Which coordinate frames are commonly used in integrated navigation and how are they related?

• A. Body frame, navigation frame (north-east-down or local-level), and Earth-centered Earth-fixed (ECEF); transforms connect body to navigation to ECEF using attitude matrices.
• B. Body frame only.
• C. Navigation frame only.
• D. Earth-centered inertial frame only.

Answer: (A)

Three frames are commonly used in integrated navigation: the body frame fixed to the vehicle, the local navigation frame (north-east-down or a similar local-level frame), and the Earth-centered, Earth-fixed frame. The vehicle’s orientation is described by rotation (attitude) matrices that connect these frames. Measurements from the IMU sit in the body frame, while GPS/GNSS data are typically in the ECEF frame. To fuse them, we rotate data between frames using attitude matrices. First, we rotate from body to the navigation frame to express the vehicle’s motion in local terms. Then, we rotate from the navigation frame to ECEF (or from ECEF to navigation) to place data in a global reference. The combination of these rotations gives a direct body-to-ECEF transformation when needed. The local-frame choice (north-east-down) is convenient for describing motion near the Earth’s surface, while ECEF provides a stable, global reference tied to the Earth’s center. This framework lets IMU, GNSS, and other measurements be represented consistently for accurate navigation.