Fig. 1: Constructional features of a laser-guided bomb
Fig. 1: Constructional features of a laser-guided bomb

Laser-guided munitions are the most widely exploited electro-optically-guided precision-strike munitions on various land based, sea based and airborne military platforms, such as main battle tanks, armoured fighting vehicles, ships, fighter aircrafts and attack helicopters. In continuation of part 1 of the article, focus in this part is on laser-guided munitions. Different aspects of this class of electro-optically-guided munitions, such as delivery concept, deployment configurations, performance parameters, international status and emerging trends, are covered in this part.

Laser-guided munitions, due to high precision, efficacy and lethality, constitute an important weapon system in the arsenal of the armed forces worldwide. These have proven their lethality and efficacy beyond any doubt during several conflicts, which include their limited use by British forces in the Falklands war in 1982, during operation Desert Storm in the Gulf War in 1991 by the coalition forces against Iraq and during Kosovo War in 1999. Laser-guided munitions were used in large numbers to great effect during Gulf and Kosovo wars. These are of great tactical importance in the contemporary battlefield scenario.

Fig. 2: Laser-guided munitions delivery
Fig. 2: Laser-guided munitions delivery

Operational basics
Laser-guided munitions use a type of opto-electronic position sensor subsystem known as seeker unit that determines in real-time the position of the weapon with respect to the target and feeds this information to a servo control subsystem. The servo control sub-system ensures that the weapon maintains its orientation in the desired direction of the target so as to ultimately hit it precisely. Fig. 1 shows constructional features of laser-guided munitions.

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In laser-guided munitions delivery operation, the target is illuminated, called target designation, by a pulsed solid-state laser producing high peak power pulses with a known pulse repetition frequency (PRF). Peak power, pulse width and PRF are typically in the range of 5MW to 8MW, 10ns to 20ns and 5Hz to 20Hz, respectively. The laser-seeker head in the weapon makes use of laser radiation scattered from the target to generate information on the angular error, which, in turn, is used to generate command signals needed to guide the weapon to the source of scatter, which is the target (Fig. 2).

Before the weapon locks on to the radiation scattered from the target, it makes sure that the radiation is the intended one. To achieve this, the laser target designator and the laser seeker used in the guided-weapon delivery mission use the same PRF code, and the PRF code compatibility check forms the basis of identification of the desired radiation. PRF code compatibility is therefore essential to the weapon’s functionality and mission success. The code is generally chosen to an accuracy of ±1µs to ±2µs in the time interval between two successive laser pulses in a nominal value that is usually in the range of 50ms to 200ms.

The opto-electronic position sensor in the front-end of the seeker unit determines the orientation of the weapon with respect to the target. But before it does that, it deciphers the PRF of the received radiation, and only if the PRF matches with the chosen PRF value within the specified tolerance, it is further processed to extract information on the angular position of the weapon with respect to the target. Information on angular error in azimuth and elevation is fed to the servo control sub-system in the specified format, which, in turn, controls the flight trajectory of the weapon with the help of front canards and a tail unit.

The position sensor
The opto-electronic sensor employed for the purpose of position sensing is usually a quadrant photo sensor. It is the heart of the laser seeker. A two-dimensional array of photo sensors is also used in some cases.

Fig. 3: Principle of operation of a quadrant photo sensor
Fig. 3: Principle of operation of a quadrant photo sensor
Fig. 4: Position of a laser beam spot with (a) laser beam aligned with the receiver’s optical axis, (b) laser beam shifted in positive X-direction, (c) laser beam shifted in negative X-direction, (d) laser beam shifted in positive Y-direction and (e) laser beam shifted in negative Y-direction
Fig. 4: Position of a laser beam spot with (a) laser beam aligned with the receiver’s optical axis, (b) laser beam shifted in positive X-direction, (c) laser beam shifted in negative X-direction, (d) laser beam shifted in positive Y-direction and (e) laser beam shifted in negative Y-direction

Fig. 3 explains the principle of operation of the quadrant photo sensor when used for the position-sensing application in laser-guided munitions.

The quadrant photo sensor is placed before the focal plane of front-end optics. The focal spot is symmetrical to the centre of the quadrant photo sensor when the perpendicular to focal plane of the detector is collinear with the axis of received laser radiation scattered from the intended target as shown in Fig. 4(a). This is the case when the weapon is pointing precisely towards the target.

