Thursday, July 09, 2009
Trackman BaseballPosted by Mike Fast
The Trackman Baseball system makes its public debut for the All-Star home run derby in St. Louis. ESPN is calling its implementation of the Trackman feature "Ball Track". Trackman is a portable Doppler radar system already in use for tracking golf ball trajectories and now being deployed in baseball.
The Doppler feature of the radar measures the radial velocity of the ball relative to the radar detector by looking at the frequency shift of the reflected signal relative to the emitted signal. An array of three detectors arranged orthogonally is used to gather phase information from the reflected signal. The difference in the phase of the reflected signal between a pair of detectors can be used to determine the angle to the ball in that dimension. Thus, with two such pairs of detectors, the angle to the ball in two dimensions can be measured. This can be accomplished most optimally with three detectors, with one pair in a line at a right angle to the other pair. If the initial distance between the ball and the radar is known (for example, by measuring the distance from the radar to home plate by some other means), by combining the two angles and the radial velocity information, the exact position of the ball is established in three dimensions.
In addition, a unique feature of this system is that radar reflections from the spinning stitches on the ball can be used to determine the spin rate by measuring the separation of the sideband peaks produced by the signals off the stitches.
That's probably more than most of you want to know about how the system works, but I'm guessing a few physics nerds out there are as jazzed by the possibilities of this system as I am. Many thanks to Dr. Alan Nathan and a few other folks who will remain nameless for their explanations of the Trackman system. You can see a cool graphic of the Doppler returns from a batted ball in a presentation (7 MB Powerpoint, see slide 37) by Alan Nathan on the subject of the physics of baseball.
The bottom line is that this system can be used to reconstruct the full trajectory of a batted ball. It gives us a direct measurement of spin magnitude that we can't get from HITf/x or PITCHf/x. It's nice to see this system getting some public air time.
Hat tip to JBrew at Beyond the Box Score. Thanks also to Alan Nathan for some corrections to my initial post.
Mike Fast is a Royals fan who enjoys investigating baseball questions using data of many sorts. He is a member of Complete Game Consulting. He welcomes comments via e-mail.