Overview
These are the conventions used by Autonomy.
Units
All units are in SI.
Angles are in radians.
Coordinates
World space
AKA global or absolute.
WGS 84 :
Axes: Latitude [decimal degrees], longitude [decimal degrees], altitude [metres]
Origin: Equator, IERS reference meridian (IRM) (prime meridian), mean sea level
https://en.wikipedia.org/wiki/World_Geodetic_System
Local space
North east down (NED):
Axes (Euclidean): North [metres], east [metres], down [metres]
Origin (usually takeoff point): Latitude of home position, longitude of home position, altitude of home position (e.g. the ground)
If the home position is on the surface of a horizontal plane:
down < 0m is above the plane
down == 0m is at the plane
down > 0m is below the plane
https://en.wikipedia.org/wiki/Local_tangent_plane_coordinates
Rotation
The coordinate axes follow the right hand rule. When the axis is pointing towards you and axes are fixed, the angle increases in the counterclockwise direction.
Which multiaxis rotation system is used in code is specified as a comment.