Why are ballistic missile reentry vehicles difficult for radar to detect?

Ballistic missile reentry vehicles (RVs) are difficult for radar to detect primarily because they are engineered to have a very low radar cross section, meaning they produce a minimal echo when illuminated by radar signals.1

These vehicles were the original targets designed for low-observability, predating modern stealth aircraft by more than two decades. Beyond their inherent design, detection is further complicated by the presence of "confusion targets" released by the attacking missile, such as decoys, metallic chaff, and debris from the missile structure, which can mimic the radar signature of the warhead.1

To counter these challenges, ballistic missile defense systems must be highly sophisticated. They utilize large, high-power phased-array radars to track these small targets at long ranges and must employ advanced processing to distinguish the actual warhead from the surrounding clutter. While exoatmospheric engagement allows for earlier detection, it requires complex methods to identify the warhead among extraneous objects; conversely, endoatmospheric engagement relies on the fact that lighter decoys and debris slow down significantly upon hitting the atmosphere, allowing the heavier warhead to be more easily identified.1