[IGSMAIL-1838] ITRF96 IGS Product Change

[AC]

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IGS Electronic Mail      Fri Mar 20  7:57:44 PST 1998      Message Number 1838
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Author: AC coordinator
Subject: ITRF96 IGS Product Change

Subject: ITRF96 IGS products change

Dear Colleagues,

Starting on March 1, 1998 (GPS Wk 0947; MJD 50873), according to the
recommendations of the 1998 AC Workshop, held at ESOC, Darmstadt, Germany,
Feb. 9-11, 1998, all the IGS products, including the IGS Rapid and Final
combined orbits/EOP, are based on ITRF96. This change of reference frame,
from ITRF94 to ITRF96, will introduce only very small discontinuities,
since ITRF96 reference frame is supposed to be (nominally) the same as that
of ITRF94.  More specifically, the following changes were estimated, based
on the ITRF94 and ITRF96 coordinates of the 13 ITRF stations used for the
ITRF realizations of IGS up to Feb 28, 1998 (GPS Wk 0846; MJD 50872):


(IGS(ITRF96)-IGS(ITRF94); epoch 1998.16 (March 1/98, Wk 0947, MJD 50873);
(the transformation parameters are consistent with the IERS conventions) 

    IGS Orbits/EOP    T1(cm) T2(cm) T3(cm) D(ppb)  R1(mas) R2(mas) R3(mas)
                                                    PM y    PM x   (-dUT)
    Parameter          0.0   -.1     .1    -.4    -0.21    -.01    -.22
    Sigma               .3    .3     .3     .4      .10     .10     .10

    Rates per year      .02  -.09    .02   -.07    -.02     .01     .01
    sigma               .06   .06    .06    .09     .03     .02     .03 

As see above the expected ITRF94/96 changes are indeed very small, in most
cases they can be considered statistically (and most likely also
practically) not significantly different from zero. However, the new
reference frame realization is expected to be much more precise and robust
than the previous ITRF94 realization of IGS. This is so since from March 1,
1998 the new ITRF realization of IGS is based on a much larger set of up to
47, highly precise and consistent, ITRF96 station positions and velocities,
provided by the ITRF section of IERS. The new 47 ITRF96 station set,
adopted by IGS for the ITRF96 realization (of IGS) is also available at the
IGS CB Archives (IN SINEX Format):

ftp://igscb.jpl.nasa/gov//igscb/station/coord/ITRF96_IGS_RS47.SNX.Z,

or in short version of SINEX format (SSC), (the same as the SINEX but
without the matrix, i.e. only the solution standard deviations are
included):

ftp://igscb.jpl.nasa/gov//igscb/station/coord/ITRF96_IGS_RS47.SSC.Z
 
If required, the above transformation should be well suited for all
IGS products. The transformations for individual AC solutions may vary
slightly, mainly due to observation/station weighting, geometry and
modeling. Some ACs have estimated and  included in their Wk 0947 summary
reports separate transformation parameters that are better suited for their
solutions. To assist IGS users to bridge the past and the current
discontinuities caused by the ITRF changes a simple ftn77 program,
transformation files and a brief description have been prepared and made
available on the following ftp site:

 ftp macs.geod.emr.ca
 cd pub/requests/itrf94_96

This program has already been made available for the previous ITRF (ITRF93 to
ITRF94) change on June 30, 1996. For more details on the much more
significant changes introduced on June 30/96, please consult:

ftp://igscb.jpl.nasa/gov//igscb/mail/igsmail/igsmess.1391          

For this, ITRF94 to ITRF96, change the program has also been enhanced to
(optionally) transfer the corresponding ERP files (in the IGS ERP format).
Also added were the transformation files ITRF94=>ITRF96 and ITRF96=>ITRF94.
both are based on the above transformation estimates. Below is an updated
brief program description which has also been put into the above directory.
_______________________________________________________



    ITRF TRANSFORMATIONS (trnfsp3n.doc)

A simple transformation program was written primarily to transform IGS sp3
orbits between different realizations of ITRF's used by IGS since 1992. The
program (trnfsp3n) can also be used to transform a station coordinate set
of  the same epoch, to and from different ITRF's. Optionally, an ERP file
(in the IGS ERP format) can be transformed, to and from different ITRF's. The
sp3 orbit/ERP transformations are  required for precise processing of data
spanning the ITRF changes. The following ITRF realizations have been
employed by IGS since 1994:

