Something is Jamming GPSGNSS
A three-act story: how GPS finds you, why a cheap jammer can blind it, and how investigators trace the interference back to its source.
Act 1
How GPS Works
GPS — the Global Positioning System — is a fleet of about 30 satellites circling Earth, each carrying an atomic clock and endlessly broadcasting the exact time. Your phone doesn't talk back; it just listens.
From each satellite's timestamp it works out how far away that satellite is. One distance puts you somewhere on a sphere; listen to a few satellites at once and only one point fits all of them — and that point is your location. Drag the green marker below and watch the coordinates it produces.
3 satellites: A third sphere shrinks the overlap to just two points, and only one of them is on the Earth's surface. That point — where all three ranges agree — is your location. The receiver never "knew" where it was; it solved for it.
Distances and travel times are computed from real GPS geometry (20,200 km orbit, speed of light); the on-screen positions are schematic, not to scale.
Act 2
Why it's so easy to jam
That satellite signal is incredibly faint — by the time it reaches you it's actually weaker than the background radio noise around you. GPS only works at all because of a clever decoding trick that lifts it up out of the noise.
But that cushion is thin. Think of the noise as a water level and the signal as a line just above it: a jammer simply raises the water until it drowns the line. Because the real signal is so weak, even a cheap device nearby out-shouts a satellite 20,000 km away. Try the presets, or drag the jammer's power and distance.
The GPS signal reaches you at about -127 dBm (the spec guarantees at least −128.5 dBm) — fainter than the -111 dBm background noise in a 2 MHz band. Despreading the satellite's code adds roughly 43 dB of processing gain (about 30 dB from one 1,023-chip code period, 43 dB over a 20 ms data bit), which lifts the signal about 27 dB above the noise. A receiver needs about 6 dB of that to keep lock, so a jammer that adds roughly 37 dB over the signal (J/S) is enough to break it. That is why a tiny transmitter close by beats a satellite 20,000 km overhead. Distance uses free-space loss; real buildings and terrain add loss, so real ranges are shorter. (Simplified model for learning.)
Act 3
The Hunt
You saw how a position falls out of distances (Act 1) and how a jammer drowns the signal (Act 2). Now use two new clues — the direction a signal arrives from and how its frequency slides (its Doppler shift) — to test each suspect against the evidence and name the source.
🛰️ Detective! GPS receivers across Europe, Greenland and Canada keep losing lock at the same moment. Find the source.
Again and again, GPS reference stations spread over thousands of km report a sudden burst of interference at the same moment. The outages are too widespread and too strong to be a single ground transmitter near the border. Your job: identify where the interference is coming from.
The evidence
Direction it arrives from
Doppler signature
Coverage footprint
Lead checked off: space-weather logs were quiet during every outage — a red herring that rules out a solar cause.
Test a theory — which source fits all the evidence?
Epilogue
What if we lose GPS?
In three steps you saw the whole picture: GPS finds you by turning a handful of satellite distances into a single point (Act 1); that signal is so faint that a cheap jammer nearby can drown it (Act 2); and when interference blankets a whole region, its direction and Doppler shift help tell a ground transmitter from one overhead (Act 3 follows one reported case where the evidence pointed to space).
Here's what makes jamming serious: GPS is as much a clock as a map. The same satellites that place your blue dot keep power grids, stock exchanges, data centres and mobile networks synchronised to within billionths of a second. Knock out the signal and it isn't just navigation that drifts — timing across critical infrastructure does too. That's why losing GPS is a far bigger deal than getting lost, and why building resilient, harder-to-jam positioning and timing is an active field.
Go deeper
- 📝 The full write-up — the same story with the RF physics and a sources list.
- 📄 “Chasing Lightning” (Clements, Kriezis & Humphreys, 2026) — the research this case draws on. It is an arXiv preprint submitted for review, not yet peer reviewed, so treat its attribution as a claim from that study.
- ▶️ Veritasium: Something is jamming GPS over Europe — the video that kicked this off.