Tech Story 4
Measuring speed: The one thing you shouldn’t calculate
TrackMan’s Doppler radar technology directly measures speed. Speed is an absolute cornerstone for obtaining accurate club delivery and ball launch data. Essential club data like Attack Angle and Club Path represent the direction of speed of the club head at impact.
A direct measurement of the speed, which is done by TrackMan through use of Doppler radar, offers a technical advantage in assuring accurate data. Similarly, the ball’s speed direction – which is measured immediately after launch – offers the same advantages for TrackMan in determining the Launch Angle and Launch Direction.
The doppler principle
TrackMan is using the Doppler principle, which explains why a signal bouncing off a moving object like a golf club or ball will change the frequency of the signal. The change in frequency is linearly proportional with the speed of the moving object relative to the signal receiver and transmitter. It is exactly the same principle which explains why the audio frequency of an ambulance siren approaching is at a higher frequency compared to when the ambulance is moving away from you.
Measuring spin rate
The spin rate of a golf ball has a massive impact on the ball flight as well as how the ball behaves once it impacts the ground. For high speed drives the difference of just a couple of 100 rpm’s have a noticeable influence on the ball flight and carry/total distance. TrackMan’s patented method of measuring the spin rate by using the Doppler radar signature of a spinning ball has been confirmed by various research projects/institutes to be state-of-the-art. The accuracy is within 20 rpm.
Consider a golf ball launched with backspin and illuminated by a Doppler radar positioned behind the golfer; the top of the golf ball will move towards the radar compared to the center of the ball, and the bottom of the golf ball will move away from the radar compared to the center of the ball. This means that the Doppler radar does not only see one velocity of a flying spinning golf ball, but a range of velocities. It turns out that the range of velocities is not continuous but has discrete components being harmonics of the spin frequency and symmetrically arranged around the ball velocity in the Doppler spectrum.
The spin rate measuring principle does not rely on any markers on the ball or knowledge of ball diameter etc. Because of the accuracy and robustness of the spin rate measurement principle, TrackMan uses this method to determine spin rate of all kinds of sports balls, including baseball, tennis ball, cricket ball, football/soccer ball, and, of course, the golf ball. All sports where spin rate is one of the key metrics for controlling ball flight and ground impact behavior.
Measuring angles and distances
For every detected Doppler frequency corresponding to a moving object (golf club head or golf ball), the vertical and horizontal angles can be determined. This is done by having multiple receiving antennas, measuring the time delay time to each of the receivers. The further the distance between the receiving antennas, the higher the accuracy of the angular measurement. TrackMan 4 has almost three times greater distance between the receivers than any other manufacturer of sports radars currently on the market.
TrackMan radars also measure the absolute distance from the radar to the moving object (club head or ball). This is achieved through measuring the phase-front delay between the transmitted frequencies for the signal roundtrip: transmitter to club/ball and back to the receivers. In summary, whenever a moving object is detected and isolated in the Doppler signal, the speed, vertical and horizontal angles, and the range can be measured directly, thereby resulting in a full 3D position measurement plus speed, at any given point in time.
Measuring multiple club-head speeds at multiple locations, simultaneously
When a club head is approaching the ball, not all parts of the club head move with the same speed. This is due to two factors: 1) the further away a specific point on the club head is from the club’s rotation center, the faster the speed is of that point, 2) when the club rotates (the clubface closes, for example), the toe moves faster than the heel.
With the ultra-high velocity measurements resolution in mind, TrackMan’s Dual Radar Technology is actually measuring multiple speeds across different parts of the club head. For every detected speed on the club head, an exact 3D position data is determined by correlating the different Doppler receivers. This creates a 4D tracking system (X, Y, Z, and speed), making it possible to measure speed on the toe and the heel of the club, for example.
The entire surface of the club head reflects the radar signal. The strongest reflection comes from the part of the club head facing the radar (the rear part of it), but weaker reflection from other parts of the club head also occur, in particular the ‘edges’ of the club head. Tracking of the club head is therefore done on the 3D surface or silhouette of the club head, which in consequence means that TrackMan 4 is capable of tracking the geometric center of the club head.
Note that multiple objects like the club and ball can be tracked independently any given point in time, as long as they have different speeds. This means that a TrackMan can track multiple objects independent of each other in 3D while they are moving across the same time interval. This concept of multi object tracking is fully utilized in the new revolutionary TrackMan Range system, which captures every single shot hit on a driving range from one or more radars with multiple balls in the air simultaneously.
Measure, don’t calculate
For an optical based launch monitor, the direction and magnitude of the speed can only be determined by calculating differences between positions over time (frames). When calculating speed, small differences in the ball’s position over time come into play. For example, how far and to exactly where did the ball move in the span of a 1 millisecond?
Any uncertainty in determining positions over time will directly translate into errors in Club Speed, Attack Angle, Club Path, Ball Speed, Launch Angle and Launch Direction. As an example, a 1mm uncertainty in determining two positions 1 millisecond apart can create up to a 4.6 mph speed error.
This example illustrates why, it is highly desirable – some times mandatory – for optical launch monitors to precisely mark the club head with reflectors to provide useful data. TrackMan’s Dual Radar Technology measures sub-millimeter motion at high speeds, with great accuracy in any weather condition (rain, fog, snow, bright sun or at night), without any markers, eliminating doubt when it comes to delivering accurate data on critical performance parameters.
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