Unlike tennis or cricket, Hawk-Eye's use in football is limited to the rare occasions when there is a disputed goal.
BBC: Hawk-Eye's inventor eyes football expansion
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However, both GoalRef and Hawk-Eye's technology must now pass a stadium test before either system can be used in a match.
CNN: FIFA awards goal-line technology licenses
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Hawk-Eye's system uses seven cameras to track the movement of the ball and sends a signal -- within a second -- to watches worn by match officials when a goal is scored.