ADS-B Explained: The Technology That Powers Flight Tracking

Twice every second, the airliner cruising seven miles above your house introduces itself to anyone who cares to listen. It announces who it is, where it is, how high it is flying, how fast it is moving and where it is turning, all in a burst of radio energy that lasts 120 millionths of a second. Nobody asked it to speak. No radar beam swept across it and demanded a reply. The aircraft simply broadcasts, continuously and automatically, into the open air. That habit of compulsive self-disclosure is called ADS-B, and it is the single most important reason you can open a live flight tracking map and watch the whole world fly.

What the name actually means

ADS-B stands for Automatic Dependent Surveillance-Broadcast, and for once an aviation acronym unpacks neatly. It is automatic because it requires no pilot action and no interrogation from the ground; the equipment transmits on its own schedule. It is dependent because the whole scheme depends on the aircraft knowing its own position, which it derives from satellite navigation, primarily GPS. It is surveillance because the purpose is to let others keep track of the aircraft. And it is a broadcast because the messages are addressed to no one in particular. Anyone with a receiver tuned to the right frequency gets the same data an air traffic controller gets.

That last point deserves a pause. Older surveillance was a conversation between a radar site and a transponder, and you needed the radar to make sense of it. ADS-B is more like a lighthouse. The aircraft does not know or care who is listening, which is precisely why a global hobbyist tracking community became possible. A commercial pilot, a national air navigation service and a teenager with a 30-dollar receiver all drink from the same stream of bits.

Anatomy of a 1090 MHz message

The dominant flavor of ADS-B rides on 1090 MHz, the same frequency aircraft transponders have used to answer radar since the 1950s. The messages are formally called extended squitters, a lovely bit of engineering slang; a squitter is a transmission sent spontaneously rather than in reply to an interrogation. Each extended squitter is 112 bits long. Some of those bits carry the aircraft's unique 24-bit ICAO address, a hardware-level identity we explore in detail in our guide to Mode S and the 24-bit address system. The rest carry the payload: position in compact latitude and longitude encoding, barometric and geometric altitude, ground speed, vertical rate, heading, callsign and a set of quality indicators that tell listeners how much to trust the fix.

No single message contains everything. Position messages alternate with velocity messages and identification messages, so a receiver assembles the full picture over a second or two. Position itself is sent in a clever scheme called compact position reporting, which splits the globe into zones so that coordinates fit into 17 bits each; a decoder combines an even-format and an odd-format message to resolve the ambiguity. It is a dense, elegant protocol designed in an era when every bit was expensive.

The mandates that switched it on

ADS-B existed in the 1990s as an experiment and in the 2000s as an option. What made it universal was regulation. In the United States, the FAA required ADS-B Out equipment meeting the DO-260B standard for flight in most controlled airspace from January 1, 2020, the date written into 14 CFR 91.225. In practical terms that covers Class A, B and C airspace, airspace above 10,000 feet, and the busy veil around major airports. The FAA's ADS-B program was the centerpiece of the NextGen modernization effort, a bet that satellite-derived surveillance could be more accurate and far cheaper to operate than maintaining hundreds of rotating radar heads.

Europe followed months later. Under rules administered by EASA, aircraft heavier than 5,700 kilograms or faster than 250 knots true airspeed had to carry compliant ADS-B Out from June 7, 2020. Australia moved even earlier, and Canada has been phasing in a mandate tied to satellite reception. The net effect is that essentially every airliner, every business jet and a large share of general aviation aircraft in the developed world now self-report continuously. On a typical busy day the global fleet visible to tracking networks runs well past 15,000 simultaneous airborne aircraft; you can watch the daily rhythm rise and fall in our traffic statistics.

ADS-B Out, ADS-B In, and the two frequencies

The mandate language always says ADS-B Out, and the qualifier matters. Out is the transmitting half: the aircraft tells the world about itself. ADS-B In is the optional receiving half: equipment that listens to other aircraft's broadcasts and to ground-station uplinks, painting nearby traffic on a cockpit display. In the United States, ADS-B In also unlocks two free data services. TIS-B, Traffic Information Service-Broadcast, rebroadcasts radar-derived traffic so that ADS-B In users can see aircraft that are not yet ADS-B equipped. FIS-B, Flight Information Service-Broadcast, uplinks weather graphics, meteorological reports and airspace notices.

There is also a second frequency in the American system. Aircraft flying below 18,000 feet in the US may satisfy the mandate with a Universal Access Transceiver, or UAT, which operates on 978 MHz instead of 1090 MHz. UAT was designed with general aviation in mind and has more bandwidth per message, which is why FIS-B weather rides on it. Airliners and anyone flying in Class A airspace or internationally use 1090 MHz extended squitter, usually written 1090ES. Hobbyist receivers overwhelmingly listen to 1090 MHz, though many US enthusiasts run a second dongle for 978.

From antenna to map

How does a broadcast in the stratosphere become an icon on your phone? Range is the first constraint. A 1090 MHz signal travels by line of sight, so a ground receiver can typically hear a cruising airliner out to 200 or 250 nautical miles, but a low-flying aircraft only over a much shorter radius, and nothing at all beyond the horizon. Tracking networks solve this with volume: thousands of volunteer-hosted receivers, each decoding whatever it hears and streaming the results to central servers. Aggregators merge the feeds, deduplicate messages heard by dozens of stations at once, smooth the trajectories and attach schedule and route data. The whole pipeline, from transponder to your screen, usually takes only a few seconds, and we trace it end to end in our tour of where flight tracking data actually comes from.

ADS-B turned aircraft surveillance from a question the ground had to ask into a statement the sky volunteers freely.

The gaps in this picture are as instructive as the coverage. Oceans, deserts, polar regions and open mountain country have few receivers, which is why long-haul flights historically vanished mid-crossing. Space-based reception, in which satellites carry the 1090 MHz receivers instead of rooftops, has closed much of that gap since 2019; our companion piece on tracking flights by satellite explains how. And when an aircraft transmits without position data, networks can still locate it by timing its signals from multiple stations, a technique called multilateration. If you want to become one of the rooftops yourself, our step-by-step guide to building a Raspberry Pi ADS-B receiver will have you feeding the map in an afternoon.

Honest limits, and what comes next

ADS-B has real weaknesses. The broadcasts are unencrypted and unauthenticated, which means they can be received by anyone and, in principle, spoofed by a bad actor with transmit equipment; air navigation providers mitigate this by cross-checking against radar and multilateration rather than trusting any single source. The system also depends on GPS, so satellite navigation outages or jamming degrade it, one reason regulators deliberately keep independent radar surveillance alive as a backup layer. Privacy is a live debate too: because the broadcasts are public, programs now let some operators fly with rotating temporary addresses, a topic we cover in our guide to tracking private jets and what gets blocked.

Researchers are already sketching successors. Eurocontrol and the FAA have studied higher-bandwidth surveillance links, and academic groups working with the OpenSky Network publish regularly on securing and authenticating the existing protocol. But the installed base of DO-260B transponders is enormous, and aviation replaces avionics slowly. The 112-bit squitter chirping away on 1090 MHz will likely remain the heartbeat of flight tracking for decades. Every time you glance at a tracker to see whether a friend's Boeing 737-800 has begun its descent, you are decoding that heartbeat.

The next time a contrail crosses your evening sky, remember that the aircraft drawing it is talking the entire way, in a dialect of 112-bit bursts that you now know how to read.