MLB’s new challenge system for ‘robot umps,’ explained

Anyone who has watched a baseball game, at any level, knows this situation well.

A pivotal moment late in a close game, a pitch on the corner, a call from an umpire, a frustrated player, a manager rushing out of the dugout in full rage, and a fan in the stands calling for the robots to take over.

Starting this season in Major League Baseball, that scenario will look a little different.

Wednesday is MLB’s Opening Day, and with it a new era of baseball arrives. The Automated Ball-Strike Challenge System, powered by T-Mobile and known as ABS for short, will give some players in those types of situations a chance to challenge the call on the field.

At a general level, ABS tracks the exact location of each pitch, relative to the specific batter’s strike zone. Players — more on this in a moment — can request a challenge of a ball or strike call they feel the umpire got wrong. When they do, a graphic displaying the result is then transmitted by T-Mobile and shown on the videoboard as well as the television broadcast. In this sense, it works in a similar fashion to the challenge system in tennis.

Once the call on the pitch is confirmed or overturned, the game resumes.

Sounds simple enough, but of course, there are additional elements to consider. So as the season begins, here is everything you need to know about MLB’s new ABS Challenge System.

Who can challenge a pitch?

Here is the first interesting caveat to the new ABS Challenge System.

Only the batter, the pitcher, and the catcher can challenge a ball or strike call. Not even a manager can do so. In addition, the challenge must be made immediately, without assistance from either the dugout or a video replay. The challenge is made by the player touching their hat or helmet.

But the timing is critical. The immediacy requirement has already lead to moments like this from Cavan Biggio, who thought he drew a walk and flipped his bat, only then to try and challenge the call:

Because he flipped his bat before the challenge, he was denied the request.

In addition, because both the pitcher and the catcher are allowed to challenge pitches, teams have been using Spring Training to decide who is in the best position to make those calls. Some teams have already indicated that they will rely on the catcher’s vantage point in those situations.

How many challenges do teams get?

Each team gets two challenges per a nine-inning game. A team will lose challenges if they fail, but will retain them upon a successful challenge.

This has already led to some questions over how teams will employ those challenges, with teams hoping to avoid burning a challenge early in the game, to make sure they still have challenges available in high-leverage situations later in the game.

Should a game go to extra innings, teams that are out of challenges will get one more challenge for the tenth inning. If that team uses that challenge in the tenth, they will get another challenge for the 11th, and if that one is burned in the 11th, the team will get an additional challenge for the 12th, and so on.

A team that still has challenges at the end of nine innings will not get an additional challenge if the game goes to extra innings, but they will get an additional challenge for any subsequent inning where they do not have a challenge at the start of the inning.

How does the system work?

From a mechanical standpoint, the ABS Challenge System works thanks to Hawk-Eye cameras, placed around the field to capture data from 12 different cameras. That data, for every single pitch, is transmitted over a private 5G T-Mobile network, and when a challenge is issued, the replay is shown on the video board at the stadium, and over the television broadcast.

When it comes to the strike zone, MLB is defining the width of the strike zone as 17 inches, the same as home plate. The top of the strike zone is 53.5% of the batter’s height, and the bottom of the strike zone is set at 27% of the batter’s height.

In addition, the ABS Challenge System makes its ball-strike determination based on where the pitch is relative to the strike zone at a depth of 8.5* from the front of home plate, the exact midpoint, as you can see in this graphic courtesy of Major League Baseball:

Regarding player heights, all position players were measured by a team of independent testers during Spring Training. To standardize the process, players will be measured between 10:00 a.m. and noon on the day of a measurement

The reason?

People shrink over the course of a day.

“Because people shrink over the course of a day,” said Brewers assistant GM Will Hudgins, the club’s liaison with the ABS Challenge System. “I’m not entirely sure how much, but I’ve been told that enough times to believe that it is scientifically true.”

Is there any data from the ABS system’s use in Spring Training?

According to MLB, during Spring Training 2.6% of pitches were challenged, with an overturn rate of 52.2%.

Notably, defensive players were more successful with their challenges (54.4%) than hitters, who were successful exactly half of the time.

Perhaps another notable statistic was how the overturn rate changed over the course of the game. According to MLB, the overturn rate dropped as games went on. In the first three innings, the overturn rate was 60%. That dropped to 51% for innings four through six, 42% in innings seven and eight, and just 46% in the ninth inning.

Where will the ABS system be used?

The ABS Challenge System will be used in every single MLB ballpark. This means that almost every single game will have the ABS system available.

There are three notable exceptions: The Mexico City Series between the Arizona Diamondbacks and the San Diego Padres, the Field of Dreams Game between the Minnesota Twins and the Philadelphia Phillies, and the Little League Classic between the Milwaukee Brewers and the Atlanta Braves.

As those games are not being played in an MLB park, the Hawk-Eye cameras will not be in place for the ABS Challenge System to be used.

However, the system will be available for the playoffs.