Unmarked chopper patrols New York City from high above

NEW YORK -- On a cloudless spring 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 monitors mounted on a console. Hundreds of sightseers 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," said crew chief John Diaz, speaking into a headset over the din of the aircraft's engine.

The helicopter's unmarked paint job belies what's inside: an arsenal of sophisticated surveillance and tracking equipment powerful enough to read license plates -- or scan pedestrians' faces -- from high above the nation's largest metropolis.

Police say the chopper's sweeps of landmarks and other potential targets are invaluable in helping guard against another terrorist attack, providing a see-but-avoid-being-seen advantage against bad guys.

"It looks like just another helicopter in the sky," said assistant police chief Charles Kammerdener, who oversees the department's aviation unit.

Police Commissioner Raymond Kelly has said that no other U.S. law enforcement agency "has anything that comes close" to the surveillance chopper, which was designed by engineers at Bell Helicopter and computer technicians based on NYPD specifications.

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

The $10 million helicopter is just part of the department's efforts to adopt cutting-edge technology for its counterterrorism operations.

The NYPD also plans to spend tens of millions of dollars strengthening 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.

Police have also deployed hundreds of radiation monitors -- some worn on belts like pagers, others mounted on cars and in helicopters -- to detect dirty bombs.

Kelly even envisions someday using futuristic "stationary airborne devices" similar to blimps to conduct reconnaissance and guard against chemical, biological and radiological threats.

Civil rights advocates are skeptical about the push for more surveillance, arguing it reflects the NYPD's evolution into an 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, executive director of the New York Civil Liberties Union.

Police insist that law-abiding New Yorkers have nothing to fear.

"Obviously, we're not looking into apartments," Diaz said during a recent flight. "We don't invade the privacy of individuals. We only want to observe anything that's going on in public."

The helicopter's powers of observation come from a high-powered robotic 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 automatically 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 hand-held devices.

"The commander on the ground can see what we're seeing," Diaz said.

On this flight, the helicopter used the camera to look for signs of trouble at several key transportation sites: the decks of Staten Island ferry terminal, the stanchions of the Verrazzano-Narrows Bridge, the giant air vents feeding the Lincoln Tunnel. All of them passed inspection.

Without leaving Manhattan airspace, the chopper also was able to get a crystal-clear picture of jetliners waiting to take off from LaGuardia Airport and to survey Kennedy International Airport's jet fuel lines, which were targeted in a plot uncovered last year.

The chopper has helped track down fleeing suspects, including a recent case of a gunman who had shot his wife in Queens. As officers on the ground worried about how to approach the suspect's car, the camera in the sky hovered overhead, peeked inside the vehicle and found that he had already shot and killed himself.

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

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