In INS/GNSS integration, what best differentiates loosely coupled from tightly coupled configurations?

• A. It uses raw GNSS measurements (pseudorange, doppler) directly in the filter.
• B. It uses the GNSS solution as a measurement to the INS, only when GNSS solution is available.
• C. It operates without GNSS input and without outages.
• D. It relies on GNSS only during outages.

Answer: (B)

Loosely coupled vs tightly coupled differ in how GNSS data enter the INS filter. In a loosely coupled arrangement, the GNSS subsystem provides a full position/velocity solution, and that solution is treated as a single measurement to update the INS. This update happens only when a valid GNSS solution is available. If GNSS can’t produce a fix, there’s no GNSS update and the INS relies on inertial data until a fix returns. This is why the option describing using the GNSS solution as a measurement to the INS, only when a GNSS solution is available, is the distinguishing characteristic.

The reason this differs from the other ideas is that using raw GNSS measurements directly inside the filter—pseudorange and Doppler—defines a tightly coupled system, which is not what this item is describing. Saying the system operates without GNSS input isn’t accurate for this distinction, and claiming it relies on GNSS only during outages doesn’t capture the typical loose-coupling behavior, where updates occur whenever a valid GNSS solution exists.