First, some background: A software product that I'm documenting allows the user to position things in three-dimensional space however they choose. The three-dimensional space has x, y, and z axes (or up/down, east/west, and north/sound directions, if you prefer). The things in this space have no defined size, but do have positions defined by their co-ordinates in Euclidean space, and orientations defined by two vectors: One "forward vector" that indicates the direction towards which the thing is facing, and another vector that is perpendicular to the first vector that defines the direction to the top of the thing from its center. (The direction to the top of the thing may potentially differ from the direction that is up, as a thing may occasionally be upside-down or otherwise rotated.) By knowing both vectors, the facing, pitch, and yaw of the thing can be calculated.
I need a name for the vector that points toward the top of the thing, so that I can write sentences like "The orientation of an emitter in three-dimensional space is defined by its forward and ____ vectors."
Calling it the "up vector" would be misleading, as it ceases to be up if the thing is rotated (for example) upside-down. I also thought of "anterior," but that usually means toward the front rather than toward the top. "Dorsal" could mean towards the top or towards the back, but I'd prefer to avoid that ambiguity.
Is there are word or short phrase that unambiguously means towards the top of something that has a defined top, and which would work in this context?