Follow That Puck: The Ins and Outs of Available Tracking Technology

It happened again.

The puck probably went in. It was under the goalie, it was behind the stick, and both of those crossed the goal line. But the video review was inconclusive because they couldn’t actually see the puck across the line itself.

Plenty of sports track whatever their important objects are, and these are often held up as examples of how hockey should be able to do it as well. Fans yell “Put a chip in the puck already!”, but is it really that simple?

Visual Tracking

Most sports use visual tracking systems. Probably the most well known and mature method is the Hawk-Eye system. Originally developed for use in cricket but likely best known for it’s use in tennis, it uses high speed cameras to track whatever needs tracking from multiple angles. While it’s the most widespread, it’s not the only version of this concept in use. The NBA uses SportVU, track and field uses Eagle Eye, while golf and several racing sports use Virtual Eye.

In hockey, we briefly had the much-maligned FoxTrax, which was not quite the same type of system as the others mentioned. The puck itself had to be modified with circuitry, a battery and 20 infrared emitters embedded in the puck - 12 around the side, plus four on the top and bottom. They initially cost around $400, but the price came down to about $15 per puck (still several times the cost of a regular puck). In addition to the puck, they hung thirty special infrared cameras from the rafters in a grid pattern to track it. It was actually a pretty impressive, if somewhat over the top, set-up.

There are solid applications for visual tracking in hockey. Player tracking would be a huge asset, and it would help in goal calls where current camera angles make it difficult for a human to tell if a visible puck completely crossed the line, or if it went in off a high stick. Where it wouldn’t help, though, is a situation like Pageau’s no-goal on Sunday. If the current cameras can’t see the puck under the goalie, an automated tracking system likely can’t either.

Sensors

An alternative, or sometimes companion, are sensor based systems that do not require a clear line of sight to determine position.

FIFA, in addition to Hawk-Eye, uses the GoalRef technology. This system creates an electromagnetic field between the goal posts, and a passive circuit (and antenna loops) is embedded in the ball. When the ball passes through the electromagnetic field, the strength of the field changes and trips the system to indicate a goal. The accuracy is pretty good, with lab tests showing an error range of about 3mm on average.

The problem translating the technology to hockey, though, is that it only detects when the middle of an object passes a certain point. With a round soccer ball, that’s good enough. The distance from the middle to any edge is the same. Even with deformation if it’s flattened under the keeper it’s within the 3cm accuracy that FIFA requires. With a puck you need to track at least three points in three dimensions, because the centre of the puck doesn’t tell us enough. If the puck is flat on the ice, the centre needs to be 38mm across the line to be entirely in. If it’s in a goalie’s glove and completely vertical, it’s only 12.5mm across to be a goal.

Another alternative is something most people are familiar with - RFID technology. You can put multiple RFID tags in the puck using a similar process to putting the infrared transmitters in the FoxTrax pucks, and multiple sensors around the arena (or in each net) can be used to triangulate the position of each of them. A computer on the back end can crunch the numbers and give a model of where the entire body of the puck is. Only one problem - the most accurate commercially available RFID position tracing technology is plus or minus 10cm. For reference, that’s almost twice the width of the goal line itself. The RFID concept could work, but not until the accuracy is several times better than it is right now.

Miscellaneous Others

There are a few other technologies used in sports tracking that just don’t translate to goal line detection in hockey. In addition to visual tracking, cricket’s Umpire Decision Review System uses sound and thermal imaging to detect contact between the ball and bats/pads/etc.

The NFL has started testing out sensors in footballs, but they measure how the football itself moves and not it’s exact positioning. While that could make for some useful information in hockey when it comes to something like perfecting a slap shot or even refining rebound handling, it won’t help with goal reviews.

No Silver Bullet

In the end, there’s no technological solution that would clear up all of these “inconclusive” goal reviews. This doesn’t mean the NHL shouldn’t be pursuing them, though. Visual tracking has uses far beyond goal reviews, and could still help with some of those. Sensors don’t have the level of accuracy required to help with a puck that may have crossed the goal line by a millimetre or two, but it would catch the pucks hidden under a goalie that go far enough into the net to be “sure things.”

If there’s something that cuts down half of the inconclusive reviews instead of 99% of them, isn’t that still worth doing?