Watchdog expresses concern over using US Navy interceptor for homeland missile defense 7 hours ago Can the SM-3 Block IIA interceptor be upgraded to counter evolving ICBM threats to the U.S. homeland without breaking the bank? The Government Accountability Office is unsure. (Ryan Keith/U.S. Army) In a report released this week, the GAO said the SM-3 Block IIA could “introduce considerable cost, schedule, and performance uncertainty to a program that has just entered initial production.” The Ground-Based Midcourse Defense, or GMD, system provides homeland defense against possible ICBM threats from North Korea and Iran, but the Missile Defense Agency has proposed using SM-3 Block IIA missiles as well as Terminal High Altitude Area Defense systems as part of an underlay to cover possible gaps in homeland defense as threats continue to evolve.