WTF ADS-B field notes
Reading aircraft observations
How to read aircraft identities, positions and measurements, and distinguish a report from an explanation.
Aircraft tracking combines messages about identity and movement into a sequence of observations. The source, timing and quality of those messages determine which conclusions the record supports.
What is ADS-B?
Automatic Dependent Surveillance-Broadcast is a system in which equipped aircraft broadcast information including identity and position. Receivers collect those messages for tracking.
What does MLAT mean?
Multilateration estimates position using the arrival times of signals at several receivers. It is a different source of position information from an aircraft broadcasting its own position.
What do knots and feet describe?
A knot is one nautical mile per hour, about 1.15 miles per hour. Ground speed measures movement over the ground; airspeed measures movement through the air. Aviation commonly uses feet for altitude, but the reference matters: pressure or geometric altitude should not automatically be read as height above the ground.
How do callsigns and registrations differ?
A callsign identifies a flight or operating identity; a registration identifies an aircraft in a registry. They can look similar, but they are separate fields. An observation should say which the record contains.
What does a gap in the track mean?
A gap means there are no suitable observations in that portion of the record. Reception, equipment and data processing can affect what is available. The gap does not establish what the aircraft did between samples.
What establishes the purpose of a flight?
A track describes movement. An operator's account or other checked source may explain a mission. An aircraft's owner does not establish who was aboard.
Background: the FAA's ADS-B explanation and FlightAware's guide to MLAT.
How the event journal counts observations
One published observation episode contributes one event, even when several detection rules describe it. The daily chart uses the episode's start date in UTC. Later edits and background profiles do not add events. Totals describe the publication's selected record, not every aircraft or flight in a region.