What is holdover mode in an INS, and when is it activated?

• A. Holdover mode uses the last known good state to continue navigation when GNSS is unavailable; drift grows with time and no external updates.
• B. Holdover mode uses GNSS to recalibrate and reduce drift.
• C. Holdover mode relies on stellar navigation.
• D. Holdover mode is only used during initialization.

Answer: (B)

Holdover mode is the self-contained INS navigation state used when GNSS updates are unavailable. In this mode, the system relies on its last good position, velocity, and attitude and continues to propagate the solution using the inertial sensors (gyros and accelerometers). Because those sensors accumulate biases and noise over time, the estimated position and orientation drift gradually as time passes without external corrections. Activation happens automatically whenever GNSS data is unavailable or unreliable, so the INS can maintain continuous navigation, and it switches back to normal aided navigation as soon as GNSS becomes available again.