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Resent-date: Tue, 27 Jun 2000 12:58:08 +0200 (MET DST)
Date: Tue, 27 Jun 2000 12:58:07 +0200
From: Tim Springer <tim.springer@aiub.unibe.ch>
Subject: [IGSMAIL-2904]: IGS ITRF97 realization
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IGS Electronic Mail      27 Jun 03:58:18 PDT 2000      Message Number 2904
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Author: Tim Springer


Dear IGS colleagues,

As announced in IGSMAIL #2899 the IGS has slightly modified the way it
realizes its terrestrial reference frame. Until GPS week 1065 the
station positions and velocities were directly based on the ITRF97 solution.
Starting with GPS week 1065 the IGS has started to use the IGS realization
of the ITRF97 reference frame. As explained in IGSMAIL #2899 the main reason
for this change is to ensure a better internal consistency of the IGS
products. Note, however, that the underlying reference frame is still ITRF97.
For the users of the IGS products this means that they can continue to use
the ITRF97 station positions without any problems.

This change should not have any effect on the IGS products in terms of
translation-, rotation-, or scale-changes. The designation for the IGS
realization of the ITRF97 reference frame is IGS97.

The main advantages for the users of the IGS products are that the IGS
realization of the ITRF97 reference frame is internally consistent and may
contain some stations which are not present in the ITRF97. The coordinates
and velocities of the 51 IGS reference stations may be found at the IGSCB
in the well known SINEX format:
  ftp://igscb.jpl.nasa.gov/igscb/station/coord/IGS00P04_RS51.SNX.Z

The complete set of stations coordinates and velocities may be found at:
  ftp://cddisa.gsfc.nasa.gov/gps/products/1046/IGS00P04.snx.Z

Every week the so-called "accumulated" solution is updated by adding the
latest solution from the IGS SINEX Combination. Those users who need the
coordinates and velocities of relatively new stations may obtain these from
the accumulated solution. The accumulated solutions are all realizations
of the ITRF97 and are fully consistent with the IGS final products.
The accumulated solutions may be found at:
  ftp://cddisa.gsfc.nasa.gov/gps/products/wwww/IGSyyPww.snx.Z
    where: wwww = GPS week
           yy   = Two digit year
           ww   = Week of the year
(notice that the IGS97 is in fact the accumulated solution for week 4 of
 this year)

One small problem which has not been resolved in this scheme is the inclusion
of regional solutions (e.g. EUREF). The IGS Reference Frame working group
is currently reviewing the possibilities of including regional solutions
for densification purposes.

Kind regards,
         Tim Springer
         IGS Analysis Center Coordinator


-- 

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 Tim Springer                      Astronomical Institute, University of Berne 
 springer@aiub.unibe.ch            Sidlerstrasse 5, CH-3012 Bern, Switzerland  
 http://www.aiub.unibe.ch/         Tel: +41 31 6318592    Fax: +41 31 6313869
 ftp://ftp.unibe.ch/aiub
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