A pool at a Minnesota high school Wednesday will become the first in the nation to begin using a drowning-detection system that links underwater cameras to a computer that analyzes movement.

The St. Cloud school fistrict, home to a pool in which a freshman student drowned during a required swimming class in 1999, has equipped four middle school and high school swimming pools with the Poseidon underwater camera system. The district expects to open all four pools in the coming weeks, beginning with one at Technical High School in St. Cloud.

The Poseidon system, made by a French company called Vision IQ Inc., uses 10 cameras to ensure that two video feeds cover any location in a 25-yard-long pool. The cameras are positioned below the surface in the deeper water; in shallow sections they're either mounted on the ceiling or posted on masts along the edge. The images are taken digitally and sent over a network to a PC, where software analyzes the trajectory of individual bodies in real time.

"If it sees a swimmer not moving or sinking to the bottom, it continues to follow that trajectory and if it continues for a few seconds, it triggers the alarm," says Steve Bagby, Vision IQ president. Bagby says the cameras are placed in pairs inside eight-inch-wide watertight globes filled with nitrogen to prevent humidity from clouding the lenses. Says Bagby, "A swimming pool is a pretty nasty place to put electronics."