This attribute controls how a TimeFrame behaves when it is used (by AST_FINDFRAME or AST_CONVERT) as a template to match another (target) TimeFrame. It identifies the time scale in which alignment is to occur. See the TimeScale attribute for a desription of the values which may be assigned to this attribute. The default AlignTimeScale value depends on the current value of TimeScale: if TimeScale is UT1, GMST, LMST or LAST, the default for AlignTimeScale is UT1, for all other TimeScales the default is TAI.

When AST_FindFrame or AST_CONVERT is used on two TimeFrames (potentially describing different time coordinate systems), it returns a Mapping which can be used to transform a position in one TimeFrame into the corresponding position in the other. The Mapping is made up of the following steps in the indicated order:

• Map values from the system used by the target (MJD, JD, etc) to the system specified by the AlignSystem attribute.

• Map these values from the target’ s time scale to the time scale specified by the AlignTimeScale attribute.

• Map these values from the time scale specified by the AlignTimeScale attribute, to the template’ s time scale.

• Map these values from the system specified by the AlignSystem attribute, to the system used by the template.

TimeFrame