GNSS interference is any radio-frequency disturbance that degrades or falsifies the satellite navigation signals (GPS, Galileo, GLONASS, BeiDou) that receivers rely on. Aviation regulators separate two kinds. Jamming blocks the signal. Spoofing sends the receiver false information. Both can be observed at scale in open ADS-B data, because aircraft broadcast how accurate and trustworthy they believe their own position is.
§01What is GNSS interference?
Satellite navigation signals arrive at the receiver extremely weak, which makes them easy to disrupt. EASA puts it simply: jamming blocks a signal, while spoofing sends false information to the receiver on board the aircraft.2Source 2 · European Union Aviation Safety Agency (EASA), 2024EASA updates Safety Information Bulletin on global navigation satellite system outages and alterationseasa.europa.eu Its Safety Information Bulletin on GNSS outages and alterations describes a notable increase in both since February 2022. The affected regions include the Mediterranean, the Black Sea, the Middle East, the Baltic Sea and the Arctic, and the bulletin has been revised several times as the picture changed.1Source 1 · European Union Aviation Safety Agency (EASA)Global Navigation Satellite System (GNSS) Outages and Alterationseasa.europa.eu For crews this means degraded navigation and surveillance functions. For analysts it means measurable patterns in open data.
§02How is GNSS interference measured from ADS-B?
Aircraft broadcast quality indicators next to their position. NACp (Navigation Accuracy Category for Position, 0–11) bounds the 95% horizontal position error. NIC (Navigation Integrity Category, 0–11) gives a containment radius the true position should stay within. For example, NACp 11 means an estimated position uncertainty under 3 m.3Source 3 · Junzi Sun, mode-s.orgThe 1090 Megahertz Riddle: Uncertainty, accuracy and integritymode-s.org When many aircraft in one area report sudden low values, the shared environment is the likely culprit.
Scroll sideways to see the full diagram.
Low-accuracy share (cell, day)=(aircraft with low accuracy − 1) ÷ total aircraft in cell × 100
- − 1
- Damps false positives in cells with very little traffic
That formula is the one the public GPSJAM map uses. It colors a hexagon yellow when 2–10% of aircraft report low accuracy and red above 10%.4Source 4 · GPSJAM (John Wiseman)GPSJAM FAQgpsjam.org
§03Common misconceptions
- “Red cells prove jamming.” The data does not identify a cause. GPSJAM’s own FAQ lists other explanations, such as military trainer aircraft maneuvering hard enough to mask their GPS antennas.4Source 4 · GPSJAM (John Wiseman)GPSJAM FAQgpsjam.org
- “It shows interference on the ground.” Aircraft fly thousands of feet up and have line of sight to far more potential sources than receivers on the ground, so an affected cell says little about GNSS reception below it.4Source 4 · GPSJAM (John Wiseman)GPSJAM FAQgpsjam.org
- “Low NIC and low NACp mean the same thing.” NACp is about accuracy; NIC and SIL are about integrity, the bound and the probability of exceeding it.3Source 3 · Junzi Sun, mode-s.orgThe 1090 Megahertz Riddle: Uncertainty, accuracy and integritymode-s.org
§04How Kimo uses GNSS interference data
Kimo’s GNSS interference monitor template aggregates NIC/NACp from your ADS-B sources into daily hexagons, sets a baseline per cell and flags departures for review. Each cell shows its traffic count, so a quiet area is not mistaken for a clean one. The work is civil-safety oriented: it maps where navigation was degraded so operators and researchers can plan around it. See the detection guide and the explainer post.
§05Related terms
- ADS-B: the broadcast that carries NIC and NACp.
- Multilateration (MLAT): positions that do not depend on the aircraft’s GNSS.
- Data fusion: weighing sources by their reported quality.
Frequently asked questions
What is the difference between jamming and spoofing?
Can I see GNSS interference without special equipment?
Does low accuracy in ADS-B always mean interference?
Sources
4 references- Global Navigation Satellite System (GNSS) Outages and Alterations (opens in a new tab)European Union Aviation Safety Agency (EASA)easa.europa.eu
SIB 2022-02 series (revision R4, July 2026); increase in jamming and spoofing since February 2022; regions affected.
- EASA updates Safety Information Bulletin on global navigation satellite system outages and alterations (opens in a new tab)European Union Aviation Safety Agency (EASA)2024easa.europa.eu
Jamming blocks a signal; spoofing sends false information to the receiver.
- The 1090 Megahertz Riddle: Uncertainty, accuracy and integrity (opens in a new tab)Junzi Sun, mode-s.orgmode-s.org
NIC, NACp, SIL definitions and value ranges.
- GPSJAM FAQ (opens in a new tab)GPSJAM (John Wiseman)gpsjam.org
Method for the daily hex map, thresholds, and caveats on causes.
External sources were accessed at the time of writing. Kimo product details, customers and figures in examples are illustrative unless a source is cited.
