Automatic Dependent Surveillance-Broadcast

Automatic Dependent Surveillance-Broadcast (ADS-B) works a bit like an aircraft figuring out its own location and then shouting it out to everyone nearby. A device on the plane (a transponder) uses satellite positioning signals to calculate its coordinates, altitude, and speed, then continuously transmits this data over radio waves in all directions. Instead of a control tower on the ground hunting for the plane with radar, the plane itself announces "here I am" — which is why the name includes both "automatic" and "broadcast."

This signal isn't encrypted, so anyone with a cheap antenna can pick it up. As a result, hobbyists around the world have set up antennas on their rooftops to receive these signals, and community websites have emerged that aggregate this data over the internet and make it public. The collected data is used for everything from real-time flight tracking maps to personal projects.

In the article about the Mac app Claude built, ADS-B is described as "data collected by thousands of hobbyist antennas picking up the position signals that transponder-equipped aircraft continuously broadcast over public airwaves." A common misconception is that this is a commercial service run by a single company. Sites like adsb.lol and adsb.fi are community infrastructure where volunteers pool signals received by their own antennas and share them for free.

1. Open a real-time flight tracking site in your browser (e.g., adsb.lol, adsb.fi, or Flightradar24).
2. Zoom in on the map to your area and check whether any aircraft are currently flying overhead.
3. Click on an aircraft icon and see if altitude, speed, and route information appears in real time. All of this data comes from signals the aircraft itself broadcasts.