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I've been eyeballing a volleyball match right now (apparently, it's world championships :), and have noticed that they show "spike speed" for spike replays. How is that speed measured? Is the ball stuffed with sensors or is there an array of hi-speed cameras to measure this? How much instrumentation is there for a high-profile match like this?

The main judge has three displays at his disposals, they look like three completely different things. At at least two times teams have contested a ruling, and a rudimentary 3D animation was shown on stream. It looked bit low-res, and definitely not like something the studio responsible for the transmission would do - so it must have originated from some kind of match-local instrumentation.

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I'm not 100% sure if this particular system is used at the current World Championships, but Hawk-Eye has a system which works for volleyball as well, though it's mostly known for tennis. According to the Wikipedia page,

The system works via six (sometimes seven) high-performance cameras, normally positioned on the underside of the stadium roof, which track the ball from different angles. The video from the six cameras is then triangulated and combined to create a three-dimensional representation of the ball's trajectory.

So the ball has no sensors (which has the advantage that they can't influence the ball's trajectory); it's just that the system 'knows' how a ball looks like.

Though Hawk-Eye (and similar systems) are mostly used to check if a ball is in or not, and the 3D trajectory is definitely from such a system, it can measure ball speed as well. But for measuring the ball's speed, older systems, based on radar, exist.

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In many gyms at bigger tournaments (World Championships, Championsleague) radar guns (eg. Bushnell or Sports Ball Radar Gun) are used at the end of the court. They are used to measure serve speed and attacking speed. The problem with these numbers is, that they are only reliable if the serve or attack is directed at the speed gun as the measurements accuracy is dependent on the angle between the direction of the radar gun and the trajectory of the ball. Anyway, although radar guns are not so accurate the Hawk Eye System is very rarly in use in Volleyball, the radar gun is much more wide spread.

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