In the early 1970s, the Navy sought APL’s help to field a long-range cruise missile that could navigate heavily defended environments and strike critical targets. Such a weapon required a means to verify and adjust its location en route to a target. To solve this problem, APL engineers applied terrain contour matching (TERCOM) technology, which matches the missile’s onboard altimeter readings to elevation maps stored in its computer. APL developed algorithms to reliably predict what ground areas would provide correct matches. When greater accuracy was needed for conventionally armed Tomahawks, APL developed the performance prediction algorithms needed to apply digital scene matching area correlator (DSMAC) technology, which further increased accuracy by comparing images taken by an onboard camera to scenes stored in the missile computer. The conventional land-attack Tomahawk enabled by TERCOM and DSMAC became the world’s first long-range, autonomous, precision-guided weapon, and it has been a key element of the U.S. arsenal since the early 1990s. TERCOM and DSMAC remain operational on Tomahawk for use when GPS is not available.