Hawk-Eye Live – the End of the Tennis Line Judges?

A typical scene at the bett1HULKS tennis tournament during the last two weeks at Cologne’s LANXESSarena: Alexander Zverev (who swept both singles tournaments, by the way – great performance!) at first serve. The ball hits somewhere near the edge of the service court. A female voice is sounding through the arena. “OUT!”. The typical first reaction of a tennis fan: looking at the linesmen. Was the shot too long? Or was it too wide? I looked at the borders of the court, looking for the stretched arms of the line judges and saw: nothing! Like at the US Open before, the bett1HULKS used automated linesmen, based on the Hawk-Eye system. By the way, in contrast to the Antwerp tournament in parallel. I felt to blog about it and my experience with it.

The coverage of the bett1HULKS tournament has been done in collaboration with 

This blog entry does not necessarily reflect the view of, though.

At the end of 2020, I decided that this posting is my favorite one published 2020.


Hawk-Eye Live – How does it work technically?

Hawk-Eye is has been originally developed in the United Kingdom by Paul Hawkins, originally intended to support calls in cricket. The technology used for tennis is in general the same as you for example use for the goal line technology in soccer: multiple high-resolution cameras, typically located under the stadium roof (in Cologne and for tennis in general, they are rather placed on a comparably low level) follow the ball in every situation of the match. According to wikipedia, you work with ten cameras in tennis. In general, by comparing the ball position with fixed positions in the arena (the fixed positions are typically camera positions itself), you can technically follow the path of the ball. This also allows the system to predict the path of ball, even if it cannot completely follow it. The decision whether the ball is in or out is decided on the path, not on the footprint of the ball. You should also clearly mention that the visualization you see in TV and in the arena is not an exact camera picture, but just an illustration – this caused discussions here and there at the bett1HULKS, for example.

By the way, one of the key reasons that the technology has been used in Cologne and not in Antwerp might simply be that Hawk Eye Innovations Ltd., the company which is driving this high definition camera technological wonder, is not only having three offices in the UK, but also one in Cologne. The system is tested and used in very different sports, ranging from cricket, tennis and soccer to NASCAR racing.


Is the Hawk-Eye fully reliable?

The answer is a clear no. I guess that the technology is constantly improved, but finally, there are technical limits by numerical exactness and the number of frames you can achieve while following the ball. I found a 2016 brochure by Hawk Eye technology stating that the mean accuracy is 2.6mm – which might sound quite cool, but in fact,

  • This is a purely stochastic information (a mean) – they do not give information about the variance of the exactness. Even with many frames per second, the variances should increase with higher ball speed or adverse settings like low light / bad sunlight.
  • The dimensions of tennis are quite small in general: a sideline is between 25 and 50mm (see ITF Rules of the Game), and the ball, which roughly has a diameter 65mm may have a very odd footprint on the court. If you are interested in that topic, I highly recommend you a 2014 paper by Rod Gross, The footprint of the tennis ball, which is showing in a lot of detail the footprint of a tennis ball and the impact of the speed on it.
  • Finally, these are figures for the hawk-eye techology itself. In Cologne, they used Hawk-Eye Live, which takes decisions during a running game. Thus, even if technology has evolved over the years, I would expect that the extremely short time frame you have from the touch of the ball on court to the decision given to the chair umpire whether the ball is out or not is extremely short – which means you have less time to compute the parth of the ball aka less accuracy.


Hawk-Eye Live Is wrong sometimes!

As a mathematician like me, you must criticize the way Hawk-Eye has been implemented to the sports. It is presented in a way that you feel it gives you a definite and non-challengeable decision on whether the ball has been in court or not – but in fact it just a decision which is likely correct with a certain, potentially high reliability.

I am not bashing the system at all – I guess that it will be much better than any human linesman can ever be. If you struggle with that, I highly recommend to have a game on that website, where you can try to do line umpire calls as a test online. The people who do the line umpire in tennis tournaments must be extremely well trained, in great phyiscal / visual condition and have a lot of experience to reach an accuracy which is acceptable for higher class tennis. Even if you do a good score in the game, you should bear in mind that in reality, you additionally fight against views blocked by the player, low sun or similar challenges on top of the call itself.


Some Hawk-Eye Live Failures

Said again, I feel that the system is a a very interesting technical innovation, which is having a lot of potential to improve decisions in tennis. Still, it might just complete the picture to point out two quite prominent failures of Hawk-Eye Live.

For example, this New York Times article about the Hawk-Eye Live technology to be used at the 2020 US Open states a quite well-known example of a New York Empire vs. Chicago Smash World Team Tennis, where the line umpire finally overruled the Hawk-Eye Live decision. Unfortunately, I could not find a video about that. In contrast to that, there is plenty of footage of a non-given goal by Sheffield United vs. Aston Villa in soccer, as of June 2020. The likely cause of the fail is that Hawk-Eye lost track of the ball as it is covered by the goalie, his arms, other players and the goal cage. This is, by the way, a beautiful example suggesting that the referee has not understood how the technology is working. There are so many players and environment around the ball that it is not too surprising that views could have been blocked and thus the system becomes unreliable. To me, this is not a Hawk-Eye, but a referee (and maybe: referee training) failure.


