What happens to the navigation solution when a satellite signal is temporarily lost in a tightly-coupled GNSS/INS?

• A. The solution relies on remaining satellites and INS; if the constellation becomes too small, the solution degrades toward dead reckoning until the signal returns and re-convergence occurs.
• B. The solution immediately recovers using previous state.
• C. The INS is disabled.
• D. The system stops.

Answer: (A)

In a tightly integrated GNSS/INS, the navigation state is constantly updated by fusing GNSS observations with high-rate inertial data. If satellite signals are temporarily lost, there are no GNSS updates to correct the estimates, so the system continues to propagate the state using the IMU data alone. This causes the accumulated errors in position, velocity, and attitude to grow. If some satellites remain, their measurements can still provide limited corrections, but with reduced geometry the updates are weaker. As a result, the solution gradually degrades toward dead reckoning—the INS-only propagation—until the GNSS signal returns and the observations re-enter the filter to re-converge the state to the true navigation solution.