ADS-B (Automatic Dependent Surveillance, Broadcast) is the aviation equivalent of AIS. Every aircraft equipped with ADS-B Out broadcasts its position, altitude, velocity, and identifying information on 1090 MHz (Class 1) or 978 MHz (UAT, used by some US general aviation under 18,000 ft).
ADS-B Out has been mandatory for most US controlled airspace since January 1, 2020. Coverage is extremely good for commercial airliners and well-equipped general aviation, patchy for older GA aircraft and military traffic that doesn't transmit ADS-B (military often uses Mode S only, or runs ADS-B off).
Every ADS-B-equipped aircraft has an ICAO24 address: a 24-bit hex number (6 characters) permanently assigned to the airframe by the country of registration. ICAO24 is the primary key for joining ADS-B data to aircraft registration databases.
Country prefixes (first 1-3 hex digits) identify the registering country:
~300K active).
Our /live aircraft panel uses the ICAO24 to join live ADS-B position with the FAA Aircraft Registry. For US aircraft (any ICAO24 starting with A), the registration overlay shows N-number, make and model, owner, year built, and certificate dates. For non-US aircraft, the panel falls through to an adsb.fi community-feed enrichment which has type and operator info for much of the global commercial fleet.
ADS-B rides on top of Mode S, the air traffic control transponder protocol. The Mode S transponder has been in commercial use since the 1980s. ADS-B Out is essentially a Mode S extension that adds a position broadcast.
Older aircraft can have a Mode S transponder (which gives air traffic control its ICAO24 and altitude) without ADS-B Out (which would also give position). These show up on radar but not on our /live map.
The transponder broadcasts a 4-digit octal "squawk" code assigned by air traffic control. A handful of squawk codes are reserved and mean specific things:
Other codes (3214, 4567, etc.) are assigned by the controller for identification.
separation. Above 18,000 ft in the US it is reported as a flight level (FL370 = 37,000 ft pressure altitude).
on newer ADS-B Out installations. Differs from baro by 100-300 ft due to atmospheric pressure variation.
ground in knots. Differs from airspeed (true airspeed at altitude) by the headwind / tailwind component.
in strong crosswinds). 511 means not transmitted.
descending, typically reported in ft/min after conversion.
Our /live map polls OpenSky Network every 30 seconds for the US-airspace bounding box. OpenSky is a community-fed academic network with good US coverage. For per-aircraft enrichment we also call adsb.fi which carries operator-of-record and type-code data OpenSky does not.
/live page: real-time ADS-B map with click-to-detail andFAA registration overlay.
for deep ownership history by N-number.
and route playback by callsign.
for the underlying technical standards.