The Type 61 Salisbury class was a class of the Royal Navy aircraft direction (AD) frigate, built in the 1950s. The purpose of the aircraft direction ships was to provide radar picket duties at some distance from a carrier task force and offer interception guidance to aircraft operating in their area.

A radar picket is a radar-equipped station, ship, submarine, aircraft, or vehicle used to increase the radar detection range around a nation or military force to protect it from surprise attack, typically air attack, or from criminal activities such as smuggling. By definition a radar picket must be some distance removed from the anticipated targets to be capable of providing early warning. Often several detached radar units would be placed in a ring to encircle a target to provide increased cover in all directions; another approach is to position units to form a barrier line.