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FORT MEADE, Md. (Reuters) - The super-secret National Security Agency (NSA), which eavesdrops on communications worldwide as part of U.S. spying operations, opened its doors on Saturday to offer outsiders a rare glimpse of facilities that test antennas and print nuclear code books.

About 16,500 employees and their families were expected at the first ``Family Day创 since 1996 held at NSA headquarters, about 25 miles outside Washington, as the spy agency makes a greater effort to inform Americans about its mission. ``American people need an image of this agency so its identity is not a vacuum,创 NSA Director Lt. Gen. Michael Hayden told reporters.

There was the ``anechoic chamber,创 which looked like a science-fiction movie set, with blue foam spikes of different sizes poking out from the floor, walls and ceiling of the 42-foot-(13-meter)-high room. It is echo-free and one of five at NSA for testing antennas used to collect information. The antenna revolves on a high pedestal at one end, while information is transmitted to it from the other end of the room. The Air Force has a similar room that is big enough to test an entire airplane, an NSA official said. ``Each cone is sized to absorb a different frequency,创 said the official who spoke on condition of anonymity.

Areas opened to visitors were sanitized to ensure no classified material was lying around.

A 66,000-square-foot (6,100-square-meter) printing plant produces code books -- 160 million pages a year -- used in U.S. military operations. Nuclear code books for use in the event the United States orders a nuclear strike are produced about four times a month at the moment, compared with nearly every day during the 1991 Gulf War. The book is enclosed inside a sealed plastic pouch that has a unique pattern of pink splotches and is puffy because of air inside. If there is any change in the package such as the amount of air inside or the design of the splotches, which is videotaped before being sent out, the codes are considered compromised. ``We磍l go back and see if the splatter diagram is the same. We磛e got a video of the package,创 said Dan Shirko, chief of publishing. The nuclear code books are in the hands of nearly 1,000 people, mostly military officers such as pilots and sailors on submarines and battleships.

The printing plant also produces ``one-time pads,创 code books for military personnel in the field who are told to go to a specific page that has numbered lines of what to the untrained eye look like alphabet gibberish for that day磗 operation. That page is not reused.

TELEPHONE SECURITY

The NSA cracks codes of foreign communications, and designs products to make classified U.S. communication secure.

One new piece of equipment, which took six to seven years to develop, is a digital secure telephone that encrypts the user's voice and high-speed data almost instantly. ``By the time it takes to lift the handset and put it to your ear, it磗 secure,创 said Michael Jacobs, deputy director for information systems security. The telephone, called an STE for Secure Telephone Equipment, will over time replace the STU-3, for Secure Telephone Unit, which has been used since the mid-1980s -- the first one given to then-Secretary of State George Shultz.

There are now 300,000 to 400,000 STU-3 units in use worldwide. They require a key that, when turned, makes the telephone secure, although the sound quality has been described as similar to talking underwater. Also displayed were security equipment that identifies a user by the iris of the eye, face, fingerprint and voice that NSA produced with private industry partners. Once the computer recognizes the user by one of those methods, it allows access.

``The face system actually distinguishes between twins,创 said Dave Murley, technical director of identification and authentication research. The iris recognition, which scans the front of the eyeball, was the most accurate of the four methods, and voice recognition the least accurate, he said.

Reminders of the need for security were all around. In the cafeteria, one sign read, ``Don磘 spill the beans pardner, the steaks are too high. No classified talk.创