What is ADS-B?
ADS-B (Automatic Dependent Surveillance-Broadcast) is a system where an aircraft works out its own position by GPS and broadcasts it by radio, along with its identity, altitude and speed, about twice a second. Anyone with a receiver can pick it up, which is how live flight maps work.
The name, unpacked
- Automatic: it runs on its own, with no pilot input once it’s set up.
- Dependent: it depends on the aircraft’s own navigation, usually GPS, for its position.
- Surveillance: it lets air traffic control see the aircraft.
- Broadcast: it goes out to everyone, not to one radar station.
Radar finds an aircraft by bouncing a signal off it or asking its transponder to reply. ADS-B flips that around: the aircraft announces where it is, and anything listening can hear.
What it broadcasts
- A 24-bit address: a unique code assigned to the airframe by its country of registration, usually written as six hex characters. Aircraft registered in the US have addresses starting with A.
- A callsign: the flight’s identity, like an airline flight or a tail number. What callsigns mean.
- Position: latitude and longitude from GPS.
- Altitude, speed, direction and climb rate.
- The squawk code, including emergency codes. What 7700 means.
The signal isn’t encrypted. That is by design: it is meant to be heard.
Two frequencies
Most of the world uses 1090 MHz, a format called Extended Squitter that rides on the same frequency as transponder replies. Airliners and anything flying high use it. The United States also allows a second system on 978 MHz, called UAT, for aircraft flying below 18,000 feet. Plenty of small planes use UAT, and it has a bonus for pilots: it carries free weather and traffic data up to the cockpit.
Who has to use it
In the US, ADS-B Out has been required since January 1, 2020 in the busiest airspace: Class A, B and C airspace, within 30 nautical miles of the major airports, and above 10,000 feet over most of the country (except in the layer within 2,500 feet of the ground). A small plane that stays out of those areas doesn’t need it, which is why some Cessnas and Pipers don’t show up on maps. Europe requires it on larger and faster aircraft flying under instrument rules.
How flight maps hear it
A receiver can be as simple as a small antenna and a cheap radio dongle plugged into a computer. Thousands of volunteers run them on rooftops and in windows, and share what they hear with community networks. Put enough of them together and you get a map of the sky. The signal travels in straight lines, so a receiver can hear a cruising airliner a couple of hundred miles away but can lose a helicopter flying low behind a hill.
Filling the gaps: MLAT, TIS-B and privacy
Some aircraft have a Mode S transponder but don’t broadcast their position. When at least four receivers hear the same transponder reply, the network can compare the tiny differences in arrival time and work out where the aircraft must be. That is called multilateration, or MLAT, and it puts a lot of military and older aircraft on the map. TIS-B is different: in the US, ground stations rebroadcast radar targets to aircraft equipped to receive them. Those targets carry temporary addresses, not a real identity.
ADS-B can’t be switched off in required airspace, so the FAA offers privacy options instead. Its Privacy ICAO Address program (PIA) lets eligible US aircraft fly with a temporary address that isn’t linked to the owner in the public registry. The position is still there; the identity isn’t. More on why planes go missing from maps.
Why it matters to spotters
ADS-B is the reason you can point at a dot in the sky and know it’s a 737 heading to Denver, a KC-135 tanker on a training sortie, or an H135 air ambulance. Military aircraft often broadcast too, and the ones that don’t are part of the hunt. In Bogey, a signal that won’t say who it is shows up as a ghost.
