Which positional data sources are provided by the FMS to check INS drift?

• A. FMS only
• B. GPS and INS
• C. FMS and GPS
• D. FMS, GPS, and INS

Answer: (D)

INS drift is detected by cross-checking multiple independent position sources to reveal accumulated error. The FMS uses its own computed position (from its navigation data), GPS provides an external reference, and the INS provides dead‑reckoning data. By comparing all three—FMS position, GPS position, and INS position—you can see discrepancies and quantify how far the INS has drifted. Relying on only two sources or a single source isn’t enough to reliably identify drift, because there’s no independent reference to validate the inertial and FMS solutions. Having all three positions available lets you continuously verify and correct the INS drift.