Thanks everyone for your responses. The funny thing is, I already concluded the way to go was to hook up an RTL-SDR dongle and start poking around. Should be here this week.
I know the frequencies (based on FCC license search) and the hardware manufacturer (IPMN). AFAICT there are a variety of technologies available for AVL, so any given transit agency is likely using something different. I see no insurmountable barriers getting to the point of successful Viterbi decodes. After that, it seems quite difficult. First I have to guess the whitening polynomial and its initialization, then figure out packet framing, and possible source coding. And all of this assumes nothing is intentionally encrypted... Mark Andrew Clegg [andrew_w_cl...@hotmail.com] wrote: > 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 > > >> > > >> _______________________________________________ > > >> Discuss-gnuradio mailing list > > >> 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) > > > > > > _______________________________________________ > > > Discuss-gnuradio mailing list > > > Discuss-gnuradio@gnu.org > > > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio > > > > > > > _______________________________________________ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio > > _______________________________________________ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
