Why the concentration on Yugoslav air defences?

Nato wants to concentrate its attacks on Yugoslav Army and Special Police units in Kosovo, and the supply routes and installations that support them.

If Nato warplanes are to be effective in attacking troops on the ground, they will need to fly at lower altitudes and to operate during daylight. The initial Nato attacks have been at night.

But the Alliance has a healthy respect for Yugoslav air defences. Thus before it can really begin to do serious damage to Yugoslav units on the ground, it needs to knock out the air defence system.

Yugoslav police and army units are the ultimate target of Nato's campaign

An air defence system can be likened to the human body, with powerful radar systems as the eyes, command and control centres as the brain, and an array of missiles, fighter jets and anti-aircraft artillery as the limbs that can actually defend against any attack.

The whole thing is linked together by a variety of communication systems, transmitting information to where it can be acted upon.

Nato's initial aim is to disrupt and destroy enough of the key elements of the Yugoslav air defence system to prevent Yugoslav commanders from gaining a broad view of what is going on, and to stop them from mounting a co-ordinated response to Nato's attacks.

How does Nato set about this? Why are the attacks so extensive?

Nato has begun attacking both the eyes and the brain of this system. The attacks against an installation near Pristina in Kosovo on the first night of operations were typical. Two large radar sites were hit, along with a command centre that could co-ordinate operations over a wide area.

Yugoslavia has a chain of radar installations across the country, and this is why the attacks are so extensive.

Nato is also striking at air bases and aircraft on the ground. On the first night of operations, Yugoslav jets - including their most valuable and advanced aircraft, the Russian-built Mig 29s - sought to intercept Nato warplanes.

The Western allies have the technological advantage

But their weaponry and training do not meet modern Nato standards. Nato claims that at least three Yugoslav jets were shot down.

Nato also needs to destroy Yugoslav surface-to-air missile systems. So far there is little evidence of the success of these attacks. Many missile sites are fixed, and thus easily identified from the air.

But mobile systems, like the potent SA-6 which is carried on a tracked launcher, or the portable SA-3, can be moved around and are easily hidden.

Defeating them requires the missile batteries to be actively engaged. Once the missile radar switches on, it can be spotted by specially equipped Nato warplanes. Air-to-surface missiles like the US HARM (High-speed anti-radiation missile) can then follow the radar beam back down to its source.

Of course, Nato jets can alternatively jam Yugoslav radar systems rather than destroy them.

A large number of specially equipped jamming or electronic counter-measures aircraft are operating out of Italy. They send out signals which disrupt the Yugoslav radar operators ability to track Nato jets.

Can Nato achieve air superiority, and will Yugoslav air defences remain a threat?

Nato could, with time, destroy Yugoslav defences

There is no doubt that given sufficient time, Nato can largely destroy Yugoslavia's air defences and achieve air superiority. But given its desire to minimise the risk of civilian casualties - as well as the weather and the terrain - this could take some time.

The silence of Yugoslavia's missile systems raises many questions. In part it may be a sign of the disruption caused by the initial Nato attacks. But it may also be a sign that Yugoslavia is holding back these weapons for later in the conflict.

Despite the attacks on the integrated defence system, Yugoslav air defences will still remain a threat. Individual missile batteries with their own self-contained radar systems can operate alone in a localised area. And at lower altitudes the Yugoslav Army's variety of anti-aircraft guns and shoulder-fired missile systems could prove threatening to Nato warplanes.

How will all this affect the situation on the ground in Kosovo?

Two struggles are in effect going on at the moment.

• One is the battle for air superiority being waged between the Yugoslav air defences and Nato warplanes.
• The other is the bitter struggle going on in Kosovo itself, between Kosovar Albanian fighters, and the Yugoslav Army and paramilitary police.

This last battle seems to be intensifying with many more refugees fleeing from their homes. Nato's problem is to link the two struggles to show Yugoslav units on the ground that they are in Nato's sights and that the bloodshed in Kosovo must end.