What Is ADS-B?

ADS-B is the transponder protocol that turns every airliner, fighter, and helicopter on the planet into a free, real-time data source. Here is exactly how it works and why every live flight tracker on the web — ours included — is built on top of it.

The acronym in one sentence

Automatic Dependent Surveillance — Broadcast. Each word matters:

• Automatic. The aircraft transmits without being interrogated by ground radar — no one has to ping it first.
• Dependent. The position the aircraft reports is its own position from its own GNSS receiver (GPS or Galileo), not a position derived externally. Hence "dependent on the source".
• Surveillance. Air-traffic surveillance, originally a ground-based radar function. ADS-B replaces ground radar for tracking aircraft outside terminal control zones.
• Broadcast. The transmission is omnidirectional and unencrypted. Anyone with a $20 receiver in line-of-sight to the aircraft can decode it.

What gets broadcast

Once per second, each ADS-B-equipped aircraft transmits a short digital message on 1090 MHz (or 978 MHz in the United States for general aviation) containing:

• ICAO 24-bit address — a unique 24-bit identifier permanently assigned to the airframe. This is the only field that never lies.
• Callsign — the flight number for airline traffic (e.g. DAL459), or a tail number for general aviation. Pilots type this in before flight, so it can be wrong or blank.
• Position — latitude and longitude from the aircraft's GNSS receiver, accurate to within roughly 10 metres.
• Barometric altitude — altitude in feet above sea level, measured by a barometer (which is why it is reported relative to a 1013.25 hPa standard rather than the local sea level).
• Ground track — the heading the aircraft is moving along, in degrees from true north.
• Ground speed — the speed across the ground in knots (different from airspeed; tailwinds add to ground speed).
• Vertical rate — climbing or descending, in feet per minute.
• Mode-S category — a hint about the aircraft type (A1 = light, A2 = small, A3 = large, A4 = high vortex, A5 = heavy, A6 = high performance, A7 = rotorcraft).

How the data reaches your browser

1. The aircraft transmits. The transponder broadcasts on 1090 MHz roughly once per second.
2. Volunteer ground stations receive. Hobbyists, aviation enthusiasts, and ATC contributors worldwide run software-defined radios (SDRs) with a small antenna to decode the broadcasts.
3. The decoded data feeds an aggregator. The receivers stream their decoded messages to free public aggregators like ADSB.lol and adsb.fi, which deduplicate and combine the streams from thousands of receivers into a single global picture.
4. The aggregator serves an API. The aggregators expose free public APIs that any web application can query for "all aircraft within X nautical miles of these coordinates".
5. Your browser queries the API. When you open our live radar, it pulls the local picture from the aggregator every 10 seconds and renders the markers on a map.

Why ADS-B is mandatory

ADS-B Out (the transmit side) is now mandatory in most controlled airspace worldwide:

• United States. FAA required ADS-B Out in most Class A, B, C, and the airspace above 10,000 ft from 1 January 2020.
• European Union. EASA requires ADS-B Out for aircraft with MTOW > 5,700 kg or max speed > 250 kt since June 2020.
• Australia. Mandatory in controlled airspace since February 2017.
• Canada. Nav Canada is rolling out a phased mandate through 2026.

The mandate exists because ADS-B is the foundation for "Next Generation" air-traffic control. It is more accurate than radar (10 m vs 100 m), updates faster (1 second vs 4-12 seconds), and works in regions where ground radar would be uneconomical (oceans, polar routes, mountain valleys).

What does NOT broadcast ADS-B

You will not see everything on the radar:

• Most military aircraft over hostile or sensitive airspace. Military transponders can be switched off or set to broadcast an obfuscated ICAO address. Trainers, transports, and tankers usually broadcast normally over peacetime airspace; combat aircraft often don't.
• Small general aviation aircraft in uncontrolled airspace. If the flight stays below 10,000 ft outside Class B/C/D airspace, the FAA mandate doesn't apply.
• Aircraft with a failed transponder. Rare, but it happens.
• Aircraft outside receiver coverage. Polar regions, mid-Pacific, and Africa have sparse volunteer coverage. The mid-Atlantic and over Greenland are typically backfilled by satellite-based ADS-B providers (Aireon), but those feeds are commercial — the free public aggregators don't include them.

Privacy and ADS-B

Every aircraft you see on the radar is broadcasting its position by international regulation, in clear, every second. There is no opt-out for civil aircraft above the mandates' thresholds. The FAA does run a Limiting Aircraft Data Displayed (LADD) programme that asks aggregators to suppress specific tail numbers; participation is voluntary and only blocks the human-readable callsign, not the raw ICAO address.