What Is ADS-B? How Live Flight Tracking Works
Every aircraft you see moving on a live flight tracker is, in most cases, telling you where it is — literally broadcasting its position over radio, several times per second, to anyone with a receiver on the ground. That technology is called ADS-B, and it is the backbone of modern flight tracking.
The name, decoded
ADS-B stands for Automatic Dependent Surveillance – Broadcast:
- Automatic — it transmits on its own, no pilot action or ground interrogation needed.
- Dependent — it relies on the aircraft's own navigation systems (GPS) to know where it is.
- Surveillance — it exists so air traffic control and other aircraft can watch the sky.
- Broadcast — it sends the message to everyone in range, not to one specific receiver.
What an aircraft actually broadcasts
On the 1090 MHz frequency, a transponder emits short digital messages containing the aircraft's ICAO24 address (a unique hex identifier like a835af), its GPS position, barometric and geometric altitude, ground speed, track angle, vertical rate, callsign, and squawk code. Different message types carry different fields, which is why a tracker sometimes shows a callsign before it has a position.
From radio waves to the map
ADS-B signals are line-of-sight: a receiver on the ground can typically hear aircraft up to about 250 nautical miles away, depending on antenna height and terrain. Thousands of volunteers and organizations run receivers and feed the decoded positions to public aggregators. FlyRadar combines those feeds, deduplicates the aircraft, and renders them on the map — usually with only a few seconds of delay between transmission and what you see.
Why some aircraft don't appear
ADS-B is mandatory in most controlled airspace (the US since 2020, Europe earlier), but it isn't universal. Some aircraft only have older Mode S transponders, which don't broadcast GPS position — their location has to be computed differently (see position sources explained). Military and government aircraft can block their data, and aircraft over oceans or remote areas may simply be out of receiver range.
ADS-B In: the other direction
The same system also works in reverse. Aircraft equipped with ADS-B In receivers can see the broadcasts of nearby traffic and receive uplinked weather and traffic information from ground stations (in the US, via the 978 MHz UAT band). It's both a surveillance technology for the ground and a collision-awareness tool for the cockpit.
Curious what it looks like in practice? Open the live map and click any aircraft — the position source ring around it tells you whether its location came from ADS-B or another technology.