On May 7, the first of the new SBIRS (Space-Based Infrared System) early warning satellites was launched. SBIRS will replace BMEWS (ballistic missile early warning system), a half century old system using radars and satellites to monitor the planet for ballistic missile launches (specifically ICBMs, but any large missile launch is detected.) 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 was augmented by DSP (Defense Support Program) 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).
The BMEWS satellites covered the entire planet, while the radars only covered most of the northern hemisphere. In all, 23 DSP satellites have been launched (the latest four years ago). The 2.3 ton DSP birds are being replaced by SBIRS, a network of four stationary orbit (like DSP) and 24 low orbit, heat sensing (infrared) satellites that will provide more detail than the DSP/SBIRS birds. The first of the low orbit sensors went up five years ago.
SBIRS should be fully operational in another two years. That's three years later than the original schedule (drawn up in the late 1990s.) SBIRS is more reliable, accurate and sensitive than the older system. It's also more expensive, behind schedule and over budget. There have also been quality control problems.