“Compass” vs. “Math” Angles

Sailors and flight control systems may use different coordinate systems. Sailors and pilots often use a North-East-Down coordinate system, while flight controllers may use other conventions such as East-North-Up.

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 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 left).

Sailors often 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 right).

For convenience, in addition to Math-coordinate outputs, the ROVL receiver outputs vehicle heading and bearing to the transmitter in a North-East-Down coordinate frame compatible with compass conventions. For convenience we call these “Compass” angles.