Replenishment at Sea (RAS)

The transfer of fuel, ammunition, stores, and personnel between ships underway, letting a fleet stay at sea without returning to port. The supplying ship and the receiving ship steam side by side on a steady course and speed (connected replenishment) while rigs span the gap, or stores move by helicopter (vertical replenishment). RAS is the NATO and Commonwealth term; the US Navy calls it underway replenishment (UNREP). It is the foundation of sustained blue-water operations and is the core mission of combat logistics ships.