HAWKEYE

In a small booth overlooking Arthur Ashe Stadium, a team of technicians sits hunkered amid a tangle of wires, monitors and whirring computers. Although they are more than 10 stories above the match being played below, in a sense this is the best seat in the house.

The network of cameras and computers they control can track the flight of a tennis ball with an accuracy measured in millimeters — 3.6 to be exact. Overseeing it all is Paul Hawkins, the thin, sandy-haired, 30-something Englishman who had the crazy idea a few years ago to do for tennis what no other professional sport seems to have managed: create an instant-replay system that works.

“I have a technology background,” said Hawkins, who holds a doctorate in artificial intelligence. “I love sports. So I kind of had an opportunity to combine my two passions.”

The result was Hawk-Eye, probably the most successful instant-replay system in sports. Since its introduction at the United States Open three years ago, Hawk-Eye has won over fans, players and even officials.

In fact, about the only criticism of Hawk-Eye is that it is not used more widely. Currently, instant replay is available only on the two larger show courts: Ashe and Louis Armstrong Stadium. This was an issue twice for the top-10 player Andy Murray, whose second- and third-round matches were marred by numerous questionable line calls and umpire overrulings.

In addition to the five-figure cost of the system each year, the infrastructure improvements — like a video-enabled scoreboard — make it cost prohibitive to put the system on all Open courts.

But Hawk-Eye has been so well received, the United States Tennis Association is seriously considering investing $100,000 to add the system next year to a third court — the Grandstand —said David Brewer, tournament manager for the Open.

“Even Hawk-Eye, when we first looked at it six or seven years ago, wasn’t accurate enough, but it is now,” he said.

The big breakthrough, Hawkins said, was not relying on optical devices to determine where a given shot lands — a surprisingly difficult spot to measure accurately. Hawk-Eye uses a system of 10 cameras to track the speed and trajectory of a ball in flight, but that is only part of the magic. The rest is done exclusively through computer modeling.

Because no tennis court is exactly flat and no line precisely straight, before the tournament. Hawkins’s team takes thousands of precise measurements of the dimensions and contours of each court, which are then converted into a three-dimensional computer model. Hawk-Eye’s virtual world takes into account other real-world factors that can affect accuracy, like the amount a ball compresses when it hits the court and even the temperature of the court.

“During warm days, the court actually changes size as it heats up or cools down,” Hawkins said.

When the ball flight data is fed into the computer model, the result is a system that is so precise it’s difficult to measure.

“The published stuff says 3.6 millimeters, but our system is actually more accurate than that,” Hawkins said.

But perhaps the biggest key to its acceptance among officials is that it matches an existing appeal process in the rules. On clay, the ball leaves clear marks where it strikes the court        [ THIS IS A TENNIS MYTH! The ball can strike the line , but leave the footprint after the line], and the umpire is allowed to use that mark to overrule incorrect calls. Hawk-Eye brings that same procedure to grass and hard courts.

One big positive for officials has been to show that they are right the vast majority of the time. Only about 30 percent of instant-replay appeals result in overruling of the original call.

NY TIMES

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