Goal-line technology gives the referee an answer without a review. The system decides on its own and signals directly to a watch, which is why the game does not stop while it works.
Two different physical approaches
One family of systems uses banks of high-speed cameras mounted in the stadium roof, several trained on each goal. Software identifies the ball in every frame and reconstructs its position in three dimensions.
The other embeds a coil in the ball and generates a magnetic field around the goalmouth. Distortions in that field reveal where the ball is, without any dependence on a clear view.
The camera approach struggles when players block the ball entirely; the magnetic approach requires a specially built ball and wiring in the goal frame. Competitions choose between those trade-offs.
Calibration defines the goal line
Neither system knows where the goal is until it is told. Before a match the goal frame and line are surveyed precisely, and that geometry is loaded as the reference the software compares against.
The decision is whether the whole ball has crossed the whole line, so the ball's diameter matters as much as its centre. The software models the ball as a sphere of known size.
Calibration is checked on matchday and re-checked if a goal frame is knocked out of position. A shifted post invalidates the reference, and officials are told when confidence drops.
The alert is designed to be unmissable
When the system confirms a crossing it sends an encrypted signal to a watch worn by the referee. The watch vibrates strongly and displays a short confirmation on its face.
Vibration is chosen because a stadium is loud and a referee is usually running. A sound alert would be lost in crowd noise at exactly the moment it is needed most.
The signal travels on a dedicated radio link rather than the venue's general network. Isolation keeps it out of the congestion that builds when tens of thousands of phones are in use.
Speed is the whole point
The full chain, from the ball crossing to the watch buzzing, is designed to complete in well under a second. That threshold exists so the referee can act while play is still live.
Video review takes far longer because a human has to look at footage and reach a judgement. Goal-line calls are treated differently precisely because they can be automated without interpretation.
The narrow scope is deliberate. The system answers one binary question and refuses every other, which is what allows it to be trusted without a second opinion.
Failure has a defined fallback
If the system reports reduced confidence or drops out, the match officials are informed before kick-off or during a stoppage. The referee then reverts to unaided judgement for the rest of the match.
Competitions require testing on the day, with balls fired across the line from multiple angles and the results logged. A failed test means the technology is not used rather than used cautiously.
That all-or-nothing rule prevents the worst outcome, which is a system trusted while it is quietly wrong. Officials are told its status explicitly, not left to infer it from silence.
