Sensor, sensor in a ball? Which is the best goal-line technology of them all?

As many of you will already be aware, FIFA and particularly Sepp Blatter have recently indicated that goal line technology does have a role in football (something we’ve argued for some time). Frank Lampard’s non-goal against Germany in the last world cup has been cited as the turning point, a mistake which must not be repeated. For those not familiar with the concept of goal line technology, this decision essentially heralds the introduction of technological officiation in football (of some capacity). As as example, a system of sensors could be used to measure the position of the ball within the goal mouth and inform the referee when the ball crosses the line and a goal is scored.

Line calling has so far been left to human judgement in football. That may be about to change. (Photo Courtesy of flickr user: Diego’s sideburns)

Technological line judging is already used in tennis (Hawk-eye), although there are significant differences in its use compared to what is being proposed in football. In tennis, a player is able to appeal the umpire’s decision after a line call which they don’t agree with. An attractive, computer generated recreation of the ball’s flight path then informs the players, audience and match official’s whether the (effectively) infallible computer agrees with the decision. The whole process takes several seconds and some commentators now believe players query a line call to give themselves valuable respite during particularly long, arduous games. This is an interesting point, whenever new rules or methods are introduced to a sport, the nature of competition will inevitably produce unforeseen behaviour from players seeking an advantage. This may explain FIFA’s reluctance to introduce technology before now, and why they have made clear stipulations for a technology to be deemed suitable.

The words ‘infallible’ and ‘instant’ have been mentioned when discussing FIFA’s requirements for goal line technology. What is clear, is that any system that is chosen must be seen to be highly accurate, and able to deliver a decision to the referee very quickly (in less than a second according to various news sources). From an original line-up of eight competing systems, two have managed to meet FIFA’s exacting standards on accuracy and speed (and I’m sure there are many other criteria). One of the systems has been developed by Hawk-eye who already have experience in cricket, tennis and snooker. The other is a system called Goalref and uses a radio transmitter situated within the ball, as opposed to the cameras of Hawk-eye. What is interesting is that both systems use quite different technologies.

Hawk-eye, as a camera based system, is entirely passive. Once the cameras are installed around the stadium no changes need to be made to the ball or the goal itself. It is the job of highly powerful image processing algorithms which will identify the ball and its position within each cameras viewpoint and calculate its 3D position within the goal mouth. Precise camera positions and fastidious calibration will ensure that the system can reliably gauge when the ball crosses the line. The difficulties arise when you have to consider the effect of differing weather conditions, lighting levels and potentially, a maelstrom of players also being present in the goalmouth (a more complex and unpredictable scenario than tennis). An automatic computer system must track the position of the ball (even if obstructed) and signal the referee within a second of it passing the line. The fact that Hawk-eye is still being considered suggests that it hasn’t failed yet.

Goalref operates quite differently and is an active sensor system. A radio transmitter situated in the ball generates a signal which is picked up by antenna attached to the goalposts (see this patent document for more details). There are some distinct advantages to this technology, the signal transmitted by the ball shouldn’t be significantly affected by weather, lighting or the presence of players. As long as the antenna receive the radio waves, a computer system can theoretically calculate the ball’s position. A disadvantage, is the necessity of placing a radio transmitter within the football. Firstly, the exact point from which the signal is transmitted will determine the position of the ball. This seems obvious, but if the transmitter moves within the ball, the system will deem the ball to be moving, even if it isn’t. This has significant implications when millimetre accuracy is demanded and the ball deforms significantly when struck. Secondly, the perception of players can have a big role on the acceptance of technology. New football designs already face extended criticism without electronics in the middle of them. If a significant number of influential players think the ball is behaving strangely because of the addition of electronics it may be hard convincing them otherwise, even if no scientific evidence arises. In addition, this type of system will obviously only work with a ball which contains a sensor. So, Goalref may need to collaborate with individual ball manufacturers and accuracy may become dependent on the make and model of the ball.

I think the addition of technological line calling in football is a good thing. While no technology is infallible or instant, deferring to a system which uses radio antenna or multiple high-speed cameras gives a very high level of reliability compared to human judgement. A lot of prestige and money rides on the outcome of football matches, a lot of pressure is put on a referee to make decisions in difficult circumstances. These technologies will help ensure that the score line at the end of the match is the right one.

With regards to which system may come through on top? I don’t have a strong opinion, my judgement sways me towards the passive Hawk-eye system but time will tell for sure.

Simon Choppin


About wiredchop

Simon Choppin Simon’s sports engineering career began at the age of six when he loosened the wheels of his skateboard in order to make it go faster. While the experiment was chalked up as his first failure, his resulting dimpled skull has provided an aerodynamic advantage in more recent sporting pursuits. Academically, Simon completed a degree in Mechanical Engineering with Mathematics at Nottingham University before joining the Sports Engineering Research Group at Sheffield to start his PhD. His main interests include work with high speed video, mathematical modelling of various sorts and experimental work involving machines with big buttons. As a sportsman, Simon has an unfortunate lack of talent for anything requiring skill, tactical awareness or the ability to learn from mistakes. He does however seem to posess the ability to move his legs around for a long time until other people get tired, for this reason you’re most likely to see him on a bike of some sort or running up a hill in offensively small shorts. Simon was fortunate enough to have a stint at the Guardian newspaper as part of the BSA’s media fellowship, which gave him the idea for this blog. Other than this, his writing experience includes his PhD thesis and various postcards to his Mum.