If the laser radiation is impinging on the laser seeker cross-section at an angle, which will be the case when the weapon is not pointing towards the intended target, the centre of the focused laser spot will shift depending upon angular error in azimuth and elevation as shown in Figs 4(b) to 4(e).

The beam position in X and Y directions is calculated using the following equations; X and Y represent angular errors in azimuth and elevation directions:

729_x

 

Fig. 5: Operating principle of a two-dimensional array based position sensor
Fig. 5: Operating principle of a two-dimensional array based position sensor

where A, B, C and D are electrical voltages corresponding to laser power falling on the four quadrants. In the case of precise pointing towards the target, all four power levels are equal, and therefore X=0 and Y=0. A+B+C+D represents total power, and division by total power ensures that the calculated position error is independent of laser-intensity variations.

Output analogue signals proportional to the magnitude of laser power falling on four quadrants are digitised and then processed to compute X and Y. Error signals X and Y are then used to guide the weapon towards the desired position of null with canards driven by a servo control system. The maximum value of the proportional field-of-view offered by the laser seeker of this type in X and Y directions is proportional to ±R where R is the radius of the focused laser spot. Larger spot size gives a larger field-of-view but a lower angular resolution. Radius of the focused spot can at the most be equal to half of the radius of the quadrant active area.

In another position sensing technique that uses a two-dimensional array based sensor, each active element in the array is identified by a unique azimuth and elevation angle. The focused spot at any time covers more than one active element, and the angular error in azimuth and elevation is computed from following equations. Fig. 5 explains the concept.

Fig. 5: Operating principle of a two-dimensional array based position sensor

where θ is the proportional field-of-view of each miniature quadrant. n and m are constants, depending upon the illuminating quadrant. A, B, C and D are electrical voltages corresponding to laser power falling on the four quadrants.

Important parameters
Major performance parameters of laser-guided munitions include sensitivity, field-of-view, PRF code compatibility and response linearity. In addition, immunity to false codes and response to desired code in the presence of false code are other important parameters relevant to assessing performance efficacy of laser-guided munitions.

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Sensitivity is the minimum value of laser power density impinging on the laser seeker cross-section in a plane orthogonal to the optical axis of the seeker head to which it can respond satisfactorily. It is a characteristic of the seeker’s front-end optics and photo sensor. Sensitivity of the seeker decides the maximum guidance range for known values of laser target designator parameters, target reflectivity, height of laser target designator (in case of airborne laser designator) and laser seeker above sea surface and visibility conditions.

Fig. 6: Laser-guided munitions delivery with target designated from another aircraft
Fig. 6: Laser-guided munitions delivery with target designated from another aircraft
Fig. 7: Laser target designator operation and laser-guided munitions delivery from ground based platforms
Fig. 7: Laser target designator operation and laser-guided munitions delivery from ground based platforms
Fig. 8: Laser target designator operation and laser-guided munitions delivery from same airborne platform
Fig. 8: Laser target designator operation and laser-guided munitions delivery from same airborne platform

Field-of-view determines the probability of the weapon finding itself within the laser basket at maximum guidance range. Laser-guided munitions using a seeker head having larger field-of-view would have a higher probability of finding themselves in the laser basket and subsequently precisely hit the intended target.

PRF code compatibility is the primary requirement for the weapon to function. This means the PRF code of the received laser radiation would be the same as the PRF code programmed in the guidance unit before the start of mission. The code is usually expressed as time interval between two successive laser pulses in milliseconds, usually up to the third decimal place. Two codes are considered compatible if the difference in time periods of the two codes is less than a certain specified value.

Response linearity predominantly decides the circular error probability (CEP). Immunity to false PRF codes and capability to stay locked to the desired code in the presence of false PRF codes enhances the probability of target hit. The former test is performed by irradiating the seeker head with a PRF code different from the programmed PRF code and later by irradiating the seeker head simultaneously with radiations of correct and false PRF codes.

