In the context of INS/GNSS integration, what is the difference between absolute navigation and relative navigation?

• A. Relative navigation estimates motion between two states or along a path.
• B. Absolute navigation estimates motion between two states or along a path.
• C. Absolute navigation estimates gravity vector.
• D. Absolute navigation estimates global position in a fixed reference frame; relative navigation estimates motion between two states.

Answer: (D)

The key idea is how the navigation output is framed: absolute navigation gives you the global position and orientation in a fixed world reference frame (like latitude, longitude, height, heading in a standard Earth frame). It anchors the solution to real coordinates on the Earth using external references such as GNSS. Relative navigation, on the other hand, provides the motion between states—how far and in what direction you moved, and how your attitude changed from one moment to the next—without tying those numbers to a global frame until you reference them later.

In an INS/GNSS system, GNSS measurements help establish absolute position in the fixed frame, while the inertial sensors continuously supply the relative changes, enabling smooth propagation between GNSS updates. The option about gravity vector isn’t what differentiates the two; it’s about whether you’re reporting an absolute position in a global frame or the incremental motion between states.