Flight-tracking glossary

1

1090ES

1090 MHz Extended Squitter, the international ADS-B data link used by airliners, business jets and most aircraft worldwide. In the US it is required for flight at and above 18,000 ft.

A

ADS-B

Automatic Dependent Surveillance–Broadcast. A system in which an aircraft determines its own position by GPS and broadcasts it, with its identity, altitude and velocity, about twice a second to ATC and anyone with a receiver.

ADS-B In

Equipment that lets an aircraft receive ADS-B, TIS-B and FIS-B broadcasts, giving the pilot a display of nearby traffic and, on 978 MHz, weather.

ADS-B Out

Equipment that broadcasts an aircraft's own ADS-B messages. Required since 1 January 2020 for most US controlled airspace under 14 CFR 91.225.

Air traffic control (ATC)

The service that separates aircraft and keeps traffic flowing, using towers at airports, approach facilities (TRACONs) and en-route centers (ARTCCs).

ARTCC (Center)

Air Route Traffic Control Center. One of 20 FAA facilities in the contiguous US that handle aircraft en route between airports, usually at higher altitudes. Pilots call them "Center," as in "Chicago Center."

B

Bank (hub bank)

A wave of arrivals followed by a wave of departures at an airline hub, timed so passengers can connect. Banks create bursts of traffic visible in national counts.

Barometric altitude

Altitude measured from air pressure. Above 18,000 ft in the US it is referenced to standard pressure (29.92 inHg). This is the altitude ATC uses and the one most trackers show.

C

Callsign

The identifier a flight uses with ATC and broadcasts in ADS-B, such as DAL1520 for an airline or N123AB for a private aircraft.

Class A airspace

In the US, the airspace from 18,000 ft MSL up to FL600. All flights must be IFR with an ATC clearance and carry 1090 MHz ADS-B Out.

Class B airspace

Controlled airspace around the busiest US airports, shaped like an upside-down wedding cake. Every aircraft needs an ATC clearance to enter.

Class C airspace

Controlled airspace around medium-sized US airports with radar approach control. Pilots must establish two-way radio contact before entering.

E

Emergency squawk

One of the three transponder codes reserved for emergencies: 7700 general emergency, 7600 lost radio communications, and 7500 unlawful interference.

Emitter category

A code in ADS-B messages describing what is transmitting, such as A1 (light aircraft), A5 (heavy), A7 (rotorcraft) or B2 (balloon).

F

Flight level (FL)

Altitude based on standard pressure, expressed in hundreds of feet. FL350 is 35,000 ft. In the US, flight levels start at 18,000 ft.

Fractional ownership

A business aviation model in which owners buy a share of an aircraft and fly on demand. NetJets (EJA) and Flexjet (LXJ) are the largest US operators.

G

Geometric altitude

Altitude from GPS, measured relative to a mathematical model of the earth. It can differ from barometric altitude by hundreds of feet depending on temperature and weather.

Ground speed

An aircraft's speed over the ground, in knots. It includes wind, so a jet with a strong tailwind can show a ground speed well above its airspeed.

H

Heading

The direction an aircraft's nose is pointing, in degrees. It differs from track when there is a crosswind.

Heavy

An aircraft with a maximum takeoff weight of 300,000 lb or more, such as a 777 or 787. Pilots add "heavy" to the callsign on the radio so controllers apply wake turbulence spacing.

Hex code (ICAO address)

The permanent 24-bit address of an aircraft's transponder, written as six hexadecimal characters, such as A1B2C3. US addresses begin with A.

Holding pattern

A racetrack-shaped pattern an aircraft flies over a fixed point while waiting for clearance to continue, often because of weather or congestion at the destination.

I

IFR

Instrument Flight Rules. Flying under ATC clearance and separation, navigating by instruments. Nearly all airline flights operate under IFR.

Indicated airspeed

The airspeed shown on the cockpit instrument, based on air pressure. It is what pilots fly by and decreases relative to true airspeed as altitude increases.

K

Knot

One nautical mile per hour, equal to about 1.15 mph or 1.852 km/h. Aviation speeds are given in knots.

L

LADD

Limiting Aircraft Data Displayed, an FAA program that lets owners ask for their aircraft to be hidden or limited in FAA-sourced flight tracking data.

M

Mach

Speed as a fraction of the local speed of sound. Most airliners cruise around Mach 0.78 to 0.85.

MLAT

Multilateration. Finding an aircraft's position by measuring when its transponder signal arrives at four or more ground receivers. Used for aircraft without ADS-B.

Mode C

An older transponder mode that replies with a squawk code and pressure altitude, but no unique aircraft address.

Mode S

A transponder mode in which each aircraft has a unique 24-bit address and can be selectively interrogated. ADS-B on 1090 MHz is built on Mode S.

N

N-number

A US aircraft registration, beginning with N, such as N123AB. For US civil aircraft it maps directly to the hex address.

NORDO

No radio. An aircraft that cannot communicate with ATC by voice, either because it has no radio or because of a failure. NORDO aircraft with transponders squawk 7600.

NOTAM

Notice to Air Missions. An official notice of a change or hazard affecting flight, such as a closed runway, an inoperative navigation aid or a temporary flight restriction.

O

Orbit

A circling flight path over one area. Common for news and police helicopters, survey aircraft, and tankers on refuelling tracks.

P

PIA

Privacy ICAO Address, an FAA program that lets eligible owners use a temporary 24-bit address not publicly linked to their registration.

R

Red-eye

An overnight passenger flight, typically from the West Coast to the East Coast, departing late evening and arriving early morning.

Registration

An aircraft's national identifier, painted on the airframe. Each country has a prefix: N for the US, C for Canada, G for the UK.

S

Squawk

The four-digit transponder code an aircraft is set to, each digit 0 to 7. Most are assigned by ATC; a few, like 1200 and 7700, have fixed meanings.

T

TFR

Temporary Flight Restriction. Airspace closed or restricted for a limited time, for example around wildfires, presidential movements, stadium events or disaster areas.

TIS-B

Traffic Information Service–Broadcast. FAA ground stations rebroadcast radar targets of non-ADS-B aircraft so ADS-B In users can see them.

Track

The direction an aircraft is actually moving over the ground, in degrees from true north. ADS-B reports track rather than heading in most messages.

TRACON

Terminal Radar Approach Control. An FAA facility that handles departures and arrivals within roughly 30 to 50 miles of busy airports.

Transition altitude

The altitude at which pilots switch from local altimeter settings to standard pressure. In the US it is 18,000 ft everywhere.

Transponder

An aircraft radio that replies to radar interrogations with a code and altitude, and in Mode S, a unique address. ADS-B Out is often built into it.

True airspeed

The aircraft's actual speed through the surrounding air, corrected for altitude and temperature. An airliner at cruise typically flies 450 to 490 knots true airspeed.

U

UAT

Universal Access Transceiver, the US-only 978 MHz ADS-B link allowed below 18,000 ft and popular in general aviation for its free weather and traffic services.

V

Vertical rate

How fast an aircraft is climbing or descending, in feet per minute. Positive values are climbs, negative values descents.

VFR

Visual Flight Rules. Flying by looking outside, with the pilot responsible for avoiding other aircraft. US VFR flights not talking to ATC squawk 1200.

W

Wake turbulence

Swirling air trailing behind an aircraft's wings. Its strength depends on weight and wingspan, which is why controllers space aircraft further apart behind heavy jets.