Which source is NOT listed as part of how the FMS determines position?

• A. GPS position data
• B. IRS heading data
• C. ADC true airspeed and altitude reference
• D. Magnetic sensor in the right wing

Answer: (D)

Position in an FMS comes from combining GPS data with inertial navigation data (from the IRS) and supporting air-data information. GPS provides accurate position, the IRS tracks movement and heading when GPS isn’t perfect, and the air data computer supplies true airspeed and altitude to help determine speed and vertical position. A magnetic sensor in the wing would only give magnetic heading information and does not contribute to calculating where you are. Because heading alone isn’t enough to determine position and magnetometer readings can be affected by interference, that sensor isn’t part of the FMS position solution.