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

[Matt Ettus]

> 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.
>
>   

Very cool.

> 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
> more.
>   

Those are a good place to look. 

> 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
> symbols.
>   

Yes, there are a few blocks which do similar things.  Look at the GMSK 
code in particular.


> 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?
>   

Yes, you are.  Let us know how it is going!

Matt