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         Gazette IERS Gazette IERS Gazette IERS Gazette IERS Gazette
                                     _______________________________________
         No 27, 28 January 1998     /
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         Title:  Nomenclature and use of UT-like arguments derived from 
                 astronomical or satellite geodesy observations
         Author: IERS
                

   With the increasing role of satellite geodetic determinations of Earth 
spin angles or angle rates that are related to universal time, such as length 
of day (LOD) or orientation angle referred to the satellite orbit, it is
useful to recall some basic characteristics of these UT-like arguments and
to recommend an unambiguous nomenclature.

   The Earth orientation parameter universal time, UT1, is defined by the 
time-varying angular direction of the zero meridian of the International
Terrestrial Reference System (ITRS) as expressed in the International
Celestial Reference System (ICRS). As a consequence, the name 'UT1' should 
be used only for those results that relate the orientation of the Earth to 
a celestial frame that has no rotation offset or rate relative to the ICRS
(within measurement error).

   As the primary realization of the ICRS, the International Celestial 
Reference Frame (ICRF), is based on Very Long Baseline radio Interferometry 
(VLBI) observations of extragalactic radio sources, the most direct method
to determine UT1. Other techniques (e.g., Lunar Laser Ranging, LLR) may
estimate the Earth's orientation relative to a celestial frame which can
be related to ICRS. Keeping in mind that the ICRF is now realized at the
level of about 0.020 mas and 0.005 mas/year in offset and rate (or
equivalently, 0.0013 ms and 0.0003 ms/year), the significance of their
contribution to universal time will be limited by the quality and stability
of the frame tie.

   In the case of artificial satellite techniques, the celestial frame 
orientation can be considered as being attached to the longitude of the 
orbital ascending node. The latter is influenced by geophysical phenomena 
and other complex processes that cannot be modelled completely. As a result,
the UT-like parameters derived from satellite observations have time varying
systematic errors that reflect the unmodelled part of the orbit node motion.
Nevertheless, satellite-derived determinations of UT or LOD are useful for
densifying or extending to recent dates the time series of VLBI-derived UT1,
provided that appropriate care of the error spectrum of the satellite
results is taken.
   
   In this situation, the IERS recommendations are as follows.
   
   . The designation "UT1" be reserved exclusively for those
     measurements which are referred to a celestial reference system
     that has no rotational rate relative to the ICRS, in particular
     VLBI observations of extragalactic compact radio sources.

   . The UT-like quantity derived by satellite techniques, directly or 
     from the integration of length of day measurements should be referred 
     to generically as "UT ", but not as "UT1".

   . When using time series of satellite-derived determinations of UT in 
     conjunction with series of UT1, one should take proper account of the 
     fact that their errors are highly correlated in time and therefore 
     should not be modelled as temporally independent measurements.

   . In all IERS publications, tables which display UT1 and/or UT
     quantities will have appropriate column headings. Tables which
     display UT results will be accompanied by a notation which
     briefly explains the distinction from UT1.
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