sentences:  At first the plan was to install a twin-barreled Vertical Gun for Advanced Ships (VGAS), developed from experiments on advanced projectiles for the "Iowa"-class battleships, but this was dropped in favour of a conventional 5" gun and two 64-cell VLS.It would also feature a revolutionary cross-layer active sonar.An Operational Requirements Document was signed in November 1997, and an Advanced Development Memorandum on 11 December.