Appropriate UCD for (Cartesian) unit vectors on the celestial sphere

[James Tocknell]

Hi everyone

I'm currently defining UCDs for a survey we're ingesting, and came across
something that stumped me.  There are three rows representing the x,y,z
coordinates of the unit vector on the celestial sphere. There does not
appear to be anything appropriate to represent these columns (`pos.cartesian`
is close but doesn't include the spherical nature of this system). Any
suggestions for what UCD should be used?

Thanks
James
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