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.