NEW YORK - On a cloudless day, the NYPD helicopter soars over the city, its sights set on the Statue of Liberty.

A dramatic close-up of Lady Liberty's frozen gaze fills one of three flat-screen computer screens mounted on a console. The hundreds of sightseers clustered below are oblivious to the fact that a helicopter is peering down on them from a mile and a half away.

"They don't even know we're here," crew chief John Diaz says.

The unmarked helicopter's corporate, silver-and-white motif belies what is inside: an arsenal of sophisticated surveillance and tracking equipment powerful enough to stealthily read license plates, or even pedestrian's faces, from high above.

Police officials say the helicopter's sweeps of the city landmarks and other potential targets are invaluable to the NYPD's post-Sept. 11 mission of guarding against another attack. They also argue that keeping it an "unmarked ship" provides a see-but-not-be-seen advantage against lurking terrorists and ordinary bad guys alike.

"It looks like just another helicopter in the sky," said Assistant Chief Charles Kammerdener, who oversees the aviation unit.

Police Commissioner Raymond Kelly has said that "no other law-enforcement agency in the country has anything that comes close" to the surveillance helicopter, designed by a team of engineers at Bell Helicopter and computer technicians based on specifications of the NYPD.

The helicopter is named simply "23," for the number of police officers killed in the Sept. 11 attacks.

The decision to customize the helicopter at a cost of $10 million reflects a new frontier in cutting-edge counterterrorism technology at the nation's largest police department.

Elsewhere, the NYPD plans to spend tens of millions of dollars to strengthen security in the Lower Manhattan business district with a network of closed-circuit television cameras and license-plate readers posted at bridges, tunnels, and other entry points. It also has deployed hundreds of radiation detectors - some worn on belts like pagers, others mounted on cars and in helicopters, to detect possible "dirty" bombs.

Kelly envisions someday using futuristic "stationary airborne devices" similar to blimps to conduct reconnaissance and help detect chemical and biological threats.

Civil rights advocates are not as enthusiastic about the surveillance, arguing it reflects the NYPD's evolution into ad hoc spy agency. "From a privacy perspective, there's always a concern that 'New York's Finest' are spending millions of dollars to engage in Peeping Tom activities," said Donna Lieberman of the New York Civil Liberties Union.

Police insist the law-abiding have nothing to fear. "Obviously, we're not looking into apartments," Diaz said. "We only want to observe anything that's going on in public."

The helicopter's powers of observation reach far and wide, thanks largely to a robotic, high-powered camera mounted on a turret projecting from its nose like a periscope. The camera has infrared night-vision capabilities and a satellite navigation system that allows police to zoom in on a location by typing in the address on a computer keyboard.

The surveillance system can beam live footage to police command centers or even to wireless handheld devices.

During this sweep, the helicopter circumvented Manhattan, using the camera to look for signs of trouble. The decks of the Staten Island ferry terminal, the stanchions of the Verrazzano-Narrows Bridge, the giant air vents feeding the Lincoln Tunnel, the aviation fuel lines stretching toward John F. Kennedy International Airport - targeted in a plot uncovered last year - all passed inspection.

During the recent visit of Pope Benedict XVI, 23 patrolled the skies, at one point receiving a call from officers who had looked up and spotted a suspicious man with a camera on a Fifth Avenue rooftop near the pontiff's residence. Diaz radioed back that it was a false alarm.

"There was a modeling shoot going on," he said.