How does goal-line technology work?
By determining, automatically and within a second, whether the whole of the ball has crossed the whole of the goal line — a decision that is frequently impossible for a human to make from any available angle.
The problem it solves. The laws require the entire ball to cross the entire line. At speed, with players and the goalkeeper obstructing the view, and with the ball sometimes crossing and returning within a fraction of a second, officials positioned metres away cannot reliably judge it. High-profile errors — a disallowed goal in a World Cup match being the most cited — made the case unanswerable.
The camera-based approach. Multiple high-speed cameras, typically seven per goal, are mounted around the stadium roof, each tracking the ball. Software uses triangulation to compute the ball's three-dimensional position many times per second, and knows the precise surveyed position of the goal line. When the calculation shows the whole ball fully across, it sends an encrypted signal to the referee's watch, which vibrates and displays "GOAL" within about a second.
The cameras can reconstruct the ball's position even when it is partially obscured, provided enough cameras see part of it — which is the key technical requirement, since a goalmouth scramble obstructs most views.
The magnetic field approach. An alternative system uses a ball containing a sensor and a magnetic field generated by wires embedded around the goal area. When the ball fully crosses, the field change is detected. This requires a special ball and modification to the pitch, which is why camera systems became more widespread in football.
Why it is trusted when other technology is argued about. The decision is binary and objective — the ball either crossed or it did not — with no interpretation involved. This contrasts sharply with offside or foul decisions, where technology can supply precise positions but a human still applies judgement. The argument about video review is almost never about accuracy; it is about interpretation and delay, neither of which applies here.
It is expensive, which is why it is used only in wealthier competitions.