The default TimeScale value depends on the current System value; if the current
TimeFrame system is
" Besselian epoch
" the default is
" , otherwise it is
Note, if the System attribute is set so that the TimeFrame represents Besselian Epoch,
then an error will be reported if an attempt is made to set the TimeScale to anything
other than TT.
Note, the supported time scales fall into two groups. The first group containing UT1, GMST, LAST and LMST define time in terms of the orientation of the earth. The second group (containing all the remaining time scales) define time in terms of an atomic process. Since the rate of rotation of the earth varies in an unpredictable way, conversion between two timescales in different groups relies on a value being supplied for the Dut1 attribute (defined by the parent Frame class). This attribute specifies the difference between the UT1 and UTC time scales, in seconds, and defaults to zero. See the documentation for the Dut1 attribute for further details.
" - International Atomic Time
" - Coordinated Universal Time
" - Universal Time
" - Greenwich Mean Sidereal Time
" - Local Apparent Sidereal Time
" - Local Mean Sidereal Time
" - Terrestrial Time
" - Barycentric Dynamical Time
" - Barycentric Coordinate Time
" - Geocentric Coordinate Time
" - Local Time (the offset from UTC is given by attribute LTOffset)
An very informative description of these and other time scales is available at http://www.ucolick.org/sla/leapsecs/timescales.html.
’t matter, there are potential problems in on-line data acquisition systems and in critical applications involving taking the difference between two times.