Beginner's guide to software-defined radio for SIGINT

I love radio, particularly when it bleeds into signals intelligence. Since the onset of the sweeping power grid blackouts across the Iberian Peninsula and into Western Europe, I’ve been surfing the airwaves and learning new things about the fascinating world of radio frequencies. That’s why I bought an RTL-SDR, or Realtek Software Defined Radio.

I am a survivor of the 1994 Northridge Earthquake, which rocked Los Angeles and caused around two million people to lose power. In fact, I was literally in the epicenter when it hit with a magnitude of 6.7. The quake caused immeasurable damage to critical infrastructure, including power substations, transformers, and transmission lines.

During this time, we suffered a blackout that lasted more than a day, and we had nothing to inform us what was going on other than what we could witness ourselves. Today, whether we’re dealing with a natural disaster, war, or a technical failure, people who know how to use radios have something to fall back on when their internet connections and mobile devices go dark.

Whatever the case may be, whether you’re preparing for a disaster or looking to decipher coded messages being transmitted from foreign state-run news agencies catering propaganda to Western listeners, the airwaves are filled with fascinating information just waiting to be intercepted.

Plus, traditional radio handsets don’t offer visual waterfall displays or spectrum analysis. This means that while a spectrum analyzer can reveal weak signals, a radio handset might only play static, which gives no indication that a signal is present or how to troubleshoot it. Without that visual feedback, you wouldn't know whether to move closer to the source or build an antenna booster to improve reception.woman in brown jumpsuit jumping to a hole of radios

In other words, whether it's tuning in to air traffic control, decoding pager transmissions, capturing satellite weather images, or hunting for covert comms, SDR turns your computer or device into a powerful, customizable surveillance and exploration tool for traversing the airwaves around us.

I bought the RTL SDR Blog V4 kit for just £59 on Amazon, which comes with pretty much everything you’re going to need, except for the software, which is free for Windows, Linux, and macOS.

What is SDR?

SDR is a radio communication system in which software, rather than expensive hardware, performs key signal processing tasks, like modulation, demodulation, filtering, and decoding. In other words, SDR lets your computer become the radio.

Instead of fiddling with a traditional radio handset that usually has frequency range limitations, SDR opens the full radio spectrum to you with just a USB dongle and some software.

Traditional radios are often hardware-limited to specific bands, such as FM, CB, VHF/UHF, etc. I have several radio handsets, each with its limitations. Obviously, a Citizen-Band radio can’t listen in on Ham Radio frequencies. An FM-only radio can’t listen to shortwave radio broadcasts from around the world. SDRs can scan massive spectrum ranges.

RTL-SDR V4 kit from Amazon. Type C to USB adapter and phone not included.

Rather disappointingly, iOS doesn’t have driver support for the SDR. But that’s okay, because if you really just have to go mobile like I do, hopefully you’ve got a spare Android lying around, which means you can download the driver from the Google Play Store, and the store also offers a few decent options for SDR apps like SDR Touch.

This means I can go fully mobile without lugging around a laptop.

Signals intelligence: intercepting the codes

Signals Intelligence or SIGINT is essentially a key discipline within intelligence gathering, and refers to intercepting and analyzing electronic signals and communications to collect information.

A handful of state-sponsored shortwave radio stations have gained notoriety for transmitting coded messages, usually in the form of number stations. These are mysterious broadcasts which typically consist of unexplained sequences of numbers, tones, or sounds believed to be encrypted instructions to intelligence operatives.

Curiously, most stations are not openly acknowledged by governments, but some have interesting histories centered around broadcasting coded messages over shortwave, and a few are still active.

Throughout most of the 1990s, broadcasts from Radio Havana, Cuba, specifically the numbers station known as HM01 (Hybrid Mode 01), were commonly intercepted by everyday radio listeners across the United States.

These strange broadcasts featured sequences of spoken numbers encrypted with a one-time pad, a method which was commonly used by Cuban intelligence operatives operating in the US in receiving covert instructions.

Remarkably, this intriguing numbers station is still active. Better yet, it continues to transmit coded messages. Reports confirm broadcasts on such frequencies as 1-345 kHz, 10860 kHz, and 13435 kHz, usually occurring between 5 a.m. and 7.30 a.m. UTC.

