 Initially planned to carry out passive operation with sonobuoys laid by helicopters and towed array sonar (TASS) as sensors, but because development of TASS was delayed, it was retrofitted later on only four ships.OQS-4 hull sonar was Japanese equivalent of American AN/SQS-56, and TASS was of AN/SQR-19.Four ships of this class have been re-purposed as training vessels