Early on, BMEWS consisted of long range radars that could spot warheads coming over the north pole (from Russia). When SSBNs (ballistic missile carrying nuclear subs) entered the Russian arsenal in the 1970s, BMEWS had been augmented by satellites equipped with heat sensors that could detect the enormous amount of heat generated by a ballistic missile launch (or any large explosion, like an above-ground nuclear weapons test). These satellites cover the entire planet, while the radars only cover part of the Middle East.  In all, 23 of these DSP (Defense Support Program) satellites have been launched (the latest last Fall). The 2.3 ton DSP birds are  being replaced by SBIRS (Space-Based Infrared System), a network of four stationary orbit (like DSP) and 24 low orbit, heat sensing (infrared) satellites that will provide more detail than the DSP birds. SBIRS should be fully operational in another five years.