rtl-power-fftw VS op25

https://github.com/AD-Vega/rtl-power-fftw can help. This can be easily piped for various outputs and works with GNUPlot off the bat. Use this on an Android based SDR for foxhunting and Signal seeking.

I like to use rrl_power_fftw

You could use rtl_power_fftw to better record the wider band over a time period. This is similar to rtl_power from librtlsdr, but is more efficient and can dwell for as little as 100ms on any 2.4MHz window (24MHz done in 1 second, f.e.). You can then plot this data through GNUPlot to get visual representations.

These are made using rtl-power-fftw and GNUPlot

I have a rooted Android with an RTL-SDR attached, which I'd take with me if buying a new laptop. I'd sit near the laptop's on display with a stubby duck in the SDR and use rtl-power-fftw to rapid scan 27-1724MHz (fastest is 100ms dwell at 2.4MSps). Takes a minute or two and can be fed into GNUPlot to give you a pretty graph of signals seen. Do it at a low gain and any spikes you see are likely from those sources.

rtl-power-fftw

Look into rtl_power_fftw. It can be run for hours on any host (including a Raspberry Pi) and the spectrum results examined later.

rtl_power_fftw is likely more well suited. It's multithreaded, so can sample IQ and process the data as it samples. It's also faster and uses libfftw for processing (faster and more accurate). Outputs more friendly, too. I use this in part with a signal heatmap project; https://www.reddit.com/r/RTLSDR/comments/ifer8j/progress_on_the_android_sdr_linuxdeploy_rtl_power/

looks like it may not be able to do what I want it to do according to this. https://github.com/boatbod/op25/issues/184

I have no experience with DMR myself, but my understanding is that the Boatbod fork of OP25 has experimental DMR support, and I believe SDRTrunk software does, too.

Using OP25[1] with RTL-SDR dongles to implement scanners for trunking P25 radio systems on a computer. And as a sidebar to that, listening in on geographically remote public safety radio systems using Broadcastify[2]. It's basically the old "radio scanner" culture, but taken online and distributed worldwide. People listen to all sorts of stuff, but given my background as a former firefighter and former 911 dispatcher, my main interest is listening to public safety dispatch stuff.

That said, another commenter below mentioned modern "phreaking" and how it's become more radio centric... and while I won't admit to doing anything illegal, let's just say that there's some interesting stuff you can do / look at / listen to these days, especially with ubiquitous and inexpensive SDR hardware and related resources. See the recent story about the KrakenSDR passive radar stuff, and some of the papers that are out there about P25 security flaws, some of the automobile hacking stuff that's RF based, etc., etc. There's a fascinating world out there buzzing around on invisible electromagnetic fields... and you can tap into it with a $30 dongle and a Raspberry Pi (or your PC).

[1]: https://github.com/boatbod/op25

If it's P25 Phase I or Phase II then SDRTrunk might work. boatbod's op25 might be another option, but more complicated to set up.

I don't have a full guide for you, but I use https://github.com/boatbod/op25 for receiving and decoding P25 Phase 1 digital signals on the command line.

op25

If you are handy with Linux you can use an RTL-SDR and OP25 on a Raspberry Pi or really any PC. This is the page for the Fairfax trunked system https://www.radioreference.com/apps/db/?sid=6957. Any talkgroup with mode D can be listened to, mode DE is encrypted. If you want I already have config files made up for the police dispatch and fire/ems dispatch channels.