Appendix – Coordinate Systems

Sensor Frame

The sensor coordinate frame is shown in the diagram below. Rotations (not shown) are right-hand rule, with pitch being rotation around the Y axis, and roll being rotation around the X axis. Pitch and roll are independent (uncorrelated). This North-East-Down (NED) coordinate frame matches the frame used by ArduSub and other flight controllers.

Sensor NED coordinate system. Note the cable penetrator axis is parallel to the -X axis.

Vehicle Frame

Vehicle Frame (or “ROV Frame” is standard North-East-Down (NED) for flight controls, with +X forward, +Y to port, and +Z to surface.

Standard NED vehicle frame. BlueROV Heavy shown.

Transforming Sensor Frame to Vehicle Frame.

Readings computed in Sensor Frame can be transformed in three axes as specified in the SET-SENSOR-ORIENTATION command. This can used to compensate the data outputs to correct for mounting positions and orientations so the data lines up with the vehicle axes (e.g., you mount the sensor rotated by 90 degrees from "normal" to get a better placement for the cable exiting the sensor head).

Right-Hand Rule

Right-hand rule is used to describe rotations around an axis in Cartesian coordinate system. In right-hand rule, you conceptually grasp an axis in your right hand, with your thumb laid along the axis pointing in the axis-positive direction. Your fingers curl around the axis, and they point in the direction of increasing (more positive Euler angles) rotation.