Precision Guidance Kit

The M1156 Precision Guidance Kit, formerly XM1156, is a U.S. Army-designed precision guidance system to turn existing 155 mm artillery shells into smart weapons. The prime contractor is Alliant Techsystems and the industry team includes Interstate Electronics Corporation.

In operation the PGK screws into the nose of the projectile much like conventional fuzes. In addition to the fuzing function it provides a GPS guidance package and control surfaces to correct the flight of the shell. This is analogous to the addition of a Joint Direct Attack Munition (JDAM) tail-kit to a dumb iron bomb, creating a precision guided munition. The system began production in 2009, was first expected to be in service by 2010, but later was fielded in spring 2013.

A conventional unguided M549A1 155 mm artillery projectile has a circular error probability (CEP) of 267 m (876 ft) at its maximum range, meaning that half of the rounds can be expected to land within 267 meters of their intended target. This has made unguided artillery dangerous to use in close combat for fear of friendly fire and collateral damage. The M982 Excalibur was fielded as a guided shell that effectively hit within 6 m (20 ft) of a target, but the Army developed the XM1156 as a cheaper alternative. The PGK fuse can be screwed onto existing M549A1 and M795 projectiles, be fired from M109A6 Paladin and M777A2 Howitzer artillery systems, and hits within 50 m (160 ft) of the target at any range.

Small aerodynamic fins allow the system to steer the shell on target. Its GPS receiver compares the PGK's flight pattern to the coordinates of where it should hit, and the fins adjust its path to match where the round will actually impact. A fail safe exists where if the shell does not impact within 150 m (490 ft) of the intended target, it will land but not explode; the PGK "decides" five seconds after launch whether it expects to impact close enough to detonate. This safety feature is expected to give soldiers more confidence when calling in artillery support close to their position. The PGK fuze weighs 3 lb (1.4 kg), one pound more than a standard fuze because of the addition of fins and an alternator. The self-contained system does not need a battery since the alternator inside generates power in flight. Not only is the PGK fuze cheaper to produce than whole purpose-built guided artillery shells, its purpose to turn standard shells into more accurate rounds enables the millions of rounds already in inventories to be upgraded, while new smart shells have to be built to create a stockpile.

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