anyone know a thing or two about p25?

[Rob Seastrom]

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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