GNSS interference, explained
What satellite navigation is, why it is so easy to disrupt, and the five terms that describe how it goes wrong. No background needed.
How GNSS works
GNSS (Global Navigation Satellite System) is the general name for satellite navigation. There are four global systems: GPS (United States), Galileo (European Union), GLONASS (Russia) and BeiDou (China). Most modern receivers use several of them.
Each satellite broadcasts its position and a precise time. A receiver measures how long each signal took to arrive and works out its distance to each satellite. With four or more satellites it can solve for its position and correct its own clock.
That is why GNSS is also the world's most widely used clock. Phone networks, power grids and banks use it to keep time, not only to find places.
A whisper from 20,000 km
GPS satellites orbit about 20,200 km above the Earth. Their signals reach the ground at around −130 dBm, weaker than the background radio noise. Receivers can only pick them out because they know exactly what pattern to listen for.
A transmitter on the ground or in the air needs very little power to overpower that signal. Aircraft at cruise altitude can be affected by a jammer hundreds of kilometres away, because nothing blocks the line of sight.
Position uncertainty
Every GNSS fix comes with an error estimate. When it suddenly grows, something is interfering.
No satellite position is exact. Receivers estimate how far off they could be. Aircraft broadcast this estimate continuously as NACp (accuracy) and NIC (integrity).
Normal airliner fixes are accurate to under 30 m (NACp 9 or better). Weak geometry, ionospheric activity, multipath or deliberate interference make the error grow.
The receiver still has a position but reports lower confidence: NACp drops below 8 (error 93 m or more) or NIC below 7.
This is what we measure. A cell turns orange or red when a share of aircraft in it report degraded accuracy at the same time.
Jamming
Radio noise on GNSS frequencies drowns the satellite signals, so receivers lose their fix.
A transmitter broadcasts noise or carrier signals on GNSS frequencies (for example GPS L1 at 1575.42 MHz). Satellite signals arrive at about −130 dBm, roughly a ten-millionth of a billionth of a watt, so even a small jammer overpowers them over a wide area.
Military jammers protect territory from GNSS-guided drones and missiles; illegal personal 'privacy' jammers in vehicles cause local outages. Aircraft at altitude see jammers from hundreds of kilometres away.
Accuracy collapses or the fix is lost entirely (NIC and NACp fall to 0). Aircraft fall back to inertial and ground-based navigation.
Labelled Jamming: clusters of red cells where many aircraft lose accuracy at once, while their tracks stay plausible. Typical around conflict zones and some borders.
Around the Baltic Sea, persistent interference has affected flights since 2022. In 2024 Finnair paused its Helsinki–Tartu route for about a month because GPS interference blocked the approach it relied on.
Spoofing
Counterfeit satellite signals trick a receiver into computing a false position or time, while it appears healthy.
A transmitter broadcasts fake GNSS signals that look genuine but are stronger than the real ones. The receiver locks on and calculates whatever position and time the spoofer chooses.
Spoofing often starts with jamming to break the lock on real signals, then offers counterfeit ones. It is harder to build than jamming, but cheap software-defined radios have made it more common.
The position can jump hundreds of kilometres or drift slowly. Clocks shift. The receiver may still report high accuracy, which makes spoofing more dangerous than jamming.
Labelled Spoofing when several aircraft jump to positions they could not have flown to, usually converging on one false point (shown with a dashed line). We count them where they really are, not where they claim to be. A spoofer that moves positions slowly and plausibly can still go unnoticed.
In 2017, around 20 ships in the Black Sea reported GPS positions at an inland airport about 30 km away. Since 2023, aircraft over the Middle East have reported spoofed positions, some leading to navigation errors.
Meaconing
Real satellite signals are recorded and rebroadcast with a delay, so the receiver works out the wrong place or time.
The term comes from 'masking beacons': receiving navigation signals and rebroadcasting them on the same frequency to confuse navigators. For GNSS, an attacker relays or replays genuine signals.
Because the rebroadcast signals are authentic, meaconing can defeat protections that only check whether a signal is genuine. The added delay changes the computed position and time toward the location of the rebroadcasting antenna.
The receiver reports a plausible but wrong position, often the location of the relay antenna, or a time offset. Accuracy indicators may stay normal.
Labelled Possible meaconing when positions collapse toward a nearby point, or stop matching the aircraft's own speed and heading, while accuracy looks normal. ADS-B data alone cannot prove meaconing, so we never mark it as high confidence.
GNSS denial
Any situation where satellite positioning is unavailable or untrustworthy, deliberate or not.
A 'GNSS-denied environment' is any place or time where systems cannot rely on satellite positioning. Jamming, spoofing and meaconing are deliberate causes.
Natural and accidental causes include solar storms and ionospheric scintillation, faulty equipment emitting on GNSS bands, terrain, tunnels and dense cities. Operators plan for denial by keeping independent backups.
