This afternoon at about 4:00 PM eastern US Time, a Boeing E-6B "Mercury" took off from Washington, DC. This is the U.S. "Looking Glass" - airborne National Command Post to command US nuclear forces worldwide in the event ground stations are all destroyed.
As seen in the flight radar image below, the aircraft took off, flew south into southern North Carolina, turned southeast out into the Atlantic Ocean, and turned its Transponder OFF.
The Boeing E-6 Mercury (formerly Hermes) is an airborne command post and communications relay based on the Boeing 707. The original E-6A manufactured by Boeing's defense division entered service with the United States Navy in July 1989, replacing the EC-130Q.
This platform, now modified to the E-6B standard, conveys instructions from the National Command Authority to fleet ballistic missile submarines (see communication with submarines), a mission known as TACAMO ("Take Charge And Move Out").
The E-6B model deployed in October 1998 also has the ability to remotely control Minuteman ICBMs using the Airborne Launch Control System.
The E-6B replaced Air Force EC-135Cs in the "Looking Glass" role, providing command and control of U.S. nuclear forces should ground-based control become inoperable. With production lasting until 1991, the E-6 was the final new derivative of the Boeing 707 to be built.
Looking Glass (or Operation Looking Glass) is the (historic) code name for an airborne command and control center operated by the United States.
In more recent years it has been more officially referred to as the ABNCP (Airborne National Command Post). It provides command and control of U.S. nuclear forces in the event that ground-based command centers have been destroyed or otherwise rendered inoperable.
In such an event, the general officer aboard the Looking Glass serves as the Airborne Emergency Action Officer (AEAO) and by law assumes the authority of the National Command Authority and could command execution of nuclear attacks.
The AEAO is supported by a battle staff of approximately 20 people, with another dozen responsible for the operation of the aircraft systems.
The name "Looking Glass," which is another name for a mirror, was chosen for the Airborne Command Post because the mission operates in parallel with the underground command post at Offutt Air Force Base.