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. The term 'picket' is an old military term for a sentry placed forward of a defensive line, in order to provide an advanced warning. 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.