Definitions

“Apparent” angles are relative to the casing of the receiver (i.e., are in the receiver coordinate frame).

“True” angles are transformed into the world coordinate frame using an internal Inertial Measurement Unit (IMU).

“Heading” or “Yaw” is the direction in which the boat is pointed (assuming the receiver is mounted to the boat in the configuration show in the diagrams). "Heading" is always given in NED conventions. "Yaw" is always given in ENU conventions.

“Bearing” is the direction (in the horizontal plane) to the transmitter (or “target”), relative to the direction the boat is pointed or to the earth’s coordinate frame, as noted in the modifiers “true” or “apparent” given with the angle. Bearing is usually in NED conventions but we will always say for sure whether it is ENU or NED.

"Azimuth" is angle in the horizontal plane. Azimuth is only used in interface message definfitions and always is given in ENU conventions.

“Elevation” is the angle up or down from the receiver to the transmitter as measured from the horizontal plane, with positive up and negative down.

Angles in interface messages are always given in degrees.