those officers in pay grades O-1 to O-3, while field grade officers are those in pay grades O-4 to O-6, and general officers are those in pay grades of O-7 and above. Air Force officer promotions are governed by the Defense Officer Personnel Management Act of 1980 and its companion Reserve Officer Personnel Management Act (ROPMA) for officers in the Air Force Reserve and the Air National Guard. DOPMA also establishes limits on the number of officers that can serve at any given time in the Air Force. Currently, promotion from second lieutenant to first lieutenant is virtually guaranteed after two years of satisfactory service. The promotion from first lieutenant to captain is competitive after successfully completing another two years of service, with a selection rate varying between 99% and 100%. Promotion to major through major general is through a formal selection board process, while promotions to lieutenant general and general are contingent upon nomination to specific general officer positions and subject to U.S. Senate approval. During the board process, an officer's record is reviewed by a selection board at the Air Force Personnel Center at Randolph Air Force Base in San Antonio, Texas. At the 10 to 11-year mark, captains will take part in a selection board to major. If not selected, they will meet a follow-on board to determine if they will be allowed to remain in the Air Force. Promotion from major to lieutenant colonel is similar and occurs approximately between the thirteen year (for officers who were promoted to major early "below the zone") and the fifteen year mark, where a certain percentage of majors will be selected below zone (i.e., "early"), in zone (i.e., "on time") or above zone (i.e., "late") for promotion to lieutenant colonel. This process will repeat at the 16-year mark (for officers previously promoted early to major and lieutenant colonel) to the 21-year mark for promotion to full colonel. The Air Force has the largest ratio of general officers to total strength of all of the U.S. Armed Forces and this ratio has continued to increase even as the force has shrunk from its Cold War highs. Warrant officers Although provision is made in Title 10 of the United States Code for the Secretary of the Air Force to appoint warrant officers, the Air Force does not currently use warrant officer grades, and, along with the Space Force, are the only