Which measurements are typically used during the update step in an integrated navigation loop?

• A. GNSS measurements alone are used for the update.
• B. Accelerometer data alone are used for the update.
• C. GNSS measurements and accelerometer data are used during the update.
• D. Only gyroscope data is used during the update.

Answer: (C)

In the update step of an integrated navigation loop, you fuse measurements that relate directly to the current state to correct the predicted trajectory. GNSS provides absolute position and velocity references from satellites, giving global corrections to drift. Accelerometer data, captured at high rate, describe the body's specific force and how velocity is changing in the short term, helping tighten the dynamic consistency of the estimate and, when included in the update, aiding the observability of biases in the IMU.

Using both together makes the update robust: GNSS anchors the solution to the world, while accelerometer measurements inject high-rate motion information that continuously refines the state and suppresses drift, even when GNSS quality is imperfect. Relying on accelerometers alone would lead to rapid drift without a fixed reference, and relying on GNSS alone would miss the detailed motion dynamics captured by the IMU. Gyroscope data alone mainly informs orientation and does not provide the direct position correction that GNSS offers.