input:
kk         = equation number (1, 2, or 3)
gamat3     = -(Gauss**2)*Mass(ncentr) for zero mass spacecraft
plus time variation of gravitational constant Gauss
sbforc(kk) = kk coordinate of satellite / r**3

zsbmsc(1-3,j) = vector pointing from masscon j to satellite in inertial
frame (masscon coordinates transformed from central
body frame for this computation)
*****************************