What is it?
Goalline technology is essentially a bundle of technologies that together monitor the path of the ball and detects when it crosses the goal line.
How does it work?
There are several systems currently approved for use by Fifa and the International Football Association Board (IFAB), broadly based either on camera-tracking or magnetic field sensors:
German-produced GoalControl is the system currently in use at the World Cup in Brazil. It uses 14 high-speed cameras mounted around the stadium – seven pointed at each goal – to track the flight of the ball and detect whether it has cross the goal line or not.
Hawk-Eye is already used in cricket, tennis and snooker, and like GoalControl employs high-speed video cameras dotted around the pitch to triangulate and track the position and flight of the ball. Seven cameras are required per goal mouth, allowing the system to still operate when several cameras are blocked. Hawk-Eye was used in the 2013-2014 Premier League season in the UK.
Cairos GLT system
Cairos Technologies and Adidas produce a system that does not rely on cameras, instead using a magnetic field to track the ball. A sensor is embedded inside the ball, which detects the magnetic field produced by thin wires run underneath the penalty box. A computer tracks the position of the ball via the sensor and detects when the ball crosses the goalline.
Another camera-based system, Goalminder uses high-speed cameras built into the goal posts and crossbar to deliver visual evidence only to the referee, leaving it to the judgement of the officials.
GoalRef uses a similar magnetic field-based technology to Cairos GLT, but instead of the ball acting as a sensor, the goal frame detects the passing of the ball.