as TV channels need a faster radio, and OnAir says next to the source what the selected radio can do.
Is it legal to receive FLARM?
Receiving aircraft broadcasts is legal in many countries, but some restrict listening to or using what you hear. OnAir is a receiver. Check the rules where you live.
Can I try FLARM 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.