High-tech U.S. bombs are precise but not perfect

In 1945, the armed forces needed 100 B-19s and an average of 650 bombs to reliably destroy a single ground target. Today, they can do it with one plane and one or two bombs.

Yet after more than 50 years of development, “precision bombing” is still subject to mistakes -- electronic, mechanical, human -- that can exact a deadly civilian toll and have far-reaching political consequences.

The imperfection of the system was painfully on display again Tuesday, as the Pentagon disclosed that three bombs had gone astray over the weekend in its 17-day-old military campaign in Afghanistan.

On Saturday, a U.S. Navy F-14 inadvertently dropped two 500-pound bombs on a residential area northwest of the capital, Kabul, officials said. The fighter had been aiming at military vehicles parked about half a mile away. And on Sunday, a U.S. Navy F/A-18 Hornet, targeting an army vehicle storage facility near the western city of Herat, instead struck a field 300 feet from a home for the elderly.

Errors could erode support for campaign

Casualty figures and other details of these accidents remain unclear. But experts say that despite the sophistication of modern weaponry, such accidents can happen for several reasons.

A bomb may fall off its aircraft rack prematurely; a loose tail fin may send it veering miles off course. Dew may briefly blind its laser-guidance system. Or a tired, stressed air crew member may punch the wrong target numbers into a satellite guidance system.

After years of effort and billions of dollars in investment, today’s bombing system “is a lot better, a lot safer” than in previous conflicts, said retired Adm. Stephen Baker, a former Navy pilot and weapons tester.

But, he added, “It’s not perfect.”

U.S. officials are well aware that mistakes that result in civilian deaths could erode vital support for the U.S. military effort from Afghan groups, allies in neighboring countries and moderate Muslims elsewhere in the world.

The U.S. effort to develop precision-bombing techniques made its first appearance with the “smart bombs” of the Vietnam War era and came fully into its own in the 1999 air war against Yugoslavia.

Fog, dust and dew can send ordnance astray

In the Balkan conflict, only 20 of the 23,000 munitions used hit unintended targets and inflicted so-called “collateral damage,” according to the Pentagon’s official tally. Yet those incidents attracted worldwide attention -- especially when a bomb fell on the Chinese Embassy in Belgrade, the Yugoslav capital, killing three people -- and repeatedly threatened to sunder the fragile European coalition that backed the war effort. The bombing of the embassy resulted from an erroneous U.S. map, an example of how faulty intelligence can lead to such mistakes.

When the war was over, the Pentagon intensified its effort to make its bombing techniques more precise.

Increasingly, U.S. forces have used bombs with highly precise laser-guidance systems and those that rely on satellite guidance gear. Satellite-guided bombs can be used more effectively than others in bad weather.

One new weapon, the Joint Direct Attack Munition, or JDAM, is a free-falling bomb that has been fitted with a satellite-guidance “kit” and a special tail section that allows it to correct itself and maneuver -- albeit for relatively short distances -- to its target. It still needs to be dropped by an aircraft into a small target area, called a “basket,” in much the same way that old-fashioned gravity bombs are deployed.

A JDAM can correct for sudden wind in midflight, but it can be thrown off if hit by heavy crosswinds near the ground. And the electronic signals used by the satellite guidance system can be interrupted by jamming by a sophisticated enemy or inadvertently by other transmitters used by U.S. forces.

Laser-guided bombs are the most precise of all, because they follow a thin beam to a precise point on the ground.

Yet they can be thrown off by rain, clouds or even heavy fog or dust.

Such bombs are often used by tactical fighter jets that aim a laser beam on a target as they fly at 500 mph at a low or medium altitude.

In a laser-guided system, the bomb relies on a device called a laser seeker to find and hit a target that has had a beam shone on it from an aircraft. But a number of things can go wrong in the process.

The seeker may not be able to find the beam if the concentrated light is diffused by moisture or dust, or by light reflecting from another surface.

The systems can also be knocked off if the aircraft hits turbulence.

On other occasions, smoke from the explosion of one bomb will drift in front of a second target, interrupting the laser beam and throwing off the bomb, said Baker, now an analyst with the Center for Defense Information think tank in Washington.

A number of mechanical glitches can also cause a bomb to be dropped on the wrong target.

Racks holding the bombs under the wings of a fighter jet sometimes fail, releasing the weapons prematurely. No laser system would be able to correct inadvertent drops, analysts said.

Human mistakes also play a role

The military has spent billions of dollars over the years trying to automate target systems in ways that would make them less subject to glitches.

In the Vietnam War, targeting information was passed between controllers and pilots and headquarters in a system that was labor intensive and subject to occasional errors.

Now the process has been computerized. Yet humans are called on to perform several tasks.

They can make mistakes, as occurred recently when a U.S. service member apparently transposed two digits in targeting data. The mistake sent a 2,000-pound bomb crashing into a row of homes in Kabul, killing at least four people.

Errors can also occur during the complicated process of converting maps into numerical targeting data, a process called “coordinate drift.”

Nick Cook, a consultant for Jane’s Defense Weekly in London, notes that the Pentagon has continued to make changes designed to make the process more accurate.

It has been reducing the standard bomb size, which has been 1,000 pounds. And it has developed a backup system for laser bombs so that if their guidance fails, the munitions can be directed by satellite, he said.

Analysts caution that such weaponry will never be glitch-free.

“In the real world, this stuff works fairly well most of the time, but not perfectly well all of the time,” said John Pike, a defense analyst with GlobalSecurity.org, a research organization in Virginia.