[Boost-users] Creating Boost unit system for astrodynamic equations

I am starting to convert code I have written using standard types (e.g. double) which calculates the orbits of objects in space. Common man's way of describing astrodynamic equations. The equations fall into two types: dimensionless and with dimensions. For those that have a dimension they fall into three types: Length (earth radii) Time (minutes) Angle (radians) So derived types would be: Velocity (radians/minute) Now that I have described what types are required I have a few questions: 1. Where is the length type for already defined for earth radii? 2. What advantage is there to creating my own unit system? It seems that my unit system would have: - earth radii - kg (useless in the equations) - minute - radians Stephen