The Physics Of Radar Guns.docx

The Physics of Radar Guns by David Kagan May 12, 2015

The radar gun can only accurately measure pitch velocity from behind the plate. (via Chris J. Nelson) In 1940 a young outfielder by the name of Danny Litwhiler joined one of the worst teams in the history of the National League, the Philadelphia Phillies. From 1940 through 1942 the Phils finished last each year, compiling a combined record of only 135 wins against 323 loses. Yes, that required three consecutive 100-loss seasons. Despite his dismal team in 1942, Litwhiler accomplished the rare feat of completing the season without a single error, and he was the first ever to do so. For this reason, his glove is in the Hall of Fame. During an interview in 2005, he stated it may have been the first glove on which the fingers were stitched together with rawhide. Few have disputed his claim. True or not, Litwhiler was a prodigious baseball innovator. In 1956 he created Diamond Grit to dry infield dirt after a rain. In 1962 he was awarded a patent for the batting cage. Finally and to the point, in 1974 he brought the idea of a handheld radar gun to the JUGS Sports company. We know radar was invented in World War II, but by the early 1970s police were routinely using it to catch speeders. Litwhiler, by then the baseball coach at Michigan State, either read

a story about radar use by the campus cops or drove by and saw them using it, or perhaps he was pulled over for speeding. The details are murky. In any case, he immediately saw the potential of radar as a teaching tool for baseball. He was particularly interested in helping pitchers increase the speed differential between their fastball and change-up. Of course, the JUGS gun has become an indispensable tool for scouts and coaches throughout baseball. The reading from a gun is displayed after ever pitch in almost every major league stadium. Radar is not actually a word, it is an acronym. It stands for radio detection and ranging, which sort of describes how it works. Just like an echo is a sound wave that bounces off something and returns, radar is a radio (electromagnetic) wave that does the same thing. Radio waves all travel at the same speed; 186,000 miles per second (the speed of light) or about one foot in a billionth of a second. So, first measure the time for the wave to go from the radar gun to the ball and back. Then multiply the time by the speed of the wave to find the distance to the ball and back to the gun. Divide by two and you have the distance to the ball. Actually, handheld radar guns don’t measure distance because all you really expect from them is the speed of the ball. It turns out the frequency of the waves that return from the ball is slightly different than the frequency of the waves emitted by the gun. This is called the “Doppler Effect,” the same Doppler as in a weather reporter’s Doppler radar. You can hear the Doppler Effect for sound waves if you stand by a busy highway. The sound of a vehicle coming toward you has a higher frequency than it does when it is heading away from you. You can really notice the change just as the vehicle passes. Last year Mike Trout of the Angels got his first, much anticipated, trip to the postseason. It began poorly and didn’t end well. He went 0-for-8 in the first two games of the American League Division Series against Kansas City. In the first inning of the third game, he crushed a blast into the fountains at Kauffman Stadium, but it wasn’t enough. Kansas City won the series and eventually went on to the World Series. Let’s start to understand the Doppler Effect by thinking about that home run ball bobbing up and down in the fountain. Each oscillation creates a circular wave that travels away from the ball. Figure 1A shows the ball bobbing up and down (in this view, into and out of the page). The crests of the resulting waves move away from the ball in perfect circles.

Figure 1B shows the bobbing baseball moving toward the right. This motion causes the waves to be closer together in front of the ball and further apart behind the ball. So if the ball is moving toward you, will measure the frequency of the waves to be higher. If the ball is moving away from you, the frequency will be lower. From Figure 1 you might guess this change in frequency depends upon how fast the ball is moving, and you would be correct. The Doppler radar uses the change in frequency to find the speed of the ball.

The ball in the fountain makes waves by pushing water as it moves up and down. For a pitch heading toward the plate, the ball doesn’t make waves. Instead, it reflects the waves sent at it by the radar gun as shown in Figure 2. If the ball is at rest, the waves reflect off it at the same frequency as they arrive as shown in Figure 2A.

When the Best Hitter Isn’t the Best Hitter by Joe Distelheim Most years, advanced statistics would argue that the batting champion really wasn't. When the ball comes toward the radar gun as shown in Figure 2B, the waves it sends back to the gun have a higher frequency. The radar detector inside the gun sends the change in frequency to an internal computer that calculates and displays the speed of the ball. One downside of a basic radar gun is the speed it records is only the part of the speed along the direction you point the gun. That explains why people using radar guns congregate behind home plate. That’s the only spot where it measures the correct pitch speed.

TrackMan TrackMan is a Doppler radar-based product of a Danish company. Its web site doesn’t really tell you much about how it works. The best information about Trackman can be found at Alan Nathan’s Physics of Baseball site.

A radar gun can measure speed only in the direction of the gun because the source of the radio waves and the detector are both housed in the gun. A full TrackMan installation includes a source and three widely spaced detectors giving the full three-dimensional motion of the ball. TrackMan can collect this data for pitches as well as hit baseballs. TrackMan can track not only the trajectory, speed, and direction of the ball but also the spin of the ball. The radar signal reflected by the ball flickers. The frequency of the flickering matches the spin rate of the ball. There are two suggestions as to the source of the flickering. Different parts of the ball probably reflect different amounts of the radar waves. Perhaps the seams are more or less reflective than the horsehide. Maybe the lettering or logo on the ball reflects differently. The more likely explanation is that different parts of a spinning ball are moving at different speeds. Imagine a ball with topspin moving directly at you. The top of the ball would be moving toward you a little faster than the middle of the ball which in turn is moving a bit faster than the bottom of the ball. Since the Doppler Effect depends upon the speed of the reflecting surface, the variation in speed across the ball could be causing the flickering.

