EOPDATA Name
EOPDATA
Author
Craig B. Markwardt, NASA/GSFC Code 662, Greenbelt, MD 20770
craigm@lheamail.gsfc.nasa.gov
UPDATED VERSIONs can be found on my WEB PAGE:
http://cow.physics.wisc.edu/~craigm/idl/idl.html
Purpose
Read and interpolate tabulated earth orientation parameters
Major Topics
Geometry
Calling Sequence
EOPDATA, JDUTC, PMX, PMY, UT1_UTC, DPSI, DEPS, $
/RESET, FILENAME=, ANGUNITS=, TBASE=
Description
The procedure EOPDATA reads, interpolates and returns Earth
orientation parameters used for precision earth-base astronomy
applications.
** 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).
DATA FILES and MAINTENANCE
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.
ROTATION MATRICES
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.
OPEN QUESTIONS
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.
Inputs
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.
Outputs
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.
Example
See Also
HPRNUTANG, TAI_UTC (Markwardt Library)
PRECESS, PREMAT, JPRECESS, BPRECESS (IDL Astronomy Library)
References
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.
http://maia.usno.navy.mil/conventions.html
McCarthy, D. \& Gambis, D. 1997, "Interpolating the IERS Earth
Orientation Data," IERS Gazette No. 13,
http://maia.usno.navy.mil/iers-gaz13
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,"
http://hpiers.obspm.fr/iers/bul/bulb/explanatory.html
Explains meanings of earth orientation parameters used and
returned by this procedure.
Definition of Final EOP data format
ftp://maia.usno.navy.mil/ser7/readme.finals
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