Baquba, Iraq ­ They sound like a loud mosquito buzzing above your head or a lawnmower rumbling in the distance. And you'll only see them by chance ­ day or night
- no matter how hard you look. But the machines producing those sounds play a key role in the war in Iraq and will do so in future conflicts around the world.

Unmanned aerial vehicles (UAV), as they are called, are the remotely piloted drones that prowl the skies above Iraq and Afghanistan, and if terrorists and insurgents haven't yet learned to fear them they should.

They give US military commanders real-time surveillance and intelligence images and data 24 hours a day, in most weather and atmospheric conditions. Some also kill, as Mussab al-Zarqawi, the former head of al-Qaeda in Iraq (AQI) learned in 2006.

Zarqawi, the object of a major manhunt by coalition forces, was killed while riding in a vehicle in Diyala province north of Baghdad by a Hellfire missile fired from a Predator UAV.

More recently, Predators in separate incidents killed two Saudi Arabian AQI leaders in Nineva province in March after military forces received intelligence on the vehicles in which the two travelled and the Predators' highly-sophisticated sensors and cameras detected them on the road.

The deaths of the two have unbalanced al-Qaeda efforts in the city of Mosul to forge a common-cause alliance with nationalist insurgent groups in the city for coordinated attacks on Iraqi and US forces.

US intelligence sources said the first Saudi killed was the key player in the outreach effort and the second, entering Iraq surreptitiously from Syria, was to be his replacement.

The Predator is the best known UAV. But two smaller UAVs are also in use: the Shadow and the Raven, which are non-lethal but vital to soldiers monitoring their area of operations.

"They haven't been able to arm (the Shadow) yet because of the weight, but we do a lot of coordination with the air weapons teams, the Apaches (attack helicopters)," Lieutenant Jason Siler said. "We're kind of the hunter and they're the killers."

Siler is with Delta Troop, 2nd Squadron, 1st Cavalry Regiment, attached to 4th Stryker Brigade Combat Team of the Army's 2nd Infantry Division. His unit flies four Shadows from Forward Operating Base (FOB) Normandy near the city of Baqubah.

One aircraft - which only performs reconnaissance, surveillance and intelligence-gathering tasks ­ is aloft at all times, giving the brigade real-time information and pictures of activity in their sector, such as movements of al-Qaeda members, suspect vehicles, possible weapons caches and hideouts or bird's-eye and closer views of terrain.

Siler says each Shadow is aloft for about six hours and features a digitally enhanced electro-optic camera with infrared capability that transmits images to the ground control station, which are then relayed to other receivers.

The aircraft is powered by 38-horsepower engine and flies at a speed of between 50-60 knots and at an altitude as low as 500 feet (160 metres). It weighs just 375 pounds
(170 kilos), has a 14-foot (4.5-metre) wingspan and is made of wood and composite materials. It's also painted grey, making it difficult to see.

Delta Troop's flight command centre is a container unit on the back of a Humvee. Two soldiers sit side by side in the unit in front of image panels, one "piloting" the UAV and the other operating its camera and other sensor systems.

"We're not rated as pilots," Sergeant Thomas Oberman, of Portsmouth, Virginia, said as he controlled a UAV in flight. "We have no illusions about that, but we're well-versed in air operations, weather, everything that comes into getting the bird up and on mission."

Oberman said the two men working the Shadow's joy stick and mission package switch off at the four-hour point.

"Yeah," said Corporal Andrew Currier, from Missoula, Montana. "You kinda go crazy looking at the same thing so we switch off to go crazy looking at something else."

The Shadow serves a brigade. For smaller units in the field, such as a battalion, there is also the Raven. It weighs just four and a half pounds and is 38 inches long.

The Raven comes in sections that fit into a single suitcase, with a hand-held controller and small image receiver. The Sparrow needs a hydraulic catapult for launch, but the Raven needs a man, his arm and a running start to toss it into the air once its tiny electrical engine is turned on.

The Raven can stay aloft for an hour. Its optics are not as sophisticated as that of the Shadow or Predator but while flying at an altitude of 1,000 feet it can still give a clear image of a man carrying a rifle. (dpa)