What must a pilot use to determine their lateral position relative to the localizer?

To determine their lateral position relative to the localizer, a pilot must rely on onboard instruments. The localizer is part of the Instrument Landing System (ILS) and provides lateral guidance to the runway centerline during an approach. The onboard instruments, specifically the aircraft's navigation display and its associated indicators, show the aircraft's position in relation to the localizer signal.

When the aircraft is on approach to the airport, the onboard instruments will indicate whether the aircraft is too far left or right of the localizer. The pilot can then make the necessary corrections to align the aircraft with the runway centerline. This capability is critical for safe instrument approaches in low visibility conditions when visual landmarks are not available, making onboard instruments the primary tool for maintaining proper lateral navigation during localizer approaches.

Ground control instructions can provide additional situational awareness or instruction, but they do not directly show the pilot's lateral position with respect to the localizer. Similarly, while visual landmarks may assist in general navigation when conditions allow, they are not a reliable method for determining lateral position in relation to the localizer, especially in the context of instrument flying. A flight computer does not serve this purpose of lateral positioning; rather, it may be used for performance calculations or flight planning