Deployment configurations

Fig. 9: Diving top attack in the case of Krasnopol
Fig. 9: Diving top attack in the case of Krasnopol
Fig. 10: Krasnopol laser-guided projectile
Fig. 10: Krasnopol laser-guided projectile

Different deployment configurations are used for laser-guided munitions delivery, depending upon the nature of mission. The one illustrated in Fig. 2 uses a ground based laser target designator and aerially-delivered bomb. There can be other possible scenarios. For example, in another case, the laser target designator is located on another aircraft, which is not the one carrying the laser-guided bomb. Such a deployment scenario is depicted in Fig. 6.

In yet another deployment configuration, the target is designated from a ground based designator and guided munitions are also launched from a ground based platform. This is true in case of cannon-launched laser-guided projectiles. This is depicted in Fig. 7.

Yet another possible deployment configuration can be the one in which the laser target designation and laser-guided munitions delivery is executed from the same airborne platform as shown in Fig. 8.

While the delivery configuration shown in Fig. 7 is applicable to canon-launched laser-guided projectiles, the other three cases shown in Figs 2, 6 and 8 are different options used in the case of delivery of laser-guided bombs and missiles.

Laser-guided munitions delivery parameters

Fig. 11: Attack profile of Copperhead projectile
Fig. 11: Attack profile of Copperhead projectile
Fig. 12: Paveway-II
Fig. 12: Paveway-II
Fig. 13: Paveway-IV
Fig. 13: Paveway-IV

For a given value of sensitivity of the laser-seeker head (minimum power density required to be present in the seeker plane orthogonal to its optical axis for it to function satisfactorily), the guidance range depends upon the power density actually available at that plane. Available power density depends upon a number of parameters, prominent among them being peak transmitted power from the laser target designator, transmitted beam diameter, transmitted beam width full angle, laser target designator-to-target distance, target-to-receiver distance, atmospheric attenuation coefficient, target reflectivity, angle between transmitter line-of-sight and normal to the target, angle between receiver line-of-sight and normal to the target, angle between receiver line-of-sight and normal to receiver aperture, and target surface area.

Transmitted beam diameter and full-width angle decide the laser beam spot area at a given distance from the transmitter. The laser beam spot area is directly proportional to the laser beam width full angle and distance from the transmitter. Smaller divergence (consequently, lower laser beam width full angle) produces a relatively smaller laser beam spot area up to a longer distance and therefore is always desirable.

Power density at target location is given by laser power available at the target location, divided by the laser beam spot area at that location. Laser power reaching the target location depends upon transmitted laser power, atmospheric attenuation coefficient at operating wavelength and transmitter-to-target distance. Further, attenuation coefficient depends upon visibility conditions and the height of transmitter above sea-level.

Target irradiance, which is the laser power density on the target surface, is not the same as power density at the target location. This is due to the angle made by the transmitter line-of-sight and normal-to-the-target surface. Target irradiance is less than the power density at the target location by a factor given by cosine of this angle.

Target brightness is the laser power density per unit solid angle in the reflected beam. This depends upon target reflectivity and mechanism of reflection, whether it is specular or diffused or a combination of the two. Target brightness together with the solid angle subtended by the receiver aperture and the attenuation coefficient from target to receiver decide the power density available at the plane normal to the receiver cross-section. The actual power density available is further reduced by a factor equal to cosine of angle between receiver line-of-sight and normal-to-receiver aperture.

Received power is the product of power density multiplied by receiver aperture area. The value needs to be further multiplied by loss coefficient due to front-end optics.

Capabilities and limitations

Fig. 14: Hellfire-II fired from a land vehicle
Fig. 14: Hellfire-II fired from a land vehicle

Laser-guided munitions, with their diverse variants in surface and aerially-launched laser beam riders, canon-launched projectiles, aerially-delivered bombs and helicopter-launched missiles, are today the preferred weapons of precision attack by Armed forces worldwide. This has happened due to the constant upgradation of their capabilities over the years since World War II.

One of the most important attributes of laser-guided munitions is their precise weapon delivery. In World War II, it required 9000 bombs to hit a target of the size of an aircraft shelter; in Vietnam, the number was reduced to 300 and today only one laser-guided munition can achieve this objective.

Reduced collateral damage is another big advantage of precision-attack weapons in general and laser-guided munitions in particular. Since the days of World War II, when a single air attack could kill tens of thousands of human lives without raising any moral outcry, attitudes towards both enemy and friendly or neutral have undergone a remarkable transformation.

