Re: [Discuss-gnuradio] Help decoding Bell 202

[En Shih]

Thanks Albin and Tim.

I've uploaded a new file of the input signal here:  "dump3"
<https://drive.google.com/file/d/0B5T4PFVUKZgmVTZoZTNZNmo1Smc/view?usp=sharing> 
 
(I hope the distortion problem will be better)


After removing the LPF ( with this flow graph
<https://drive.google.com/file/d/0B5T4PFVUKZgmcmFWSmI0cFpoeU0/view?usp=sharing> 
), this is what I get:
<http://gnuradio.4.n7.nabble.com/file/n57532/2015-12-29_18-38-56.png> 
<http://gnuradio.4.n7.nabble.com/file/n57532/2015-12-29_22-31-04.png> 
<http://gnuradio.4.n7.nabble.com/file/n57532/2015-12-29_22-58-16.png> 
which looks better. However, I'm unsure how to identify the frames, or how
to use the HDLC deframer correctly.

Concerning the HDLC specification, does it mean that I have to identify the
start and end of packet transmission by 0x7e (or 01111110, in NRZI)?

The transmission is from an iMet radiosonde, and the packet specifications
are described in  this letter
<https://drive.google.com/file/d/0B5T4PFVUKZgmV2JHWnVaMjd2QUU/view?usp=sharing> 
, although it only deals with the demodulated and deframed data (as in the 
"decoded"
<https://drive.google.com/file/d/0B5T4PFVUKZgmRTRsNkN6N2pNZVU/view?usp=sharing> 
 
file I put up in Google drive), and not the underlying physical layer.

I am continuing a project at National Taiwan University physics department
mentioned in the letter. We got some radiosondes from the atmospheric
department for transmitting GPS and PTU data. We are also going to put some
particle detectors on top of the balloon (which will use a different system
for transmission).

The goal is a easily deployable reception system based on SDR and GNU radio
for multiple ground stations. Furthermore, I find decoding the transmission
data interesting to do :)

En Shih



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