Hawk-Eye Live – How did it impact the game?

I could not find written rules of tennis under Hawk-Eye Live, but in general, the chair umpire must not overrule the system. The only exception are (obvious or reported) technical malfunctions. There are also no challenges against the system – you could for example imagine that a player challenges the call and then it is re-calculated with more computation time and thus higher accuracy. Most articles from sports magazines I read about this topic did not discuss this potential that you could in fact challenge Hawk-Eye Live by Hawk-Eye itself. Close calls at the end of a rally were indeed illustrated on the screens.

The good thing about using Hawk-Eye Live is that (most) players also seemed to be happy that they can do their job again and in fact do not really know how the system works (or they feel it is needless to complain about it…). The atmosphere on close calls in LANXESSarena was much more calm and relaxed than what I saw in earlier time in human-umpired matches, either on TV or at courtside. There were, however, players, who did complain about calls. Two I remember were Alexander Zverev and Denis Shapovalov in their singles matches. Here and there, the voice of the chair umpire had to replace Hawk-Eye Live, which is giving an automated voice (there are different characters, male and female, which is quite entertaining).


Foot Fault

Just a side note on the impact of Hawk Eye Live: many players seem to feel offended when a (human) line umpire calls a foot fault at serve. The acceptance of these calls, which can also be made by Hawk Eye Live, has been significantly higher. I saw more foot fault calls than I have been used to – and less discussion about it. Just assuming that players feel offended somehow by human umpires reveling this rule violation.


The Challenge System

As said, there were no challenges allowed against Hawk-Eye Live (in contrast to human calls in the past). I just personally feel to add that I feel that I always hated the way challenges in tennis are presented, especially in TV. Very often, after a successful challenge, TV showed the “faulty” line umpire in full screen. Ain’t that some sort of medieval pillory and you virtually spit on this person for his “terrible” behavior for a second? I can just refer to the umpire game again. Try it out and see how difficult the job of the people is – and the conditions under which line umpires are working for professional tennis are quite poor to what the people have to invest into it and to the money made in the sports.


Hawk-Eye Live – The down-side

Even though I state that I feel you should pay line umpires better than you do today, I feel the introduction of Hawk-Eye Live also has a strong human factors. The people you see in the sponsor-branded sportswear each and every day and the edges of a professional tournament tennis court are the ones which drive the sports. You need line umpires for challengers and futures, there will not be Hawk Eye Live at a 10k ITF future out of nowhere. There will be people who do the job – and I guess part of the motivation they have to line judge or chair umpire matches at these tournaments is that they want to be at a grand slam or at least a professional tour event one day.

Removing human line umpiring in highly professional tennis might cause a big impact lower level events, down to amateur and junior level. These people drive the sports without any kind of publicity. Can you name any line judge on a US Open Men’s Final (if you are not related to one?). From my personal refereeing background, I strongly guess this has been the moment of their lives for the men and women out there.

Respect the Umpires!

Another argument why I feel negative about Hawk-Eye Live is that it is blurring a problem we got in professional tennis nowadys: I feel that the respect of many professional players towards the rules and the umpires of the match is too low. If you for example followed the recent discussions about Djokovic’s ball of a line umpire hit in Flushing Meadows, you read comments about his default which make you feel like it was the lady’s fault to get hit by a ball. I remember Serena Williams threatening a line umpire after a call she did not agree to. In other sports, this behavior is absolutely unacceptable.

You remove an issue you have by removing the line umpires – and giving the chair umpire just very limited potential to really judge on the plays. In mid term, I am sure you will make things worse – because you will have worse chair umpires with a shorter record of critical situations they solved well (or bad and learned from them).


You take a Risk

At least the Hawk-Eye technology has proven to be overall reliable – though you had situations here and there, where it was not working properly in the past. If you generally replace human line judging by algorithms, you do not have a fall-back. The only fall-back is that the chair umpire is taking the calls, which is even for experienced people, leading to a massive increase of the potential of faulty calls compared to human line judges, just due to unfortunate perspective. This is a risk you need to accept when you go the route.


Hawk-Eye Live – My View

At the moment of the pandemic, Hawk-Eye Live is just a welcome opportunity to reduce the number of people on and around court. I would definitely not say that it is a bad thing – but I would clearly love to see that tennis is reflecting more about the system: understanding that it is not perfect, understanding the stochastic behind these kind of decisions, but more than that discussing what professional tennis with Hawk-Eye Live would mean. Yes, Hawk-Eye Live would have prevent scenes like the parallel tournament in Antwerp for example had in the Khachanov match (see below), but sometimes you have accept human failure as well. You do not replace players by robots to reduce the number of mistakes on court as well. Umpires are part of the match, in almost any sports. Thus, I am very mixed in my thoughts about this development in tennis.


Postings about Tennis

Here are all postings which are related to tennis on


All “classic” blog posts

No reviews – just debating and certain topics – these are my “traditional” blog posts in the categories Just blogging, Just blogging on music and Sports Blogging:


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