et body T at 
      epoch et+lt, where 'lt' is the one-way light time. 'lt' is the 
      solution of the light-time equation 
 
                | T(et+lt) - O(et) | 
         lt = ------------------------                            (3) 
                          c 
 
      Subtracting the geometric position of the observer, O(et), 
      gives the position of the target body relative to the 
      observer: T(et-lt) - O(et). 
 
                 SSB --&gt; O(et) 
                / |    *  
               / 