The positions in the formation are called
station assignments. A ship's position depends on its abilities. Many modern warships can fight several ways, but some are better at certain things. AAW and ASW are the important defensive properties. ASuW conduct is usually offensive.
A standard formation provides a number of layers of defence, designed to give maximum protection to the fleet's
high value units (HVUs) or
main body. Furthest out are the
picket ships,
Combat Air Patrol (CAP) craft and
airborne early warning (AEW) aircraft. These units operate at 200 nautical miles (370 km) or more out from the main body. The units of the
outer screen operate between 12 and 25 nautical miles (22 and 46 km) from the main body. The
inner screen is within 10 nautical miles (19 km) of the HVUs.
The ships of the
outer screen are intended to detect and engage any enemy units that have bypassed the pickets. These ships must be multi-role, but there is usually an emphasis on ASW, especially passive detection. It is quieter out there than near the HVUs and so detection is easier. Preferably there are
helicopter ASW assets for 'stand off' engagement. The ASW ships are usually assigned to specific sectors which allows a 'sprint and drift' detection of submarines - the ship 'sprints' to the front edge of its sector, then slowly moves back across the sector. Passive towed
sonar arrays operate very efficiently on the return leg. AAW ships in the outer screen operate to protect ASW operations and to attack enemy aircraft before they reach their weapons-launching points, so range of defensive weapon is more important than rate of fire here.
The
inner screen emphasis is on AAW. The central task is to engage any airborne threats that penetrate that far. This means the threat is almost certainly a
missile so AAW rate of fire is important. The more defensive firepower in the air the more enemy threats will be destroyed. For ASW the inner screen needs good active sonar. Any threat this close is too serious for passive sonar as immediate targeting is needed. Checking the area around and under HVUs for submarines is called 'delousing'. If possible at least one ASW helicopter is always airborne, to target detected contacts as quickly as possible.