Ball Tracking in Cricket: How DRS Predicts the Ball's Path
Ball tracking systems like Hawk-Eye use multiple cameras and physics models to reconstruct and predict the ball's trajectory, powering the LBW prediction in the Decision Review System.
Ball tracking in cricket is the technology that reconstructs and predicts the path of a cricket ball from the moment it leaves the bowler's hand to the point of impact and beyond. It is the core component of LBW decisions in the Decision Review System (DRS). The system — most commonly Hawk-Eye, though VirtualEye and other systems exist in some competitions — uses multiple cameras, trigonometry, and physics models to achieve this in near real-time.
The Physics Behind the Prediction
After measuring the ball's trajectory from release to impact, the system must predict where the ball would have continued. This involves modelling the ball's spin rate (measured from the camera footage), the pitch of the delivery, and the laws of physics governing deceleration and deviation. Leg-spin pitching on the leg-stump line and turning into the stumps is modelled differently from a straight seam delivery. The system's accuracy is highest for deliveries that pitch close to the impact point because there is less distance over which to extrapolate.
The Three Tests for LBW Ball Tracking
For an LBW review to succeed using ball tracking, three conditions must be met: pitch (the ball must pitch in line with the stumps or on the off-stump side), impact (the ball must strike the pad in line with the stumps), and trajectory (the ball must be predicted to hit the stumps). Ball tracking contributes to the pitch and trajectory checks; Hawk-Eye data determines whether the ball landed in line (using camera data) and whether it would have gone on to hit (using the trajectory model).
Controversy and Ongoing Development
Ball tracking has attracted controversy in specific edge cases: very full deliveries that pitch immediately in front of the batter give Hawk-Eye little space to measure pre-impact trajectory, reducing its accuracy. Deliveries with extreme reverse swing or drift have also challenged the models. System providers continuously refine algorithms based on match data. All DRS ball-tracking systems remain subject to ICC certification before use in international cricket.
Frequently asked questions
How does the DRS predict where the ball would have gone?
Ball tracking systems like Hawk-Eye measure the ball's speed, spin and direction from multiple cameras, then use physics models to extrapolate the projected path from impact to the stumps, accounting for spin deviation and deceleration.
What is the umpire's call zone in DRS ball tracking?
Umpire's call applies when the ball is projected to clip the very edge of the stumps. In that marginal zone, the on-field decision stands, because the system's inherent margin of error could put the ball either side of the stump.