Laser guidance is a technique of guiding a missile or other projectile or vehicle to a target by means of a laser beam. Some laser guided systems utilize beam riding guidance, but most operate more similarly to semi-active radar homing (SARH). This technique is sometimes called SALH, for Semi-Active Laser Homing. With this technique, a laser is kept pointed at the target and the laser radiation bounces off the target and is scattered in all directions (this is known as “painting the target”, or “laser painting”). The missile, bomb, etc. is launched or dropped somewhere near the target. When it is close enough that some of the reflected laser energy from the target reaches it, a laser seeker detects which direction this energy is coming from and adjusts the projectile trajectory towards the source. As long as the projectile is in the general area and the laser is kept aimed at the target, the projectile should be guided accurately to the target.
Note that laser guidance is not useful against targets that do not reflect much laser energy, including those coated in special paint which absorbs laser energy. This is likely to be widely used by advanced military vehicles in order to make it harder to use laser rangefinders against them and harder to hit them with laser- guided missiles. An obvious circumvention would be to aim the laser merely close to the target.
Missiles differ from rockets by virtue of a guidance system that steers them towards a pre-selected target. Unguided, or free-flight, rockets proved to be useful yet frequently inaccurate weapons when fired from aircraft during the World War II. This inaccuracy, often resulting in the need to fire many rockets to hit a single target, led to the search for a means to guide the rocket towards its target. The concurrent explosion of radio-wave technology (such as radar and radio detection devices) provided the first solution to this problem. Several warring nations, including the United States, Germany and Great Britain mated existing rocket technology with new radio- or radar-based guidance systems to create the world's first guided missiles. Although these missiles were not deployed in large enough numbers to radically divert the course of the World War II, the successes that were recorded with them pointed out techniques that would change the course of future wars. Thus dawned the era of high-technology warfare, an era that would quickly demonstrate its problems as well as its promise.
The problems centered on the unreliability of the new radio-wave technologies. The missiles were not able to hone in on targets smaller than factories, bridges, or warships. Circuits often proved fickle and would not function at all under adverse weather conditions. Another flaw emerged as jamming technologies flourished in response to the success of radar. Enemy jamming stations found it increasingly easy to intercept the radio or radar transmissions from launching aircraft, thereby allowing these stations to send conflicting signals on the same frequency, jamming or "confusing" the missile. Battlefield applications for guided missiles, especially those that envisioned attacks on smaller targets, required a more reliable guidance method that was less vulnerable to jamming. Fortunately, this method became available as a result of an independent research effort into the effects of light amplification.
Dr. Theodore Maiman built the first laser (Light Amplification by Stimulated Emission of Radiation) at Hughes Research Laboratories in 1960. The military realized the potential applications for lasers almost as soon as their first beams cut through the air. Laser guided projectiles underwent their baptism of fire in the extended series of air raids that highlighted the American effort in the Vietnam War. The accuracy of these weapons earned them the well-known sobriquet of "smart weapons." But even this new generation of advanced weaponry could not bring victory to U.S. forces in this bitter and costly war. However, the combination of experience gained in Vietnam, refinements in laser technology, and similar advances in electronics and computers, led to more sophisticated and deadly laser guided missiles. They finally received widespread use in Operation Desert Storm, where their accuracy and reliability played a crucial role in the decisive defeat of Iraq's military forces. Thus, the laser guided missile has established itself as a key component in today's high-tech military technology.
