Last Sunday, during the World Cup game between France and
Holland Honduras, FIFA's goal line detection technology passed its first field test. It was early in the second period when the French striker Karim Benzema nailed a shot right into the post that sent the ball blasting back across the goal line and bobbling into the hands of the Honduran goal tender. The crowd gasped and then went silent. It wasn't immediately apparent whether the ball had crossed the line.
Plays like this are decided by a lone referee on the field, and in the past, whether he made the right call depended entirely on whether he had a good line of sight through a throng of frenzied athletes (this 2010 World Cup goal goes to show that they don't always get it right). Now, however, the refs are getting backup from a system of high speed cameras that follow the ball in 3-D and send an alert when it crosses the goal line. Seconds after Benzema made his attack, the ref received a vibrating signal on his smartwatch, a point went up on the board, and the crowd broke into a jubilant chant.
FIFA's goal line technology has improved the accuracy of point scoring to such a degree that one has to wonder whether other sports besides soccer might benefit from similar ball tracking systems. Now, it seems, American football may provide the next test case. Engineers at North Carolina University and Carnegie Mellon have teamed up with Disney Research to design a sensor-based system for determining the location and position of a football on the field and are beginning to test its accuracy. They describe the technology in IEEE's Antennas and Propagation Magazine.
American football presents certain challenges that do not exist with soccer. Sometimes, the most important calls a ref can make happen on plays where the ball is buried beneath a scrum of sweaty, 150-kilogram men. In such cases, a visual detection system, such as the one now being used at the World Cup, would be useless.
Instead, the NCU researchers are opting to send radio signals from transmitters hidden within the layers of the football and track it with receivers positioned around the field. According to the paper, engineers have tried similar approaches in the past, but ran into trouble because they were using a high frequency wave, of a kind that is easily absorbed by the human body. The researchers claims that results from these previous attempts failed because they did not produce a clear enough signal.
This time around, engineers are using extremely low frequency radio waves (producing a wavelength that is hundreds of meters long) that are able to pass through the human body unmolested. The design they came up with requires that a transmitter and battery, weighing less than 30 grams, be embedded into the football. Eight receivers, placed on the perimeter of the field then calculate the location and orientation of the ball in 3-D.
Unfortunately, using low frequency waves solves one problem while creating another. This time, the interference comes from the ground which absorbs the signal and then re-emits it in a pattern called an eddy-current. In order to reduce the noise from this secondary signal, the engineers rigged the field with a set of optical instruments. While the receivers are keeping tabs on the signal transmitted from inside the football, the optical instruments collect information about the position of the receivers. This data can then be used to scrub out the signals coming from the ground.
According to the researchers, their technique must be accurate to within half the length of a football—about 14 centimeters—in order to be considered reliable. Right now they report having it down to 77 centimeters.
For those of you wondering how the performance of a football might change after it's been stuffed with sensors, David Ricketts, one of the authors of the study, says that footballs are actually rather asymmetrical objects to begin with. "The American football is already unbalanced," he explained by email. "The laces cause a weight mis-distribution, so they counterweight it to compensate. The added weight of the transmitter would be handled in the same way."
If you'd like to see how the tracking system would work in a real game, the group has posted a video of their trials here.
Editor's note: The game referenced in the first sentence was between France and Honduras, not Holland as originally reported.