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Re: [Discuss-gnuradio] ADS-B and Mode-S Transponders

From: Eric A. Cottrell
Subject: Re: [Discuss-gnuradio] ADS-B and Mode-S Transponders
Date: Sun, 25 Mar 2007 12:59:13 -0400
User-agent: Thunderbird (X11/20060911)

Eric Blossom wrote:
> This is great!  Looking forward to what you come up with!
> Eric

I was able to modify the oscilloscope sink to trigger at a signal level.
 I noticed a lot more activity after the proper triggering.  I do not
get really strong signals except when aircraft are nearby. :)

This is a good example for the USRP since the data is at 1 Megabit per
second and not possible over a sound card interface.

The Mode A and C transponders uses Pulse AM with the presence of pulses
in defined timeslots determining the data with framing pulses at the
start and end.  Mode S uses Pulse Position Modulation (PPM) with a
preamble.  I included a png of the first part of a Mode S packet.  The 4
pulses at the start are the preamble.

Most of my thinking will go into how to implement this within the
GNURadio style.  Do I make a more general case or a specialize case? I
noticed GNURadio has some framing and bit sync blocks I need to look at

PPM is decoded by comparing the amplitude values in the 1 time period to
the 0 time period.  This is like FSK but uses time instead of frequency.
The timing is such that each 1 uS of time is a symbol.  There are 4
symbol states.  I am remapping 0 and 1 as the valid data values.  I need
some decision logic to determine the presence of a pulse.  The block
also needs pulse sync logic. So the block takes in samples and outputs

The next block looks for the preamble pattern and valid data frames by
looking at the timings of valid (0, 1) versus invalid (2,3) symbols.  It
outputs valid data frames.

I get the impression that this is the general flow for digital decoders
in GNURadio although the framing is usually separate from the decoding.
 Am I on the right track with this?

73 Eric

PNG image

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