Callsign — what the crew typed in
The callsign is the most prominent field, and the least permanent. It is entered by the crew before departure and identifies the flight, not the aircraft. Tomorrow the same jet will carry a different one.
Airline callsigns are built from a three-letter ICAO airline code plus a flight number. The three-letter code is often nothing like the airline's name, and the code spoken on the radio is different again:
| Seen on radar | Airline | Spoken on the radio as |
|---|---|---|
BAW | British Airways | "Speedbird" |
EZY | easyJet | "Easy" |
RYR | Ryanair | "Ryanair" |
VIR | Virgin Atlantic | "Virgin" |
EXS | Jet2 | "Channex" |
TOM | TUI Airways | "Tomjet" |
So BAW117 on the map is British Airways flight 117, and the controller is calling it
"Speedbird one one seven". If you are listening to ATC alongside the map, that mismatch is the single most
confusing thing for newcomers.
Private and general aviation aircraft usually broadcast their registration instead — GABCD
for a UK aircraft registered G-ABCD. Registrations are national: G- is the United Kingdom,
EI- Ireland, D- Germany, F- France, N
the United States.
If the callsign field is empty, the receiver has heard the aircraft's position but not yet caught an identification message. They arrive less frequently than position updates. Wait a few seconds and it will usually fill itself in.
Hex code — the one that never changes
The hex code (also called the ICAO 24-bit address, or Mode S address) is a six-character hexadecimal
identifier such as 406B9A. It is assigned to the airframe when it is registered and stays with
it for life, unless the aircraft changes country.
Blocks of addresses are allocated by country, so the hex code alone tells you where an aircraft is
registered. UK aircraft mostly begin 40 or 43; US aircraft begin A.
Because it is stable, the hex code is what you want when researching a specific aircraft — it will find the same airframe regardless of what flight it happens to be operating today.
Altitude — two different numbers
ADS-B can carry two altitudes and they are not the same thing.
Barometric altitude
This is the one normally displayed, and it comes from the aircraft's pressure sensing. Above the transition altitude, every aircraft sets its altimeter to the standard pressure setting of 1013.25 hPa — not the real local pressure. The result is a flight level: FL350 means the altimeter reads 35,000 ft on the standard setting.
This is deliberate. If everyone uses the same reference, aircraft are correctly separated from each other even when the whole set of readings is a few hundred feet away from true height. In the London Terminal Control Area the transition altitude is commonly 6,000 ft, though it varies by region.
Geometric altitude
This is height derived from GNSS — closer to true height above the ellipsoid. On a day with unusual pressure the two figures can differ by several hundred feet. Neither is wrong; they are answering different questions.
Two aircraft that appear to overlap on a flat map are almost always thousands of feet apart vertically. A 2D map projection cannot show separation, and vertical separation is the kind that actually matters.
Speed — not the one in the cockpit
The speed on a tracker is ground speed: how fast the aircraft's shadow moves across the earth. The crew are far more interested in airspeed, which is speed through the air mass — and the difference between the two is the wind.
An aircraft cruising at a constant airspeed will show a much higher ground speed heading east across the North Atlantic than heading west, because the jet stream flows west to east. Ground speeds above 700 knots happen regularly in winter and mean a strong tailwind, not an unusually fast aeroplane.
Speeds are given in knots — nautical miles per hour. One knot is roughly 1.15 mph or 1.85 km/h. A typical airliner cruises around 450–500 knots ground speed in still air.
Track, heading and vertical rate
Track is the direction the aircraft is actually moving over the ground, in degrees from north. Heading is the direction the nose is pointing. In a crosswind these differ — an aircraft flying due north in a westerly wind points slightly into the wind to hold its track. Most trackers display track, because that is what the position data gives.
Vertical rate is in feet per minute: positive climbing, negative descending. Airliners typically climb at 1,500–2,500 fpm early in a departure and descend at around 1,000–2,000 fpm on approach. Sustained figures far outside that range are usually a data glitch rather than a dramatic event.
Why the aircraft glides between updates
Position messages arrive at intervals, not continuously. If a tracker only redrew the icon when a message landed, aircraft would visibly hop across the screen. Instead the map extrapolates — it takes the last known position, track and speed, and advances the icon smoothly until real data arrives to correct it.
Most of the time you cannot tell. Occasionally you will see an aircraft twitch or jump slightly: that is a fresh message correcting an extrapolated guess. If the signal is lost entirely, the prediction is only trustworthy for a short window, after which the aircraft should be dropped rather than flown on fiction.
Reading the whole picture
Once the individual fields make sense, patterns start to stand out:
- A tight circle or racetrack at a steady level — a holding pattern, usually traffic flow management into a busy airport.
- A long straight descent lined up with a runway — an ILS approach, typically joining the final approach around 2,000–3,000 ft some miles out.
- Slow, low, wandering, no airline callsign — general aviation, a training flight, or a survey aircraft.
- Very high, unusual track, no route data — often business aviation or a ferry flight.
- Tight orbits over one spot at low level — police, air ambulance or news helicopters.