If GNSS signals are unavailable and the navigation solution relies entirely on the inertial sensors, this mode is commonly referred to as:

• A. Holdover or dead reckoning
• B. Real-time correction
• C. Batch processing
• D. Differential correction

Answer: (A)

When GNSS signals are unavailable, navigation must rely on the inertial sensors alone. This mode is called holdover or dead reckoning. An inertial navigation system uses gyroscopes to measure orientation and accelerometers to sense motion, then integrates those measurements to propagate position, velocity, and attitude from a known initial state. Because the sensors have biases and noise, errors accumulate over time, causing the estimate to drift away from the true path. Holdover describes holding the last valid solution and continuing to update it with inertial data until GNSS returns. The other options involve corrections or processing tied to GNSS data, which isn’t applicable when GNSS is offline.