3. SEMI ACTIVE RADAR HOMING
Semi-active radar homing, or SARH, is a common type of missile guidance system, perhaps the most common type for longer range air to air and surface-to-air missile systems. The name refers to the fact that the missile itself is only a passive detector of a radar signal – provided by an external (“off board”) source — as it reflects off the target. The basic concept of SARH is that since almost all detection and tracking systems consist of a radar system, duplicating this hardware on the missile itself is redundant. In addition, the resolution of a radar is strongly related to the physical size of the antenna, and in the small nose cone of a missile there isn't enough room to provide the sort of accuracy needed for guidance. Instead the larger radar dish on the ground or launch aircraft will provide the needed signal and tracking logic, and the missile simply has to listen to the signal reflected from the target and point itself in the right direction. Additionally, the missile will listen rearward to the launch platform's transmitted signal as a reference, enabling it to avoid some kinds of radar jamming distractions offered by the target. Contrast this with beam riding systems, in which the radar is pointed at the target and the missile keeps itself centered in the beam by listening to the signal at the rear of the missile body. In the SARH system the missile listens for the reflected signal at the nose, and is still responsible for providing some sort of “lead” guidance. The disadvantages are twofold: One is that a radar signal is “fan shaped”, growing larger, and therefore less accurate, with distance. This means that the beam riding system is not accurate at long ranges, while SARH is largely independent of range and grows more accurate as it approaches the target, or the source of the reflected signal it listens for. Another requirement is that a beam riding system must accurately track the target at high speeds, typically requiring one radar for tracking and another “tighter” beam for guidance. The SARH system needs only one radar set to a wider pattern.
4. MISSILE COMPONENTS
Guided missiles are made up of a series of subassemblies. The various subassemblies form a major section of the overall missile to operate a missile system, such as guidance, control, armament (warhead and fuzing), and propulsion. The major sections are carefully joined and connected to each other. They form the complete missile assembly. The arrangement of major sections in the missile assembly varies, depending on the missile type.
The guidance section is the brain of the missile. It directs its maneuvers and causes the maneuvers to be executed by the control section. The armament section carries the explosive charge of the missile, and the fuzing and firing system by which the charge is exploded. The propulsion section provides the force that propels the missile.
4.1. Guidance and Control Section
The complete missile guidance system includes the electronic sensing systems that initiate the guidance orders and the control system that carries them out. The elements for missile guidance and missile control can be housed in the same section of the missile, or they can be in separate sections.
There are a number of basic guidance systems used in guided missiles. Homing-type, air-launched, guided missiles are currently used. They use radar or infrared homing systems. A homing guidance system is one in which the missile seeks out the target, guided by some physical indication from the target itself. Radar reflections or thermal characteristics of targets are possible physical influences on which homing systems are based. Homing systems are classified as active, semiactive, and passive.
In the active homing system, target illumination is supplied by a component carried in the missile, such as a radar transmitter. The radar signals transmitted from the missile are reflected off the target back to the receiver in the missile. These reflected signals give the missile information such as the target's distance and speed. This information lets the guidance section compute the correct angle of attack to intercept the target. The control section that receives electronic commands from the guidance section controls the missile’s angle of attack. Mechanically manipulated wings, fins, or canard control surfaces are mounted externally on the body of the weapon. They are actuated by hydraulic, electric, or gas generator power, or combinations of these to alter the missile's course.
In the semi active homing system, the missile gets its target illumination from an external source, such as a transmitter carried in the launching aircraft. The receiver in the missile receives the signals reflected off the target, computes the information, and sends electronic commands to the control section. The control section functions in the same manner as previously discussed.
In the passive homing system, the directing intelligence is received from the target. Examples of passive homing include homing on a source of infrared rays (such as the hot exhaust of jet aircraft) or radar signals (such as those transmitted by ground radar installations). Like active homing, passive homing is completely independent of the launching aircraft. The missile receiver receives signals generated by the target and then the missile control section functions in the same manner as previously discussed.
4.5 ARMAMENT SECTION
The armament system contains the payload (explosives), fuzing, safety and arming (S&A) devices, and target-detecting devices (TDDs).
The payload is usually considered the explosive charge, and is carried in the warhead of the missile. High-explosive warheads used in air-to-air guided missiles contain a rather small explosive charge, generally 10 to 18 pounds of H-6, HBX, or PBX high explosives. The payload contained in high-explosive warheads used in air-to-surface guided missiles varies widely, even within specific missile types, depending on the specific mission. Large payloads, ranging up to 450 pounds, are common. Comp B and H-6 are typical explosives used in a payload. Most exercise warheads used with guided missiles are pyrotechnic signaling devices. They signal fuze functioning by a brilliant flash, by smoke, or both. Exercise warheads frequently contain high explosives, which vary from live fuzes and boosters to self-destruct charges that can contain as much as 5 pounds of high explosive.
