Trunking Police Scanning With SDRTrunk

There was a time when it was easy to eavesdrop on police and other service radio networks. Police scanners fans can hear live police, fire, and ambulance calls. However, it isn’t as easy as it used to be because nearly all radios now are trunked. That means conversations might jump from channel to channel. However, P25 can unscramble trunked radio calls intercepted by a cheap SDR dongle and let you listen in. [SignalsEverywhere] shows you how to set it up for Windows or Linux and you can see the video below.

Trunking radio makes sense. In the old days, you might have a dozen channels for different purposes. But most channels would be empty most of the time. With trunking radio, a radio’s computer is set to be in a talk group and a control channel sorts out what channel the talk group should use at any given time. That means that one channel might have several transmissions in a row from different talk groups and one talk group might hop to a new channel on each transmission.

P25 is the APCO ( Association of Public Safety Communications Officials) Project 25 standard used for public service trunking radios. You can, of course, get commercial equipment to monitor these radios, too, but what fun is that?

With everyone spending more time at home these days, radio monitoring is a great way to live vicariously. Not the first time we’ve seen an SDR dongle scanner, of course. Just watch out for kiddy toys.

19 thoughts on “Trunking Police Scanning With SDRTrunk

      1. At least the EU TETRA radios are cheaper than the public safety grade Motorola P25 handhelds.
        Like one tenth the price cheaper.
        Airbus Defence THR880i terminals is like 750eur new and Hytera TETRA terminals are about the same.
        Compare that to 5000-8000usd for an APX8000 from Motorola Solutions Inc.

        I do wish the TETRA radios had analog FM fallback, they’d be more usefull hobbyist radios if they did that.

  1. How is this a “hack” or even news? We’ve been using RTL-SDR to listen to trunked radio since the RTL first started being used for general RF purposes.
    Can you find something newsworthy to write about? The amount of filler on this site is getting rather excessive

    1. I know this is a couple years later but just imagine being able to scroll past a story you don’t want to read. Imagine the possibilities. Wow, just imagine. Also, there are people out here like myself that have just stumbled onto this so just relax and consider the posibility that there are other people in the world other than yourself. Thank you, have a nice day and thank you to the author for putting this out there for people that don’t know about this tech yet.

  2. People listened to police radio before there were scanners.

    You had a choice, a tuneable radio or crystal control. The former let you find signals, but since transmission is generally short, you might need to tune a lot. Crystal control always put you on the right frequency, but you needed to know it first, and you needed a crystal for each frequency, and they were a few dollars each.

    Some police band radios were tuneable but had the option of one crystal position, the best of both worlds.

    Scanners appeared in 1970 or 71, six or maybe eight channels, a crystal needed for each. Qnd eqch scanner would cover only one band.

    It was a big thing when synthesized scanner arrived, late seventies or early eighties. Get all the channels with precision, and scan through them. I think even the earliest tuned the low and high VHF bands, and maybe the UHF band. Air band was rare for a long time, except for scanners that only scanned the airband. Since it was AM rather than FM, that’s likely the reason.

  3. I have fond memories of one of my grandparents house having one of the old crystal based scanners on all the time. It had 8 or 10 channels that had frequencies chosen by crystal so you couldn’t change it easily. My grandpa was on the volunteer fire department so some of those were for that but he had police and ambulance too I think. Most of the stuff they said was in code but you’d always hear some activity. The daily radio test for the fire department was neat to hear too. I have the radio since they’ve passed but it’s obviously not very usable now. Even if the frequencies were still in use, I don’t live where they did.

    Once things are back to normal, if you have a scanner then tune it to whatever school bus frequencies are in use. They always have good information about road conditions in the winter, or if it is foggy.

  4. I used to design police and fire dispatch radio systems at Motorola back when trunking first started. Those were the Syntor and SyntorX radio days. The “handytalkies” were often used by police to club people on the head. When they routinely got returned to the factory for service, the dried blood had to be cleaned out of them. I designed the Orange County Fire Dispatch system back in the early 80s. It was a trunked and simulcast system that used rubidium frequency standards to put the repeaters on exactly the same frequency so vehicles could drive almost anywhere in the county and talk without losing comms.

      1. The old system we built in the late ’80s used phase blocks to vary the delay before we adapted digital channel banks. That was a nightmare because of the varying freq response of the audio and how it played with the delays. Ah, the good ‘ol days!

    1. Hi, I was a PM on the LA County FD Simulcast system (and others) in the late ’80s. Programmed and wrote software for the CCII & +/Embassy. Indeed, the STX portables were great bricks! I also witnessed an MTX2000 fall off a guy up a tower abut 50′ and it landed on concrete. Battery flew off but I snapped it back on and made a call. Bulletproof! And, Mt. Lee was a crazy site!

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