Aircraft announce themselves
ADS-B stands for Automatic Dependent Surveillance–Broadcast. Automatic because the aircraft sends it without being asked; dependent because it depends on the aircraft's own navigation system (GPS) for the position; broadcast because it is sent to everyone rather than answered to a particular radar. Roughly twice a second the transponder transmits the aircraft's identity, position, altitude, ground speed, heading and vertical rate on 1090 MHz. In the United States, many light aircraft use a second frequency, 978 MHz (UAT), instead.
Since 2020 ADS-B Out has been mandatory in most controlled airspace in the United States and Europe, so essentially every airliner and most business jets transmit it. That is the reason public flight tracking became so complete so quickly.
From a rooftop antenna to a world map
The signal is unencrypted and the receiver is cheap: a software-defined radio dongle, a small antenna and a Raspberry Pi will pick up every aircraft within one to two hundred miles of a rooftop. Volunteers run tens of thousands of these receivers and feed what they hear to aggregation networks. Open networks such as adsb.lol and airplanes.live, which power this site, merge the feeds, remove duplicates and publish the combined picture; commercial trackers run their own receiver networks and add satellite reception.
Because the position comes from the aircraft's GPS, it is accurate to a few metres. The lag you see on a map, typically a few seconds, is network and rendering time, not measurement error.
MLAT: locating aircraft that don't say where they are
Older transponders (Mode S without ADS-B) still broadcast identity and altitude but not position. If four or more receivers hear the same transmission, the tiny differences in arrival time let the network calculate where it came from, a technique called multilateration, or MLAT. It is how trackers show many military, government and older private aircraft. MLAT positions are less precise and need dense receiver coverage, so they appear mostly over populated areas.
Why some aircraft are missing
- Out of range. Receivers are on the ground, so coverage ends a few hundred miles offshore. An aircraft that vanishes mid-Atlantic has simply flown beyond the last receiver and will reappear near the far coast. Satellite-based ADS-B fills the gap but is not part of the open networks.
- No ADS-B. Some light aircraft, helicopters and older types do not transmit position, and without enough receivers for MLAT they stay invisible.
- Privacy programs. In the US, the LADD and PIA programs let operators block their aircraft from FAA-sourced data feeds or fly under a temporary anonymous identity. Open receiver networks still hear the raw broadcast, which is why they often show aircraft that commercial trackers hide, though an anonymised identity will not resolve to a registration.
- Military and state aircraft may transmit only in limited modes, or not at all, especially on operational flights.
Reading the fields
- Callsign is what the crew uses with air traffic control (DAL123, not DL123); see How to track a flight.
- Hex is the aircraft's permanent 24-bit ICAO address, the surest way to identify an individual airframe.
- Altitude is barometric altitude referenced to standard pressure, so it can differ from GPS altitude by a few hundred feet; trackers show it in feet.
- Squawk is the four-digit transponder code; three codes mean emergencies, explained in Squawk 7700, 7600 and 7500.
- Ground speed is speed over the ground, which includes wind; it is not the airspeed the pilots fly by.
- Type is the ICAO type designator, such as A20N or B38M, decoded in our type code list.
Limits worth remembering
Open ADS-B data is a best-effort community picture. Coverage depends on where volunteers happen to live, data can lag or drop briefly, and nothing on a public tracker is suitable for navigation or operational decisions. For watching the sky, following a friend's flight, or spotting what is overhead, it is remarkably good; our overhead page is built on exactly this data.