Precision Guided Munitions [PGM]
A Precision Guided Munitions [PGM] is a missile, bomb or artillery shell equipped with a terminal guidance system. It contains electrical equipment that guides it in the last phase before impact. The terminal guidance unit is designed to sense emitted or reflected EMR (electromagnetic radiation) within its field of view.
Target Acquisition (TA) systems must be used to employ a PGM, and include the human eye, which is the most commonly employed TA system. Others include radar, TV and forward-looking IR sensor display systems, and systems using laser quidance.
The classic target acquisition cycle was composed of the following five steps:
(1)Detection of target area,
(2) Detection of the target itself,
(3) Orientation of the target,
(4) Target recognition,
(5) Weapon release,
(1) Seeker or Guidance Unit. The seeker discriminates differences in energy received by its sensor. Each sensor has a threshold contrast level, and contrast levels below the threshold will not activate the unit. Each sensor has a maximum energy level. Too much energy will saturate and perhaps damage the unit.
(2) Tracker or Control Unit. The tracker controls the flight of the PGM to the target. "Lock-on" refers to activation of the tracker unit. The two types of trackers are as follows:
(a) Edge Tracker - flies the PGM to the area of more intense contrast between target and background,
(b) Centroid Tracker - flies the PGM to the point of maximum or minimum emitted or reflected EMR. For example, a visual centroid tracker will fly towards the center of a bright or dull spot. An IR centroid tracker will fly towards the center of a hot or cold spot.
Active guidance systems respond to energy which originates from the PSM and was reflected from the target, Semiactive guidance responds to energy which originates at a source other than the PGM and was reflected from the target (e.g. it may respond to EMR reflected from a laser designator at a remote location), Passive guidance responds to naturally emitted or naturally reflected EMR from the target. Most systems are passive.
In 1944, it took 108 B-17s dropping 648 bombs to destroy a point target. In Vietnam, similar targets required 176 bombs. Now, a few precision guided munitions (PGM) can do the job. Precision munitions also enhance strategic agility. For example, just over three C-5 sorties per day could have supplied every PGM used by the Air Force during the Gulf War. But the types of weapons in the US inventory remained largely unchanged since the end of the Vietnam War. During the 1980s a variety of "transitional" weapons were acquired in small numbers, carried on a limited number of platforms. Desert Storm demonstrated the current weapons' effectiveness, and revealed their shortcomings.
Since Desert Storm, the Air Force has:
- Tripled the number of precision-capable platforms since the war
- Boosted PGM inventories by 25 percent above pre-war levels
- Developed new generations of PGMs with enhanced accuracy, standoff, and adverse weather capabilities
Although a number of these new-generation precision munitions are entering production, as of late 1998 only relatively trivial numbers were actually available for combat. While tens of thousands of these weapons are slated for delivery over the coming decade, no more than a few dozen were combat ready.
Beginning in the mid-1980s, the Air Force and Navy began development of "next generation" weapons to fulfill the shortcomings of the earlier weapons.
- Joint Stand-off Weapon (JSOW) is an adverse-weather, short-range, stand-off anti-armor/SEAD dispenser weapon. A small number of these weapons became operational with the Navy in December 1997.
- Joint Direct Attack Munition (JDAM) is an Inertial Navigation System (INS)/GPS guidance tail kit that converts dumb bombs into accurate adverse-weather capable weapons. JDAM was certified as operational capable on the B-2 in July 1997, and achieved operational status with other selected Air Force units in late 1998, including Limited Initial Operational Capability which was achieved on the B-52 in December 1998. JDAM modification kits will be installed on an initial block of seven B-1B bombers by January 1999.
- Wind Corrected Munitions Dispenser (WCMD) provides a similar capability for cluster munition dispensers. Achieving an accuracy of less then 30 feet in tests, the munition was expected to enter general service by April 1999 following approval on 03 August 1998 for initial production. WCMD Limited Initial Operational Capability was achieved on the B-52 in November 1998.
- The Sensor Fused Weapon follow-on (SFW P3I), which will increase the accuracy, enlarge the pattern, and offer greater kills-per-pass than the original SFW, was slated for deployment around the turn of the century.
- Joint Air-to-Surface Stand-off Missile (JASSM) provides long-range, precision strike with a limited hard target penetration capability. Currently in development, it will enter the inventory by around 2001.
These new weapons are all autonomously guided and have adverse weather capability. These weapons are being integrated into virtually every American combat aircraft.
Since Desert Storm, the Navy's ability to attack targets with precision weapons has increased more than five-fold. Every air wing has the capability of employing standoff and through-the-weather precision weapons, such as the Stand-off Land Attack Missile-Expanded Response, Joint Stand-Off Weapon and Joint Direct Attack Munition. The Navy's focus has shifted from the number of sorties per target to the number of aimpoints per sortie. Yet, challenges remain, including the number of precision weapons, pods, training ordnance and the availability of ranges.
Improving Coalition Forces’ ability to hold hard or deeply buried targets at risk requires a reliable fuze that can sense when it enters a void. The voidsensing capability mitigates a substantial part of the intelligence uncertainty surrounding targets because the weaponeering no longer needs to select a single time delay based on the estimated thicknesses of target layers and estimated concrete strength. The previous projects, Hard Target Smart Fuze (HTSF) and Multi-Event Hard Target Fuze (MEHTF) successfully tested void sensing/layer counting ability.
The FMU-143B/B Fuze System was an electromechanical fuze system that provides impact delay detonation for penetrating warheads. The fuze contains an explosive train, which was mechanically and electrically out-of-line until specific weapon launch cycle events have occurred.
The FMU-157/B Hard Target Smart Fuze (HTSF) enables precision bombs with penetrating warheads to detonate at a desired point inside buried or reinforced concrete targets, such as underground bunkers and command centers. Similar to HTVSF, detonation occurs after a sensor tells the fuze that the weapon has passed through a pre-programmed number of hard layers or voids in the target. The HTSF, designated the FMU-157/B, was an active decision-making, accelerometer-based fuze system capable of counting layers and voids (floors), as well as calculating distance travelled. When the weapon reaches the pre-determined floor, it tells the bomb to explode. The HTSF was compatible with a variety of penetrating warheads.
The FMU-152A/B, Joint Programmable Fuze (JPF) was a multifunction, multi-delay tail fuze system with hardened target capability for use in general purpose and penetrating unitary warheads. The JPF operates with a wide variety of guidance kits, high and low drag fins, and with all configurations of the DSU-33 proximity sensor to provide an airburst capability.
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