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  Craig B. Markwardt, NASA/GSFC Code 662, Greenbelt, MD 20770
  UPDATED VERSIONs can be found on my WEB PAGE:


  Read and interpolate tabulated earth orientation parameters

Major Topics


Calling Sequence



  The procedure EOPDATA reads, interpolates and returns Earth
  orientation parameters used for precision earth-base astronomy
  ** NOTE: The user is responsible for downloading and maintaining an
  up-to-date file of earth orientation parameters from the
  International Earth Rotation Service. See below. **
  This interface is somewhat provisional. See OPEN QUESTIONS below.
  The values returned are described below. These descriptions are
  taken from the Explanatory Supplement to IERS Bulletins A and B.
    * PMX and PMY, the coordinates of the Celestial Ephemeris Pole
      (CEP) relative to the earth-fixed International Reference Pole
      (IRP). The x-axis is in the direction of the IERS Reference
      Meridian (IRM), the y-axis is in the direction 90 degrees West
      longitude. The time series of PMX and PMY is referred to as
      "polar motion."
      These are the coordinates of the earth rotation pole, as seen
      in an *earth-fixed* coordinate system. A station whose
      coordinates are given in earth-fixed coordinates referred to
      the ITRS can be transformed to the earth-fixed coordinates
      referred to the true rotation pole of date using the following
      matrix transformation:
            R_TRUE = RX(PMY) ## RY(PMX) ## R_ITRS
      where the matrices RX and RY are defined below.
    * UT1, the the rotation angle about the pole. It is related to
      the Greenwich mean sidereal time (GMST) by a conventional
      relationship (Aoki et al., 1982). It gives access to the
      direction of the International Reference Meridian IRM in the
      ICRS, reckoned around the CEP axis. It is expressed as the
      difference UT1-UTC. Thus, the value of UT1 is computed as:
          UT1 = UT1_UTC + UTC
      where UTC is the UTC time, expressed in seconds.
    * DPSI and DEPS are the offsets in longitude and obliquity of the
      celestial pole with respect to its direction defined using the
      conventional IAU precession/nutation theory. An a priori
      correction model is available in the IERS Conventions (1996),
      (McCarthy, 1996). The expressions to compute the nutation
      angles are:
          DEPS_TRUE = DEPS_1980 + DEPS ;; Nutation in obliquity
          DPSI_TRUE = DPSI_1980 + DPSI ;; Nutation in longitude
      where DPSI_1980 and DEPS_1980 are the nutation values
      determined from the IAU 1980 Nutation Theory; and DPSI_TRUE and
      DEPS_TRUE are the nutations to be used as arguments to further
      precession and nutation computations.
  For requested times which are between tabular values, a linear
  interpolation is performed. This is not exactly the correct
  procedure, and can result in errors of +/- 0.1 mas in the earth
  polar motion and 1 usec in UT1 (see McCarthy & Gambis 1997).
  The user is responsible for downloading and maintaining the earth
  orientation parameters file as supplied by the IERS. The format
  of the files is the "Final" EOP data ASCII format. They can be
  downloaded here:
      ftp://maia.usno.navy.mil/ser7/finals.all ;; from May 1976-present
      ftp://maia.usno.navy.mil/ser7/finals.data ;; from Jan 1992-present
  The user must place this file in a known location, and in *at
  least the first call*, this filename must be passed using the
  FILENAME keyword.
  EOPDATA will load the data once on the first call, and keep a
  cached copy for subsequent calls. On a daily basis the file will
  be reloaded in case the quantities have been updated from the
  server. A reload of data can be forced using the RESET keyword.
  The rotation matrices RX(T) and RY(T) mentioned above in relation
  to polar motion are:
      RX(T) =EQ= [[1,0,0], [0,cos(T),sin(T)], [0,-sin(T),cos(T)]]
      RY(T) =EQ= [[cos(T),0,-sin(T)], [0,1,0], [sin(T),0,cos(T)]]
      RZ(T) =EQ= [[cos(T),sin(T),0], [-sin(T),cos(T),0], [0,0,1]]
  and are meant to be applied to a vector R as, RX(T) ## R.
  How will the transition to a new IERS EOP series be accomplished?
  Using a keyword?
  Should there be a quality flag? The EOP file contains a
  "predicted" flag, and also there are rows which contain no value
  at all. These should probably be flagged somehow.


  JDUTC - a vector or scalar, the UTC time for which earth
          orientation parameters are requested, expressed in Julian
          Days. The value of the keyword TBASE is added to this
          quantity to arrive at the actual Julian date.


  PMX, PMY - the earth-fixed angular coordinates of the celestial
              ephemeris pole, measured in ANGUNITS units.
  UT1_UTC - the value of UT1 - UTC, expressed in seconds.
  DPSI, DEPS - the corrections to the IAU 1980 theory of Nutation,
                for nutation in longitude and obliquity, expressed in
                ANGUNITS units.

Keyword Parameters

  FILENAME - scalar string, on the first call, the name of the file
              from which earth orientation parameters will be read.
              Default value: (none)
  TBASE - a fixed epoch time (Julian days) to be added to each value
          of JDUTC. Since subtraction of large numbers occurs with
          TBASE first, the greatest precision is achieved when TBASE
          is expressed as a nearby julian epoch, JDUTC is expressed
          as a small offset from the fixed epoch.
          Default: 0
  ANGUNITS - scalar string, output units of angular parameters.
              Possible values are 'ARCSEC' or 'RADIAN'.
              Default value: 'RADIAN'
  RESET - if set, forces EOP file to be re-read.


See Also

  HPRNUTANG, TAI_UTC (Markwardt Library)


  Aoki, S., Guinot, B., Kaplan, G.H., Kinoshita, H., McCarthy, D.D.,
    Seidelmann, P.K., 1982: Astron. Astrophys., 105, 359-361.
  McCarthy, D. D. (ed.) 1996: IERS Conventions, IERS T.N. 21.
  McCarthy, D. \& Gambis, D. 1997, "Interpolating the IERS Earth
    Orientation Data," IERS Gazette No. 13,
    Instructions for high precision EOP data interpolation, not done
    in this procedure.
  Ray, J. & Gambis, D. 2001, "Explanatory Supplement to IERS
    Bulletins A and B,"
    Explains meanings of earth orientation parameters used and
    returned by this procedure.
  Definition of Final EOP data format

Modification History

  Written, 30 Jan 2002, CM
  Documented, 14 Feb 2002, CM
  Add default message, 01 Mar 2002, CM
  More robust handling of input file, 10 Mar 2002, CM

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