There are a handful of others, but this one is even more interesting: Russia’s UZB-76, commonly referred to as The Buzzer, a name derived from its distinctive buzzing sound.

The station was first reported in the 1970s and continues to this day, transmitting a monotonous buzzing noise around 25 times per minute, 24 hours a day. It does not appear to serve any other purpose.

On rare occasions, the buzzing is interrupted by either a live Russian male or female voice transmitting a structured message in a format or template such as “Monolith” or “Uzor,” which are believed to follow classified military protocols. These formats are often prefaced by phrases like “UZB-76. UZB-76. 180 04 BROMAL 74 27 99 14.” BROMAL could be a call sign or a coded phrase.

Operating on the frequency 4625 kHz, the unexplained transmissions broadcasting out of UZB-76 have led listeners to speculate that the station possibly operates as a kind of dead man’s switch for military units within Russia’s Western Military District, to maintain military communications.

How to use AI to analyze radio streams

Using ChatGPT, I collected a list of every mysterious UZB-76 transmission from 1982 to the present day, only to find several discernible patterns, which follow a highly structured operational protocol.

For example, nearly all voice messages follow this strict, repeating pattern: [Callsign] [5-digit identifier] [Codeword] [4-digit number] [4-digit number]

Also, certain code words have been repeated throughout the years, which are likely persistent operational identifiers. However, the analysis is rather lengthy, but the results were fascinating.

I’m not trained to understand or interpret anomalous tones or noises I hear whenlistening to radio signals. Using ChatGPT as an interpreter to analyze patterns has been a fruitful endeavor, especially since it’s easy to record RTL-SDR streams and upload them to ChatGPT for deeper insights.

Users can easily record these streams and save them in a variety of formats, since ChatGPT can convert audio recordings into spectrograms, decode embedded patterns, and even identify signal types or anomalies using acoustic analysis tools.

This means you do not need formal training in signal analysis if you want to investigate broadcasts. I use ChatGPT as an OSINT tool to analyze linguistics by identifying accents and dialects, translating foreign messages with contextual nuance, detecting emotion, urgency, or stress patterns in speech transcripts, extracting keywords or themes from conversations, and spotting anomalous cues and patterns.

Spying the airwaves over free virtual streams

If you’re thinking about going down the rabbit hole of SDR, there’s abundant software support and plenty of apps to explore, but you don’t need to commit to hardware right away, only to find out it’s not for you.

Many users host publicly shared SDR streams online, which act as virtual radio receivers that you can freely control and listen to in real time without needing to subscribe to anything.

This means they act as remotely accessible SDRs, allowing anyone with an internet connection to monitor frequencies, investigate unusual signals, and even take part in open-source SIGINT efforts from virtually anywhere in the world.

Here’s a list of remote streams you can hop on:

• https://www.receiverbook.de/ (secure HTTPS)
• https://rx-tx.info/ (secure HTTPS)
• http://www.websdr.org/ (Insecure HTTP)
• http://kiwisdr.com/public/ (insecure HTTP)

This is essentially a niche radio hobbyist community, and it shows in the infrastructure. While there are several awesome remote SDR stream directories you can hop on, many do not depend on secure HTTP, because many of these sites are run by individuals or small groups rather than commercial entities.

Therefore, security isn’t always a priority. Consequently, you should avoid entering any personal information on these platforms.

For Android and iOS users, you can download MagicSDR, which is also available for Windows. I prefer this over browsing the web pages, since everything you need is centralized in one app.

These web-based SDRs function like virtual radio receivers, giving anyone with internet access the ability to monitor global frequencies, analyze strange signals, and even contribute to open-source SIGINT efforts – all without owning any physical equipment.

Desktop SDR Software

I use SDR++ on my Windows box, macOS, and Linux machines. It’s free and one of the easiest-to-use SDR software programs I’ve encountered.

If you don’t want to chase down a bunch of apps, you can do what I did at first, and download DragonOS, which is a Lubuntu-based Linux distribution .iso specifically designed for SDR. Mount it on a USB thumb drive or spin up in a virtual machine.

Whatever your motivation, whether you’re like me and driven by a strong wind of curiosity or want to do some disaster prep for a potential long-haul communications blackout, SDR is an excellent option, especially over traditional handsets any day of the week.