The Flight Tracking Glossary: 60 Terms Explained Simply

Every hobby guards its gates with vocabulary, and flight tracking is worse than most: a live map greets newcomers with squawks, flight levels, MLAT targets and Zulu timestamps as if everyone were born knowing them. The good news is that the whole dialect fits in an evening. These sixty terms, grouped by theme and defined in plain English, are the working vocabulary behind every guide on this site and every data panel on the live map. Read them once, keep the page bookmarked, and the map stops speaking in code.

The signals: How planes announce themselves

• ADS-B. Automatic Dependent Surveillance-Broadcast. The aircraft works out its own position by satellite and broadcasts it about twice a second, unencrypted. The backbone of public tracking, mandated in most US controlled airspace since 2020 by the FAA, and worth a full read in our ADS-B deep dive.
• Transponder. The radio box that answers ground interrogations and sends ADS-B. When people say a plane "went dark," they usually mean its transponder stopped transmitting.
• Mode A/C. Older transponder modes sending only an identity code and altitude. No position, which is why such targets need MLAT.
• Mode S. Selective interrogation mode with a unique address per airframe; the foundation ADS-B is built on.
• ICAO 24-bit address. A hexadecimal code like A360A5, permanently assigned to one airframe. Its digital fingerprint, about 16.7 million possible values worldwide.
• Squawk. The four-digit code a controller assigns to a flight, set on the transponder. Also a verb: "squawk 3471."
• 7500 / 7600 / 7700. The three emergency squawks: unlawful interference, radio failure, general emergency. The full story is in our guide to transponders and squawk codes.
• MLAT. Multilateration. Four or more receivers time the same transmission and triangulate the sender, positioning aircraft that never broadcast where they are.
• 1090 MHz. The radio frequency carrying transponder replies and ADS-B for airliners worldwide; 978 MHz (UAT) serves some US general aviation.
• Primary radar. Radar that detects raw echoes off metal: position, but no identity. Secondary radar (SSR) interrogates transponders and gets identity plus altitude back.
• ADS-C. A contract-based cousin of ADS-B: the aircraft reports its position over satellite datalink to specific control centers at set intervals, the workhorse of oceanic tracking.
• Ground station / feeder. A receiver, often a hobbyist's, that hears aircraft and feeds a tracking network. Networks like OpenSky are built from thousands of them.

Identity: Names, numbers and fingerprints

• Registration / tail number. The painted identity of one airframe, like N301DN or D-AIMA. Follows the plane through its whole life with an owner.
• Callsign. The radio identity of a flight, like DAL123 or SPEEDBIRD 287. Differs from the ticketed number often enough that we wrote a whole piece on flight numbers versus callsigns.
• Flight number. The commercial label sold to passengers: DL123. One flight can carry several via codeshares.
• IATA code. Two-letter airline codes (DL) and three-letter airport codes (ATL) used in booking systems.
• ICAO code. The operational equivalents: three-letter airline codes (DAL) and four-letter airport codes (KATL). The K in KJFK simply marks a United States airport in the ICAO scheme.
• Telephony designator. The spoken airline name on the radio: Speedbird, Shamrock, Dynasty.
• Codeshare. One operated flight sold under multiple airlines' numbers.
• Livery. The paint scheme. Special liveries (retro, alliance, anniversary) are prize sightings for spotters.
• Type code. ICAO's shorthand for an aircraft model: B738 for the 737-800, A21N for the A321neo. Trackers use them everywhere, so the common ones are worth recognizing on sight.

Motion: The numbers on the data panel

• Knot (kt). One nautical mile per hour, about 1.15 mph. Aviation's speed unit.
• Ground speed. Speed over the Earth's surface, wind included. What trackers display.
• Airspeed. Speed through the air itself. A jet with a 100-knot tailwind shows ground speed far above its airspeed.
• Mach number. Speed as a fraction of the speed of sound; long-haul jets cruise near Mach 0.85.
• Altitude (barometric). Height measured by air pressure, broadcast by the transponder in feet.
• Flight level. Standardized pressure altitude in hundreds of feet: FL350 is 35,000 feet on a standard altimeter setting. Jets cruise mostly between FL310 and FL410 because thin air is cheap air.
• Vertical rate. Climb or descent speed in feet per minute; plus or minus 2,000 is a brisk but normal value.
• Heading vs track. Heading is where the nose points; track is where the plane actually travels once wind shoves it. Trackers show track.
• Great circle. The shortest path over a sphere, why Chicago to Beijing arcs over the Arctic on a flat map.
• Waypoint. A named point in the sky, often five pronounceable letters (BARBQ, WHALE), that routes are built from.
• Airway. A highway in the sky connecting waypoints.
• NAT tracks. The North Atlantic organized track system, redrawn twice daily around the jet stream.
• Holding pattern. A racetrack-shaped queue in the sky, instantly recognizable on a map trail.
• Squawk 1200 / 7000. The "nobody assigned me anything" VFR codes in the US and Europe respectively; a hint you are watching casual general aviation.

