Hawk-Eye v GoalRef: The pros and cons of the over-the-line systems FIFA are testing
Football is in uproar after the ghost goal awarded to Chelsea in their FA Cup semi win over Tottenham.
The clamour for the implementation of technology to help referees decide whether all of the ball has crossed all of the line has returned, but how would that work?
Here, Martin Lipton looks at the two rival systems that could prevent Sunday's embarrassing gaffe happening again ...
FIFA conducted test trials for a total of eight systems, involving cameras and chip-in-ball technology, over a four-month period.
Previous systems, which were tried out at the 2005 Under-17 World Cup in Ecuador and 2007 Club World Cup in Japan, were deemed to have failed.
Initially, 12 companies applied to FIFA, before a short-list was compiled last summer.
The two systems that met the standards set by FIFA - 100 per cent reliability, accuracy and speed of the decision-making process - were approved by the International FA Board (which includes the four Home Nations, making up half of the eight-member body) for final testing.
A second testing stage begins this month and will last a month, before a special meeting of IFAB in Kiev on July 2, which should give assent for the two systems to be introduced worldwide.
However, logistical issues may mean a delay in its introduction in the Premier League until the 2013-14 season.
THE TWO SYSTEMS...
HAWK-EYE - British-designed computerised system based on the principle of triangulation, using the visual images and timing data provided by high-speed video cameras at different locations around the area of play.
The system uses six cameras to triangulate and track the ball in flight, meaning installation costs will be high.
Software calculates the ball's location for each frame by identifying the pixels that correspond to the ball through at least two cameras.
The margin of error for the system is 3.6 millimetres.
The FA and Premier League have long backed Hawk-Eye, believing it provides accurate and swift decisions within FIFA's "one-second" demand.
It would be television-friendly, giving fans at home - and in the stadium - visual "proof" of the validity of the decision, and high-speed cameras allow the ball to be tracked even if they only cross the line for a fraction of a second.
However, software can only track the ball and predict the flight path as long as 25 per cent of it is visible - no decision could be given if the ball was "buried" under a keeper's body, for example.
GOALREF - A joint Danish-German project, initially pioneered by former FIFA ref Peter Mikelsen and developed in Copenhagen, which uses magnetism to determine whether the ball is over the line.
Unlike the previously-considered Cairos system, which required the chip to be inserted in the exact middle of the ball, the electronic probes are attached between the inner ball and the inside of its leather outer lining.
Sensors are installed on the inside of the posts and crossbar and send out bursts of electronic waves, based on the "Doppler Effect" you get when an ambulance or F1 car passes and the signal moves through 180 degrees.
The system means an "instantaneous" signal (quicker than one tenth of a second) would be sent to the referee when it is detected the ball has crossed the line - faster, in effect, than the assistant referee could flag for a goal.
Because a magnetic signal is used, there is no need for the ball to be in sight for a decision to be made.
GoalRef insist the system is compatible with any ball - unlike Cairos, which was an Adidas project.
Would not be great for TV - there is just a signal to the referee and other officials - but far cheaper to install than Hawk-Eye, with a mass production version already in the pipeline.