a   the equatorial earth radius
 f   the "flattening" parameter ( = (a-b)/a ,the ratio of the
     difference between the equatorial and polar radii to a;
     this is a measure of how "elliptical" a polar cross-section
     is).

e^2 = 2f - f^2 ,  or  e = sqrt(2f-f^2) .

a   = 6378137 meters
   (1/f) = 298.257224

1
    C =  ---------------------------------------------------
         sqrt( cos^2(latitude) + (1-f)^2 * sin^2(latitude) )

S = (1-f)^2 * C .

x = (aC+h)cos(lat)cos(lon)
   y = (aC+h)cos(lat)sin(lon)
   z = (aS+h)sin(lat)