NED or “Compass” vs. ENU or “Math” Angles

For historical and common practice reasons, we use two different coordinate conventions in this manual: North-East-Down (NED) and East-North-Up (ENU). Sailors and pilots often use a NED coordinate system, while mathematicians and some IMUs use an ENU coordinate system.

The Cerulean ROVL coordinate system is described below and essentially passes the East-North-Up coordinate frame used by the internal IMU through with no transformations. For convenience we sometimes call this the “Math” coordinate frame. In this coordinate frame, the yaw (heading) angle is zero when the vehicle X-axis points due east, and the angle increases as the vehicle rotates counterclockwise (i.e., turns to port).

Sailors prefer a North-East-Down coordinate frame compatible with ordinary compasses, with the heading (yaw) angle zero when the vehicle X-axis points north, and the angle increasing as the vehicle rotates clockwise (turns to starboard).