      ITRF92      Jan  2, 1994(Wk 0730) to Dec 31, 1994 (Wk 0781)
      ITRF93      Jan  1, 1995(Wk 0782) to Jun 29, 1996 (Wk 0859)
      ITRF94      Jun 30, 1996(Wk 0860) to Feb 28, 1998 (Wk 0946)             
      ITRF96      Mar 01, 1998(Wk 0947) to present

Using the above program, sp3 orbit (and ERP) files can be instantaneously,
as required, transferred into ITRF92/93/94/96 and vice versa. Since all IGS
sp3 orbit/ERP files contain epochs, the proper transformation of the date
is utilized.

The  ftn77 code source (trnfsp3n.f) has been put onto our ftp directory.


    PROGRAM USAGE:

to run the program use:

    trnfsp3n  inputfile(sp3)  outputfile  transfile [input(erp) output(erp)]

where the  'transfile' is a file containing appropriate transformation
parameters including rates and [input(erp) output(erp)] are the optional
ERP input and output files in the IGS ERP format. 

The following transformation files  have also been put into the same
directory:

     ITRF96.itr93             transforms itrf94-> itrf96
     ITRF94.itr93             transforms itrf93-> itrf94
     ITRF93.itr92             transforms itrf92-> itrf93
     ITRF92.itr93             transforms itrf93-> itrf92
     ITRF93.itr94             transforms itrf94-> itrf93
     ITRF94.itr96             transforms itrf96-> itrf94

For example, to transform the orbit file IGS09470.sp3/IGS09477.erp (in
ITRF96(of the current date)) into IGS09470.itr94/IGS09477.erp_94 in
ITRF94(of the current date) simply run:

 trnfsp3n IGS09470.sp3 IGS09470.itr94 ITRF94.itr96 IGS09477.erp IGS09477.erp_94 
 
or to transform the 13 ITRF94(1993.0) station coordinate set in the file
ITRF94.13 into ITRF96(1993.0) in ITRF96.out, simply run:

 trnfsp3n  ITRF94.13  ITRF96.out  ITRF96.itr94

The files ITRF94.13, ITRF96.out, and this write up (trnfsp3n.doc) were also put
into the directory for your convenience.


    PROGRAM LIMITATIONS:

All the above ITRF transformations are based on the Analysis Coordinator
reports published in the 1994, 1995, 1996 IGS Annual Reports, which are
applicable to the respective IGS ITRF realizations (the ITRF96/ITRF94
transformation will be included in the 1997 IGS Annual Report). Since the
above IGS estimates are very close to the nominal (official) transformation
values published by IERS, the above transformation files can also be used
for all the applications related to ITRF92/93/94/96 with little or no
precision compromise.

Only sp3 orbits are transformed, no corresponding transformation (i.e. no
correction is done to the corresponding ERP file), unless it is
specifically requested by the optional ERP input and output files as
demonstrated in the first example. Even when only an ERP file
transformation is desired,  the first two coordinate files must be
specified (e.g. "dummy files" such as the included ITRF94.13 & ITRF96.out)
along with the 'transfile', all must precede the ERP input and output files.
I.e.
  trnfsp3n ITRF96.out ITRF94.out ITRF94.itr96 IGS09477.erp IGS09477.erp_94

The epoch of the dummy file is then irrelevant to the ERP file
transformations as MJD's are used for the transformation epochs. The
original ITRF94.13 and ITRF94.out must then be identical in this example.

There are only a few syntax limitations, and they all pertain to the  station
transformations (they are imposed by the sp3 format) namely: The first line
must have an epoch (YYYY MM DD) in a free format starting after col #3, the
station coord (in mm, m, or km) must start with "P " or "V " in col. 1. sta
name of only 3 chars and coordinates must start in, or after col.6.

WARNING: Only ERP's after MJD 50243 (Wk 0857) is processed by the program,
since only on this date the IGS ERP files has been changed to the current
IGS ERP format version. If ERP transformation prior this date is needed,
the corresponding IF statement in the ERPRDWR subroutine will have to be
removed and the ERP format will have to be tested. 

The ftn77 source code of the program trnfsp3n contains HP UX system
routines IARGC() and IGETARG() which enable command line input. For
different systems/installations the local corresponding routines will have
to be substituted in the source code.


[Mailed From: Jan Kouba <kouba at geod.emr.ca>]