e light time corrections are not anti-symmetric, the correct 
      target-observer vector CANNOT be found by negating the light 
      time corrected position of the observer as seen from the 
      target body. 
 
 
      Target body's orientation 
      ------------------------- 
 
      Using the definitions of `et' and `lt' above, the target 
      body's orientation at et-lt is used.  The surface 
      normal is dependent on the target body's orientation, so 
      the body's orientation model must b