PGM technology

In 1999 during the Kargil war India felt very acutely the need for air-delivered precision guided munitions. Freefall bombs, as the name implies would fall all over the craggy mountain except where the Pakistan Army’s Northern Light Infantry men were ensconced in their sanghars-stone embankments.

By trial and error the Indian Armed Forces finally arrived at the perfect solution-the Bofors weapon fired like a line-of-sight gun instead of a high-trajectory howitzer-and bringing up the straight-firing Indian designed Pinaka multi-barrel rocket launcher for area saturation. A total of 2,50,000 tons of metal was delivered before the Pakistani game plan was nipped in the bud.

In hindsight it could be said that air-delivered precision guided munitions could have reduced the number of sorties required to achieve the final victory. They may even have prevented the loss of one fighter aircraft and one helicopter by being able to be delivered accurately where the Pakistanis were waiting with US-supplied Stinger shoulder-fired missiles by increasing the stand-off distance and staying out of reach of the missiles.

Precision guided munitions have been developed to be able to hit a target with the first shot and thus obviate the need for a revisit attack where the air defence systems of the enemy may be in dense configuration and could pose an existential threat to the attacking aircraft.

The experience at Kargil forced the Defence Ministry and the Defence Research and Development Organisation to look for ways to improve the accuracy of the huge quantity of “dumb” or “freefall” or “gravity” bombs in its arsenal. These weapons, ranging in yield from 200 kg to 1000 kg, are among the few weapons that can be produced in bulk within the country.

Laser guidance

The fitment of a laser guidance apparatus on the nose cone turned a bomb that would land dozens of meters away from the target and require that many be launched to be able to hit one target is now able to achieve the same objective with lesser unwanted collateral damage and greater cost-effectively. One bomb per target is very easy to achieve with precision guided munitions.

Even as the news gained currency that the Defence Research and Development Organisation had fashioned and successfully tested a laser guidance kit to the dumb bomb and photographs showing it being released from an aircraft, the other development was that India had bought laser guided bunker busters” from the US.

Though they are both laser guided there is a big difference between the two. The laser guided bomb is intended to destroy targets on the surface, tanks, vehicles, arms dumps, railways, airfields, etc. the bunker buster is designed and intended to dig deep into the earth, even several feet of concrete, and explode.

There are several different ways of busting an underground bunker or fortification. One is to ensure that the projectile hits close to the outside of the suspected location, digs deep into the ground and explodes. The explosion would cause a huge crater undermining the foundations of the concrete structure, causing it to fall into the crater with adverse consequences for the inhabitants. Some bunker busters drill through the concrete and explode when the fuze senses a wide open space like a room and explodes.

Somewhat more sophisticated is the “counter” which counts the number of floors in the underground structure and the fuse explodes at a pre-determined number, causing severe damage to the whole concrete edifice.

This type of bunker buster requires previous intelligence about the type of bunker to be attacked. It can also be made to explode at a pre-determined depth thereby ensuring that maximum damage is inflicted on any kind of structure buried beneath the earth. The “drilling effect” is a factor of the height from which the bomb is released-making use of the kinetic force acquired by a falling body-and the mass and strength of the bomb.

The body of the bomb itself is hardened steel, the earlier versions being slices of artillery barrels which can withstand a lot of internal pressure caused by the passage of the shell when fired. It can slice through more than a dozen feet of cement concrete aggregate. The kinetic force is enhanced with the firing of the rocket motor which gives the rocket warhead an added kinetic force that can slice through rock and cement like a hot knife.       

Delivery method

Not all precision guided missiles are laser guided. The DRDO product Astra air-to-air missile which is to be tested from a Sukhoi 30 is led to its target by terminal radar homing. Its attack is with a warhead that is pre-fragmented triggered by a proximity fuze when the enemy aircraft is within reach. The Astra can be used both for dogfights at up to 20 km and for beyond visual range attack up to 80 km largely because the warhead is in self-seeking mode.

The use of in-the-nosecone guidance system has given rise to the possibility of fire-and-forget because the missile will guide itself to the target be it land based or airborne.

In many ways this is a more preferred method of delivery of weapon and is less complicated than the laser-guided bomb or missile which depends on an external targeting system to illuminate the target before the laser guided seeker can home onto it.

In air warfare this must be done either by launching aircraft or another accompanying aircraft or by a land based infantryman with a target designator used to “paint” the target for the aircraft launched missile to attack.

India has another indigenous ‘fire and forget’ missile that can be launched by helicopters flown either by Army pilots or Air Force flyers. The Nag fire-and-forget missile has been in the making for many years and has only recently been inducted into active service. It is believed that the users prefer a longer-ranged weapon than the Nag and have put a question mark on the future of this indigenous weapon.

The US has long used the television guided, air-launched bombs and missiles as well as imaging infrared and electro-optical warheads to good effect.

Given that there is severe attenuation in the laser system during rainfall and heavy cloud cover the US has married its laser guidance systems to the satellite navigation network through the Global Positioning System (GPS) to ensure accurate delivery of warheads. Nonetheless the GPS is amenable to enemy jamming and hence the guidance has to shift to inertial navigation which is somewhat less accurate than the laser guided system leading to a larger circular error probable or the ‘miss distance’ of a weapon.

However, even given the high accuracy modern warheads can achieve there have been major errors of judgment like the bombing of the Chinese embassy in Belgrade during the operations against the former Yugoslavia or the massacre of Iraqi children during the first Gulf war.