oint 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 `abcorr'.

               `trgepc' is expressed as seconds past J2000 TDB.