Logging the airwaves and dealing with a big change

Logging the airwaves and dealing with a big change

Back in the summer of 2011, I wrote something which would just keep running on my Linux box to log radio traffic. It's still there, and it's still running, having collected over 1.6 million calls, all nicely organized and arranged for easy access.

As nice as this system is, it won't last forever. Wheels are turning in local governments, and the world of public safety radio is going to change yet again. This time, it's going to leave a lot of listeners behind. If I don't find a way to stay ahead of it, that will mean my system as well. Here's why.

Right now, the city system I monitor most of the time uses a trunking scheme called SmartNet. Essentially, one of their channels is nailed up transmitting "control channel" data for a day or so before rotating. The other 7 channels are used for call audio. The radios listen to the control channel for frequency assignments, and when their "talkgroup" is active, they will switch to the right audio channel and will start receiving. Likewise, when you go to transmit, your radio chats with the controller and gets a channel assignment.

The software I run here tracks that control channel so it knows which talkgroup is active on any given audio channel. Without that, there would just be a bunch of recordings but you'd have no idea who was talking without getting really familiar with all of the users. On a big regional system, that would be an impossible task.

The problem is that the city is going to change to a new system. This one is based around a series of standards called Project 25, or just P25. P25 itself isn't particularly new, but it does introduce a degree of complexity which previously hasn't been known in this area. At the moment, none of the local (South Bay) agencies run this kind of stuff.

The new hotness these days is running P25 in the 700 MHz band. Phase 1 of this new system, the Silicon Valley Regional Communication System (SVRCS) will cover Sunnyvale and Santa Clara, and it's supposed to grow as more money is allocated. Eventually, it should cover the entire county and will also tie in with the equivalent systems of the Peninsula, East Bay, and beyond. Ultimately, there will be a radio network where different agencies can talk to each other directly.

Unfortunately, some of the technologies involved are not the easiest thing to monitor. According to the specs, this new system will be "phase 2" P25, and that means the potential for audio channels which use TDMA and manage to cram two digital voice paths down a single 12.5 kHz channel. The days of just having ordinary FM audio are numbered, in other words.

All of my research suggests that only one specific scanner model can follow this kind of audio channel... and it's no longer being produced for some reason. That means if you didn't already have one and can't buy one somewhere (used), there will be no way to listen to the new system. Even if you do manage to find one, it would still be subject to the annoyances of traditional scanners.

Personally, I've been spoiled by my logging system, so going back to a traditional handheld scanner is not something I want to do.

So this is the state of affairs: this new system is coming online later this year, and it's using a scheme which has never been used in this area. I'm going to have to come up with a way to track it, tune it, and decode it in order to continue having a usable site. Otherwise, once they're done with the testing and acceptance and switch over, all of my existing tools will stop working.

I don't intend to let my hard work become useless. Unless they wind up encrypting the traffic, there should be a way to monitor this. I just need to get it going before they finally move away from their current systems.

This should be another interesting summer.