 variable would be defined by loading a PCK file.


      et          is the epoch for which target surface data will be
                  selected, if the surface is modeled using DSK data.
                  In this case, only segments having time coverage that
                  includes the epoch `et' will be used.

                  [1,1] = size(et); double = class(et)

                  `et' is ignored if the target is modeled as an
                  ellipsoid.

                  `et' is expressed as TD