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Pin-point accuracy offered by precision laser-guided munitions gives military planners the comfort and confidence of attacking intended targets even in the midst of major cities. The precision of attack and near-zero collateral damage gives military planners and decision makers the freedom and flexibility to use military force closer to non-combatant-inhabited areas in enemy homeland or enemy-occupied territory than at any previous time in military history.

Yet another advantage of laser-guided munitions is their resistance to jamming and electronic countermeasures. Ability to operate at night, simplicity, reliability and maintainability are other positive attributes.

While laser-guided munitions are highly accurate and have demonstrated their efficacy in several conflicts in the past, the precision of attack and reliability of operation are guaranteed only under certain conditions. Factors that have a large bearing on the overall performance of a laser-guided munitions delivery mission include accurate and uninterrupted target designation, atmospheric attenuation due to prevailing environmental conditions and mode of weapon release.

Laser designation of the target should be such that the weapon’s seeker head finds itself within the reflected radiation basket and also the received laser power is greater than its sensitivity. This is the first major challenge. Laser designator to target path length is sometimes an issue under adverse environmental conditions. Laser under these conditions is attenuated more than it would have under ideal or normal conditions. This may lead to laser power as received by the seeker falling below its threshold, which further causes failure of the guidance system if the received power is inadequate.

Improper designation may also lead to mishits. This was observed during the Gulf war when the laser radiation was reflected off sand surface rather than the intended target. Temporary blockage of laser radiation due to smoke, fog and dusty conditions and increased moisture content is another reason for guided munitions missing targets.

Correct weapon release is another challenge. The guidance system of earlier laser-guided munitions, such as Paveway-II, resulted in a recti-linear flight path that had a tendency to lag below the sight line. Currently, weapons are released on an unguided ballistic flight path. The weapon release should be such as to allow the weapon on a ballistic trajectory find itself within the laser basket at the time of start of terminal guidance. Relatively narrow field-of-view of the laser seeker further complicates the problem.

Uninterrupted target designation is yet another challenge. The concept of terminal guidance reduces the overall designation time and improves reliability of uninterrupted designation. In the case of autonomous laser designation, that is, designation from weapon-carrying aircraft, uninterrupted target designation is a big problem in the presence of smoke, fog, clouds and dusty conditions. In addition, it leaves the aircraft vulnerable to enemy attack by ground fire or air support.

Optimum weapon release height poses another challenge. Optimum altitude for weapon release is in the range of 6km to 9km. This makes attack aircraft seriously vulnerable to surface-to-air missile (SAM) attack. During their 1981 raid on the Iraqi nuclear reactor at Osirak, the Israeli Air Force chose to use unguided Mark 84 bombs rather than laser-guided weapons during a raid because they felt that the need to designate the target would leave attackers unacceptably vulnerable.

Major laser-guided weapon systems
Laser-guided munitions are broadly categorised as surface-launched projectiles, aerially-delivered bombs and surface-to-surface, surface-to-air and air-to-surface missiles. A large number of laser-guided weapon systems in these categories are manufactured by international giants, including Lockheed Martin, SAAB Bofors Dynamics Israel Aerospace Industries, Matra, Raytheon and KBP Instrument Design Bureau. The more common and established weapons, including Krasnopol/Krasnopol-M and Copperhead (cannon-launched laser-guided projectiles), Paveway-series and Griffin (aerially-delivered bombs), LAHAT, RBS-70/RBS-70NG and Hellfire (laser-guided missiles) are briefly discussed in the following paragraphs.

Laser-guided projectiles
Krasnopol and Copperhead are cannon-launched fin-stabilised terminally laser-guided explosive projectiles designed to engage small, hard, point-ground targets, such as tanks, armoured vehicles, self-propelled artillery systems and other high-value targets, such as bridges, defensive fortifications, C4I (command, control, communications, computers and intelligence) centres, etc.

Krasnopol projectile is produced in two variants, namely, Krasnopol and Krasnopol-M. The former is a 152mm two-section projectile designed to operate with both towed and self-propelled guns and howitzers. It, however, has a shortcoming that it is incompatible with 2S19 auto-loader due to the projectile’s length.

