Build a Trunked Radio Scanner
~130 minIntermediateDecode P25, DMR, and Motorola trunked radio systems used by public safety agencies using two RTL-SDR dongles and SDRTrunk. Follow live calls across dozens of talkgroups.
Hardware
- •Two RTL-SDR dongles (RTL-SDR Blog V4 recommended)
- •Two VHF/UHF antennas (700/800 MHz whip or discone)
- •Powered USB hub for both SDRs
- •Computer with USB 2.0+ (Windows, Linux, or macOS)
Software / Accounts
- •Java 17 or later (adoptium.net)
- •SDRTrunk (latest release from GitHub)
- •RTL-SDR drivers installed
- •RadioReference.com account (free tier sufficient to start)
A trunked radio system dynamically assigns radio channels to users on demand, rather than each agency having a permanently assigned frequency. A computerized controller manages channel allocation in real time, allowing many agencies to share a small pool of channels with minimal wait time.
How trunking works
- All radios monitor a control channel — a dedicated data channel broadcasting system state
- When a radio transmits, it sends a request to the controller via the control channel
- The controller assigns an available voice channel and broadcasts the assignment to all radios in the talkgroup
- Radios instantly retune to the assigned voice channel for the call
- After the call, the channel is returned to the pool
Common trunking protocols
Motorola Type I / II
Legacy analog trunking. Type II is most common. Uses MDC-1200 or IDEN signaling on control channel.
EDACS (GE/Ericsson)
Enhanced Digital Access Communication System. Analog voice, digital control channel at 9600 baud.
LTR (Logic Trunked Radio)
Decentralized — no dedicated control channel. Trunking info embedded in each voice channel.
P25 Phase I / II
APCO-25 digital standard. Most modern public safety. Phase II uses TDMA for double capacity.
DMR Tier III
Digital Mobile Radio trunking. Commercial and some public safety use.
Control channel vs. voice channels
The control channel broadcasts continuously at high speed (3600–9600 baud). It carries group call grants, individual call grants, affiliation updates, and system parameters. Decoding the control channel is the key to following trunked traffic — SDRTrunk does this automatically and follows calls across all voice channels simultaneously.
Most US public safety agencies (police, fire, EMS) migrated from conventional to trunked radio between 1995 and 2015. In a trunked system, 10 agencies can share 10 channels with far less dead air than 10 conventional channels each assigned to one agency.