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Time - IAU Resolutions A4,C6

On the Establishment of a Relativistic Coherent Reference Frame - Send comments to: Dr. Dennis McCarthy.

RESOLUTION B3

• BIPM: Bureau International des Poids et Mesures 
• IAG: International Association of Geodesy 
• IERS: International Earth Rotation Service 
• IUGG: International Union for Geodesy and Geophysics

RESOLUTION B6

Discussion

• The CCDS has a working group "Application of General Relativity to Metrology".
• The International Astronomical Union (IAU) has a Working Group on Astronomical Standards and its Sub-WG on Relativity in Celestial Mechanics and Astrometry (RCMA).
• The International Association of Geodesy (IAG) has a number of Special Commissions and Special Study Groups, notably Special Commission 3 (Fundamental Constants).
• The International Earth Rotation Service (created by the IAU and the International Union of Geodesy and Geophysics) publishes every 3-4 years a monograph on the "IERS Conventions".

However a number of questions still have no satisfactory answer.

1. The IAU Resolution A4 (1991) sets a coherent frame to define reference systems centered at the barycenter of an ensemble of masses, but it is not sufficient for some applications (e.g. dynamical astrometry). On this matter, the RCMA Working group has explicitly considered the terms needed to expand the metric. But no decision is taken among the possible options.  It is subject to ambiguous interpretations. 
   • For example the considering a) of Recommendation IV, which implies TT for terrestrial metrology, has been misused: it is one reason why the IERS Conventions (1996) do not conform to the IAU Resolution. For this reason, although the scale unit of the spatial coordinates of the terrestrial reference frame is clearly defined by IAU and IAG resolutions, there is an uncertainty on its realization by the IERS, the ITRF. Recommendation IV itself, in introducing TT as a reference for geocentric ephemerides, did much to cause the ambiguity between TT and TCG.
2. Definitions and conventions for many quantities and concepts are needed:
   • First in the framework of Resolution A4 (1991): An example is the definition of TT to an accuracy of 1x10-17, for which the treatment of tides is to be specified.
   • Then in view of the extension of the metric: An example is the need for a relativistic definition of the geoid.
   • The meaning of some astronomical constants remains unclear: For example the IERS Standards (1992) provide two values for GME, then the IERS Conventions (1996) retain only one, but one which is not compatible with Resolution A4 (1991).
3. There is no conventional system of notations to allow unambiguous expressions in the framework of general relativity:
   to distinguish between proper and coordinate quantities to express the relationship between time coordinates (e.g. avoid "in seconds, TCB-TDB = LB x (MJD - 43144.0) x 86400" as in IERS Conventions (1996))
   to express the constants (e.g. avoid "GME in TT units" as in IERS standards (1992), or "The SI second, the basic unit of the TT time scale, is specifically assumed" as in IERS Conventions (1996))

B) Proposals
All the questions mentioned above are cross-disciplinary by nature. Therefore a Joint Committee should be established under the auspices of the BIPM (for metrology) and the IAU and IUGG (for space-time references in astronomy and geodesy).

The work of the Joint Committee should cover three areas:

1. Establish definitions and conventions to provide a coherent frame for all activities in space-time references and metrology.
   • Specify higher order metric
   • Define the basic concepts and quantities (e.g. TT, geoid, center of mass, Astronomical Unit, ...) in the framework of the new metric.
   • Define the fundamental constants appearing in the theory and establish procedures to obtain their numerical value.
2. Establish a uniform system of notations for quantities and units and apply it to future official texts and to rewrite past texts if necessary. 
   • This system should solve the questions related to the expression of proper / coordinate quantities the relationships between coordinate scales the expression of fundamental constants (e.g. the case of GME above)
   • Areas 1 and 2 should give rise to Recommendations which should be passed by all interested bodies, as simultaneously as possible.
3. Provide practical applications of the adopted definitions and conventions, to be at the disposal of interested users:
   • Models for the treatment of measurements (ranging, doppler, VLBI).
   • Procedures for the realization of time scales (e.g. transformation between proper and coordinate time) and for the transformation between reference systems (e.g. pulsar analysis).
   • Interpretation of the results (coordinates) of the different realizations of space-time references (e.g. terrestrial reference frames, solar-system ephemerides).
   • Interpretation of the results (determination of fundamental constants and other parameters) resulting from the different realizations of space-time references.
   • Interpretation of models which are based upon space-time references (e.g. precession, nutation...)
     

Draft Resolution

• the IAU Resolution A4 (1991) has set up a general relativistic framework to define reference systems centered at the barycenter of the solar system and at the geocenter,
• the Sub Working Group on Relativity in Celestial Mechanics and Astrometry, established by IAU Resolution C6 (1994), reports that relativity has to be taken into account for all astronomical and geodynamical observations but that the framework of IAU Resolution A4 (1991) is not sufficient for some applications, and that the current terminology should be changed to be consistent in the general relativistic framework,
• a consistent system of notations is desirable and should be used in all fields of astronomy, geodesy and metrology that deal with space-time references, noting that
• work on these matters is also being carried out in several other organizations of different types; in the BIPM (an intergovernmental organization), in the IAG (a scientific union), in the IERS (a service of IAU and IUGG),
• it is of utmost importance that all interested parties adopt consistent definitions and conventions in a coherent general relativistic framework,
• the BIPM has proposed a collaboration with the IAU to realize this goal, recommends that
• a Joint Committee of the BIPM and the IAU should be formed, its tasks being to establish definitions and conventions, to provide a coherent relativistic frame for all activities in space-time references and metrology at a sufficient level of uncertainty, to establish a uniform system of notations for quantities and units, and to develop the adopted definitions and conventions for practical application by the user,
• the IUGG should be invited to participate in this Joint Committee to ensure that a coherent system is agreed by the scientific community,
• the organizations taking part in the Joint Committee should adopt Resolutions or Recommendations, each following its own procedures, with the aim of having identical definitions, conventions and notations based on the conclusions of the Committee.