Sheet 01 · Officiating technology

How technology decides in sport: Hawk-Eye, goal-line systems, VAR, the TMO and DRS

Notes on the tracking cameras, replay rooms and review rules that sit behind officiating decisions in tennis, cricket, football and rugby. For each system we ask what it measures, what it has to infer, and where the protocol rather than the hardware settles the outcome.

Open the five sheets Compare the protocols

Fig. 01 · Camera geometry

Contents · five sheets

The five systems, one sheet each

Each sheet takes one system, explains its mechanism in plain words, and separates the questions it answers cleanly from the ones it only narrows. Where a figure is quoted it is a published threshold or a documented date, not an estimate of our own.

Fig. 02 · Bounce reconstruction

Sheet 02

02 · Ball tracking

Hawk-Eye: how a bounce mark is reconstructed

Developed at Roke Manor Research in 2001 and first used for player challenges at a Grand Slam at the 2006 US Open, Hawk-Eye triangulates a ball from several calibrated cameras and fits a flight model to the result. The sheet covers the ITF approval process, the shift to Hawk-Eye Live and fully electronic line calling, and why the graphic is an estimate drawn with hard edges.

Resolves: in or out on a tennis line, with no margin category
Cannot resolve: anything after the ball stops being measured, which is why cricket needs umpire's call

Read the Hawk-Eye sheet

Fig. 03 · Whole ball over line

Sheet 03

03 · Goal-line technology

Goal-line technology: one question, answered in under a second

Law 10 defines a goal as the whole ball crossing the line between the posts and under the crossbar, and goal-line systems answer only that. This sheet compares camera-based systems such as Hawk-Eye and GoalControl with the magnetic GoalRef approach, walks through the IFAB's 2012 approval and the FIFA Quality Programme's one-second signal requirement, and looks at the Premier League occlusion failure of June 2020.

Resolves: whether the ball wholly crossed the line
Cannot resolve: whether the goal should stand, which stays with the referee and VAR

Read the goal-line sheet

Fig. 04 · Check, then review

Sheet 04

04 · Video assistant referee

VAR: the check, the review and the "clear and obvious" standard

VAR entered the Laws of the Game in March 2018 and was used at that summer's World Cup in Russia. The sheet sets out the four reviewable categories, the difference between a silent check, a VAR-only review and an on-field review, how offside lines were drawn by hand before semi-automated offside, and why "clear and obvious error" is a threshold that cannot be measured.

Resolves: factual matters such as offside position and ball out of play
Cannot resolve: subjective calls, which it can only refer back to the referee

Read the VAR sheet

Fig. 05 · On-field decision first

Sheet 05

05 · Television match official

The rugby TMO: how the referee frames the question

Rugby union has used a television match official since 2001, first for in-goal decisions only and, after World Rugby's 2012 trials, for the phases before a try and for foul play. The sheet explains why the referee announces an on-field decision before asking, how the Foul Play Review Officer or "bunker" introduced in 2023 fits in, and what a camera cannot see when the ball is grounded under three bodies.

Resolves: grounding, touch and clear foul play, when the angle exists
Cannot resolve: forward passes across a side-on camera, or a ball hidden by players

Read the TMO sheet

Fig. 06 · Pitching, impact, wickets

Sheet 06

06 · Decision Review System

Cricket's DRS: umpire's call and the three-zone LBW review

The Decision Review System, trialled in 2008 and adopted by the ICC in 2009, bundles ball tracking, stump-microphone edge detection and infrared Hot Spot under one protocol. The sheet follows a review in the order the third umpire must take it, explains the pitching, impact and wickets zones, and works through why "umpire's call" exists and why it produces different results from the same delivery.

Resolves: no-balls, run-outs, stumpings and edges with a synchronised sound spike
Cannot resolve: a projected path to the stumps beyond the umpire's call margin

Read the DRS sheet

Comparison · protocol structure

What each protocol actually decides

The hardware is the least interesting part of a review. What matters is who may ask for one, what question the system is allowed to answer, and how much of the original decision survives when the answer is close. Set out side by side, the five look less like one idea and more like five different bargains between speed and certainty.

System	Who triggers a review	Question answered	Decision standard	What stays with the official
Hawk-Eye (tennis)	Player challenge, or automatic under Hawk-Eye Live	Did the ball's footprint touch the line?	Binary: in or out, no margin shown	Nothing on the call itself; the chair umpire manages the challenge count
Goal-line technology	Automatic on every attempt	Did the whole ball cross the line?	Binary signal to the referee's watch within one second	Whether the goal stands: fouls, offside and handball beforehand
VAR (football)	The VAR checks every incident in four categories; the referee may also ask	Was there a clear and obvious error or a serious missed incident?	High threshold for subjective calls; factual calls decided outright	The final decision, and every subjective judgement
TMO (rugby union)	Referee referral, or TMO intervention for foul play	Is there clear evidence to change the on-field decision?	On-field decision stands unless clearly wrong	The referee, who frames the question and gives the decision
DRS (cricket)	Batter or fielding captain within 15 seconds; umpires for certain matters	Was the on-field decision wrong on the evidence available?	Umpire's call preserves the original decision when less than half the ball is inside a zone	The third umpire's reading, and the on-field umpire's marginal calls

Reading note

A recurring pattern: the systems that settle disputes most quietly are the ones asked the narrowest question. Goal-line technology is rarely argued about because it decides one physical fact and hands everything else back. VAR is argued about constantly because it was asked to police judgement. That is not a fault of the cameras, and better cameras will not fix it.