I didn't look at your equations, but if the radius of curvature of your curve gets larger than 500 m, then that could probably result in your error message. That would put the center of the radius more than 500 m from the origin, which is a no-no for SW.
Thanks for the response Jerry. The isn't a single radius of curvature because the curve isn't a circle. In the sense of analytical geometry, yes, the local radius of curvature (the inverse of the second derivative) might get rather large, but I haven't calculated it. From what I read in the book "SolidWorks 2011 Parts Bible", it looks as if the curve will be approximated by a spline, so I can't see why the local radius of curvature would matter.
I just discovered that SW2013 does conics. Now if I can just figure out how to get from eccentricity to rho, I'll be all set.