The Navy rejected safety experts' recommendations that U.S. submarines be required to use "active" sonar before surfacing to avoid collisions like the one that sank a Japanese trawler off Hawaii. Active sonar sends out ping-like signals that bounce off objects and detects their range. Passive sonar uses a device called a hydrophone to amplify noises in a broader area. Contrary to a 1990 National Transportation Safety Board recommendation, commanding officers have complete discretion to decide which system to use under the circumstances, the Navy said. "We don't have a requirement to use active sonar," said Lt. Cmdr. Cate Mueller, a Navy spokeswoman at the Pentagon.