anyone know a thing or two about p25?

[Mark Whittington]

If you do decide to look through ULS, look for things licensed at 16600
Courage Ct, Leesburg, VA 20175.

I'm 95% sure that Loudoun holds its own licenses.


On Mon, Aug 19, 2019 at 3:15 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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