PETERSON AIR FORCE BASE, Colo. - Lots of military secrets are hidden behind the gleaming walls of NORAD'S headquarters building, including this one: Just how do they get Santa's flight path onto their computer screens every Christmas Eve?

Tracking Santa's travels is a celebrated tradition at the North American Aerospace Defense Command, and it unfolded Friday for the 55th year.

NORAD insiders drop hints about how they do it - "ultra-cool, high-tech, high-speed digital cameras"; radar; satellites. and Canadian Forces fighter jets. They happily release a flurry of facts: They answered 74,000 phone calls and 3,500 e-mails from around the world last year asking for Santa's location.

But any inquiry into the technological particulars is met with a polite rebuff and a cryptic explanation involving the magic of Christmas.

NORAD Tracks Santa, the official name of the exercise, began in 1955 when a Colorado Springs newspaper ad invited children to talk to Santa on a telephone hotline. But a typo in the phone number connected dozens of kids to the Continental Aerospace Defense Command in Colorado Springs, the predecessor to NORAD.

The officers on duty played along and began passing on reports on Santa's progress. It's now a cherished ritual at NORAD, a joint U.S.-Canada command that monitors the North American skies and seas from a control center at Peterson Air Force Base.

"It's really ingrained in the NORAD psyche and culture," said Canadian Forces Lt. Gen. Marcel Duval, the deputy commander of NORAD, who pitches in to field French-language calls on Christmas Eve. "It's a goodwill gesture from all of us, on our time off, to all the kids on the planet."

It's also one of the few modern additions to the centuries-old Santa Claus story that have stuck, said Gerry Bowler, a history professor at the University of Manitoba and author of Santa Claus: A Biography.

NORAD takes an essential element of the story - Santa's travels on Christmas Eve - and looks at it through a technological lens, he said.

"It brought Santa into the 20th century," Bowler said.

And into the 21st century. NORAD Tracks Santa now has a Twitter account, a Facebook page, a YouTube channel, and apps for mobile phones, along with a website, www.noradsanta.org, and the phone line, 877-446-6723.

More than 13 million visitors went to the website last December. NORAD Tracks Santa had almost 530,000 "likes" on Facebook by Friday and more than 39,000 followers on Twitter.

It takes four months of planning to marshal the 1,200 volunteers, 100 phones, 30 laptops, and two big projection TV screens that the exercise requires, NORAD spokeswoman Joyce Frankovis said. All the labor is volunteer. Google, Verizon, Air Canada, defense contractor Booz Allen Hamilton, and others chip in.

The phone line is still at the core of NORAD Tracks Santa. Volunteers answer calls in two-hour shifts from 2 a.m. Mountain Time on Christmas Eve until 3 a.m. Christmas Day.

On Friday, volunteers answered calls and e-mails from two conference rooms in a building not far from NORAD's headquarters. In a separate room, a three-member team fired out tweets and Facebook updates, checking against a schedule marked with a secrecy warning that read, "Santa's Eyes Only." From vacation in Hawaii, first lady Michelle Obama helped to answer calls, too.

Civilian and military staff wore blue Santa hats with "Special Operations Elf" written on the white trim.

"It is tremendously fun," said Jim Jenista, NORAD's deputy chief for joint training exercises, who has volunteered to answer the phones for nearly a decade.

Sometimes the line is silent until a parent's voice encourages a shy child to speak up, Jenista said. Or "sometimes it's an overnight, and all the cousins are there. They get so excited," he said.

The phone volunteers don't get specific about Santa's ETA, Jenista said, but they do encourage children to get to bed soon. "We get e-mails from parents thanking us," said Mike Wilkerson, a network engineer.

Duval, the deputy commander, is careful to say that tracking Santa does not interfere with NORAD's other job, watching out for enemy threats to North America.

And neither he nor others think it's out of character for one of the world's most technologically sophisticated military commands to make time and space available for a venerable folk hero.

"It's a magic season," said Bowler, the Santa historian. ". . . It makes people childlike, which is wonderful."