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D Suggested Time Scale Specification

  [Not part of formal DATExxxx agreement]

Use of the keyword TIMESYS is suggested as an implementation of the time scale specification. It sets the principal time system for time-related keywords and data in the HDU (i.e., it does not preclude the addition of keywords or data columns that provide information for transformations to other time scales, such as sidereal times or barycenter corrections). Each HDU shall contain not more than one TIMESYS keyword. Initially, officially allowed values are:
Coordinated Universal Time; defined since 1972.
Universal Time, equal to Greenwich Mean Time (GMT) since 1925; the UTC equivalent before 1972; see: Explanatory Supplement, p. 76.
International Atomic Time; ``UTC without the leap seconds''; 31 s ahead of UTC on 1997-07-01.
International Atomic Time; deprecated synonym of TAI.
Ephemeris Time, the predecessor of TT; valid until 1984.
Terrestrial Time, the IAU standard time scale since 1984; continuous with ET and synchronous with (but 32.184 s ahead of) TAI.
Terrestrial Dynamical Time; = TT.
Barycentric Dynamical Time.
Geocentric Coordinate Time; runs ahead of TT since 1977-01-01 at a rate of approximately 22 ms/year.
Barycentric Coordinate Time; runs ahead of TDB since 1977-01-01 at a rate of approximately 0.5 s/year.

For reference, see: Explanatory Supplement to the Astronomical Almanac, P. K. Seidelmann, ed., University Science Books, 1992, ISBN 0-935702-68-7, or
Use of Global Positioning Satellite (GPS) time (19 s behind TAI) is deprecated.

By default, times will be deemed to be as measured at the detector (or in practical cases, at the observatory) for times that run synchronously with TAI (i.e., TAI, UTC, and TT). In the case of coordinate times (such as TCG and TCB) and TDB which are tied to an unambiguous coordinate origin, the default meaning of time values will be: time as if the observation had taken place at the origin of the coordinate time system. These defaults follow common practice; a future convention on time scale issues in FITS files may allow other combinations but shall preserve this default behavior. The rationale is that raw observational data are most likely to be tagged by a clock that is synchronized with TAI, while a transformation to coordinate times or TDB is usually accompanied by a spatial transformation, as well. This implies that path length differences have been corrected for. Note that the difference TDB - UTC, in that case, is approximately sinusoidal, with period one year and amplitude up to 500 s, depending on source position. Also, note that when the location is not unambiguous (such as in the case of an interferometer) precise specification of the location is strongly encouraged in, for instance, geocentric Cartesian coordinates.

Note that TT is the IAU preferred standard. It may be considered equivalent to TDT and ET, though ET should not be used for data taken after 1984. For reference, see: Explanatory Supplement, pp. 40-48.

If the TIMESYS keyword is absent or has an unrecognized value, the value UTC will be assumed for dates since 1972, and UT for pre-1972 data.

Examples. The three legal representations of the date of October 14, 1996, might be written as:

DATE-OBS= '14/10/96'           / Original format, means 1996 Oct 14.
TIMESYS = 'UTC     '           / Explicit time scale specification: UTC.
DATE-OBS= '1996-10-14'         / Date of start of observation in UTC.
DATE-OBS= '1996-10-14'         / Date of start of observation, also in UTC.
TIMESYS = 'TT      '           / Explicit time scale specification: TT.
DATE-OBS= '1996-10-14T10:14:36.123' / Date and time of start of obs. in TT.

The convention suggested in this Appendix is part of the mission-specific FITS conventions adopted for, and used in, the RXTE archive, building on existing High Energy Astrophysics FITS conventions. See:
The VLBA project has adopted a convention where the keyword TIMSYS, rather than TIMESYS, is used, currently allowing the values UTC and IAT. See p. 9 and p. 16 of:

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