MODE C decoding

Sylvain AZARIAN sylvain.azarian at
Mon Feb 17 16:11:03 UTC 2014

Hi ,

just one comment : the interrogation is periodic and stable.. the request
is transmitted by a rotating antenna turning at constant speed. If you can
get the direct path to the interrogation system, you may estimate the
round-trip time and then when the interrogation pulse is sent


2014-02-17 16:46 GMT+01:00 Allan Healy <allan.healy121 at>:

> Hi Pete
> Thanks for the reply. Yeah I can see the MODE C transmissions with the
> rtl_adsb alright but I don't know how to see the interrogation
> transmissions from an SSR. Like you said there are many decoders available
> for MODE S. Hopefully my supervisor will understand the gravitas of the
> project and will accept MODE S as a suitable solution. Still I will try do
> some more research. If I am successful I will provide my solution to this
> forum. Thanks again.
> Allan
> On 17 Feb 2014, at 15:31, Pete Zaitcev <zaitcev at> wrote:
> > 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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