[R] spherical coordinates and quadratic regression

[Spencer Graves]

	  What functions are available to support working in spherical 
coordinates, e.g., transforming (x, y, z) on a unit sphere to 
(longitude, latitude) and vice versa?

	  More generally, is anything available on quadratic regression, i.e., 
obtaining a least squares solution to a system of n equations quadratic 
in 3 unknowns?  The sum of squares of residuals (SSE) can have either 
one or two local minima.  With polar coordinates, I can tell if a 
solution is unique and find the other solution if it is not.

	  I can write my own, but I'd rather use more standard utilities if 
they exist.

	  Thanks,
	  Spencer Graves