Which combination of data sources is used by the FMS to provide positions for drift checks?

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

Answer: (C)

Drift checks rely on cross-checking multiple independent position sources to verify the aircraft’s actual location and catch navigator errors. The FMS maintains a position estimate from its navigation data, GPS provides an accurate external position reference, and the INS offers continuous dead-reckoning position from inertial sensors. By using all three, the system can quickly detect discrepancies among them—any drift in the INS, GPS anomalies, or inconsistencies in the FMS data show up as a mismatch. This three-way combination gives the most reliable validation of position for drift checks, so the correct approach uses the FMS, GPS, and INS.