Is it possible to properly receive mode A/C?

is it possible to properly receive mode A/C?

I use ver.7.2. downloaded from GitHub - flightaware/dump1090: Dump1090 is a simple Mode S decoder for RTLSDR devices, I can observe flights from 220 nm. I have got mlat.
Lately I witnessed a conversation between a flight controller with a pilot. The conversation went something like this:
Controller: Unfortunately I do not see you within a radar.
Pilot: Hmm, a colleague turned on only mode a. We are going to change it now.
Controller: OK, I see you now.
Pilot: Super, now we are on c mode.

In both cases I didn’t see this plane in the table. What steps could I take take to help with it?


You need to have mode A/C decoding enabled in dump1090 (–modeac switch), however the information it provides to a casual observer is limited. You can see that there is an aircraft squawking a particular code, or that there is an aircraft at a particular altitude, but you can’t really relate the two very easily since neither protocols have ID transmissions associated with them. This isn’t a problem for ATC, since the transmissions are in response to an interrogation which is directional from the radar head. In addition, mode A/C has no error correction so there will be quite a lot of errors and spurious data seen.

If you want to do more with mode A/C messages, you could have a look at something like planeplotter which has some kind of mlat system for them, but I have no idea how effective it is.


I have heard about planeplotter, but I guess it’s not available for RaspberryPi.
PiAware - Advanced Configuration Settings - FlightAware - I have found settings for A/C mode here, but my file has less options:
Where exactly should I be looking for “allow-modeac” option? Majority of commands just do not show in my location.

sudo nano /etc/piaware.conf

allow-modeac yes

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Besides receiving those AC “answers”, you would have to be synchronized with the ATC rotating radar position and know what interrogation ID code they have sent out.
It’s really not used anymore, except some military flights, not sure in what part of the world are you (and why are you interested).

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He’s in Belarus, so probably interested in military flight over Ukraine.

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Most likely - Poland and Lithuania. No military aircraft over Ukraine uses ads-b


The conversation quoted in the first post wasn’t about any kind of military aircraft, but was from a Cessna in training. If it was being used by army, that would just be an addition to that.

I didn’t manage to edit the line that geckoVN sent, but I can see in logs something about A/C:



I’ve found a description of planeplotter for rasberrypi but have not tried it out yet: | Updated ppup1090 - supporting Mode A/C MLAT

This is how I enabled MODEAC on my dump1090-fa 8.2 PiAware SD card install:

sudo nano /usr/lib/piaware-support/generate-receiver-config
change EXTRA_OPTIONS {} to EXTRA_OPTIONS {–modeac}
Then do a restart or reboot

My CPU jumped up from 20 to about 25% usage which is fine. Of course the message rate jumped up a lot. :wink:


Does anyone know if modeac returned locally via a different port? adsb is obviously 30003/5 and mlat is 30105, but can modeac be returned via a port or not?

modeac data will be returned in beast format on port 30005.
The port 30003 format cannot display modeac data.

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That’s What does enabling modeac actually mean for me? I’m in the UK where we have a lot of mode A military jets do I get those returned to skyaware or not?

It’s of minimal value in my opinion. Mode A gets you a squawk code for a particular aircraft, but there’s no way to identify that aircraft unless you happen to have info from elsewhere like ATC to correlate it with.

You will see that an aircraft is present, but have no information regarding its position or what it is.

Planeplotter apparently has some kind of mlat system that can work with mode A, but I don’t accurate or useful it is.


There’s not really anything for skyaware to display.

Mode A/C tells you, specifically, “some aircraft claimed it had this squawk or altitude”. It does not include the aircraft address, so you don’t know who is making that claim. It does not include the aircraft position. In many cases, you can’t tell whether the message is a squawk or an altitude. There is no error checking.

You could mlat the transmissions (and I actually built a prototype of that), but for a passive receiver the data is not all that much use except for very coarse situational awareness - you know there’s something transmitting at a particular approximate location, but you don’t know what the aircraft is or even if it’s the same aircraft from one transmission to the next.


In my coverage area I see a lot of military training so when they turn off the ModeC and or ADSB I can still see them on my local tar1090 display. I don’t know what flightaware does with the information and this is very new to me in the past two days.

Here in the USA the regulations have been changed from requiring -everyone- that is equipped with ADSB OUT to “transmit at all times” to now exempting the military and government. Fancy that.

I don’t have such a folder:

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