What Is ADS-B? How Flight Trackers Know Where Every Plane Is
When you open a live flight map, hundreds of planes move in real time. Nobody types those positions in. They come from the planes themselves, through a system called ADS-B.
The plane tells everyone where it is
ADS-B stands for Automatic Dependent Surveillance–Broadcast. That long name describes something simple:
- The plane works out its own position using satellite navigation (GPS).
- About twice a second, its transponder broadcasts that position by radio, together with its altitude, speed, heading and callsign.
- Anyone with a receiver in range can listen. Air traffic control does, other planes do, and so do thousands of hobbyists on the ground.
ADS-B is required for most airliners in many parts of the world, including Europe and the United States, which is why almost every commercial flight can be tracked.
Where flight trackers come in
A cheap receiver with a small antenna can hear planes up to about 250 to 400 km away, depending on height and terrain. Volunteers around the world run these receivers and share the data with networks such as FlyItalyADSB and adsb.lol. fly24 combines several of these networks with commercial data from Plane Finder, so you see as many planes as possible on one map.
Why coverage differs between countries
Because the data comes from receivers on the ground, a region is only well covered when people there run receivers. Europe and North America have thousands. Parts of South Asia, Central Asia and Africa have far fewer, so a plane can disappear from the map over those areas and reappear when it comes back into range.
Over oceans there are no ground receivers at all. Some trackers fill these gaps with satellite ADS-B, which is expensive, or estimate the position from the last known speed and heading.
What ADS-B tells you
| Data | What it means |
|---|---|
| Callsign | The flight's radio name, like PIA301 |
| Altitude | Height in feet, from the plane's own instruments |
| Ground speed | Speed over the ground in knots, so it includes the wind |
| Heading | The direction the plane is pointing |
| Vertical rate | How fast it is climbing or descending |
| Squawk | A four-digit code set by air traffic control |
Is it private?
ADS-B signals are unencrypted radio broadcasts by design: being visible is what keeps planes safe. Some private aircraft owners can ask for their aircraft to be hidden or anonymised on public trackers, and military aircraft often switch ADS-B off.