Date: Tue, 26 May 2015 06:28:44 -0700
Subject: Re: [Discuss-gnuradio] Question about reverse-engineering a new mode
On 26 May 2015 03:28, "Robert McGwier" <address@hidden> 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.
> On Tue, May 19, 2015 at 3:04 PM, Mark Haun <address@hidden> 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.
>> Discuss-gnuradio mailing list
> 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)
> Discuss-gnuradio mailing list
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