In tightly coupled GNSS/INS systems, which information is used that is not used in loosely coupled systems?

• A. Raw GNSS measurements (pseudorange and carrier-phase) are integrated with the INS rather than relying only on a GNSS position solution.
• B. Only the GNSS position solution is used.
• C. Only Doppler measurements are used.
• D. GNSS signal strength is used for the navigation solution.

Answer: (A)

In a tightly coupled GNSS/INS system, the navigation filter fuses raw GNSS measurements directly with inertial data to estimate all states at once. This means pseudorange and carrier-phase observations are treated as measurements alongside the IMU data, and the filter solves for position, velocity, receiver clock bias, and IMU biases within a single estimation framework. Because the raw measurements carry information about distance to satellites and phase ambiguities, the system can maintain a reliable solution even when the GNSS geometry is poor or only a few satellites are visible. That capability—using the raw GNSS observations inside the state estimator—is what distinguishes tight coupling from loose coupling, where only the GNSS position (and sometimes velocity) from the receiver is used to update the INS.

The other options describe aspects of loosely coupled approaches or less direct data use: relying solely on the GNSS position solution omits the inside-the-filter use of raw measurements; using only Doppler or focusing on signal strength does not provide the integrated, state-estimating input that tight coupling relies on.