Possible solutions:

RFID - could be used for very short distances only. The effective range is not much more than a few feet. This would require sensors near and around the ball to identify and triangulate the location. Accuracy would still likely be inches at best.

Bluetooth beacons - These are used in retail stores to locate your cell phone as you shop. They require beacons about every 25ft. Accuracy is still only about a meter.

GPS - sensors and transmitters can be very small but accuracy once again is still probable more than a yard.

Laser/light - probably wouldn’t be too difficult to get he accuracy, but it would be line of sight which is the biggest issue in placing the ball. Light can’t see through a pile.

Even if you could solve the accuracy problemsto locate the ball, you would still Ned to determine when the player is down. This is not trivial as the player is down when some part of his body touches the ground not always the ball.

Any of these possible solutions would be incredibly expensive, probably unreliable, and would fundamentally change the game with the necessity of equipment in the ball, in uniforms, and on the field. In the short term, the best solution is probably a video review to let someone try to determine if the spot is correct.