[IGSMAIL-7399] Upcoming switch to IGS14/igs14.atx

Paul Rebischung Paul.Rebischung at ign.fr
Wed Dec 21 05:39:07 PST 2016


Dear colleagues,

The IGS will soon adopt a new reference frame, called IGS14, as the basis of its
products. IGS14 is closely related to ITRF2014, released in January 2016. An
updated set of satellite and ground antenna calibrations, igs14.atx, will also
become effective at the same time and should be used together with IGS14. The
IGS intends to switch from IGS08/igs08.atx to IGS14/igs14.atx starting with
products of GPS week 1934 (29 January 2017). The IGS08/igs08.atx framework has
been in effect since GPS week 1632 (17 April 2011) and was also used for the
second IGS reprocessing campaign (repro2) back to 1994.

The following files can already be downloaded from either
ftp://igs-rf.ign.fr/pub/IGS14 or ftp://igs-rf.ensg.eu/pub/IGS14:
- IGS14.snx                  : IGS14 SINEX file
- IGS14.ssc                  : IGS14 SINEX file without covariance matrix
- IGS14.png                  : map of the full IGS14 network
- psd_IGS14.snx              : post-seismic deformation models to be used with
                               IGS14
- soln_IGS14.snx             : discontinuity list to be used with IGS14
- IGS14_core.txt             : list of IGS14 core stations
- IGS14_core.png             : map of the IGS14 core network
- igs14_1926.atx             : latest version of igs14.atx (will be updated, if
                               needed)
- ITRF2014_to_IGS14.txt      : position offsets applied to the ITRF2014
                               coordinates of IGS14 stations affected by ground
                               antenna calibration updates
- igs08_to_igs14_offsets.txt : position offsets estimated for all IGS stations
                               affected by ground antenna calibration updates
- lat_models.txt             : latitude-dependent models of position offsets
                               expected from ground antenna calibration updates

Below, you will find some details about the definition and use of IGS14 and
igs14.atx. IGS14 and igs14.atx were defined using a similar strategy as IGS08
and igs08.atx. Therefore, additional useful information can be found in the
IGS08/igs08.atx documentation:
- [IGSMAIL-6354] Upcoming switch to IGS08/igs08.atx
  (https://igscb.jpl.nasa.gov/pipermail/igsmail/2011/006346.html)
- [IGSMAIL-6355] Upcoming switch to IGS08/igs08.atx - Details on igs08.atx
  (https://igscb.jpl.nasa.gov/pipermail/igsmail/2011/006347.html)
- [IGSMAIL-6356] Advice for coping with the change to IGS08+igs08.atx
  (https://igscb.jpl.nasa.gov/pipermail/igsmail/2011/006348.html)
- IGS08: the IGS realization of ITRF2008
  (http://doi.org/10.1007/s10291-011-0248-2)
- Absolute IGS antenna phase center model igs08.atx: status and potential
  improvements (http://doi.org/10.1007/s00190-015-0876-3)

Also note that, starting with the release of GPS week 1934 (IGS17P05.snx), the
IGS cumulative solution will be replaced by a new version based on:
- the weekly combined repro2 solutions (ig2) of GPS weeks 0730-1831,
- the weekly combined operational solutions (igs) of GPS weeks 1832-present,
- a revised discontinuity list, consistent with the ITRF2014 discontinuity list,
  which will replace the current list at:
    ftp://igs-rf.ign.fr/pub/discontinuities/soln.snx
    ftp://igs-rf.ensg.eu/pub/discontinuities/soln.snx
This new IGS cumulative solution will also incorporate the ITRF2014 post-seismic
deformation models. It will be made consistent with the igs14.atx ground antenna
calibrations and aligned to the IGS14 reference frame.

Please send any further questions to:
- igs-rf at ign.fr for the reference frame aspects,
- schmid at tum.de for the antenna calibration aspects.

