An NDB used in conjunction with an ILS is called a(n) ______.

An NDB, or Non-Directional Beacon, used in conjunction with an Instrument Landing System (ILS) is referred to as a compass locator. This term is specific to an NDB that is typically located near the runway of an airport and serves as a backup navigation aid or assists pilots in transitioning from en route navigation to the final approach phase of landing.

The compass locator operates in the low frequency (LF) range and provides a reference for pilots as they make their final approach, helping to confirm their position relative to the runway. In this role, it complements the precision guidance of the ILS, which provides both lateral and vertical guidance.

Other terms in the answer choices are not applicable in this context. For example, an ILS locator might imply a connection directly to the ILS, but that is not a standard term used in aviation. A VOR locator refers to a VHF Omnidirectional Range system which is a different type of navigation aid altogether and does not specifically pertain to ILS approaches. Directional locator could refer to devices that provide navigational guidance but does not specifically identify the function of an NDB as it relates to ILS. Thus, "compass locator" is the precise term used in aviation to describe