How does goal-line technology work?

The new football season will see the English Premier League trialling goalline technology. How does it work?

It happens from time to time in football: thousands of people filling a stadium watch the ball cross the goal line, but the most important of them all – the referee – hasn’t seen it. Those thousands are aggrieved, the goal is not given, and the whole course of a tournament or league competition is not what it might have been.

Or perhaps the referee does award a goal – one that the defending side insists shouldn’t stand. The German language even has a specific term for it: a “Wembley-Tor”, after the goal that gave England a 3-2 lead in the 1966 World Cup final. The story that the “Russian” linesman, actually from Azerbaijan, when asked how he could be so sure that the goal was legitimate, replied simply: “Stalingrad” is probably apocryphal.

Now, technology is being introduced in an attempt to overcome human error with mathematical certainty. The two main systems being trialled are Hawk-Eye and Goalref.

Hawk-Eye

Hawk-Eye is the system already employed by other sports such as tennis and cricket, and will be tested in football for the 2013–14 Premier League season.

It uses a network of high-speed video cameras to track a ball’s position at a given time via triangulation. The trial set to take place in the Premier League will use seven cameras placed around the stadium.

Knowing the ball’s position, Hawk-Eye can tell when it’s crossed the goalmouth, and the software alerts the match officials via a radio transmission to the referee’s watch.

The system’s software can also predict the future path of a ball – it’s often used like this in cricket to determine whether the ball would have hit the wicket had it not hit the batsman first, and the predicted path can be drawn on-screen.

Because it can create this kind of visual display it’s expected to be popular among viewers of televised games.

As used in tennis, Hawk-Eye has a margin of error of just 3.6 mm, better than the 3 cm required by football’s governing body, FIFA. However it needs to be able to see at least a quarter of the ball to work – not always possible if there are multiple legs in the way.

Hawk-Eye has previously been tested in the Hampshire Senior Cup final and at a friendly between England and Belgium at Wembley – although the system’s results were only used for evaluation, and the match officials didn’t have access to them.

Goalref

The Goalref system, which has been tested in the Danish Superliga and at the 2012 Club World Cup, makes use of electromagnetic induction.

A passive electronic circuit is embedded into the ball between the leather outer layer and the inflatable centre. An electromagnetic field is created in the goalmouth and its strength monitored by computer. (It’s perfectly safe – so there’s no need to worry about goalkeepers getting their brains fried.)

When the circuit in the ball crosses the goal line, the electromagnetic field changes, and the computer automatically notifies the referee, again by a radio signal transmitted to his or her watch.

Because the technology behind Goalref is inexpensive compared to Hawk-Eye, it’s easier to implement at the lower levels of the game – addressing concerns that goal-line technology would exacerbate the gulf between the grassroots and megabucks.

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