epoch at which an observation or remote 
      sensing measurement is made, and let et - lt (&quot;lt&quot; stands 
      for &quot;light time&quot;) be the epoch at which the photons received 
      at `et' were emitted from the body (we use the term &quot;emitted&quot; 
      loosely here). 
 
      The correct observer-target vector points from the observer's 
      location at `et' to the surface point location at et - lt. 
      The target-observer vector points in the opposite direction. 
 
      Sinc