The Boeing E-4 Advanced Airborne Command Post, the current "Nightwatch" aircraft, is a strategic command and control military aircraft operated by the United States Air Force. The E-4 series are specially modified from the Boeing 747-200B for the National Emergency Airborne Command Post program. The E-4 serves as a survivable mobile command post for the National Command Authority, namely the President of the United States, the Secretary of Defense, and successors. The four E-4Bs are operated by the 1st Airborne Command and Control Squadron of the 595th Command and Control Group located at Offutt Air Force Base, near Omaha, Nebraska. An E-4B when in action is denoted a "National Airborne Operations Center".