What Is ADS-B? A Complete Beginner’s Guide to Flight Tracking

If you have ever opened a flight tracker and watched little aircraft icons crawl across a map, you have used ADS-B — whether you knew it or not. ADS-B is the technology behind nearly every live flight tracking service on the planet, including Planes.FYI. And once you understand how it works, aviation gets a lot more interesting.

What does ADS-B stand for?

ADS-B stands for Automatic Dependent Surveillance–Broadcast. That is an intimidating name, so let us break it apart:

• Automatic — the aircraft transmits without a human pressing anything
• Dependent — it depends on the aircraft knowing its own position (usually from GPS)
• Surveillance — the purpose is to let others see where the aircraft is
• Broadcast — the signal goes out to anyone listening, not to a specific recipient

In plain English: the plane figures out where it is using GPS, then shouts its position out into the air about once per second so that controllers, other aircraft, and hobbyists on the ground can all see it.

What does an ADS-B signal contain?

Each broadcast packs a lot of information into a very short radio burst on 1090 MHz (or 978 MHz in parts of the United States). A typical transmission includes:

• A unique 24-bit ICAO aircraft address — the plane’s digital fingerprint
• Latitude and longitude from the onboard GPS
• Altitude, both barometric and GPS-derived
• Ground speed and heading
• Vertical rate (climbing or descending)
• Flight number or callsign, when the crew has entered it

Everything you see on a flight tracker — the aircraft symbol, the trail behind it, the altitude badge, the flight number — ultimately comes from stitching these little broadcasts together.

ADS-B Out vs ADS-B In

You will often see aviation documents split ADS-B into two halves:

• ADS-B Out — the transmitter. The aircraft is shouting its position into the world.
• ADS-B In — the receiver. The aircraft is listening to other aircraft around it.

ADS-B Out is mandated in most controlled airspace worldwide. ADS-B In is optional — it gives pilots traffic awareness but is not legally required. When you run a flight tracker, you are effectively being a giant crowd-sourced ADS-B In receiver.

How your phone sees live flights

Your phone does not have an ADS-B receiver — and it does not need one. A global network of volunteers and sites operates small ground stations (many of them no bigger than a Raspberry Pi with an antenna) that listen to ADS-B broadcasts and upload what they hear to aggregator networks. Services like Planes.FYI combine those feeds, merge them with airline schedule data, and send the result to your phone over the internet.

This is why you can sometimes see gaps over oceans or remote areas: no receivers on the ground means no one is listening. Over the continental United States and Europe, coverage is now close to 100 percent.

What ADS-B does not show

ADS-B is powerful but not omniscient. A few things to keep in mind when you are staring at a tracker:

• Military aircraft frequently turn off or block their transmitters
• Some private jets opt out via the LADD program (Limiting Aircraft Data Displayed)
• Older aircraft without ADS-B still fly below certain altitudes and may not appear
• Signals can be blocked by terrain, buildings, or lost over oceans

If a flight suddenly disappears from the tracker, the plane almost certainly did not — it just flew out of receiver coverage or into a filtered list.

Why this matters for casual plane spotters

Understanding ADS-B transforms the experience of watching the sky. That plane overhead is not a mystery anymore — it is a machine broadcasting its name, destination, and altitude at you, a dozen times a minute. Flight trackers are just the translator. Once you know the language, the sky gets a lot more alive.