srvr, spoint

   returns:

      trgepc   is the &quot;target surface point epoch.&quot; `trgepc' is defined
               as follows: letting `lt' be the one-way light time
               between the observer and the input surface point
               `spoint', `trgepc' is either the epoch et-lt or `et'
               depending on whether the requested aberration correction
               is, respectively, for received radiation or omitted.
               `lt' is computed using the method indicated by `ab