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Tuesday, 16 October, 2001, 20:53 GMT 21:53 UK
Why bombing can go wrong
Mazar-e Sharif Taliban base
Accurate target information is crucial
By BBC News Online's Gary Eason

Precision guided munitions - smart bombs - have greatly improved the accuracy with which aircraft can attack targets on the ground. But they are far from infallible.

The weapons work in various ways, but all require certain "release conditions" to be met - outside those parameters, their chances of hitting what they are aimed at are reduced.

It is certainly not a computer game

Defence studies academic David Jordan
Bombs and missiles with TV or infrared "seekers" in their noses show the target to the attacking pilot on a screen in the cockpit.

The pilot fixes cross-hairs on the target, marking it for the weapon to aim at, and fires.

But the view from the weapon has been described as "looking through a straw".

And David Jordan, of the Centre for Defence Studies, at King's College, London, UK, makes the point that attacking pilots still have to display considerable skill to meet the weapons' release conditions - typically in stressful, dangerous situations.

"It is certainly not a computer game," he said.

Relative precision

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.

US attack on Afghan missile facility
Cross-hairs mark a target for the weapon to fire at
Again it has to be released in such a way that it can latch on to and follow the cone of light.

But 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.

Because the electronics involved are so complicated, they can simply malfunction.

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

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.

The latest laser-guided bombs have a "circular error probable" of about three metres - but for many of those being used in Afghanistan it is 10 m (33 ft).

That means that half of those you drop will land within 10 m of their targets - and half will not.

The new JDAM system, which is a guidance kit that can be clamped to ordinary, unguided bombs to make them into precision weapons, is not quite as accurate - quoted as 13 m, although the US Air Force claims it has performed better in action.

GPS navigation

JDAM bombs work by using global satellite positioning (GPS) navigation. They are one of the military's chief solutions to the problem of laser guidance being affected by the weather.

US Navy Hornet dropping JDAM guided bomb
US Navy Hornet dropping JDAM guided bomb
The worldwide GPS system is funded by the US Department of Defense.

About 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. If it is released and functions correctly, it will hit the target.

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

That is the theory. In practice, errors of more than 10 m (33 ft) can creep in due to atmospheric conditions and electronic "noise".

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

If that happens with a JDAM, its "circular error probable" rises to 30 m (98 ft).

Human factor

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.

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

The US military uses the World Geodetic System of co-ordinates, similar to the latitude and longitude shown on maps.

The co-ordinates of the target 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.

Intelligence failure

But the problem goes even further back in the process.

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

Preliminary indications are that the accident occurred from a targeting process error

US statement

Their target was supposed to have been 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.

And the increased use of such weapons means the whole targeting process has to be more painstaking than ever.

Laser and TV or infrared-guided weapons have a person "in the loop", as the jargon puts it, of aiming them.

Quality targeting

But GPS-guided weapons make their own way to the target.

Their accuracy is determined mainly by the accuracy of the target co-ordinates. Once released, there is no stopping them.

So their increased use means that, more than ever, the targeting information has to be good.

See also:

16 Oct 01 | South Asia
US deploys lethal low-flying gunship
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