anyone know a thing or two about p25?

[Martin]

P25 signals are pretty distinctive on the waterfall.  I would scan around
800 and even 900 MHz for signals with GQRX.

FWIW the DC police and others were pretty good at keeping their encryption
working so not so interesting to listen in on.  Interested in how Loundoun
rolls.

Thanks for the post - I didn't know about Phase 2.

--Martin

On Mon, Aug 19, 2019 at 12:15 PM Rob Seastrom <rs at seastrom.com> wrote:

>
> Hi folks,
>
> Can’t remember if I sent this here - I sent it to a couple of people who I
> thought might know a thing or two about the internal workings of APCO-25,
> but no joy so far.
>
> I’ve done on-again off-again fiddling around with the RTL-based USB DVB-T
> dongles.  Mostly I’ve been using them for receiving ADS-B and AMR squawks
> from my power meters (yes, plural, I have a personally owned Form 2S meter
> on the car charger).
>
> Recently I became aware of a P25 receiver that claims to even do Phase 2.
> http://osmocom.org/projects/op25/wiki
>
> Went ahead and installed it on random beater laptop.  Admittedly the
> antenna I have on the USB dongle is pretty lame (and it has a front end
> that is all but nonexistent), but I figured that the signal strength has to
> be good enough to work on random HTs that first responders are carrying, so
> that shouldn’t stand stand between me and… seeing at least something on the
> waterfall, right?
>
> Went to Radioreference for Loudoun’s 800 MHz frequencies.  Tried having
> op25 look at what was flagged as control frequencies, but didn’t have any
> luck so obviously the next thing to do was to break out http://gqrx.dk and
> see what kind of spectral energy I could see on the frequencies that
> radioreference mentioned.
>
> Nothing.  I figure set for narrow FM demodulation even if I don’t hear
> something (I think I should), I should at least see something on the
> waterfall, and the control channel should be chattering more or less
> constantly (right?) but there doesn’t seem to be anything on these
> frequencies.  I see stuff nearby, so I’m reasonably sure that the software
> is working, and I’m using a stick which I’ve used with dump1090 and rtlamr
> before so I’m reasonably confident in the hardware, and besides, it works
> just fine decoding fire dispatch on VHF low band and receiving broadcast FM
> so...
>
> I have not gone digging through all of ULS for likely-looking frequencies,
> and given all the mutual aid stuff that goes on in the greater Washington
> area I figure it’s entirely likely that I’ll find the control channel for
> Loudoun is actually on a shared frequency that is licensed to someone else
> entirely.  But that’s merely blind speculation.
>
> Thoughts or ideas?
>
> -r
>
> Sent from my iPad
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