# 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 the boat is pointed (assuming the receiver is mounted to the boat in the configuration show in the diagrams).

“Azimuth” or “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.

“Elevation” is the angle up or down from the receiver to the transmitter as measured from the horizontal plane.

Important: Many angles in this document are expressed in mathematical conventional rotations referenced to the X axis (or “bow”) pointed East being 0 degrees. See “Compass vs. Math Angles” for more information.

Angles in interface messages are always given in degrees.

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