Passive Underwater Fire Control Feasibility System

It was designated AN/BQG-4 and was primarily installed on United States Navy conventional submarines built in the 1950s beginning with the Tang class, and also those converted to GUPPY III or otherwise modernized in the 1960s.

It was also installed on the USS Thomas A. Edison (SSBN-610) but never achieved operational status.

It was associated with long-range passive detection of targets for the Mark 45 nuclear torpedo and other weapons.

Most submarines backfitted with it were also lengthened 12–16 feet (3.7–4.9 m) to accommodate additional electronics and plotting rooms.

It was also planned for Thresher and Sturgeon class nuclear submarines, but was not fitted on them except Micropuffs experimentally on Barb and Haddock.

USS Harder (SS-568) with the three distinctive shark-fin PUFFS domes
Fin and central section of HMAS Onslow . The three orange covers on the casing are protective sheathes over the submarine's Micropuffs sonar