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Introduction Smart
bombs are weapons capable of destroying enemy targets with out the need for a
launch aircraft to penetrate the envelops of the air defense systems. These essentially
comprise a terminal guidance unit that guides them in the last phase to achieve
pinpoint accuracy. Increased accuracy means that a single, moderate-size bomb
can give a better result than multiple strikes with larger, non-guided bombs. Smart
bombs are desirable both from ethical and tactical standpoints. On ethical grounds,
the military desires that each warhead deployed should strike only its indented
target so that innocent civilians are not harmed by misfire. From tactical standpoint,
it wants weapons with pinpoint accuracy to inflict maximum damage on valid military
targets and minimize the number of strikes necessary to achieve mission objectives.
Gravity bombs with laser or GPS/INS guidance are smart bombs that have changed
the face of modern warfare Laser-Guided
Bombs
Laser-guided bombs have an internal
semi-active guidance system that detects laser energy and guides the weapon to
the target illuminated by an external laser designator.Laser
designators radiate a narrow beam of pulsed energy in the near infrared wavelength
spectrum. These are semi active illuminators used to tag the desired spot. These
can be aimed such that laser energy precisely designates the chosen spot on the
target. Laser designator can be located in the delivery aircraft, another aircraft
or on a ground source. Typical LGB seekers
comprise an array of photo diodes to decode the laser pulsed repetition frequency
(PRF) and derive target position signals. Laser designators and seekers use a
pulse coding system to ensure that a specific seeker and designator combination
work in harmony. By setting the same code in both the designator and the seeker
will track only the target designated by the chosen designator. Laser
designators provide precision target marking for air support. LGBs with TV or
infrared seeker in their nose show the target to the attacking pilot on a screen
in the cockpit. The pilot fixes cross hairs on the target and marks it for the
weapon to aim at. The target scatters the received laser energy in all the directions.
LGB seeker having a limited field of view receives a small part of the scattered
energy and decodes it. If the received PRF code matches the preprogrammed code,
the pilot fires the bomb. It then horns onto the reflected laser energy to attack
the target Some LGBs require laser target
illumination before launch or release and/ or during the terminal portion of flight.
The LGB flight path can be divided into three phases: ballistic, transition, and
terminal guidance. During the ballistic phase,
the weapon continues on the unguided trajectory established by the flight path
of the delivery aircraft at the moment of release. In this phase, the delivery
altitude takes on additional importance, since maneuverability of the unguided
bomb is related to the weapon velocity during terminal guidance. Therefore, airspeed
lost during the ballistic phase equated to a proportional loss of maneuverability.
So the closer to the target the release of the bomb, the more the accuracy of
the bomb The transition phase is where initial
acquisition of the designating laser takes place. During this phase, the weapon
attempts to align its velocity vector with the line of sight vector to the target.
Once a lock has been achieved, the guidance system uses the canards to try and
keep the bomb within the cone of the reflected laser energy.
The terminal phase is when the reflected laser centers on the seeker causing the
bomb to dive to the target. LGBs are excellent performers in dive deliveries initiated
from medium altitude. A steep, fast dive attack increases LGB maneuvering potential
and flight ability. Medium-altitude attacks generally reduce target acquisition
problems and more readily allow for target designation by either ground or airborne
designation platforms. Medium-altitude LGB dive delivery tactics are normally
used in areas of low to medium threat. LGBs
miss the target if the laser is turned on too early: During certain delivery profiles
where the LGB sees laser energy as soon as it is released, it can turn from its
delivery profile too soon and miss by failing short of the target. To prevent
this, the laser designator must be turned on at the time that will preclude the
bomb from turning down toward the target prematurely. Normally, the pilot knows
the proper moment for laser 'on'. The specific LGB and the delivery tactics of
the attacking aircraft dictate the minimum designation time required to guide
the weapon to the intended target. The four
basic requirements for effective use of laser designators with LGBs are:
1.
The PRF code of the laser designator and the LGB must be compatible
2. A direction
of attack must be determined because the LGB must be able to sense sufficient
laser energy reflecting from the target being designated.
3. The laser designator
must designate the target at the correct time.
4. The delivery system must
release the weapon within the specific weapon's delivery envelope.
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