Learn thirty nouns and the sky starts speaking in sentences: a squawk, a flight level and a vertical rate are a complete story in three numbers.

Time, schedules and status

• UTC / Zulu time. Aviation's single worldwide clock, written 1435Z. Every timestamp in raw tracking data is UTC; your app quietly converts it.
• ETA / ETD. Estimated times of arrival and departure. A tracker's ETA is computed from live position and speed.
• Block time. Gate to gate, the time airlines schedule and crews are paid on. Off-block means pushback; on-block means parked.
• Wheels-up / wheels-down. The actual takeoff and touchdown moments, what position data measures best.
• Diversion. Landing somewhere other than the planned destination, for weather, medical or technical reasons. Usually undramatic.
• Go-around. An abandoned landing followed by a climb and another approach. On the map: a low pass, a climb, a loop. Routine and safe.
• Ferry flight. A repositioning flight with no passengers, often with an odd callsign and no schedule match.
• Turnaround. The ground pit stop between a landing and the next departure, choreographed to the minute by fueling, catering, cleaning and loading crews all working at once.
• Slot. A permitted takeoff, landing or airspace-entry window. Miss it and the delay cascades.
• Curfew. Hours when an airport bans or restricts operations, usually overnight for noise.

Map, network and spotter slang

• Trail / track log. The breadcrumb line behind an aircraft icon, and the downloadable history of the flight.
• Estimated position. A dashed or faded segment where the tracker is projecting rather than receiving, common mid-ocean, as explained in why flights disappear.
• Coverage. The territory within range of a network's receivers. Radio horizon: the line-of-sight limit, about 230 nautical miles for a jet at cruise.
• LADD. The FAA's Limiting Aircraft Data Displayed program, which withholds enrolled aircraft from FAA-fed public displays; details at faa.gov.
• PIA. Privacy ICAO Address: temporary, rotating electronic identities for US aircraft seeking anonymity.
• Blocked aircraft. Any target a given site declines to identify or display, for legal, contractual or program reasons.
• Feeder network. The crowd of volunteer receivers supplying a tracking site. Feeding your own data usually earns a free subscription.
• Heavy. Radio suffix for aircraft above 300,000 pounds maximum weight ("Delta 123 heavy"), warning of wake turbulence. Super is reserved for the A380.
• Wake turbulence. The horizontal tornado of disturbed air behind a large aircraft, the reason for spacing between arrivals.
• METAR / TAF. Coded airport weather observation and forecast, decodable with practice and available through NOAA's aviation weather services.
• NOTAM. Notice to Air Missions: official bulletins about runway closures, airspace restrictions and hazards.
• GA. General aviation, everything that is not airline or military, from Cessnas to Gulfstreams.
• Spotter. Someone who watches, photographs and logs aircraft for the joy of it. If you have read this far, the diagnosis is probably positive.

How to actually learn all this

Do not memorize the list. Instead, open the live map, click one flight, and account for every field in its data panel using the definitions above: the callsign, the squawk, the flight level, the ground speed in knots, the vertical rate, the type code, the registration. One flight, fully decoded, teaches more than three read-throughs. Tomorrow, decode a second one over the ocean and watch for the estimated-position trail. By the weekend, glance at the statistics dashboard and notice you now understand every column. The vocabulary sticks because the map keeps quizzing you, gently, forever. When you are ready to go beyond definitions, the full walkthrough of tracking a flight end to end puts the whole dictionary to work.

Sixty terms sounds like a language course, but it is really a map key. Keep this page open beside the sky for a week and the jargon dissolves into what it always was: short words for a system that runs on precision, spoken by machines that never stop introducing themselves.