Definition of Reference Frames Within SPAN

The reference frames that are most frequently used throughout the SPAN documentation are the following:

The Local-Level Frame (ENU)

The definition of the local level coordinate frame is as follows:

  • z-axis – pointing up (aligned with gravity)

  • y-axis – pointing north

  • x-axis – pointing east

Local-Level Frame (ENU)

The IMU Body Frame

The definition of the IMU Body frame is given by the physical axes of the IMU and represents how the sensors are mounted inside the IMU. If your IMU is enclosed, both the IMU Body frame axes and center of navigation are marked on the enclosure. Refer to IMU Technical Specifications, PwrPak7 Mechanical Specifications, CPT7 Mechanical Specifications or SMART7 Mechanical Specifications for illustrations of the IMU Body frame axes.

The axis marking on the CPT7 indicates the orientation of the X, Y and Z axis only. It does not indicate the center of navigation. For the location of the center of navigation, refer to Figure: CPT7/CPT7700 Center of Navigation.

The origin of this frame is not the enclosure center, but the center of navigation (sensor center).

IMU Body Frame Marking

The Vehicle Frame

The definition of the vehicle frame is as follows:

  • z-axis – points up through the roof of the vehicle perpendicular to the ground

  • y-axis – points out the front of the vehicle in the direction of travel

  • x-axis – completes the right-handed system (out the right-hand side of the vehicle when facing forward)

Vehicle Frame

The User Output Frame

The user output frame is an arbitrary frame, which can be optionally defined to transfer inertial output to a more useful position or reference frame.

By default, SPAN position and velocity output is given at the center of navigation of the IMU. To output position and velocity at a different location, specify a user offset translation using the SETINSTRANSLATION command.

By default, SPAN attitude output is the rotation from the Local-Level frame to the Vehicle frame. If attitude output is desired relative to another frame, specify a user offset rotation using the SETINSROTATION command.