OnAirDecode Mode A/C with an SDR
Mode A/C: squawks and altitudes of aircraft answering secondary radar.
- Group
- Aviation
- In the mode list
- Squawk and altitude, 1090 MHz
What Mode A/C is
Mode A / C transponder replies on 1090 MHz: the squawk (Mode A, four octal digits) and the altitude (Mode C, 100 ft steps) of aircraft that answer secondary radar, with the ident pulse, shown beside the Mode S aircraft of the same capture on the ADS-B map and matched to them by squawk or altitude. Needs the same radio as ADS-B (2 Msps or more, 4 Msps recommended).
Frequencies and settings
OnAir takes any frequency you type; the numbers above are the mode's starting point and the range its table describes. In the app the shortcut label is Squawk and altitude, 1090 MHz.
What OnAir shows
Mode A / C transponder replies on 1090 MHz: the squawk (Mode A, four octal digits) and the altitude (Mode C, 100 ft steps) of aircraft that answer secondary radar, with the ident pulse, shown beside the Mode S aircraft of the same capture on the ADS-B map and matched to them by squawk or altitude. Needs the same radio as ADS-B (2 Msps or more, 4 Msps recommended).
Limits and what it was checked with
Checked with generated signals and impairments; recordings of real signals 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.
Receiving and the law
Questions about Mode A/C
Can I decode Mode A/C 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 A/C?
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 A/C 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.