What's New With Smart Weapons
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 is composed of the following five
(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 is reflected from the target, Semiactive guidance responds to energy which originates at a source other than the PGM and is 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. All of these are now in development, or in the initial stages of production.
- 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 is 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, is 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.
Laser Guided Bombs
Laser guided bombs [LGBs] remain the most numerous precision guided munition, with roughly 25,000 in the current inventory. During Desert Storm, the F-111F and the F-117 accounted for the majority of the guided bomb tonnage delivered against strategic targets. The Navy's A-6E, which is no longer in service, and other aircraft, were used to deliver LGBs was used only sparingly. Demonstrated accuracies are estimated at between three and eight meters. Subsequent improvements include:
- In Desert Storm, 229 US aircraft were capable of delivering laser-guided munitions. By 1996 the expanded installation of low-altitude navigation and targeting infrared for night (LANTIRN) pods on F-15Es and block 40 F-16s had increased this capability within the Air Force to approximately 500 platforms.
- Only four AAS-38 Nite Hawk laser target designator systems were available for the Navy's F/A-18 Hornet during the Gulf War. The improved AAS-38A Laser Target Designator/Rangefinder (LTD/R) was cleared for Fleet service on Hornet-C/Ds in January 1993.
- The LANTIRN laser guided bomb designator system, modified for the Navy's F-14 Tomcat Strike Fighter, became operational in June 1996. While only a few aircraft deployed aboard the USS Enterprise (CVN-65) are currently so equiped, eventually all 210 F-14 Tomcat fighter aircraft will receive the LANTIRN upgrade.
Although laser-guided bombs have demonstrated the ability to destroy point targets with only a few rounds per aim point, their employment faces several constraints. The primary limitation on their use is the requirement for a clear line of sight between the bomb's laser seeker and the laser spot-beam designating the target, which is not possible under adverse weather conditions [rain, clouds, dust, etc]. Additionally, laser designators are deployed on only a limited number of aircraft, and the number of platforms that can deliver LGBs is much larger than the number that have independent target designation capabilities.
Global Positioning System Munitions
The impending massive expansion of precision munition inventories is largely a product of the introduction of relatively inexpensive and highly accurate guidance systems incorporating receivers for the Navstar Global Positioning System [GPS]. These new munitions will provide accuracies comparable to LGBs, while overcoming adverse weather limitations, and eliminating the need for laser target designation systems.
These GPS munitions will also facilitate accurate delivery of area munitions from the higher altitudes that are characteristic of post-Cold War air operations. Low level employment is one of the most demanding tasks facing fighter/attack crews, but during the Cold War these tactics were dictated by the nature of the Warsaw Pact air defense threat. The major disadvantage of a low level delivery is the requirement to fly over the target and its associated air defense weapons. During the Gulf War air campaign initial aircraft losses early in the air campaign resulted from low-altitude munition deliveries. Subsequently the majority of bombs were released from aircraft flying above 12,000 to 15,000 feet. Higher altitudes provided a relative sanctuary from most air defenses but resulted in a major compromise in terms of bomb accuracy and, ultimately, effectiveness. Although quite inexpensive and less restricted by low visibility, unguided munitions cannot reliably be employed against point targets from medium and high altitudes. The addition of JDAM and WCMD will solve these problems.
But pending the arrival of these new munitions, American air operations in Kosovo during early 1999 largely depended on the same precision munitions used [or available for use] in Desert Storm in 1991, or Deliberate Force in 1995. These were supplemented, though not yet replaced, by the small numbers of more sophisticated "transitional" weapons that entered the inventory in the early 1990s, as well as very limited numbers of the newer "next generation" weapons now transitioning to operational units.
Environmental effects of weather can influence the success of a mission by affecting the use of Precision Guided Munitions. In order to brief weather conditions that may affect PGMs, an understanding of their operation is important.
Visible Systems. Passive
systems which respond to naturally emitted or reflected EMR in the visible
spectrum. Environmental limitations are as follows:
(1) Clouds or Fog,
(2) Precipitation, blowing snow or blowing spray,
(3) Poor illumination,
(4) Low sun angle.
Near-IR Passive Systems. A TV-silicon vidacon which senses radiation between .5 and 1.2 microns. The longer wavelength of these systems enhances the sensor's ability to penetrate atmospheric aerosols such as haze. Near-IR systems cause an increase in contrast between natural and painted objects than at visual wavelengths. Environmental limitations are the same as for visible systems except that atmospheric aerosols are less of a problem.
IR Semiactive Systems. These employ
a laser designator operating at IR wavelengths. The point of maximum reflected
energy is sensed and tracked using a centroid tracker. These systems have
the advantage of day or night operations. Environmental limitations:
(1) Clouds/Fog, other than very thin (they absorb IR energy),
(2) Haze and other dry aerosols (for near IR systems only),
(3) High absolute humidity (for far IR and far IR systems only).
Middle and Far IR Passive
Systems. These systems respond to naturally emitted EMR from terrestrial
objects in the middle and far IR wavelengths. Threshold thermal contrasts
must be met for IR lock-on. These systems may be used for day or night
operations. Resolution is poor due to long wavelength. Environmental limitations:
(1) Clouds/Fog other than very thin,
(2) High Absolute Humidity (for far IR only).
Millimeter Wave/Microwave Systems. These systems respond primarily to emitted, but also to reflected
energy at millimeter wave or microwave wavelengths. Most of these systems are passive, such as the "SHRIKE" anti-radiation missile which has a microwave sensor and homes in on microwave or radar emissions. Day or night operations
are possible, but resolution is poor due to long wavelength. Environmental limitations:
(1) Dense clouds of high liquid water content (e.g. CR or TCU).
(2) Precipitation causes attenuation.
Environmental conditions can affect target acquisition
anywhere in the cycle:
(1) Severe (or greater) turbulence can break lock-on,
(2) Icing of any kind can coat sensor cover which blinds it or the ice can cause aerodynamic problems (changes shape and adds weight). Icing can also jam the controls of the tracker,
(3) Ablation or erosion of the sensor cover by pitting may occur when the PGM passes through hard atmospheric particles such as hail or sand,
(4) Lightning or Triboelectrification (static charge buildup) can foul electronic circuitry. Lightning can cause detonation of PGM,
(5) Sun angle: (a) Mie scattering (path radiance) causes problems for TV and near-IR systems, (b) Low sun angle can create shadows which may cause false lock-on (c) Thermal contrast is affected as the different parts of the target heat up and cool down at various rates as the sun rises and sets.
(6) Target Size and shape determines the range at which target acquisition and lock-on can be accomplished,
(7) Soil moisture content and precipitation, (a) Change in moisture content of soil can change its color and the inherent contrast. It can also change the soil temperature, and thus affect thermal contrast (moist soil warmer than dry), (b) Rain or snow in the target area will affect the temperatures of targets and backgrounds, affecting thermal contrast, (c) Snow cover has a strong influence on contrast. 1 The inherent contrast can decrease, 2 Path radiance problem for VIS sensors, 3 Patchy snow cover creates alternating hot/cold spots in background, affecting thermal contrast.
(8) Radiative Temperature Crossover is a function of the diurnal temperature cycle. It occurs when the target and background achieve the same radiative temperatures and is caused by the different rates at which the target and background heat up and cool down,
(9) Strong surface winds can decrease thermal contrast,
(10) Thermal clutter is hot or cold spots surrounding the target which can confuse or distract the IR sensor by momentarily alterinq or wipinq out the thermal contrast.
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