What is an ICAO code?
It is the standard for aircraft registration code. Every aircraft in member countries should have a unique registration code. This includes military aircraft. The standard is set by the International Civil Aviation Organization. In layman's terms, it's the "tail numbers" you see on anything in the air. The organizations website is here https://www.icao.int.
How do you find the code on a flying aircraft?
Well, they're in big letters on the tail of the craft or underneath. Sometimes, they're easy to see:
And sometimes, not so easy:
How else can we identify aircraft?
We started focused on the visual electromagnetic spectrum. This limits our ability to see what's flying around us to the visual range, and within reach by binoculars or zoom lenses. On other frequencies (visible light is 380 to 740 nanometers or 405-790 THz), thanks to ICAO, there are standards for electronic broadcast of identity, including the aircraft registration. They use the ADS-B standard. The military can shut off their transmitters if needed for operational reasons. Generally, when flying in commercial airspace, they broadcast over ADS-B. Sometimes the information is "interesting". This is where we begin.
How do you listen to these signals?
I have line of sight visibility over SF Bay and into San Pablo Bay. I can see a lot of aircraft coming and going across the bay in all directions. At first, I set up a simple software defined radio (SDR) and used a TV receiver to listen to ADS-B frequencies. All of this ran on a raspberry pi 3b+. It worked fine for a year. After spending some time writing an ADS-B decoder and trying to build a map myself, I found tar1090. This lead me to ADSBXChange. The map image above shows the possible range and shape of area my antenna can receive various signals. With this knowledge, and the tar1090 software, I started to record everything that flew overhead. I then ordered a Raspberry Pi 4 Model B and used the ASDBXchange image to contribute back to the world. Why the XChange over other alternatives? Simply because it's not for profit and unfiltered. The signals are out there, might as well record them all.
Here's a gallery of interesting aircraft I've "seen" fly within range of my antenna, aka "interesting icaos".
A Raspberry Pi?
Yes, it easily handles the load.