`Alpha Sierra Two, come in." It is 10.55 p.m. and the woman's voice at Garda control interrupts coffee and a video of operations with the real thing. There has been a robbery at an Xtra-vision shop in Castleknock 10 minutes earlier.

By 10.59 p.m. the helicopter is in the air. Without two extra passengers they can do it in less than three minutes.

The radio code for the Garda Air Support Unit helicopter, Alpha Sierra Two, is already well known to those who drive stolen cars around the city.

One youth is said to have pulled up to the pavement in a stolen BMW when he heard on his scanner that the helicopter was being scrambled. Gardai said he simply got out of the car and walked away.

Thursday had been a good day. Earlier they had been called to follow two people suspected of stealing a motorcycle. The suspects were arrested. The Kawasaki was returned to its owner. He hadn't even realised it had been stolen.

Flying over Castleknock, the sergeant in charge, Dermot Mann, and his colleague, Garda Joe O'Flynn, have more details of the suspects. On the ground the patrol-car has patched into the helicopter and is asking them to search an area of waste ground.

The weather has deteriorated and the thermal imaging camera is showing a murky grey picture on the screen. Anything that gives off heat is picked out as a white blob.

Some white glows are quickly identified as chimney stacks. Then the helicopter moves round to get a better view of a large white object. It could be two or three people.

The powerful night-sun searchlight is switched on. "It's a horse," three voices say in unison as the animal starts to run.

"Kilo Charlie One, there's nobody in the back of the graveyard," Sgt Mann says, talking to the unit on the ground.

Then a mist comes down and the helicopter is forced to return to base at Baldonnel. They are frustrated, but the weather rules.

In the five months since it started operations the Garda Air Support Unit has been involved in 1,115 tasks. One hundred and seven people have been arrested as a result of joint operations with Air Support and gardai on the ground. Seven missing people have been found and 65 vehicles recovered.

On a daylight flight yesterday it was possible to focus on one street, using both daylight camera and thermal imager.

The thermal image could show which car had parked most recently by the heat of the engine. Those houses that had their heating on could also be picked out.

The twin Squirrel helicopter accounts for most of the tasks, with the fixed-wing Defender aircraft, referred to as Alpha Sierra One, used for longer flying operations outside Dublin.

They have started compiling a video of successful operations for training gardai on how the air unit works. One of the videos shows the thermal imager picking out a white figure on a river bank as two other figures move towards it.

All three move away from the bank together. This was the rescue of a disturbed woman from the bank of the Liffey on a cold night last month.

Gardai were searching the banks after a 999 call that a woman had been seen in a distressed state. Located by the helicopter hovering over the Strawberry Beds, the gardai in the air talked their colleagues on the ground to the woman.

In a daylight operation on a Sunday morning a BMW is shown speeding down the M50, passing cars on the inside lane before hitting the verge and crossing the central reservation on to the opposite dual carriageway.

Two men jump out and run across fields, followed by the helicopter. A voice in the helicopter directs gardai into the housing estate. One of the youths is caught in a back garden. The other hides behind a shed until he is arrested. The garda on the ground gives a thumbs-up to the helicopter.

Fifteen men and one woman, four sergeants and 12 of garda rank, make up the unit, led by Supt Nacie Rice.

The helicopter and plane are flown by Air Corps pilots assigned to the unit. The camera is on a pod at the front of the helicopter. At the rear there is a night sun-torch, said to be powerful enough to light up a football pitch.

They work in shifts from 7 a.m. to 1 a.m., with people on call the rest of the time. Both aircraft are able to communicate with individual gardai, patrol-cars and stations.

An encrypted radio signal is used for secure contact with Garda units like the Emergency Response Unit. They do not patrol in the helicopter as it can only fly for hours on a tank of fuel. "We could be up for two hours and 20 minutes and then get a call and not be able to respond because we have to refuel."

Two videotapes, a master and a back-up, record operations from the sky. The master is sealed as evidence to be used in any court proceedings, and the copy can be shown to gardai on the ground in advance of a prosecution.

It is a different kind of policing. As the voices from the air, the unit gardai are not part of the camaraderie of a station, where a successful operation might be celebrated that night.

"There is an adrenalin rush from being responsible for someone getting caught that you know would have got away. You get the rush, but it's in a different form."

And, yes, the Garda Air Support Unit can see your face from the sky, if they are at the right angle. The camera can read number plates, also depending on the angle.

There have been complaints from people who may have been woken by the noise and light of a search at night.

"We reply to them in writing," Sgt Mann said. "Usually when we explain that there was a robbery in their area and we were looking for the suspects people have no problem."