The principal U.S. radars composing this class are listed. While this chapter is focused on BMD, the MW and SSA missions are also addressed in the introduction because a given sensor may perform aspects of all three missions. In the United States, the Integrated Tactical Warning and Attack Assessment (ITWAA) system detects and tracks strategic ballistic missile threats to the United States. The current generation of early warning radars (EWRs) supporting the ITWAA mission employs active electronically scanned arrays (AESAs) operating in the ultrahigh frequency (UHF) band. While the United States has deployed geosynchronous satellites to detect missile launches from the short-wave infrared (SWIR) signature of the boost phase, EWRs are critical to verifying and characterizing the attack as well as estimating the impact points of the individual reentry vehicles (RVs). The first generation of missile warning radars employed fixed-beam search radars and mechanically scanned dish radars and was one of the first billion-dollar development projects undertaken by the U.S. defense establishment. The current generation of AESA EWRs provides superior traffic handling and reliability.