t;lt&quot; stands
         for &quot;light time&quot;) be the epoch at which the photons
         received at `et' were emitted from the surface point `spoint'.
         Note that the light time between the surface point and
         observer will generally differ from the light time between
         the target body's center and the observer.


         Target body's orientation
         -------------------------

         Using the definitions of `et' and `lt' above, the target body's
         orientation 