A tracker that suddenly goes dark, a fleet that loses GPS lock in the same corner of the yard, a machine whose position freezes or drifts — these can be a glitch, or they can be a sign that someone is jamming the GPS signal on purpose. For plant, machinery and high-value vehicles, the difference matters a great deal, because jamming is how a lot of them get stolen.
What GPS jamming actually looks like
GPS (and the other satellite systems grouped under GNSS) relies on extremely weak signals from space. They're so faint that a tiny transmitter nearby can drown them out completely — that's all a jammer does. When it's running, the symptoms are distinctive:
- A tracker or GPS unit that drops offline suddenly, then comes back later for no obvious reason.
- Several assets losing signal at once in the same place — far more telling than one device misbehaving.
- Lock that returns the moment a particular vehicle leaves the area.
- Position that freezes, jumps or drifts rather than simply going blank.
It's worth separating two things. Jamming is denial — it floods the band so the receiver gets no fix at all. Spoofing is trickier — it feeds the receiver false signals so it reports the wrong position. Theft almost always uses simple, cheap jamming; spoofing is rarer and more sophisticated. Either way, the receiver stops telling you the truth.
Why plant and vehicles are the targets
A GPS jammer that plugs into a 12V socket costs around £30 on the grey market. To a thief, that's a trivial outlay to defeat a tracking system worth far more — switch it on in the cab or a vehicle parked nearby, and the asset can be driven, lifted or shipped away with no live trace. It's why plant theft recovery rates are so low, and why the tracking industry has moved to backup technologies specifically to survive jamming. The same trick is sometimes used by an operator who simply doesn't want their movements logged.
Jamming or just a genuine fault? How to tell
Not every lost fix is sinister. Before assuming the worst, look at the pattern:
- One device only, intermittently — more likely a fault, an aerial problem or a flat backup battery.
- Several devices, same place, same time — that points strongly at an external jammer, because a fault wouldn't hit them all at once.
- Loss that tracks a specific vehicle or person being present — a strong indicator the jammer is travelling with them.
- Loss in tunnels, deep indoors, or tight urban canyons — usually just natural signal blockage, not jamming.
The honest truth is that the symptoms overlap enough that you can't be certain from the behaviour alone. Proving it takes looking at the radio itself.
How an RF engineer proves it
GPS sits at a known frequency — the L1 band around 1575 MHz. When a jammer is active, a spectrum analyser shows it plainly: the noise floor across that band lifts dramatically, with a signature quite unlike a natural dropout. From there, direction finding traces it to the source — a device in a particular cab, a vehicle across the road, or a unit concealed on a machine. The result isn't a hunch that "it might be jamming"; it's measured evidence of what happened, when, and where it came from.
The one question that narrows it fast
Next time it happens, ask: did more than one asset lose signal in the same spot at the same time? If yes, a fault is unlikely and external jamming moves to the top of the list. Note the time and what vehicles were on site — that single observation is often what turns a suspicion into a provable case.
See GPS jammer detection →The legal bit — and why it matters for you
Using, supplying or even possessing a GPS jammer is a criminal offence in the UK. So if jamming is happening on your site, it isn't just an inconvenience — it's evidence of a crime, often a precursor to theft. That's exactly why independent, documented proof matters: an insurer assessing a theft claim, or the police, will take a neutral engineer's measured evidence far more seriously than a guess. If you ever do find a suspected device, don't operate it or move it about — leave it be and get it documented properly.
When to get someone in
If a yard keeps losing tracker visibility, if a theft claim hinges on whether jamming occurred, if high-value plant or vehicles are involved, or if losses keep coinciding with a particular vehicle on site — that's the point to bring in RF test gear. Detecting, locating and documenting a jammer is precise, fast work for the right equipment, and it puts certainty where there was only suspicion.