Why some aircraft are not shown on the map
The plane you see in the sky but not on the map is rarely anything mysterious. About ADS-B, volunteer receivers and the gaps between them.
A plane comes in to land right over your head, but is not on the map. A helicopter is visible on one service and gone on another. The explanation almost always lies in how aircraft positions are actually collected — by volunteers, with radio, with no central authority.
The aircraft says it itself
Modern aircraft broadcast their own position several times a second on 1090 MHz. The system is called ADS-B, and the message contains identity, position, altitude, speed and vertical rate. It is not encrypted and not addressed to anyone in particular. Anyone with a simple receiver can read it.
There is no official, open source that collects this for the public. Instead, thousands of private individuals have set up receivers at home and share the signals onwards. Kartradar uses adsb.lol, one such community-run network. Coverage is therefore a mirror of where people live and have equipment set up.
Why aircraft are missing in some areas
The signal follows the line of sight. An aircraft at cruising altitude can be heard several hundred kilometres away, while an aircraft just above the ground needs a receiver almost within sight. That gives some clear patterns:
- Low-flying aircraft and helicopters disappear between receivers, especially in mountain areas and sparsely populated inland regions.
- The open sea is thinly covered. Far out in the Norwegian Sea or the Barents Sea there is no one listening, even though the aircraft is transmitting as normal.
- Cities and airports have very good coverage, often from many receivers at once.
We saw this clearly when a user found a helicopter on another service that was missing from ours. The difference was not that one service was "better", but that the two networks had different receivers in that area at that moment.
Those that do not want to be seen
Some aircraft do not transmit a position at all, or transmit under a hidden identity:
- Military aircraft often switch off ADS-B during exercises and missions. Sometimes they fly with only older transponder replies, which give altitude but not position.
- Older aircraft without ADS-B equipment, typically light aircraft, gliders and vintage aircraft, are invisible even when flying in the middle of the best coverage.
- Privacy-protected owners. Some aircraft are permitted to operate with an anonymised identity so they cannot be linked to their owner.
- Voluntary filtering. Many data communities hold back certain categories themselves, for example military aircraft, regardless of what the receivers pick up.
An aircraft you see in the sky but not on the map is therefore not necessarily anything mysterious. Most often, no receiver nearby heard it, or the aircraft is not transmitting.
What the fields in the panel mean
When you tap an aircraft, the panel shows what the aircraft itself broadcasts:
- Callsign is what the pilots have entered for the flight, not the registration. "SAS4531" is a callsign, "LN-RGB" is the registration painted on the fuselage.
- Altitude is barometric, measured with air pressure, not GPS. That is a story of its own, which you will find in the guide to altitude measurement.
- Speed is ground speed, how fast the aircraft moves over the ground. With a strong tailwind, an aircraft can have a much higher ground speed than its speed through the air.
- Vertical rate is given in feet per minute, the way pilots measure it.
The track behind the aircraft
The line behind a selected aircraft is made up of the positions actually received. If coverage is thin, there are fewer points and the line is straighter than the real route. Dense coverage gives many points, and then the line follows the turns as they were flown.
Between two received messages, the aircraft symbol glides smoothly along its own course and speed so the movement does not stutter on screen. That is smoothing of the last known measurement, not a prediction of what the aircraft will do.
One more thing: the ground
Aircraft on the ground still transmit, but without a meaningful altitude. They are shown as "on the ground", and you will find them at the terminal areas of large airports. Taxiing aircraft at Gardermoen and Flesland are therefore real observations, not misplaced symbols.