Radar picket

A radar picket is a <a href="/facts/Radar/jk9BTbFA">radar</a>-equipped station, ship, submarine, aircraft, or vehicle used to increase the radar detection range around a nation or military (including naval) force to protect it from surprise attack, typically air attack, or from criminal activities such as <a href="/facts/Smuggling/BdPclLAp">smuggling</a>. The term '<a href="/facts/Picket_(military)/NQevrTqq">picket</a>' 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.
Radar picket units may also be equipped to direct friendly aircraft to intercept any possible enemy. In British terminology the radar picket function is called aircraft direction. A ship performing this function is termed a fighter direction ship. Airborne radar pickets are referred to as Airborne early warning and control (AEW&C) or simply airborne early warning (AEW), depending on capabilities. 
In a sense radars intended to track <a href="/facts/Ballistic_missile/Pl6juY73">ballistic missiles</a> can be thought of as radar pickets (the early US <a href="/facts/Ballistic_Missile_Early_Warning_System/cl1vhFX6">Ballistic Missile Early Warning System (BMEWS)</a> was originally termed as such), but because such systems also came to be used for tracking orbital <a href="/facts/Satellites/PTX8c68t">satellites</a> and <a href="/facts/Space_debris/X7YSKlXl">space debris</a> the current preferred term for them is <a href="/facts/Space_domain_awareness/CC3nQoUw">space domain awareness systems</a>.

Radar picket open-in-new

Radar picket