## Attributes which Qualify Time Coordinate Systems

Time coordinate systems require some additional free parameters to identify a particular coordinate system from amongst a broader class of related coordinate systems. For example, all TimeFrames are qualified by the time scale (that is, the physical process used to define the flow of time), and some require the position of the observer's clock.

In AST, these free parameters are represented by additional TimeFrameTimeFrame attributes, each of which has a default appropriate to (i.e. defined by) the setting of the main SystemSystem attribute. Each of these qualifying attributes may, however, be assigned an explicit value so as to select a particular coordinate system. Note, it is usually best to assign explicit values whenever possible rather than relying on defaults. Attribute should only be left at their default value if you “don't care” what value is used. In certain circumstances (particularly, when aligning two Frames), a default value for an attribute may be replaced by the value from another similar FrameFrame. Such value replacement can be prevented by assigning an explicit value to the attribute, rather than simply relying on the default.

The main TimeFrame attributes which qualify the System attribute are:

TimeScaleTimeScale
  This specifies the time scale.

LTOffsetLTOffset
  This specifies the offset from Local Time to UTC in hours (time zones east of Greenwich have positive values). Note, AST uses the value as supplied without making any correction for daylight saving.

TimeOriginTimeOrigin
  This specifies the zero point from which time values are measured, within the system specified by the System attribute. Thus, a value of zero (the default) indicates that time values represent absolute times. Non-zero values may be used to indicate that the TimeFrame represents elapsed time since the specified origin.

For further details of these attributes you should consult their descriptions in Appendix C and for details of the System settings for which they are relevant, see the description of the System attribute (also in Appendix C).

Note that it does no harm to assign values to qualifying attributes which are not relevant to the main System or TimeScale value. Any such values are stored, but are not used unless the System and/or TimeScale value is later set so that they become relevant.