[Spice_discussion] Earth orientation accuracy and lack thereof

[Nat Bachman]

Folks,

NAIF has IMHO rather thoroughly documented the known limitations
of the IAU Earth rotation data provided in the IAU/IAG Working
Group Report. See for example p.7 of the "Lunar Earth PCK FK" tutorial
on the NAIF web site at URL

    ftp://naif.jpl.nasa.gov/pub/naif/toolkit_docs/Tutorials/pdf
    /individual_docs/25_lunar-earth_pck-fk.pdf

and the discussion in the NAIF "generic" text PCK itself, which
I've copied in part below. This discussion has been included
since the 2004 version of the generic text PCK.

I encourage SPICE users to read the pertinent documentation;
tedious as this may be, it's less work than solving mysteries
caused by unexpected data characteristics.

   -Nat Bachman (JPL/NAIF)

Nathaniel.Bachman at jpl.nasa.gov



Known Limitations and Caveats

      Accuracy
      --------

      In general, the orientation models given here are claimed by the
      IAU/IAG Working Group Report [1] to be accurate to 0.1 degree
      ([1], p.158). However, NAIF notes that orientation models for
      natural satellites and asteroids have in some cases changed
      substantially with the availability of new observational data, so
      users are urged to investigate the suitability for their
      applications of the models presented here.

      Earth orientation
      -----------------

      NAIF strongly cautions against using the earth rotation model
      (from [1]) for work demanding high accuracy. This model has been
      determined by NAIF to have an error in the prime meridian location
      of magnitude at least 150 arcseconds, with a local minimum
      occurring during the year 1999. Regarding availability of better
      earth orientation data for use with the SPICE system:

         Earth orientation data are available from NAIF in the form of
         binary earth PCK files. NAIF employs an automated process to
         create these files; each time JPL's Tracking Systems and
         Applications Section produces a new earth orientation parameter
         (EOP) file, a new PCK is produced. These PCKs cover a roughly
         10 year time span starting at Jan. 1, 2000. In these PCK files,
         the following effects are accounted for in modeling the earth's
         rotation:

            - Precession:                   1976 IAU model

            - Nutation:                     1980 IAU model, plus interpolated
                                            EOP nutation corrections

            - Polar motion:                 interpolated from EOP file

            - True sidereal time:

                   UT1 - UT1R (if needed):  given by analytic formula
                 + TAI - UT1 (or UT1R):     interpolated from EOP file
                 + UT1 - GMST:              given by analytic formula
                 + equation of equinoxes:   given by analytic formula

              where

                 TAI    =   International Atomic Time
                 UT1    =   Greenwich hour angle of computed mean sun - 12h
                 UT1R   =   Regularized UT1
                 GMST   =   Greenwich mean sidereal time

         These kernels are available from the NAIF web site

            http://naif.jpl.nasa.gov

         (follow the links to Data, generic_kernels, and PCK data) or

            ftp://naif.jpl.nasa.gov/pub/naif/generic_kernels/pck

         or via anonymous ftp from the server

            naif.jpl.nasa.gov

         The kernels are in the path

            pub/naif/generic_kernels/pck

         At this time, these kernels have file names of the form

            earth_000101_yymmdd_yymmdd.bpc

         The second and third dates are, respectively, the file's
         coverage end time and the epoch of the last datum.

         These binary PCK files are very accurate (error < 0.1
         microradian) for epochs preceding the epoch of the last datum.
         For later epochs, the error rises to several microradians.

         Binary PCK files giving accurate earth orientation from 1972 to
         2007 and *low accuracy* predicted earth orientation from
         2007 to 2037 are also available in the same location. See the
         aareadme.txt file at the "pck" URL above for details.

         Characteristics and names of the binary kernels described here
         are subject to change. See the "pck" URL above for information
         on current binary earth PCKs.