What is the difference between coarse alignment and fine alignment of an INS?

• A. Coarse alignment uses the gravity vector to initialize attitude.
• B. Fine alignment uses rough initial attitude.
• C. Coarse alignment is performed after moving to compute attitude.
• D. Coarse aligns using initial attitude rough estimate from known location; fine alignment refines attitude using sensor data and gravity vector, often during stationary or low-movement conditions.

Answer: (D)

In INS alignment, you start with a rough orientation and then tighten it as you gather better data. Coarse alignment provides a quick, approximate attitude using a rough initial guess (often from the known location) and uses the gravity vector to establish the direction of down, helping set roll and pitch. Fine alignment then improves that estimate by continuously fusing sensor measurements—accelerometers, gyros, and gravity data—often while the platform is stationary or moving very slowly to minimize disturbances. This refinement reduces errors and biases, yielding a much more accurate attitude.

The main idea is that coarse alignment gets you a usable starting point, and fine alignment uses detailed sensor information to polish the attitude. Saying that coarse alignment is done after moving or that fine alignment uses a rough initial attitude goes against how these steps are typically performed, since coarse alignment relies on an initial rough estimate (from location) and fine alignment relies on refined sensor data under low-dynamics conditions.