Where the data comes from
Most airliners and many smaller aircraft broadcast their position, altitude and identity about twice a second over ADS-B (Automatic Dependent Surveillance–Broadcast). Volunteers around the world receive these broadcasts with ground stations and share them through open networks. The map uses two of them, adsb.lol and adsb.fi.
Every 3 hours, a collector divides the world from 72°S to 72°N into about 1,000 overlapping circles of 250 nautical miles and asks the feeds which aircraft are in each one. Busy circles are polled first and quiet ones, such as open ocean, at least once a day. Each poll is a snapshot of the aircraft in the air at that moment, not a continuous recording.
What NACp is
Alongside its position, an ADS-B aircraft reports how accurate it believes that position to be. This is the Navigation Accuracy Category for Position (NACp), a number from 0 to 11 set by the aircraft's own navigation system. Each value stands for a bound on the estimated position uncertainty (95% containment):
| NACp | Estimated position uncertainty |
|---|---|
| 11 | under 3 m |
| 10 | under 10 m |
| 9 | under 30 m |
| 8 | under 0.05 NM (93 m) |
| 7 | under 0.1 NM (185 m) |
| 6 to 1 | under 0.3, 0.5, 1, 2, 4 and 10 NM |
| 0 | 10 NM or more, or unknown |
An aircraft with a working GPS receiver normally reports NACp 8 or better. On this map an aircraft counts as affected if it reported NACp 7 or lower, an uncertainty of 0.05 nautical miles or more. Reports with no NACp are ignored.
Why low NACp points to interference
GPS signals are very weak by the time they reach an aircraft. A jammer drowns them out, and the receiver either loses its position or falls back to less accurate sources, such as inertial navigation or ground radio beacons; either way the reported NACp drops. When many unrelated aircraft in the same area report low accuracy at the same time, the likeliest common cause is radio interference in that area, rather than faults in each aircraft.
Spoofing, which broadcasts fake GPS signals, often comes with jamming and can also leave receivers reporting low accuracy. A receiver that is fully deceived, however, may report high accuracy for a wrong position: see the limitations.
The 24-hour window
Each aircraft seen in a hexagon is remembered for 24 hours from the last time it was seen there. An aircraft counts once per hexagon, however many times it is seen, and it counts as affected if any of its reports in that hexagon during the window had NACp 7 or lower. Each time the collector runs, observations older than 24 hours are dropped and the map is rebuilt, so it always describes the 24 hours up to its update time, shown on the map and on every page.
The hexagon grid
Aircraft are grouped into flat-topped hexagons laid out on the same Web Mercator projection as the map, so every hexagon looks the same size on screen and faces north. Each has a centre-to-corner radius of 0.8 degrees of longitude. On the ground that is about 89 km at the equator, 63 km at 45° latitude and 52 km at 54° (the southern Baltic): hexagons cover less ground towards the poles.
For each hexagon, with affected and unaffected counted as distinct aircraft:
share affected = 100 × affected ÷ (affected + unaffected), rounded to one decimal place.
- Green: under 3% of aircraft affected.
- Yellow: 3% to 10%.
- Red: over 10%.
Hexagons with fewer than 3 distinct aircraft are not drawn, so an empty area means too little data, not an absence of interference. Clicking a hexagon on the map shows its counts.
Limitations and false positives
- Low accuracy is not proof of jamming. Faulty or older avionics, a GPS receiver not wired to the transponder, or an aircraft whose antenna is briefly blocked can all report low NACp. In a hexagon with few aircraft, one such aircraft is enough to turn it yellow or red (one of three aircraft is 33%). Isolated red hexagons deserve less weight than large red areas.
- Spoofing can be missed. A receiver fully deceived by fake signals may report high accuracy for a wrong position. The map shows where aircraft report poor accuracy; it cannot detect positions that are wrong but reported as accurate.
- Hexagons show where aircraft were, not where the transmitter is. An aircraft at cruising altitude can receive a ground transmitter hundreds of kilometres away, so the affected area is usually much larger than the source, and the map cannot locate or identify it.
- Jamming and spoofing are not told apart, and neither are their causes. The map does not say who is responsible for any interference.
- Snapshots, not a continuous record. Polls every 3 hours can miss short events, and a hexagon can stay coloured for up to 24 hours after interference stops.
- Coverage depends on volunteers. Areas with few ground receivers, much of the oceans, Africa and parts of Asia, have little or no data. Aircraft that do not broadcast ADS-B, including many military aircraft, are not counted.
- Natural causes. Strong solar activity can degrade GPS accuracy, especially at high latitudes, over areas much larger than a typical jamming zone.
Sources and credits
Aircraft data: adsb.lol and adsb.fi, shared under the Open Database License (ODbL). Basemap © MapTiler © OpenStreetMap contributors. The approach of mapping ADS-B accuracy reports onto a daily grid follows the one popularised by GPSJAM.
Questions about the map are answered in the FAQ; why this matters is on the About page.