[IGSMAIL-6401] Changes in the IGS SINEX products

[Paul Rebischung]

Dear Colleagues,

Taking advantage of the switch to IGS08/igs08.atx, some other
modifications were brought to the IGS SINEX combination process. These
changes will affect the products of the SINEX combinations starting with
week 1632.

The most important change concerns the IGS cumulative solutions
(IGSyyPww.snx). Until week 1631, they have been obtained by stacking the
operational IGS weekly combined solutions. But from week 1632 on, the
IGS cumulative solutions will consist in a stacking of:
- weekly solutions based on repro1 products for 1994.0-2008.0,
- weekly solutions based on operational products for 2008.0-present.
The new cumulative solutions will thus be based on a longer, homogeneous
dataset and should be significantly improved in comparison to the
previous ones. They will be consistent with the igs08.atx antenna
calibration set and aligned to the IGS08 reference frame.

More information about the new cumulative solution and the other changes
are given below. Please contact igs-rf at ign.fr if you have any further
question.

Best regards,

Paul Rebischung, Bruno Garayt



* New cumulative solution: Input data

  New combinations of the AC's reprocessed solutions for weeks 0730-1459
  and of the AC's operational solutions for weeks 1460-1631 were
  recently performed at IGN. The obtained weekly combined solutions
  (called "igb") were exhaustively compared and showed very little
  differences with the official ig1/igs combinations. The main goal of
  this work was to obtain a homogeneous set of weekly combined
  solutions, based on the latest IGN combination strategy and covering
  the whole IGS time period.

  The "igb" solutions as well as the corresponding residual and summary
  files will be posted as soon as possible at:
  ftp://igs-rf.ign.fr/pub/igb and
  ftp://igs-rf.ensg.eu/pub/igb.

  Starting with week 1632, the IGS cumulative solutions
  (IGSyyPww.snx) will now be obtained by stacking:
  - the "igb" solutions for weeks 0730-1631,
    (after application of station-specific corrections to account for
    the igs05.atx to igs08.atx updates - see IGSMail 6356)
  - the operational IGS weekly combined solutions from week 1632 on.

* New cumulative solution: Changes in the stacking process

  The main change in the stacking process is the addition of
  constraints that impose station velocities to be equal before and
  after a position-only discontinuity (due for example to an equipment
  change). In a near future, we will add constraints that will equate
  velocities of different stations within the same site.

* New discontinuity list:

  Associated with the new cumulative solution, a new discontinuity list
  based on the ITRF2008 discontinuity list has been compiled.
  Information about velocity discontinuities was also included. This new
  discontinuity list will be continuously updated as new breaks occur
  and is available at:
  ftp://igs-rf.ign.fr/pub/discontinuities/soln.snx
  ftp://igs-rf.ensg.eu/pub/discontinuities/soln.snx

  For users of the IGS08 reference frame, we would like to remind that
  a restriction of this discontinuity list to the IGS08 stations will
  also be maintained and is available at:
  ftp://igs-rf.ign.fr/pub/IGS08/soln_IGS08.snx
  ftp://igs-rf.ensg.eu/pub/IGS08/soln_IGS08.snx

* New time series:

  All time series of coordinates, residuals, combination statistics
  available at ftp://igs-rf.ign.fr and ftp://igs-rf.ensg.eu, as well as
  the corresponding figures have been replaced by new ones extracted
  from the "igb" combinations.

  The time series of stacking residuals (res/XXXX_igs_IGS.res) now refer
  to the new cumulative solution. But their method of calculation has
  also changed. Until week 1631, one line was added each week at the end
  of each file after the latest weekly solution had been added to the
  cumulative solution. Now, the whole time series will be entirely
  re-generated each week, so that all the records will be referred to
  the latest cumulative solution and that the series will be more easily
  interpretable.

* Other changes:

  - The transformation parameters from the AC solutions to IGS08
    published in the combination summaries (igsyyPwwww.sum) are now
    estimated using the IGS08 core stations available in the AC
    solutions. The numbers of IGS08 core stations available in the AC
    solutions are given in section 5-1 of the summaries.
  - The weekly combined solutions used to be rescaled by the variance
    factor of their Helmert comparison to the cumulative solution. This
    scaling will be discontinued since it has the drawback of introducing
    seasonal variations in the standard deviations of the combined
    parameters. Consequently, the standard deviations of all combined
    parameters (station positions, ERPs, apparent geocenter) will be
    about 5 times less than before and thus certainly too optimistic.
    Users should be aware that these standard deviations are mostly
    intended to track internal variations and not a measure of absolute
    accuracy.