MODE C decoding

Pete Zaitcev zaitcev at
Mon Feb 17 15:31:37 UTC 2014

On Mon, 17 Feb 2014 14:57:44 +0000
Allan Healy <allan.healy121 at> wrote:

> My name is Allan an I am currently working on my final year project
> for college. The idea is to create a radar system that interprets
> MODE C interrogation signals and replies by aircraft and SSR and
> with this information determine where these airplanes are in your vicinity.

I never heard of anyone being able to do that so far. Mode S is easy
and there's any number of 1090 Extended Squitter receivers and decoders
out there, but Mode C relies on the illumination by the radar to
trigger the transmission. Since you cannot know where the radar
is pointing and when the impulse started (and sometimes not where
the radar is located), you cannot link Mode C squitter with the particular
primary target.

You can receive Mode C with rtl_adsb and have a look at 56-bit packets,
but they are completely useless for an independent ground station.

Some people nowadays play with a concept of building a something
like interferometer from 3 or 4 receivers, but nobody has succeeded
thus far, as far as I know.

-- Pete

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