The fuzing and firing system is normally located in or next to the missile's warhead section. It includes those devices and arrangements that cause the missile's payload to function in proper relation to the target. The system consists of a fuze, a safety and arming (S&A) device, a target-detecting device (TDD), or a combination of these devices. There are two general types of fuzes used in guided missiles—proximity fuzes and contact fuzes. Acceleration forces upon missile launching arm both fuzes. Arming is usually delayed until the fuze is subjected to a given level of accelerating force for a specified amount of time. In the contact fuze, the force of impact closes a firing switch within the fuze to complete the firing circuit, detonating the warhead. Where proximity fuzing is used, the firing action is very similar to the action of proximity fuzes used with bombs and rockets.
4.5.3 Safety And Arming (S&A) Devices:
S&A devices are electromechanical, explosive control devices. They maintain the explosive train of a fuzing system in a safe (unaligned) condition until certain requirements of acceleration are met after the missile is fired.
4.5.4 Target-Detecting Devices (TDD):
TDDs are electronic detecting devices similar to the detecting systems in VT fuzes. They detect the presence of a target and determine the moment of firing. When subjected to the proper target influence, both as to magnitude and change rate, the device sends an electrical impulse to trigger the firing systems. The firing systems then act to fire an associated S&A device to initiate detonation of the warhead. Air-to-air guided missiles are normally fuzed for a proximity burst by using a TDDwith an S&A device. In some cases, a contact fuze may be used as a backup. Air-to-surface guided missile fuzing consists of influence (proximity) and/or contact fuzes. Multifuzing is common in these missiles.
4.5.5 Propulsion Section
Guided missiles use some form of jet power for propulsion. There are two basic types of jet propulsion power plants used in missile propulsion systems—the atmospheric (air-breathing) jet and the thermal jet propulsion systems. The basic difference between the two systems is that the atmospheric jet engine depends on the atmosphere to supply the oxygen necessary to start and sustain burning of the fuel. The thermal jet engine operates independently of the atmosphere by starting and sustaining combustion with its own supply of oxygen contained within the missile.
Atmospheric jet propulsion system.
There are three types of atmospheric jet propulsion systems—the turbojet, pulsejet, and ramjet engines. Of these three systems, only the turbojet engine is currently being used in Navy air-launched missiles. A typical turbojet engine includes an air intake, a mechanical compressor driven by a turbine, a combustion chamber, and an exhaust nozzle. The engine does not require boosting and can begin operation at zero acceleration.
Thermal Jet Propulsion System
Thermal jets include solid propellant, liquid propellant, and combined propellant systems. As an AO, you come in contact with all three systems. The solid propellant and combined propellant systems are currently being used in some air-launched guided missiles. The majority of air-launched guided missiles used by the Navy use the solid propellant rocket motor. They include the double base and multibase smokeless powder propellants as well as the composite mixtures. Grain configurations vary with the different missiles. Power characteristics and temperature limitations of the individual rocket motors also vary. In some guided missiles, different thrust requirements exist during the boost phase as compared to those of the sustaining phase. The dual thrust rocket motor (DTRM) is a combined system that contains both of these elements in one motor. The DTRM contains a single propellant grain made of two types of solid propellant—boost and sustaining. The grain is configured so the propellant meeting the requirements for the boost phase burns at a faster rate than the propellant for the sustaining phase. After the boost phase propellant burns itself out, the sustaining propellant sustains the motor in flight over the designed burning time (range of the missile).
5. MISSILE CONTROL SYSTEM
The heart of a missile is the body, equivalent to the fuselage of an aircraft. The missile body contains the guidance and control system, warhead, and propulsion system. Some missiles may consist of only the body alone, but most have additional surfaces to generate lift and provide maneuverability. Depending on what source you look at, these surfaces can go by many names. In particular, many use the generic term "fin" to refer to any aerodynamic surface on a missile. Missile designers, however, are more precise in their naming methodology and generally consider these surfaces to fall into three major categories: canards, wings, and tail fins.