StatCast On Opening Day of 2015, the next generation of technology was supposed to begin operation in all 30 major league ballparks. The system uses Trackman to follow the ball throughout the ballpark. In addition, video technology from ChyronHego was to monitor the motion of every player at all times. Radar is much better for monitoring the ball because it travels so rapidly. Players, on the other hand are slow – at least compared to the ball. Video is more than sufficient to track their movements using a method similar to the way PITCHf/x tracks a pitch. MLB Advanced Media plans to use the raw data to generate quantities useful to players, coaches and fans. The numbers and graphics generated from the system for the fans is collectively known as “StatCast.” Perhaps you saw StatCast video in action during the big rollout on April 21, 2015, as the Nationals hosted the Cardinals. If not, samples are available at MLB.com. Table 1 summarizes just some of the data that are collected and displayed by StatCast: Partial Summary of Data Collected by StatCast Play Type Data Fly ball Launch velocity, launch angle, hang time, travel distance, max height 1st step time, acceleration, max speed, distance covered, route efficiency, throw Fielding release time, throw initial velocity Baserunning lead length, secondary lead length, max speed, 1st step time, time between bases Pitching Extension, velocity, spin rate, pitch type StatCast has the ability to simply overwhelm us with data and visuals. Whether or not all this enhances the game for the fans is strongly dependent upon being able to display cleanly and

clearly the data fans want to know. We’ll see what happens as MLBAM continues with the rollout.

Explained: the Doppler effect The same phenomenon behind changes in the pitch of a moving ambulance’s siren is helping astronomers locate and study distant planets. Morgan Bettex, MIT News Office August 3, 2010

Share Comment Many students learn about the Doppler effect in physics class, typically as part of a discussion of why the pitch of a siren is higher as an ambulance approaches and then lower as the ambulance passes by. The effect is useful in a variety of different scientific disciplines, including planetary science: Astronomers rely on the Doppler effect to detect planets outside of our solar system, or exoplanets. To date, 442 of the 473 known exoplanets have been detected using the Doppler effect, which also helps planetary scientists glean details about the newly found planets. The Doppler effect, or Doppler shift, describes the changes in frequency of any kind of sound or light wave produced by a moving source with respect to an observer. Waves emitted by an object traveling toward an observer get compressed — prompting a higher frequency — as the source approaches the observer. In contrast, waves emitted by a source traveling away from an observer get stretched out. In astronomy, that source can be a star that emits electromagnetic waves; from our vantage point, Doppler shifts occur as the star orbits around its own center of mass and moves toward or away from Earth. These wavelength shifts can be seen in the form of subtle changes in its spectrum, the rainbow of colors emitted in light. When a star moves toward us, its wavelengths get compressed, and its spectrum becomes slightly bluer. When the star moves away from us, its spectrum looks slightly redder. To observe the so-called red shifts and blue shifts over time, planetary scientists use a highresolution prism-like instrument known as a spectrograph that separates incoming light waves into different colors. In every star’s outer layer, there are atoms that absorb light at specific wavelengths, and this absorption appears as dark lines in the different colors of the star’s spectrum that are recorded from the light emanating from the star. Researchers use the shifts in these lines as convenient markers by which to measure the size of the Doppler shift. If the star exists by itself — that is, if there is no exoplanet or companion star in its stellar system — then there will be no change in the pattern of its Doppler shifts over time. But if there is a planet or companion star in the system, the gravitational pull of this unseen body or star will perturb the host star’s movement at certain parts of its orbit, producing a noticeable change in the overall pattern and size of Doppler shifts over time. In other words, the pattern of a star’s Doppler shifts can change over time as a result of gravity affecting the star’s

motion. “If this shift is large, then it must be caused by another star pulling it, but if this shift is small, then it is likely caused by a low-mass body like an exoplanet,” explains Joshua Winn, an assistant professor in MIT’s Department of Physics. As part of his work at MIT’s Kavli Institute for Astrophysics and Space Research, Winn studies the relationship between an exoplanet’s orbit and its parent star’s rotation for clues about how the planet may have formed. How a planet’s Doppler shift changes over time can also shed light on the planet’s orbital period (the length of its “year”), the shape of its orbit and its minimum possible mass. Recently, Kavli postdoc Simon Albrecht used the Doppler effect to detect color shifts in the light absorbed by an exoplanet, which indicated strong winds in the planet’s atmosphere. Doppler shifts are used in many fields besides astronomy. By sending radar beams into the atmosphere and studying the changes in the wavelengths of the beams that come back, meteorologists use the Doppler effect to detect water in the atmosphere. The Doppler phenomenon is also used in healthcare with echocardiograms that send ultrasound beams through a body to measure changes in blood flow to make sure that a heart valve is working properly or to diagnose vascular diseases. Police also rely on the Doppler effect when they use a radar gun to bounce radio beams off of your car; the change in frequency between the directed and reflected beams provides a measure of your car’s speed.