Computing, Tutorials

P25 Trunked Tracking and Decoding with RTL-SDR, Unitrunker, and DSDPlus

The project that got me really into experimenting with the RTL-SDR dongles is using them to decode P25 digital trunked public service radio systems.  I have been a casual scanner listener for years and like to listen to emergency calls nearby.  In college it was great to listen in on a party weekend hearing fights, disturbances, or why my street suddenly filled with cars at 3 AM.

Narrowbanding

That was when most agencies were analog.  To get more use out of the radio spectrum, the FCC decreed a narrowbanding mandate requiring a “maximum of 12.5 kHz bandwidth across the private land mobile bands between 150-174 and 421-512 MHz.”  This means going digital for much of that radio spectrum because traditional FM transmissions are 15 KHz.  Ironically they will “go digital” but move to 700/800 MHz.

As a casual listener, I wasn’t exactly thrilled with spending at least $500 for a scanner capable of digital (P25 mostly) and trunked system tracking (also Radio Reference wiki).

$40 RTL-SDR trunked scanner

Lurking around the Radio Reference forums, I saw references to being able to use the RTL-SDR dongles for trunked digital decoding.  I had to try it.  I had played around with these dongles and read about the many projects people were doing with them.  In actuality this project cost me $65.

About the project

You will need at least two RTL-SDR dongles ($20/each) and a copy of Virtual Audio Cable ($26).  I already had a premium Radio Reference account.  You can do the project with one dongle but you loose many features in Unitrunker like talkgroup priority.  Theoretically, the single dongle listens to the system control channel and then tunes to voice calls, then back to the control channel.  You will miss calls because that notification comes across the control channel while the dongle was tuned to a voice transmission.  I will cover a two dongle setup and do not plan to cover a single dongle setup.

This project is still very complicated but it is MUCH easier than it used to be.  This manly thanks to Rick, the developer of Unitrunker who implemented support for the RTL-SDR chipset in his program.  Previously, there needed to be a plug-in for both Unitrunker and SDRSharp, there were all kinds of “moving parts.”  In one respect, being able to see the signal waveform on a spectrum analyzer made it much easier to fine tune the PPM correction on-the-fly as opposed to guessing on a modulation scope.  This setup is much cleaner and the Unitrunker developer has implemented advanced features like drift correction.  It will take some time and patience to understand, research, and know the types of systems and system specifics.

There are some advantages like cost and being software based.  Changing modulation types is often as easy as changing programs.  As an example, DSDPlus will decode MotoTrbo as opposed to no standalone scanner being able to do so currently.  However, portability of this setup is limited as you have to have many pieces of equipment with you.  You’ll need an Internet connection to find sites to program and a PC to tweak settings.

Two very specific and key things to note about trunked radio systems in general:

  • You cannot tell the tower your’re listening to which talkgroup you want to monitor.  Doing so would require the ability to transmit and IS ILLEGAL because you are not authorized to do so.  If the talkgroup is not transmitted by the tower, you’re out of luck.  You can’t be in Dayton and tell the system you want to listen to a talkgroup originating from Cleveland.
  • Nothing here (and no program I know of) will defeat encryption, even if you own the keys.  Decoding encrypted transmissions is not implemented in any of these programs.  On the flip side, be aware that using this tutorial and feeding the audio to Radio Reference and Broadcastify may not make agencies happy.  You could get a take-down notice or even worse, it is trivial to turn on encryption at the system level and you just blocked reception for all scanner listeners.

Thanks to those whose tutorials I first used getting this setup working: $20 trunking police scanner and RTL-SDR Tutorial: Following Trunked Radio With Unitrunker.

Program versions

I used a Windows 7 64 bit PC. Applications and versions used in this writeup:

  • SDRSharp: 1.0.0.1330
  • Virtual Audio Cable: 4.14
  • DSDPlus: 1.51
  • UniTrunker: 1.0.30.10

Parts list

Listed below are all the parts needed to get this project working.

  • Computer with some processing and memory horse power.  It is recommended to have a computer with a recent Intel Core i5 processor and 8GB of RAM, or better.
  • Receive antenna that covers 700 & 800 MHz where P25 trunked usually resides. For an external antenna, splitters and coax runs maybe needed.  The stock RTL-SDR dongle antennas worked fine for me.
  • Two RTL-SDR Dongles.  To decode more than one voice transmission, increase the number of dongles needed.
  • Virtual Audio Cable.  Not free but trial version available.
  • Radio Reference account.  Premium account.  If you don’t want to fork over the money, become an audio feed provider.
  • Recommend a USB hub.  Couple years ago I picked up a Rosewill RHUB-300 USB 2.0 Hub 7-Port HUB.  I recommend this hub because when the dongle is plugged in, the antenna connector is pointed up.  This makes it easier to connect an adapter and a Pryme RD-98.  If available, connect the hub to an Intel USB chipset on your motherboard.  I’ve had far less issues using Intel based hardware.
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