Or, doesn't have to be LE.  Presumably any signal of a certain freq above a certain strength.

A member here wrote a plugin for one of the popular SDR programs.  (or possibly discovered it) wherein he was alerted whenever a transmission on a specified freq is above a threshold.

I remember you posting in another thread here in the Ham forum.  I think.

Anyone?

This SDR# scanner plugin will scan frequencies you specify and log activity on said frequencies.

Here's a youtube video showing it in use.

It wasn't a plugin, it was just a small python program.
Here's the thread.

The program can be modified to block out nearby transmitters that would otherwise trigger it.

Be nice if there was collaboration between emergency vehicle lighting manufacturers and car stereo manufacturers so that when fire/police/EMS is rolling code 3 it will cause nearby car stereos to announce, "Move the %&*@ Over!" Or start playing some Ludacris.

Thanks, seek.

My thread about the fast scanner plugin and milair.

https://forums.radioreference.com/military-monitoring-forum/320983-225-400-1-5-seconds-best-milair-setup-ive-encountered-yet.html

Delete--ND'd into the wrong thread

For you guys more clever than I, I was reading around about rtl_fm and came across this page for rtl_power:

http://kmkeen.com/rtl-power/

Very interesting the part about 'passive' radio receiving as radar.  It occurred to me you could possibly work this out enough to where you could tell if a transmission was/is coming towards you, or away.  And speed thereof.  (or at least vector magnitude)

Interesting stuff.

Mancow - thanks for your post as well.

I've been sitting on a few of these inexpensive SDR dongles for a while now, and am just now realizing some of the possibilities.

rtl_power is the basis of the script in my thread that I linked to. It's a really cool program
and very easy to process the output from it. (There are a few quirks, mostly around
the timing of the output frames v. time to scan them.)

You can do some very interesting things with bi-static radar, but reliable setups can be a challenge
(e.g. you get frequency drift in the dongle and it complicates processing for longer term samples.)

One thing that I discovered running rtl_power on long term emitters was about a 1dB variation
in signal strength based on time of day -- I think it relates to air density or ducting. I also was able
to figure out that the moments I was seeing signals from broadcast stations > 150 miles away were
due to aircraft reflecting the signals over the horizon. Apparently someone else has done some
studies on APRS packet reception and it's not at all uncommon to get a few packets from
way far away that propagate by this mode.

Should sell well in the hood.

There are microwave guys that have used aircraft reflection as an intentional propagation tool.

Didn't bearcat  have a scanner that would listen to the state police mobile extender input freqs and set off an alert ?

Neat

They still do. It's part of what I was emulating in the code (basically my solution is a hybrid between closecall and the beartracker alerts.)

Tried to run that under python windows, not working so well will try to spin it up on my linux laptop and see what happens

It was coded under linux, so it'll hopefully work better. I have no idea how it'd behave
on windows, or if rtl_power even exists for windows for that matter.

If you have troubles I'll try to help where I can, but I used a pretty esoteric distro compared to most.

how does that work since all the emergency and LEO coms go through repeaters? Can you block the repeater transmit frequencies and just pick up the recueve freqs that vehicles transmit on?

That's how I understood it.  Listen to the power levels of signals on the repeater input frequencies.  You don't care about content.  Only if a signal is present and how strong.

@seek2

Yes, that's exactly how it works. In practice you don't have to block much, because you're looking at wildly different power levels
for a near signal v. a far one -- for a typical nearby VHF emitter I'll see +20-+30 dB v. -10 to -20 dB for a repeater, so that's a 30-50 dB spread
between them. After some tweaking I just look for signals what are say +5 dB, and might have to manually filter out a few signals
from pager transmitters, NOAA (local transmitter is 300W and is literally line of sight for me, while miles away I can look at it
with a telescope.)

If you look at the code you can see all this ( > 5.0, and then the frequency filters)

I do additional post-processing on the output of the script I pasted, since once I have a frequency and a dB power value, I
can convert that to a named user (local fire, PD, whatever) and an approximate distance by pairing past event locations
with measured power. I'm pretty sure if you had just a few SDRs spread through the area every couple miles you could do
trilateration and have a real-time map of emitters with many square miles of coverage.

What's interesting is the "martians" that pop up from time to time. High-power local signals on unusual frequencies.
One I tracked down to a satellite tracker on a semi trailer that parks nearby. There's others that are correlated to
some federal LEO activities, and one that popped up that was from electronic news gathering that was giving
instructions from the ground to a news helicopter. One was a TACAMO that kept triggering the system
as it orbited the area and I could actually follow the orbit progress on a handi-finder.