SDRs (or how I pull radio signals out of the air)

I figured I’ll be posting things based on how frequently I used them, at least to start. Eventually, I’d like to post based on your requests! Out of all the topics listed in the Introduction post, I use (or at least have various systems & automations using) SDRs to pull data out of radio signals the most frequently. In fact, most of what I have set up runs 24×7.

First off – what is a SDR? SDR stands for software defined radio. It means you can plug in one of these USB-based devices into your computer, and instantly be able to pull radio signals out of the air. Previously, like from whenever radio became a thing, until recently, radios were analog devices made up of resistors, transistors, and other things like that. They were bought tuned to a specific frequency and further, only listening to a certain type of signal on that specific frequency.

Software defined radio (SDR) changes all of that. It can be tuned to a wide range of frequencies and just passes along the data it gets to whatever program you use to decode the signals. Luckily for us, there are many open-source programs written by very smart people that already decode these signals!

This is the broad post for SDRs. I’ll write more posts for what I’m doing with the specifics. You may also see them called RTL-SDRs. RTL is the shortened name of the chipset (RealTek). There are other, more expensive SDRs not based on RealTek chipsets that can work better in some situations. For me, the $15-25 RTL SDRs do great. RTL-SDRs were originally intended to receive over-the-air TV signals. I’m sure they do fine with that but I’ve never tried to utilize them for their intended use.

Here are some of the things I’m doing with SDRs:

  1. ADS-B aircraft signal reception. As of the beginning of 2020, all civilian aircraft within US airspace must constantly broadcast their position. With a decently positioned antenna, these signals can be received up to 250 miles away from the aircraft! Link to part one of ADS-B reception.
  2. AMR – automated meter reading. Where I live, the electric and gas utility has meters installed at my house that broadcast their usage at least once a minute. I can receive these signals from my (and my neighbors!) meters and plot them to determine electricity/natural gas usage.
  3. Temperature sensor reading. You know those basic temperature sensor kits that most dads have sitting near the kitchen sink? The ones with a screen showing temperature/humidity inside and outside? Those are very easily decoded. I have one hanging outside and a few others around the house. They can also be placed in the freezer for advanced warning if a freezer goes out!

Some other thing I’m not currently doing but have thought about:

  1. Satellite reception for weather satellites (US GOES, Russian Meteor M2, and others). They transmit when overhead for the non-geosynchronous satellites and all the time for the geosynchronous satellites. The antenna required is quite a bit bigger than the around-the-house SDR stuff but still perfectly reasonable.
  2. General amateur radio reception
  3. Police radio scanning
  4. Aviation frequency reception

This post will be much better with pictures! I’ll add them soon.