Subject: [IGSMAIL-1501] IERS Geocenter Analysis Campaign
From: Jim Ray
Date: Mon, 30 Dec 1996 12:50:21 PST

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IGS Electronic Mail      Mon Dec 30 12:50:21 PST 1996      Message Number 1501
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Author: Jim Ray
Subject: IERS Geocenter Analysis Campaign

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             INTERNATIONAL   EARTH   ROTATION   SERVICE   (IERS)
          SERVICE   INTERNATIONAL   DE   LA   ROTATION   TERRESTRE

                           CALL FOR PARTICIPATION

                                    IN AN

                           IERS ANALYSIS CAMPAIGN

                                      TO  

                    INVESTIGATE MOTIONS OF THE GEOCENTER

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Background
----------
Analyses of satellite tracking data have provided persuasive evidence that the
coordinate frame formed by the tracking stations attached to the Earth's crust
moves detectably relative to the Earth's center of mass.  This translational
motion, when viewed from a crust-fixed frame, is known as "geocenter motion"
and is caused by the mass movement of planetary fluids, primarily the
atmosphere and oceans.  It apparently has a total magnitude of up to a few
centimeters and consists of two components.  The tidal variations have mostly
diurnal and semi-diurnal periods and are predominantly caused by oceanic
motions; the non-tidal geocentric motions are thought to be due primarily to
atmospheric mass motions, ocean circulation, and surface water storage.

The IERS is charged with maintaining a geocentric terrestrial reference
frame, including the effect of the atmospheric and oceanic masses.  Currently
the International Terrestrial Reference Frame (ITRF) gives coordinates of
stations relative to the position of the Earth's center of mass averaged over
about 20 years of SLR observations.  In order to provide improved accuracy
for instantaneous geocentric coordinates, it has been proposed that the IERS
collect and distribute tabulated results for geocenter motions, analogous to
the Earth orientation service already provided by the IERS.

To better understand this phenomenon and the capabilities of the various
satellite-tracking techniques, the IERS Working Group on the ITRF Datum is
organizing this Analysis Campaign to collect appropriate products from
interested space geodetic data analysis centers and geophysical data analysis
centers, and to compare the results.  While it is likely that the results of
this Campaign will strongly influence any future IERS activity to regularly
monitor geocenter motions, participation in this Campaign does not obligate
a participant to contribute to any future operational activities.  This
Campaign does not request the collection of any new observational data not
already being collected for other purposes.  For further information on the
ITRF and the IERS Working Group on the ITRF Datum, please refer to the www
site http://schubert.ign.fr:8000/CIAG/WITRF/index.WITRF.html.

Areas of participation
----------------------
This Call for Participation seeks collaborators in the following areas.
Investigators are invited to participate in one or more areas.  In order
to facilitate intercomparison of results, guidelines have been developed
for certain common elements.  The guidelines document is available
anonymously at ftp://ray.grdl.noaa.gov/pub/gc-iers.guide.

1) Analysis of satellite-tracking data

   Observations of the Earth's center of mass are inferred by analyzing
   the dynamics of satellite orbits.  To date, the most thorough studies
   of geocenter motion have been made by researchers at the Center for
   Space Research using observations of LAGEOS, and historically these
   solutions have been used to realize the origin of the ITRF.  Recently,
   GPS and DORIS have demonstrated similar sensitivities for detecting
   geocenter motions, although different time resolutions probably apply.

   Satellite analysis results are sought from all techniques (not only
   SLR, GPS, and DORIS used by the IERS) to address either or both of
   the following types of geocenter motion:

   1.a) Tidal geocenter variations

   1.b) Non-tidal geocenter variations

   For the study of non-tidal geocenter motions, it is planned that the
   comparison campaign will cover about 2.5 years of observations with
   sampling intervals of 1 week or longer.  Data products to be compared
   will include time series of sets of site coordinates (using the SINEX
   format).

2) Analysis of atmospheric, oceanic, and/or other geophysical data

   In order to investigate the relationship between observed geocenter
   motions and the likely sources of geophysical excitation, analogous
   products are sought from analyses of atmospheric, oceanic, and/or
   other fluid mass movements:

   2.a) Tidal oceanic geocenter variations

   2.b) Non-tidal atmospheric geocenter variations

   2.c) Non-tidal oceanic geocenter variations

   2.d) Effects of other fluid motions on the geocenter

   Calculations of the integrated effects on the geocenter are sought
   for: tidal oceanic variations, non-tidal atmospheric mass motions,
   non-tidal oceanic currents, and/or any other geophysically significant
   fluid motions.  For the study of non-tidal geocenter motions, the
   comparison campaign will correspond to the same 2.5 year period as in
   1.b).

3) Comparison of analysis products

   The analysis products from 1) and 2) above will be compared and analyzed
   in parallel by several centers.

Publication of results
----------------------
Splinter meetings will probably be organized in conjunction with the EGS
XXII General Assembly in Vienna (21-25 Apr. 1997) and/or the Spring '97 AGU
Meeting in Baltimore (27-30 May 1997) to discuss the status of activities
among the Campaign participants.  It is expected that a special session will
be convened at the Fall '97 AGU Meeting in San Francisco (Dec. 1997) to 
present the final results of the Campaign.

Reports and analyses from all participants in this Campaign will be collected
and published as an IERS Technical Note.  If warranted, a group journal
paper(s) may also be recommended to summarize the significant results that
emerge.  Participants in this Campaign will not be under any obligation to
contribute to any such journal papers.

Schedule
--------
It is anticipated that the schedule of activities will be reasonably flexible.
However, for planning purposes the following schedule is suggested:

   01 Jan. 1997 -- Call for Participation released
   15 Feb. 1997 -- responses from interested participants due
   21 Apr. 1997 -- start of EGS XXII General Assembly, Vienna
   27 May  1997 -- start of Spring '97 AGU Meeting, Baltimore
   01 Jul. 1997 -- submittal of analysis products from satellite, atmospheric,
                   and oceanic data
   31 Oct. 1997 -- submittal of first round comparison results
   08 Dec. 1997 -- start of Fall '97 AGU Meeting, San Francisco
      later     -- review of results by IERS Directing Board

Response form
-------------
Investigators who are interested in participating in this Analysis Campaign
are requested to return the following reponse form by 15 Feb. 1997 to

   Jim Ray   chairman, IERS Working Group on the ITRF Datum
   e-mail:   [email-masked]

All participants will be included in an e-mail exploder list at
[email-masked].  The guidelines document for the Analysis Campaign
is available anonymously at ftp://ray.grdl.noaa.gov/pub/gc-iers.guide.

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Response to CALL FOR PARTICIPATION in IERS ANALYSIS CAMPAIGN to
            INVESTIGATE MOTIONS OF THE GEOCENTER

Agency or group:

Contact person(s):

E-mail address(es):

Areas of participation:
   1) Analysis of satellite-tracking data: Y or N (if Y, give technique)
      a) Tidal geocenter variations:       Y or N
      b) Non-tidal geocenter variations:   Y or N
   2) Analysis of atmospheric and/or oceanic effects on geocenter:
      a) Tidal oceanic:                    Y or N
      b) Non-tidal atmospheric:            Y or N
      c) Non-tidal oceanic:                Y or N
      d) Effects of other fluid motions:   Y or N
   3) Comparison of analysis products:     Y or N

Limitations on participation: (please list any restrictions)

Comments or suggestions:

Please return to [email-masked], preferably by 15 Feb. 1997
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[Mailed From: Jim Ray (NOAA 301-713-2850) <[email-masked]>]