Passage: George Denis Patrick Carlin was born on May 12, 1937, in Manhattan, New York, the younger son of secretary Mary Carlin (née Bearey) and The Sun's advertising manager Patrick John Carlin. His father was an Irish immigrant from County Donegal, while his mother was an Irish-American. Carlin's maternal grandfather, Dennis Bearey, was an Irish immigrant who worked as an NYPD officer. Carlin recalled that his grandmother's maiden name was O'Grady, but it was changed to Grady before she reached the U.S. He joked that they "dropped the O in the ocean on the way here". He named his character on The George Carlin Show O'Grady as an act of homage to her. His parents separated when he was two months old because of his father's alcoholism. Mary raised Carlin and his older brother, Patrick Jr., on her own.Carlin said that he picked up an appreciation for the effective use of the English language from his mother, though they had a difficult relationship, and he often ran away from home. He grew up on West 121st Street, in a neighborhood of Manhattan he said he and his friends called "White Harlem" because that "sounded a lot tougher than its real name" of Morningside Heights. He attended Corpus Christi School, a Roman Catholic parish school of the Corpus Christi Church in Morningside Heights. He went to The Bronx for high school but, after three semesters, Carlin was expelled from Cardinal Hayes High School at age 15. He briefly attended Bishop Dubois High School in Harlem and the Salesian High School in Goshen, New York. He spent many summers at Camp Notre Dame on Spofford Lake in Spofford, New Hampshire, and regularly won the camp's drama award. Much later in life, he requested that a portion of his ashes be spread at the lake after his death.Carlin joined the United States Air Force and trained as a radar technician. He was stationed at Barksdale Air Force Base in Bossier City, Louisiana. He also began working as a disc jockey at radio station KJOE, in nearby Shreveport. Labeled an "unproductive airman" by his superiors,...

Question: What was the branch of the United States forces that Carlin joined?
Air Force