By the late 1950s, it was clear that existing radar technology was unable to detect, track, and guide U.S. surface-to-air missiles against multiple attacking enemy aircraft or missiles. To address this critical challenge, APL developed a “phased array” radar system for the Navy. The system was designed to provide the near-instantaneous scanning, tracking, and closed-loop guidance needed to defend against simultaneous aircraft and missile raids. Advances led to a prototype phase shifter—a key element of a phased array—along with the radar-beam control algorithms and software and the complex signal processing needed to discriminate real targets from environmental clutter and signal noise. By 1969, APL had built and tested a prototype phased array radar system known as AMFAR (advanced multi-function array radar). This system was the precursor to the AN/SPY-1A, a key enabler of the Navy’s Aegis Combat System. Today, the legacy of APL’s AMFAR lives on in the AN/SPY-1 radar and its successors, which provide the continuous radar coverage needed to defend the U.S. Navy, as well as the navies of Japan, Australia, Spain, and South Korea, against multiple, simultaneous aircraft and missile attacks.