Guide
How to read flight data
Every aircraft on a live tracker is described by a handful of fields: who it is (hex, callsign, registration, type), where it is (position, altitude), how it is moving (speed, track, vertical rate) and what it is saying (squawk, category). This guide explains each one, with a worked example.
Identity fields
Hex (ICAO 24-bit address)
A six-character hexadecimal code, such as A1B2C3, permanently assigned to the aircraft's transponder by its country of registration. It is the most reliable identifier because the aircraft broadcasts it in every message. It is what our aircraft lookup uses, and what airplanes.live uses in links such as ?icao=a1b2c3. See callsigns, registrations and hex codes.
Callsign (flight)
Up to eight characters entered by the crew. Airlines use a three-letter ICAO code plus a flight number, such as UAL1234 for United 1234. Private aircraft usually use their registration, such as N123AB. Learn the common prefixes in our airlines and operators guide.
Registration
The aircraft's "licence plate," painted on the side: N-numbers in the US, C- in Canada, G- in the UK. It comes from a database linked to the hex address, not from the broadcast itself.
Type
The ICAO type designator, a code of up to four characters such as B738 (Boeing 737-800), A321 or C172. It also comes from a database. Browse all of them on aircraft types.
Position and altitude
Latitude and longitude
From the aircraft's GPS (ADS-B) or calculated by ground receivers (MLAT). See ADS-B vs MLAT.
Barometric altitude
The altitude from the aircraft's pressure altimeter, referenced to standard pressure (29.92 inHg or 1013.25 hPa). This is what air traffic control uses to separate aircraft, and it is what we show as "altitude". Above 18,000 ft in the US it is spoken as a flight level: 35,000 ft is FL350. When the aircraft reports it is on the ground, the value reads "ground" instead of a number. More in altitude and flight levels.
Geometric altitude
The GPS height, which can differ from barometric altitude by hundreds of feet depending on temperature and weather. Trackers that show both will often show a gap between them on hot or cold days.
Motion fields
Ground speed
Speed over the ground in knots (1 knot = 1.15 mph = 1.852 km/h). It includes the effect of wind, so a jet with a 150-knot tailwind in the winter jet stream can show more than 650 knots, while the same jet heading west might show 400. That is why our fastest right now list is usually led by eastbound airliners, not fighters.
Airspeed and Mach
Speed through the air. Indicated airspeed is what the pilot flies by; true airspeed corrects for altitude; Mach is speed as a fraction of the speed of sound. Most ADS-B messages report ground speed, not airspeed.
Track
The direction the aircraft is actually moving over the ground, in degrees from true north (0° north, 90° east). Heading, the direction the nose points, can differ because of crosswind.
Vertical rate
Climb or descent rate in feet per minute. Airliners typically climb at 1,500 to 3,000 ft/min and descend at 1,000 to 2,500 ft/min. Our dashboard counts aircraft climbing and descending faster than a few hundred feet per minute.
Status fields
Squawk
The four-digit transponder code, each digit 0 to 7. Most are assigned by ATC for one flight. A few are special: 7700 emergency, 7600 radio failure, 7500 unlawful interference and 1200 US VFR. See the full squawk code index.
Category
The ADS-B emitter category, such as A3 (large aircraft) or A7 (rotorcraft). It is how we spot helicopters and balloons. See emitter categories.
Seen
How many seconds since the last message or position was received. Under about 10 seconds is fresh. Larger values mean the aircraft may have flown out of range, landed or turned off its transponder.
Worked example
| Field | Value | What it tells you |
|---|---|---|
| hex | a4c2e1 | A US-registered aircraft (US addresses start with A) |
| flight | SWA2417 | Southwest Airlines flight 2417 |
| type | B38M | Boeing 737 MAX 8 |
| alt | 37000 | Cruising at FL370 |
| gs | 512 | 512 knots over the ground, likely with a tailwind |
| track | 84 | Heading roughly east |
| vrate | 0 | Level flight |
| squawk | 3512 | A normal ATC-assigned code |
| category | A3 | Large aircraft (75,000 to 300,000 lb) |
| seen | 1 | Received a second ago |
The values above are illustrative. Try a real one: open the live map, pick an aircraft, and compare its fields to this table.
Questions
What is the hex code on a flight tracker?
It is the aircraft's ICAO 24-bit address, written as six hexadecimal characters. It is assigned to the transponder by the country of registration and is broadcast in every message, so it is the most reliable way to identify an aircraft.
Why is ground speed higher than a plane's top speed?
Ground speed includes wind. An airliner flying east with a strong jet-stream tailwind can exceed 650 knots over the ground while its airspeed is normal.