OnAirAviation · Experimental
Decode Mode S uplink with an SDR
OnAir receives Mode S uplink. Mode S uplink: the 1030 MHz interrogations of radars and ACAS (Mode A/C, intermode, UF0 to UF24); needs 8 Msps.
What Mode S uplink is
On 1030 MHz, secondary radars and collision-avoidance systems interrogate aircraft transponders. Decoding the interrogations shows which radars are around and what they ask for.
Frequencies and where it is used
Radar and ACAS interrogations, 1030 MHz (needs 8 Msps).
In OnAir the tuning shortcut is Interrogations, 1030 MHz; you can enter any frequency your radio reaches.
What OnAir shows for Mode S uplink
Interrogator codes and rotation periods, aircraft and the registers asked for, TCAS coordination.
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 Mode S uplink
Can I decode Mode S uplink 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 Mode S uplink?
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 Mode S uplink 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.