Fig. 15: LJDAM integrated with F-16C aircraft
Fig. 15: LJDAM integrated with F-16C aircraft

Krasnopol-M is a 152mm/155mm projectile. It is an improvement over Krasnopol and is fully compatible with 2S19 auto-loader, which makes it usable with western produced 155mm howitzers.

Other than that, both have the same attack profile (diving top attack as illustrated in Fig. 9), targets engaged and the type of warhead used. The target ranges are similar; 20km in the case of Krasnopol and 17km for Krasnopol-M. Fig. 10 shows the photograph of Krasnopol projectile.

Krasnopol projectile follows a ballistic trajectory approaching the target once it is fired. Subsequent to its firing, a forward observer illuminates the target with a laser designator at a maximum range of 7km. The seeker in the front-end of the projectile locks on the target and the guidance and control system corrects its flight path in order to impact on the selected/illuminated point. Krasnopol projectile follows a top-attack pattern to achieve an optimised probability of kill against the armoured target.

M-712 Copperhead is a 155mm-calibre terminally laser-guided projectile with a minimum and maximum range of 3km and 16km, respectively. It has two operational modes: ballistic mode and glide mode. Ballistic mode is used when the cloud ceiling is high and visibility condition is good. Terminal guidance begins at 3km from the target. Glide mode is used when the cloud ceiling and/or visibility is too low to allow use of ballistic mode. The attack profile in the case of Copperhead projectile is laser-illuminated point attack. Fig. 11 shows Copperhead projectile in flight as it nears the target.

Copperhead projectile was successfully used during Operation Desert Storm in 1990-91 and Operation Iraqi Freedom in 2003. It is in use by various Armed forces, including Australian army, United States army, Egyptian army, Jordanian armed forces and Taiwanese army.

Laser-guided bombs
Of all the variants of laser-guided munitions, laser-guided bombs are the most widely exploited weapons if the number of user countries and if laser-guided bombs used in warfare in the past are any indication. Paveway family of laser-guided bombs has revolutionised tactical air-to-ground warfare by converting dumb bombs into smart precision-guided munitions. Paveway family of the laser-guided bomb is the preferred choice of Air Forces worldwide, as these have proven their accuracy and efficacy in almost all major conflicts in the past. The family has evolved over the years and has seen continuous capability enhancement with newer versions. It has seen four generations, namely, Paveway-I, -II, -II Plus, -III and -IV.

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Paveway-I used a gimballed seeker head, a computer control group (CCG) and a set of air foils. The seeker head operated in bang-bang mode, which meant that control surfaces were deflected either fully or not at all. This led to a sub-optimal flight trajectory. Bombs that could be fitted with Paveway-I LGB kit included M117, M118E1, MK-82, MK-83, MK-84, MK-20, CBU-74/B, CBU-75/B, CBU-79/B and CBU-80/B. More than 10,000 Paveway-I LGBs were used by the US Air Force in South East Asia with great success.

Paveway-II (Fig. 12) has a nose-mounted seeker head and fins for guidance. Manufactured by Defence contractors Raytheon and Lockheed Martin, it also uses the bang-bang guidance concept. That is, fins deflect fully or do not deflect at all.

Paveway-III is an improvement over Paveway-II and uses a more efficient proportional guidance technology. Produced by Raytheon, it was introduced into service in 1983.

Paveway-IV (Fig. 13) is an advanced and highly accurate laser-guided weapon. It is the most recent member of the Paveway family. Manufactured by Raytheon Systems Ltd, the UK, Paveway-IV entered into service in 2008. It will replace Paveway II and enhanced Paveway-II weapon systems as well as the 453.6kg (1000-pound) unguided general-purpose bomb. Paveway-IV employs a combination of semi-active laser guidance and INS/GPS guidance to combine the flexibility and accuracy of laser guidance and all-weather capability of INS/GPS to give significantly improved battlefield performance.

Griffin laser-guided bomb kit is manufactured by Israel Aerospace Industries and is designed to retrofit the existing MK-82, MK-83 and MK-84 dumb gravity bombs. The kit employs a laser-seeker head and a set of steerable tail planes for guidance. The CEP is estimated to be 5m. It is in use by Israeli Defence Forces, Indian Air Force and Colombian Air Force.

