The default TimeScale value depends on the current System value; if the current TimeFrame system is "Besselian epoch" the default is "TT", otherwise it is "TAI". 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. This attribute specifies the difference between the UT1 and UTC time scales, in seconds, and defaults to zero. See the sun210DUT1 in SUN/210 for further details.
KAPPA --- Kernel Application Package