Ground-Based Midcourse Defense (GMD) is the United States' anti-ballistic missile system for intercepting incoming warheads in space, during the midcourse phase of ballistic trajectory flight. It is a major component of the American missile defense strategy to counter ballistic missiles, including intercontinental ballistic missiles (ICBMs) carrying nuclear, chemical, biological or conventional warheads. The system is deployed in military bases in the states of Alaska and California; in 2018 comprising 44 interceptors and spanning 15 time zones with sensors on land, at sea, and in orbit. In 2019, a missile defense review requested that 20 additional ground-based interceptors be based in Alaska.