Loss of position, wrong position, or unreliable timing, depending on the cause.
Labelled Denial when most affected aircraft keep transmitting but lose their position entirely. Ground-level outages can be smaller and are not always visible from the air.
How they differ
| Jamming | Spoofing | Meaconing | Natural degradation | |
|---|---|---|---|---|
| What happens | Signals drowned in noise | Fake signals replace real ones | Real signals replayed with a delay | Signals weakened or distorted |
| Receiver output | No fix, or very poor accuracy | Wrong position or time, often 'healthy' | Wrong position or time | Lower accuracy |
| Easy to notice? | Yes | Often not | Often not | Usually |
| Typical source | Military systems, illegal jammers | Military or state actors, research tools | Relay or record-and-replay equipment | Solar storms, terrain, faulty equipment |
| Visible on our map | Clearly | Partly | Rarely | Sometimes (wide, short-lived) |
Risks and threats
GNSS interference rarely causes accidents on its own, because professionals train for it and keep backups. The danger is in what it takes away: margins, automation and trust in the data.
Aviation
Loss of satellite-based approaches, terrain-warning false alerts, and degraded air-traffic surveillance, because ADS-B position reports come from GNSS. Crews must switch to backup navigation, adding workload.
Maritime
Ships rely on GNSS for navigation and for AIS, the system that shows them to other vessels. Wrong positions raise the risk of collisions and groundings, especially in narrow straits.
Timing and critical infrastructure
Mobile networks, power grids, data centres and financial trading use GNSS as a precise clock. A shifted or lost time signal can disrupt synchronisation far from where the interference happens.
Drones and autonomy
Uncrewed aircraft, autonomous vehicles and agricultural machines depend on GNSS. Spoofing can steer them off course; jamming can force landings or stops.
Road, rail and logistics
Fleet tracking, tolling, tachographs and train control use GNSS. Interference creates data gaps and can be used to hide cargo theft.
Emergency services
Caller location, dispatch and search-and-rescue all use satellite positioning. Errors cost time when it matters most.
Not rare, not random
Interference is concentrated around conflict zones and some borders, and it is persistent: the same regions show up day after day. Industry bodies such as EASA, IATA and OPSGROUP have reported a sharp increase since 2022, and EASA has published safety bulletins on GNSS outages and spoofing.
Our own records show the pattern in numbers. See the statistics or today's report.
Reading NIC and NACp
Aircraft broadcast two numbers with every ADS-B position. NACp (Navigation Accuracy Category for position) says how accurate the position is. NIC (Navigation Integrity Category) says how far off it could be before the system would raise an alarm. Higher is better; 0 means unknown or no usable position.
| NACp | Accuracy (95%) | Status |
|---|---|---|
| 11 | < 3 m | normal |
| 10 | < 10 m | normal |
| 9 | < 30 m | normal |
| 8 | < 93 m (0.05 NM) | normal |
| 7 | < 185 m (0.1 NM) | degraded |
| 6 | < 556 m (0.3 NM) | degraded |
| 5 | < 0.5 NM | degraded |
| 4 | < 1 NM | degraded |
| 3 | < 2 NM | degraded |
| 2 | < 4 NM | degraded |
| 1 | < 10 NM | degraded |
| 0 | ≥ 10 NM or unknown | degraded |
| NIC | Containment radius | Status |
|---|---|---|
| 11 | < 7.5 m | normal |
| 10 | < 25 m | normal |
| 9 | < 75 m | normal |
| 8 | < 0.1 NM | normal |
| 7 | < 0.2 NM | normal |
| 6 | < 0.6 NM | degraded |
| 5 | < 1 NM | degraded |
| 4 | < 2 NM | degraded |
| 3 | < 4 NM | degraded |
| 2 | < 8 NM | degraded |
| 1 | < 20 NM | degraded |
| 0 | ≥ 20 NM or unknown | degraded |
We count a report as degraded when NACp is below 8 or NIC is below 7, the same thresholds used by GPSJAM.
How operators protect themselves
Multi-constellation, multi-frequency receivers
Using GPS, Galileo, GLONASS and BeiDou on several frequencies makes a receiver harder to jam and helps it spot inconsistent signals.
Signal authentication
Galileo's OSNMA adds cryptographic authentication to civil navigation messages, which helps detect simple spoofing.
Controlled-pattern antennas
Antennas that suppress signals arriving from the horizon reject many ground-based jammers.
Independent backups
Inertial navigation, ground radio aids (DME, VOR), eLoran and terrestrial time sources keep operations going when GNSS is unavailable.
Monitoring and reporting
Maps like this one, crew and ship reports, and national monitoring networks show where interference happens so others can prepare. Report suspected interference to your aviation, maritime or spectrum authority.