In a tightly coupled GNSS/INS EKF, which measurements are typically included in the observation vector?

• A. Only pseudorange measurements.
• B. Pseudorange, Doppler, and carrier-phase measurements; sometimes phase ambiguities if resolved; plus IMU data for prediction.
• C. Only carrier-phase measurements.
• D. Only Doppler measurements.

Answer: (B)

In a tightly coupled GNSS/INS EKF, you fuse multiple GNSS observables rather than relying on one type alone, and you rely on the IMU to predict motion between GNSS updates. The observation vector typically includes pseudorange (the range to each satellite), Doppler (range rate), and carrier-phase measurements. Carrier-phase is very precise but contains an integer ambiguity, which can be estimated or fixed if resolved. When ambiguities are resolved, they can be incorporated to improve accuracy. The IMU data drive the prediction step, providing high-rate motion information (accelerations and angular rates) that keep the state forecast accurate between GNSS measurements. This combination—GNSS observables for the update plus IMU data for prediction—yields robust, tight fusion. Limiting to a single GNSS measurement type or omitting the IMU input would miss the complementary information and predictive capability that tight coupling requires.