The example shown above illustrates a generic missile configuration equipped with all three surfaces. Often times, the terms canard, wing, and fin are used interchangeably, which can get rather confusing. These surfaces behave in fundamentally different ways, however, based upon where they are located with respect to the missile center of gravity. In general, a wing is a relatively large surface that is located near the center of gravity while a canard is a surface near the missile nose and a tail fin is a surface near the aft end of the missile.
Most missiles are equipped with at least one set of aerodynamic surfaces, especially tail fins since these surfaces provide stability in flight. The majority of missiles are also equipped with a second set of surfaces to provide additional lift or improved control. Very few designs are equipped with all three sets of surfaces. Most aircraft have fixed horizontal and vertical tails with smaller movable rudder and elevator surfaces, missiles typically use all-moving surfaces, like those illustrated below, to accomplish the same purpose.
In order to turn the missile during flight, at least one set of aerodynamic surfaces is designed to rotate about a center pivot point. In so doing, the angle of attack of the fin is changed so that the lift force acting on it changes. The changes in the direction and magnitude of the forces acting on the missile cause it to move in a different direction and allow the vehicle to maneuver along its path and guide itself towards its intended target.
6. THE MANUFACTURING PROCESS
6.1. Raw Materials
A laser guided missile consists of four important components, each of which contains different raw materials. These four components are the missile body, the guidance system (also called the laser and electronics suite), the propellant, and the warhead. The missile body is made from steel alloys or high-strength aluminum alloys that are often coated with chromium along the cavity of the body in order to protect against the excessive pressures and heat that accompany a missile launch. The guidance system contains various types of materials—some basic, others high-tech—that are designed to give maximum guidance capabilities.
These materials include a photo detecting sensor and optical filters, with which the missile can interpret laser wavelengths sent from a parent aircraft. The photo detecting sensor's most important part is its sensing dome, which can be made of glass, quartz, and/or silicon. A missile's electronics suite can contain gallium-arsenide semiconductors, but some suites still rely exclusively on copper or silver wiring. Guided missiles use nitrogen-based solid propellants as their fuel source. Certain additives (such as graphite or nitroglycerine) can be included to alter the performance of the propellant. The missile's warhead can contain highly explosive nitrogen-based mixtures, fuel-air explosives (FAE), or phosphorous compounds. The warhead is typically encased in steel, but aluminum alloys are sometimes used as a substitute.
6.2. Constructing the body and attaching the fins
The steel or aluminum body is die cast in halves. Die casting involves pouring molten metal into a steel die of the desired shape and letting the metal harden. As it cools, the metal assumes the same shape as the die. At this time, an optional chromium coating can be applied to the interior surfaces of the halves that correspond to a completed missile's cavity. The halves are then welded together, and nozzles are added at the tail end of the body after it has been welded.
Moveable fins are now added at predetermined points along the missile body. The fins can be attached to mechanical joints that are then welded to the outside of the body, or they can be inserted into recesses purposely milled into the body.
6.3. Casting the propellant
The propellant must be carefully applied to the missile cavity in order to ensure a uniform coating, as any irregularities will result in an unreliable burning rate, which in turn detracts from the performance of the missile. The best means of achieving a uniform coating is to apply the propellant by using centrifugal force. This application, called casting, is done in an industrial centrifuge that is well-shielded and situated in an isolated location as a precaution against fire or explosion.
6.4. Assembling the guidance system
The principal laser components—the photo detecting sensor and optical filters—are assembled in a series of operations that are separate from the rest of the missile's construction. Circuits that support the laser system are then soldered onto pre-printed boards; extra attention is given to optical materials at this time to protect them from excessive heat, as this can alter the wavelength of light that the missile will be able to detect. The assembled laser subsystem is now set aside pending final assembly. The circuit boards for the electronics suite are also assembled independently from the rest of the missile. If called for by the design, microchips are added to the boards at this time.
