What is a tightly-coupled GNSS/INS integration?

• A. The EKF fuses derived navigation solutions (position and velocity) from GNSS with INS data, not raw measurements.
• B. The EKF fuses only the GNSS-derived position solution.
• C. It uses INS data exclusively for attitude and ignores GNSS.
• D. The EKF fuses raw GNSS measurements (pseudorange, Doppler or carrier phase) directly with INS states for improved robustness, especially with limited satellites.

Answer: (D)

Tightly-coupled GNSS/INS integration means the navigation filter blends inertial measurements with GNSS observations at the measurement level, not by using a GNSS-derived position alone. In an EKF framework, the INS states—position, velocity, attitude, and sensor biases—are updated directly using raw GNSS observables such as pseudorange, Doppler, and carrier-phase measurements. This joint processing lets the filter estimate GNSS-related quantities inside the same system (like receiver clock error and carrier-phase ambiguities) and keeps navigation robust when satellites are limited or signals are degraded. Because the GNSS information is extracted from the raw measurements and fused with the inertial model, the solution remains reliable during short GNSS outages and under weak geometry, offering better continuity than relying solely on a GNSS position solution or using GNSS data without integrating the INS.