On any other night, the North American Aerospace Defense Command (NORAD) is scanning the heavens for potential threats, such as last year's Chinese spy balloon.
But on Christmas Eve, at least 100,000 kids call in to inquire about Santa's location, with volunteers in Colorado Springs fielding questions like, "When is Santa coming to my house?" and, "Am I on the naughty or nice list?".
Millions more follow online in nine languages, from English to Japanese.
The story goes that NORAD's annual tracking started with a child's accidental phone call in 1955. The Colorado Springs newspaper printed a Sears advertisement that encouraged children to call Santa, listing a phone number.
A boy called. But he reached the Continental Air Defense Command, now NORAD, a joint US and Canadian effort to spot potential enemy attacks. Tensions had been growing with the Soviet Union, along with anxieties about nuclear war.
Air Force Colonel Harry W Shoup picked up an emergency-only "red phone" and was greeted by a tiny voice that began to recite a Christmas wish list.
Realising an explanation would be lost on the youngster, Shoup summoned a deep, jolly voice and replied, "Ho, ho, ho! Yes, I am Santa Claus. Have you been a good boy?"
Shoup said he learned from the boy's mother that Sears had mistakenly printed the top-secret number. Fifty calls a day followed, he said.
When a staff member jokingly drew Santa and his sleigh over the North Pole on the agency's map of North America, a tradition was born.
Some grinchy journalists have nitpicked Shoup's story.
In 2014, tech news site Gizmodo cited an International News Service story from December 1, 1955, about a child's call to Shoup.
"When a childish voice asked COC commander Col. Harry Shoup, if there was a Santa Claus at the North Pole, he answered much more roughly than he should — considering the season:
'There may be a guy called Santa Claus at the North Pole, but he's not the one I worry about coming from that direction,'" Shoup said in the brief piece.
In 2015, The Atlantic magazine doubted the flood of calls to the secret line, while noting that Shoup had a flair for public relations, and was concerned about morale during the Cold War.
Shoup died in 2009. His children told the StoryCorps podcast in 2014 that he'd received letters from all over the world thanking him for his sense of humour.
The tradition continues to be upheld, with Air Force Lt Gen Case Cunningham explaining that NORAD radars in Alaska and Canada are the first to detect Santa.
"That's when the satellite systems we use to track and identify targets of interest every single day start to kick in," Cunningham said. "A probably little-known fact is that Rudolph's nose that glows red emanates a lot of heat. And so those satellites track (Santa) through that heat source."
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