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SDR Scanner Project I

Posted on Tuesday June 29, 2021

Over the weekend I spent some time playing around with the RTL-SDR dongle I had bought a few months ago. I want to learn more about how to use it and about what I can do with it and GNURadio.

I have the dongle hooked up to a Pine64 (the 2GB model) running Debian right now. For the antenna, I'm using the little dipole unit that came with the dongle, with its cable shut in the sliding patio door. It seems like the weather-stripping on the door is provding it enough padding so that it doesn't get too badly crimped in the doorjam. The antenna is then attatched to the apartment's rain gutter by means of a flexible tripod. I figure I'll leave it there until management says something about it; it's not hurting anyone.

Here's a photo of the antenna mounted to our apartment's rain gutter.

What initially interested me about SDR was the ability to decode APCO P25-encoded digital message traffic, such as that used by local law enforcement and public utilities. I had an idea in mind to try to build a web-based streaming police scanner for my area. A major snag I've run into while researching methods of decoding P25 though is that there isn't a lot of open-source software available that can perform that kind of analysis.

I might write another entry at some point wherein I review the pros and cons of various pieces of digital signal decoding software that I found around the web.

The tasks I think I need to accomplish in order to get this project up and running:

Receiving a signal over-the-air (accomplished via RTL-SDR)
Decode P25 trunking information from a control signal
Process that trunking information in order to follow message traffic across carrier frequencies
Convert message traffic into a digital audio signal
Compress and upload the encoded audio data to a server
Expose some way of accessing that data once it's on the server

Since the digital signal processing aspects of this project aren't my strong suit, I figured I'd start out by handling the networking and data-streaming parts first. I'll be writing a follow-up to this gemlog entry describing what that will look like. I plan on posting further gemlog entries as I continue to work on this project in my free time.

Stay tuned!

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