Satellite navigation feels like infrastructure, reliable in the way that gravity is reliable. It is not. It is a radio service, delivered by transmitters 20,000 kilometres away, and by the time it reaches the ground it is one of the faintest useful signals in regular civilian use. Anything louder nearby wins.
That fragility has become geopolitically visible over the last few years, and the fascinating part, for anyone who follows aircraft, is that the aviation system detects it automatically. Aircraft do not have to be asked whether their navigation is working. They are already broadcasting the answer.
Why GPS is so easy to drown out
The civil GPS signal is specified to arrive at the earth's surface at a power level well below the background thermal noise. If you put a spectrum analyser on the GPS band you do not see a signal; you see noise, with the signal buried inside it.
A receiver extracts it by correlation. Each satellite transmits a long, known, pseudo-random code, and the receiver multiplies the incoming noise against its own copy of that code. Everything uncorrelated averages toward nothing; the wanted signal accumulates. That trick buys tens of decibels of processing gain and is the only reason satellite navigation works at all.
It also defines the vulnerability precisely. A transmitter that raises the noise floor across the band by more than the receiver's processing gain defeats the correlation, and the receiver simply loses lock. The power required is trivial. Cheap illegal "personal privacy devices" (sold to defeat vehicle tracking, and a few watts at most) have been documented interfering with aviation GPS equipment from passing traffic on a nearby road. Purpose-built military systems, running much more power from higher ground, deny navigation across entire regions.
Altitude makes the affected area far larger than intuition suggests. Radio at these frequencies is line of sight, so a ground transmitter that reaches a car for a few kilometres reaches an aircraft at 35,000 feet for hundreds. Jamming that is a local nuisance at sea level is a regional event in the flight levels.
Jamming and spoofing are not the same problem
Jamming is noise. It denies the service: the receiver knows it has lost the signal, reports that it has lost the signal, and downstream systems fall back to other sources. It is loud, obvious and honest about itself.
Spoofing is a lie. Rather than drowning the real signal, a spoofer transmits counterfeit signals structured like the genuine ones, so the receiver locks onto them and computes a position (a wrong one) with complete confidence. Because GNSS also distributes time, a spoofed receiver can produce a wrong clock as well as a wrong place, which propagates into anything that depends on either.
Spoofing is much harder to do and much harder to detect, and reports of it affecting civil aviation have grown steadily. It is the more troubling of the two precisely because nothing complains. A jammed receiver says "I don't know where I am." A spoofed one says "I know exactly where I am," and is wrong.
The number that gives it away
Here is the elegant part. As covered in how ADS-B works, aircraft broadcast rather more than a position. Alongside it they transmit a set of quality indicators: the aircraft's own assessment of how much it trusts what it is telling you.
The most useful is NACp, the Navigation Accuracy Category for position. It is a small integer, and each value corresponds to a bound on the position error: higher is better. The threshold that matters is 8, which corresponds to an accuracy better than 0.05 nautical miles (about 93 metres) and which is the value the FAA treats as the floor for compliant ADS-B Out equipment.
An airliner with a healthy GNSS fix typically reports comfortably above that. When the fix degrades or is lost, the figure collapses, and the aircraft announces the collapse on 1090 MHz to anyone listening. No cooperation is required, no reporting scheme, no permission. The information is a by-product of a system designed for something else entirely.
From one aircraft to a map
One aircraft reporting poor accuracy is meaningless. Equipment breaks. What makes this work is volume. At any moment there are thousands of aircraft airborne, each broadcasting a position and an accuracy figure several times a minute, and the community receiver network hears a large fraction of them.
The method, which the interference map on this site uses, is:
- Take every eligible aircraft report and bin it into a hexagonal cell using the H3 geospatial grid.
- For each cell, compute the share of distinct aircraft reporting NACp below 8.
- Colour the cell by that share, and pale it out where few aircraft passed through, because a cell with three data points is not evidence of anything.
"Eligible" is doing real work in step one. Only ICAO-addressed aircraft above 10,000 feet are counted. Ground traffic reports poor accuracy for ordinary reasons; light aircraft with older equipment frequently report low categories with nothing wrong at all; and rebroadcast traffic originating from ground stations rather than from the aircraft has its own accuracy characteristics. Include those and every busy airport lights up like a jamming site.
Red does not always mean jamming
What the map shows is aircraft reporting degraded navigation accuracy in a place. That correlates strongly with known interference, but avionics faults, scheduled military exercises and equipment classes with different reporting behaviour all produce the same colour. The honest reading of a red cell is "something is degrading navigation accuracy here", not "there is a jammer at these coordinates".
What it looks like when you watch it
The picture that emerges is not random. Interference clusters into a handful of long-running regions. The Baltic, the Black Sea, the Eastern Mediterranean and the Persian Gulf have dominated the map for as long as it has existed. Within those regions it has structure. Affected areas tend to be broad and roughly centred, because they are the line-of-sight footprint of a ground transmitter seen from altitude, and they have soft edges rather than sharp ones, because aircraft at the margin sometimes hold their fix and sometimes do not.
They also have a clock. Interference switches on and off, often on daily patterns. This is the case for updating a map continuously rather than once a day: an event that begins this morning is visible this morning, in the hours when somebody might actually want to know about it, rather than in tomorrow's summary.
Occasionally something appears well away from the familiar regions: a short-lived cell over somewhere unremarkable. Those are worth noticing, and they are exactly what a daily aggregate averages away.
What it means for the aircraft
To be clear about the safety picture without overstating it: aircraft are not dependent on GNSS to fly. Airliners carry inertial reference systems that continue to produce a position without any satellite input, and the conventional ground-based navigation aid network still exists precisely so that this remains possible. Crews operating in these regions plan for it.
What interference does produce is workload and lost capability. Satellite-based approach procedures become unavailable, so a crew planning a GNSS approach needs a different plan. Navigation performance requirements that depend on GNSS cannot be met, which constrains routing. Systems that use satellite position as an input can raise spurious alerts, and a warning that is both alarming and wrong is its own kind of hazard in a busy cockpit. Aviation authorities and airline bodies have raised all of this repeatedly, and it is a serious operational issue rather than a curiosity.
And what it means for flight trackers
Return to that second letter in ADS-B: dependent. The transponder repeats what the aircraft's navigation system believes, and it has no independent way to check.
So in a jammed area, aircraft go quiet or intermittent on trackers, not because they have stopped transmitting, but because they have stopped having a position worth transmitting. And in a spoofed area, trackers faithfully draw fiction: aircraft placed at a location they are not, sometimes an entire group of them converging on the same false point. Anyone who has watched an airliner appear to be parked on an airport it is nowhere near has seen this.
Which is why we treat it as a first-class feature rather than a footnote. The GNSS interference map runs on the web and inside the app, built from the same receiver network that draws the unfiltered live map. The hexagonal-aggregation approach was pioneered by John Wiseman's gpsjam.org, which has mapped this daily since 2022; our cells use the same H3 grid at the same resolution so the two can be read against each other.
It is a strange and rather beautiful piece of accidental instrumentation. A surveillance system designed so that controllers could see aeroplanes turns out, if you read the fields nobody was thinking about, to be a worldwide sensor network for the health of satellite navigation. Reported voluntarily, in the clear, by the aircraft themselves.
