Editing Missile guidance (section)

==={{Anchor|Astro-inertial guidance|Astro-inertial|Stellar-inertial guidance}} Astro-inertial guidance===
{{See also|Inertial navigation system|Celestial navigation}}
'''Astro-inertial guidance''', or '''stellar-inertial guidance''', is a [[sensor fusion]]-[[information fusion]] of [[inertial guidance]] and [[celestial navigation]]. It is usually employed on [[submarine-launched ballistic missile]]s. Unlike silo-based [[intercontinental ballistic missile]]s, whose launch point does not move and thus can serve as a [[reference]], SLBMs are launched from moving submarines, which complicates the necessary navigational calculations and increases [[circular error probable]]. Stellar-inertial guidance is used to correct small position and velocity errors that result from launch condition uncertainties due to errors in the submarine navigation system and errors that may have accumulated in the guidance system during the flight due to imperfect [[instrument calibration]].

The USAF sought a precision navigation system for maintaining route accuracy and target tracking at very high speeds.{{Citation needed|date=July 2014}} [[Nortronics]], [[Northrop Corporation|Northrop]]'s electronics development division, had developed an [[astro-inertial navigation system]] (ANS), which could correct [[inertial navigation]] errors with [[Celestial navigation|celestial observations]], for the [[SM-62 Snark]] missile, and a separate system for the ill-fated [[AGM-48 Skybolt]] missile, the latter of which was adapted for the [[SR-71]].<ref>Morrison, Bill, SR-71 contributors, Feedback column, [[Aviation Week and Space Technology]], 9 December 2013, p.10</ref>{{vs|date=February 2012}}

It uses star positioning to fine-tune the accuracy of the inertial guidance system after launch. As the accuracy of a missile is dependent upon the guidance system knowing the exact position of the missile at any given moment during its flight, the fact that stars are a fixed [[Frame of reference|reference point]] from which to calculate that position makes this a potentially very effective means of improving accuracy.

In the [[Trident (missile)|Trident missile system]] this was achieved by a single camera that was trained to spot just one star in its expected position (it is believed{{who|date=June 2014}} that the missiles from Soviet submarines would track two separate stars to achieve this), if it was not quite aligned to where it should be then this would indicate that the inertial system was not precisely on target and a correction would be made.<ref>{{cite web|title=Trident II D-5 Fleet Ballistic Missile|url=http://www.globalsecurity.org/wmd/systems/d-5-features.htm|access-date=June 23, 2014}}</ref>