The guidance system (laser components plus the electronics suite) can now be integrated by linking the requisite circuit boards and inserting the entire assembly into the missile body through an access panel. The missile's control surfaces are then linked with the guidance system by a series of relay wires, also entered into the missile body via access panels. The photo detecting sensor and its housing, however, are added at this point only for beam riding missiles, in which case the housing is carefully bolted to the exterior diameter of the missile near its rear, facing backward to interpret the laser signals from the parent aircraft.
6.5. Final assembly
Insertion of the warhead constitutes the final assembly phase of guided missile construction. Great care must be exercised during this process, as mistakes can lead to catastrophic accidents.
Simple fastening techniques such as bolting or riveting serve to attach the warhead without risking safety hazards. For guidance systems that home-in on reflected laser light, the photo detecting sensor (in its housing) is bolted into place at the tip of the warhead. On completion of this final phase of assembly, the manufacturer has successfully constructed on of the most complicated, sophisticated, and potentially dangerous pieces of hardware in use today.
Propellants and explosives used in warheads are toxic if introduced into water supplies. Residual amounts of these materials must be collected and taken to a designated disposal site for burning. Each state maintains its own policy pertaining to the disposal of explosives, and Federal regulations require that disposal sites be inspected periodically. Effluents (liquid byproducts) from the chromium coating process can also be hazardous. This problem is best dealt with by storing the effluents in leak-proof containers. As an additional safety precaution, all personnel involved in handling any hazardous wastes should be given protective clothing that includes breathing devices, gloves, boots and overalls.
7. ADVANTAGES OF LASER GUIDED MISSILES
Laser guided weapons, such as the Lockheed Martin Hellfire, and Lahat and Nimrod, developed by IAI/MBT offer many advantages for heliborne and airborne use. The SAL seeker is relatively low cost, offering high precision operational flexibility, despite its adverse weather limitations.
This concept of operation places high priority on target designation capabilities, deployed close to the target by unmanned platforms and Special Forces. Not every laser seeker will be suitable for the task. Only the more sophisticated missiles offer the flexibility and field of regard ('side looking') capability adequate for effective lock-on after launch targeting. Such capability seldom requires their seeker to be mounted on a gimbal, to achieve adequate field of regard, something that simple, low-cost stiff-necked or static seeker assemblies may not support.
The LAHAT laser guided missile is lightweight weapon can be employed by light helicopters. It can be fired at targets over distances between 8 to 13 kilometers, with devastating effects against armor as well as softer targets. Besides its potential helicopter application, LAHAT is considered by several armies for its original role as gun-fired laser-homing munition for tanks. Nimrod, a much larger missile, has also been evaluated as a helicopter borne weapon. Utilizing its extended range (over 22 km), this missile is often used in 'lock on after launch' mode, combining inertial guidance and semi- active laser homing to strike targets at long ranges.
With the availability of such 'net centric' precision attack missiles, the role of attack helicopters is also re- examined, and several air forces and manufacturers are already considering using assault helicopters for some attack roles, employed either as a 'sky truck' or in direct support, when they are fitted with target acquisition systems.
8. THE FUTURE LASER GUIDED MISSILES
Future laser guided missile systems will carry their own miniaturized laser on board, doing away with the need for target designator lasers on aircraft. These missiles, currently under development in several countries, are called "fire-and-forget" because a pilot can fire one of these missiles and forget about it, relying on the missile's internal laser and detecting sensor to guide it towards its target. A further development of this trend will result in missiles that can select and attack targets on their own. Once their potential has been realized, the battlefields of the world will feel the deadly venom of these "brilliant missiles" for years to come. An even more advanced concept envisions a battle rifle for infantry that also fires small, laser guided missiles. Operation Desert Storm clearly showed the need for laser guided accuracy, and, as a result, military establishments dedicated to their missions will undoubtedly invent and deploy ever more lethal versions of laser guided missiles.
"In World War II it could take 9,000 bombs to hit a target the size of an aircraft shelter. In Vietnam, 300. Today we can do it with one laser-guided missile. Laser guided missile can be fired at targets ranging 8 to 13 kilometers and some like LAHAT laser guided missile up to 22 kilometers. Though many missiles are developed, they don’t find accuracy as in the reaching the target. Laser guided missile has be one of dangerous missile in war field in past and will be the future.