On the outer surface of each was another vertical structure that looked like an auxiliary rudder, but was in fact a fitting for sonar sensors.

No color video cameras were mounted on the trailing sonar sensor.

This sonar sensor, a little bundle of electronics sealed in plastic, turns on only when it is in the water.

Maybe some kind of sonar sensor?

It is wire-guided and has a passive sonar sensor, which sends back information through the wire.

They use passive sonar sensors to pick up underwater noise.

One reason for this is that towed-array sonars typically have a different frequency response than hull-mounted sonar sensors.

They had just started for the door when the detection room officer announced: "Television cameras and sonar sensors in position, Skipper."

"Tell him to have the sonar sensor pulled in before it drags bottom," Pitt said offhandedly.

He jumped on deck and greeted the two engineers connecting the sonar sensor to the recorder cable, then entered the wheelhouse.