Sudarshan laser-guided bomb kit developed by Aeronautical Development Establishment of DRDO and manufactured by Bharat Electronics is another LGB kit. It was introduced in Indian Air Force in 2013. The CEP is estimated to be 10m. In future, Sudarshan LGB kit will incorporate a GPS sensor to improve its performance.

Laser-guided missiles
Laser-guided missiles use both beam riding as well as semi-active laser-guidance concepts. RBS-70/RBS-70NG and LAHAT are examples of laser beam-riding missiles. These were briefly described in part 1 of the article. AGM-114 Hellfire-II is a combat-proven tactical surface-to-surface and air-to-surface missile system that uses semi-active laser homing. Fig. 14 shows Hellfire-II fired from a land vehicle.

Fig. 16: Prototype of the laser-guided bullet developed at Sandia National Lab
Fig. 16: Prototype of the laser-guided bullet developed at Sandia National Lab

Hellfire family comprises Longbow Hellfire and Hellfire-II missiles. Hellfire-II missile has a maximum range of 7km (direct fire) and 8km (indirect fire). The missile can be launched from multiple air, sea and ground platforms, either in autonomous mode or with remote designation. A variant designated AGM-114L uses millimetre-wave radar guidance. Manufactured by Lockheed Martin and introduced into service in 1984, its primary use is as air-to-surface to engage and defeat individual static or moving advanced armour, mechanised or vehicular targets, patrol craft, buildings and bunkers. AGM-114K, AGM-114M, AGM-114N and AGM-114R are laser-guided variants.

New developments
While laser-guided bomb kits continue to improve in terms of hit accuracy, operational range, guidance technology and so on, in recent years, there has been emphasis to improve guidance technology to improve the weapon’s performance in adverse weather conditions. This has been made possible by combining laser guidance with global positioning system (GPS)/inertial navigation system (INS). Laser joint direct attack munition (LJDAM) is an example.

Another major development has been the use of guidance technology in smaller ammunition. In the recent past, field trials have shown encouraging results in laser-guided bullets.

Joint direct attack munition (JDAM) is a low-cost guidance kit used to convert existing unguided free-fall bombs into near-precision-guided weapons. The JDAM kit consists of a tail section that contains a GPS/INS and body strakes for additional stability and lift. JDAM is produced by Boeing.

LJDAM expands the capabilities of the JDAM by combining a laser sensor kit with a JDAM kit. LJDAM has the accuracy of a laser-guided weapon and all-weather capability and longer range of GPS/INS guided weapons. It can precisely hit both stationary and mobile targets. LJDAM has been integrated with GBU-38 and is operational on the US Air Force F-15E and F-16 and the US Navy F/A-18 and A/V-8B platforms. It is planned to integrate LJDAM with GBU-31 and GBU-32. Fig. 15 shows Boeing LJDAM on F-16 fighter aircraft (lowermost weapon in the figure).

Laser-guided bullet development at Sandia National Laboratories is making headlines as it is expected to significantly increase the range of sharp shooting. Modern bullets gain their accuracy from a technique known as rifling, in which the rifle barrel has a series of spiralling grooves etched into it. The spiralled grooves give a spin to the bullet, thereby stabilising its flight path. The laser-guided bullet developed at Sandia National Laboratories is fired from a smooth bore barrel and is stabilised by four steerable fins at its rear (Fig. 16). The fin movement is controlled by a computer chip, which, in turn, is driven by a signal from an optical sensor on the bullet’s nose. The intended target is illuminated by a laser beam and the bullet uses steerable fins to adjust its mid-flight trajectory. The operation is similar to that of a laser-guided munition, which makes a laser-guided bullet nothing but a miniature laser-guided munition.

According to one computer simulation, an unguided bullet fired at a target at 800m would miss the target by about 9m. The laser-guided bullet, on the other hand, would cut that inaccuracy to just 20cm. Knowing peculiarities of ballistics, the accuracy gets better for longer ranges.


Dr Anil Kumar Maini is former director, Laser Science and Technology Centre, a premier laser and optoelectronics research and development laboratory of Defence Research and Development Organisation of Ministry of Defence

Nakul Maini is currently pursuing Masters at University of Bristol, UK. He was working as a technical editor with Wiley India Pvt Ltd

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