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Thursday, July 09, 2009

Trackman Baseball

Posted 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.

Comments

JBrew said...

Mike, I was hoping that you or one of the other F/X analyst/physicist knew more about this.  I had thought that it might add the final pieces missing in HitF/X.

Do you know anything about the accuracy of the system since creating visual graphics and measuring accurate data are two different things.

Posted 07/10  at  09:25 AM
Mike Fast said...

I don’t know anything about the accuracy of the system.  I’ve talked with some of the people who have used the system and/or looked at the data, but it hasn’t undergone the rigorous public scrutiny that the PITCHf/x data has, nor has it even been the subject of academic study as far as I know, so it’s tough to assess its accuracy. 

Their claim is accuracy within 1 foot on batted ball trajectories, which would be great.  Technically that seems feasible, so I have no particular reason to doubt it.  However, I am very curious about other sources of error (either systematic errors or errors that come and go based on conditions) and whether Trackman can eliminate them much as Sportvision and MLBAM have had to do with the PITCHf/x system through the trial and error process over the course of collecting data on millions of pitches.

For example, Sportvision found that they had to correct for the fact that their cameras moved from the pregame calibrated position when the stadium filled with people and the concrete of the stadium began to sag under the weight at game time.  So they added the ability to correct for that on the fly.  It’s tough to know what sort of obstacles Trackman will encounter.

I believe ISG is targeting more of a scouting/coaching market for their Trackman Baseball product where they can take advantage of their portability.  I have not heard that they have any plans to make their data public in the same fashion as Sportvision/MLBAM have with the PITCHf/x and HITf/x data, although you never know.  There is certainly a LOT of value to be had in the feedback process between the data collectors and the analyst community.

Even an academic evaluation of their system would have a lot of value, but last I heard, Alan Nathan had not been able to make arrangements for one with ISG.

Posted 07/10  at  10:33 AM
Adam Guttridge said...

Trackman represents a large breakthrough. I’ve known about this for a little while, and been psyched about it.

I’ve done some in depth pitch f/x work… basically just an audit of the system just to understand how it works and it’s limitations (I never published… this was internal stuff). You have to be really careful, and I see too many people incorrectly assuming the data to be capable of doing things it realy isn’t. As I explain it in short…. Pitch f/x isn’t pitch tracking, it’s pitch estimation/recreation, with some very generous assumptions/shortcuts (average acceleration etc).

I very seriously doubt Trackman will become public anytime in the near future. I personally know of one team using it. For now, they are interested in contracting with teams, not media/the league.

Interesting backstory too… this is an outgrowth of a company whose primary endeavor had been rocket tracking, and I believe they’re based out of northern Europe (Norway?).

Posted 07/11  at  01:12 PM
Potsie Moots said...

The Trackman folk are out of Denmark, and Fredrik Tuxen is the head of the company that makes it. He has 4 patents pending in the U.S., with none granted as of yet.

I am dubious about the 1ft/100yds accuracy claim in the absence of an audit. That “radar reflections from the spinning stitches on the ball can be used to determine the spin rate” is a staple of their claims leaves me wanting for more information before I buy in to 1ft/100yds. Now, granted, it’s just baseball, so if that’s wrong, what the heck, no lives lost. But still, good questions need answering.

Posted 07/12  at  06:40 PM
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