Sounds like an interesting project. I'd like to know more about the spectrum 
aspect -- do you know which band segments in 700 MHz are used for this in the 
U.S.? Me and my spectrum analyzer want to know :)
Andy
Date: Tue, 26 May 2015 06:28:44 -0700
From: martin.br...@ettus.com
To: rwmcgw...@gmail.com
CC: discuss-gnuradio@gnu.org
Subject: Re: [Discuss-gnuradio] Question about reverse-engineering a new mode



On 26 May 2015 03:28, "Robert McGwier" <rwmcgw...@gmail.com> wrote:

>

> [...] 

> That said, hackers (the good definition) live for this, and I encourage it.
Just wanted to emphasise this. Go for it! Worst case, you learn a lot of 
interesting things. 
Cheers, 

M
>

> Bob

>

>

> On Tue, May 19, 2015 at 3:04 PM, Mark Haun <hau...@keteu.org> wrote:

>>

>> This is a bit of an idle question, but I'm hoping some knowledgable folks on

>> here can offer advice.  Mostly I'm trying to understand better what I

>> don't know, and the size of the challenge, before jumping in to a project:

>>

>> I'd like to try decoding some AVL traffic in the 700-MHz band (GPS locations

>> broadcast by transit vehicles to a central collector, where predictors are

>> used to generate the ETAs displayed on electronic bus-stop signs).  The

>> modulation is 4-FSK, similar to P25 except wider with a higher symbol rate,

>> emission designator 20K0F1D.  The particular frequency(s) should be easy

>> enough to discover.  Transmissions are short packets on shared channels with

>> some kind of slotted aloha or CSMA MAC.  A rate-3/4 convolutional code is

>> used.  The preceding is public information gleaned from the web.  I haven't

>> captured any signals yet.

>>

>> The known unknowns:  preambles and framing stuff, symbol mapping,

>> the particular rate-3/4 code used (only a couple of candidates though), and,

>> the scrambler (whitener) and its initialization.  AFAIK there is no

>> encryption per se.  The payload is supposed to be TCP/IP, so there could be

>> some sort of header compression.

>>

>> My question, then, is given this information, are there reasonable odds of

>> success?  I have some digital comms background from grad school but little

>> to no practical experience.  Wondering if this might be an excuse to pick up

>> a HackRF etc. and learn GNU Radio, or if it's likely to be a dead end.

>>

>> Thanks,

>>

>> Mark

>>

>> _______________________________________________

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>> Discuss-gnuradio@gnu.org

>> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio

>

>

>

>

> -- 

> Bob McGwier

> Co-Founder and Technical Director, Federated Wireless, LLC

> Research Professor Virginia Tech

> Senior Member IEEE, Facebook: N4HYBob, ARS: N4HY

> Faculty Advisor Virginia Tech Amateur Radio Assn. (K4KDJ)

>

> _______________________________________________

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> Discuss-gnuradio@gnu.org

> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio

>



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