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Last Updated:  Wednesday, 26 March, 2003, 19:45 GMT
Why bombing can go wrong
By Gary Eason
BBC News Online

The US-led forces attacking Iraq are using precision guided munitions far more than in any previous conflicts.

Bomb crater in Baghdad
Crater in Baghdad: was anything hit?

These "smart bombs" have greatly improved the accuracy with which aircraft can attack targets on the ground.

But they are not infallible.

All weapons require certain "release conditions" to be met - outside those parameters, their chances of hitting what they are aimed at are reduced.

And attacking pilots sometimes have to display considerable skill to meet those conditions - often in stressful, dangerous situations.

Laser limitations

Laser-guided bombs work by homing in on an inverted cone of light reflected off the target when a thin laser beam is pointed at it.

The laser light is a coded series of pulses, forming a data signal which the bomb recognises and flies down.

Pilot checks laser-guided bomb on an RAF Harrier
Pilot checks a laser-guided bomb on his RAF Harrier
So the target has to be kept illuminated by the laser while the bomb falls, and the faint light is prone to disruption by cloud, smoke, even heavy rain.

This plagued British aircraft over Kosovo.

The official Ministry of Defence "lessons learned" included: "the limitations of our current laser guided systems in poor weather were highlighted, as cloud cover often prevented laser designation of targets, accurate delivery of ordnance became impossible, and was therefore not attempted."

Its response has been to go down the US-led route of equipping its planes with weapons that have GPS satellite positioning receivers, buying "Enhanced Paveway" bombs.

Satellite guidance

And suddenly satellite-aided guidance is everywhere.

The mainstay of the United States arsenal is becoming its JDam bombs - a tail kit for an ordinary 1,000 lb or 2,000 lb bomb that gives it GPS guidance, so it "knows" where it is - with, as a fall back, inertial navigation.

Fins allow the bomb to manoeuvre as it falls, to try to stay on target.

JDams on a B-52 bomber leaving Fairford in England
JDams on a B-52 bomber leaving Fairford in England
The worldwide GPS system is funded by the US Department of Defence.

More than two dozen satellites orbiting the Earth send out signals which can be picked up by small receivers. Within the signals is a precision code reserved for military use.

By cross-referencing the data from several satellites, a receiver can fix its location on the planet with an accuracy of less than a metre.

And, by referring to atomic clocks, the GPS system can provide a time accurate to billionths of a second, and therefore the precise speed of the receiver.

This means a weapon with GPS guidance can "know" exactly where it is in relation to its intended target.

Failures

In practice, errors can creep in due to atmospheric conditions and electronic "noise" and even deliberate enemy efforts to "jam" the GPS signals.

If a GPS-guided weapon loses touch with the guiding satellites, it falls back on inertial navigation - calculating where it is in relation to its last fixed position - which is much less precise.

Because smart weapons are so complicated, they can simply malfunction.

JSow guided bombs on a US Navy Hornet
JSow guided bombs here on a US Navy Hornet can glide for up to 50 miles
In a Pentagon briefing shortly before the invasion of Iraq, a senior official said: "There are a number of both electrical and mechanical things have to happen in order for that bomb to go where it is designed.

"And you could have a power failure on a guidance unit, you could have a fin lock up, and that bomb will go somewhere we know not."

He added "that is not uncommon in 8 to 10% of the time."

But even if they work as intended, "precision" in military terms is relative.

Errors

Any weapon has what is known as its "error probability" - jargon for how close it is likely to get to the point at which it is aimed.

In tests, JDam bombs had a "circular error probable" (CEP) of 9.6 metres (31.5 ft) - in other words, half of those dropped landed within 9.6 m of their targets, half further away.

That was with GPS guidance functioning - the CEP was about 30 m (98 ft) using inertial guidance only.

The main blast from a 2,000 lb bomb extends out for about 600 feet (183 m).

Constant efforts are being made to improve accuracy, involving some very clever electronic error-correction systems.

Although adverse weather can affect a guided bomb's efforts to stay focused on its target, US Brigadier General Vincent Brooks told reporters in the Gulf that would be taken into account in planning a mission.

A new system now in development, known as Damask, would get JDam's circular error probability down to just three metres.

It provides the weapon with an image of the intended target, obtained from a photograph, infra-red or radar surveillance.

When the bomb nears the target, a seeker in its nose starts comparing what is ahead with the stored image, at 30 frames a second - and tells the guidance unit how to steer the last mile.

Human factors

But some of the most notorious instances of the wrong targets being hit appear not to be the result of weapons failing to work as intended.

If the equipment is functioning perfectly, by far the greatest potential for things going wrong comes from an age-old problem: Human error.

A missile can go down the ventilation shaft of an enemy army command bunker - but it is just as capable of going down the ventilation shaft of a hospital.

The Iraqis have exploited this, it is said, by deliberately siting military installations next to hospitals, schools or mosques.

GPS-guided weapons are not aiming for a target as such, they are aiming for a set of pre-programmed co-ordinates.

The co-ordinates have to be loaded into a GPS-guided weapon either before the aircraft carrying it takes off, or in flight.

Somebody has to do that, and they can get it wrong - "fat fingering" as it is known.

The military prefer to have the co-ordinates entered electronically, from one device to another, to reduce this risk.

Intelligence

But the problem goes even further back in the process.

On 7 May, 1999, American B-2 stealth bombers launched JDams which precisely hit a building in Belgrade, killing three people.

Their target was a Yugoslav arms procurement office. What they hit was the Chinese embassy, which had been on that site for four years.

The mistake was put down to a remarkable failure of intelligence by the CIA.

Then there is the case that nothing might go wrong as such.

When precision bombs are aimed at missile sites in a residential area, the military and political aspects of hurling around high explosives overlap.

"While the coalition goes to great lengths to avoid injury to civilians and damage to civilian facilities, in some cases, such damage is unavoidable when the regime places military weapons near civilian areas," said the US Central Command.






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