Correctly determining when soccer players are offside turns out to be surprisingly difficult. And the English Premier League turned to an unusual source for a fix: an offside-detection system powered by dozens of iPhones.

The same system might someday be used to track players in other sports, too.

iPhones used as referees to call offside soccer players

The offside rule is intended to stop soccer players from hovering around the opposing team’s goal. A player is determined to be offside if they touch the ball while closer to the opposing team’s goal than both the ball and the second-last opponent — usually the goalkeeper. Implementation turns out to be a complex problem, and there are assistant referees whose primary role is watching players for offside violations. Even so, mistakes are common … especially if you ask fans.

“[English Premier] League’s new offside-detection system is apparently able to spot a player’s position on the field, and call them offside, with more accuracy than ever — and it’s all powered by iPhones,” reported Wired on Wednesday.

The system developed by Genius Sports and its subsidiary Second Spectrum puts an array of iPhone 14 and newer models around the pitch. These use their built-in cameras and a proprietary software system dubbed Dragon to track the locations of players from many angles.

“By capturing streams of video and positioning data as a player moves on the field, that player can be re-created virtually — their movements, likeness, and hand gestures, all rendered digitally in real time,” notes Wired.

Using a large number of handsets is intended to deal with the one of the largest problems with correctly calling a player offsides — seeing the person behind other players.

This isn’t the first time a video assistant referee has been used. FIFA turned to an earlier version of the tracking system now being implemented by the English Premier League for the 2022 World Cup in Qatar.

Moving from beta to reality

The iPhone-powered soccer offside-tracking system is reportedly scheduled to go into effect later in the English Premier League’s 2024–2025 season.

And that might be only the beginning. “Genius hopes to make inroads in basketball and American football soon,” reports Wired. The exact digital recreations of players produced by Dragon might be used to show instant replays from any angle, for example.