Best regards,

Paul Rebischung, Ralf Schmid, Tom Herring



* Antenna calibration updates

  - Satellite antenna calibrations:
      Because of recent orbit modeling changes (Earth radiation pressure,
      antenna thrust), the scale of the IGS products currently differs from the
      ITRF2008 scale by about -0.3 ppb. igs14.atx therefore contains new radial
      antenna phase center offsets (z-PCOs) for all GPS and GLONASS satellites.
      These offsets were obtained from time series of z-PCO estimates,
      consistent with the ITRF2014 scale, derived from the daily repro2
      solutions of seven IGS analysis centers. Note that the z-PCO time series
      were trend-corrected to epoch 2010.0 before computing weighted averages.
      The igs14.atx satellite antenna z-PCO values are therefore expected to
      give access to the ITRF2014 scale at epoch 2010.0.
      Satellite-specific x- and y-PCOs from pre-flight calibrations were
      additionally introduced in igs14.atx for the GPS Block IIR satellites.

  - Ground antenna calibrations:
      For 17 antenna types, additional robot calibrations provided by different
      institutions could be added to igs14.atx. Moreover, 19 type-specific phase
      center corrections were updated with the results of recent individual
      calibrations. As also the AOAD/M_T antenna calibration got updated, all
      converted calibrations slightly changed. Detailed antenna names can be
      found in the header of igs14.atx.

* IGS14 definition and use

  - Station selection:
      IGS14 is a subset of 252 stable, well-performing IGS stations from
      ITRF2014. Station stabilities were assessed from the ITRF2014 results
      (WRMS of residual time series, formal errors of ITRF2014 coordinates
      propagated over the expected lifetime of IGS14). Compared to IGb08, IGS14
      contains 65 new stations, mostly in previously sparsely covered areas. On
      the other hand, 48 previous IGb08 stations did not meet the IGS14
      selection criteria.

  - Coordinate corrections due to ground antenna calibration updates:
      While ITRF2014 coordinates are consistent with the igs08.atx set of
      antenna calibrations, IGS14 needs to be consistent with the updated
      igs14.atx set. The impact of the igs08.atx to igs14.atx ground antenna
      calibration updates on the ITRF2014 station coordinates was therefore
      assessed from differential PPP analyses, and the estimated corrections
      were applied to the ITRF2014 coordinates of the affected IGS14 stations
      whenever exceeding 1 mm in either direction. The coordinate corrections
      actually applied are listed in ITRF2014_to_IGS14.txt.

  - Discontinuities:
      IGS14 station coordinates are generally represented by piecewise linear
      functions, i.e., several position+velocity sets valid over successive time
      intervals. Each set is denoted by a solution number (soln) whose validity
      period is given in the discontinuity file soln_IGS14.snx. The
      discontinuity list in soln_IGS14.snx is based on the ITRF2014
      discontinuity list but also includes recent discontinuities and will be
      continuously updated as new breaks occur. Because IGS14 does not contain
      coordinates for solns starting after 15 February 2015 (end of ITRF2014
      input data), stations affected by such recent discontinuities cannot be
      used as reference frame stations anymore.

  - Post-seismic deformation models:
      Like in ITRF2014, the reference coordinates of some IGS14 stations include
      post-seismic deformation models in the form of exponential and/or
      logarithmic functions. These models are given in psd_IGS14.snx. More
      details on their application can be found at:
        http://itrf.ign.fr/ITRF_solutions/2014/psd.php
      The IGS14 positions+velocities of the affected stations cannot be used
      without the post-seismic deformation models.

  - IGS14 core network:
      For the purpose of aligning global GNSS solutions to IGS14, a well-
      distributed sub-network of IGS14 stations, called IGS14 core network, was
      designed. It is composed of 51 clusters of stations (i.e., 51 primary
      stations, each with possible substitutes; see IGS14_core.txt) selected to
      ensure a homogeneous global distribution and the best possible
      temporal stability of the core network.

* Impact on user results

    The impact of the switch to IGS14/igs14.atx on station positions estimated
    using the IGS products will be twofold:

  - Reference frame change:
      The alignment of the IGS products to IGS14 instead of IGb08 will induce
      small global effects due to the reference frame change from ITRF2008 to
      ITRF2014. The corresponding transformation parameters can be found at:
        http://itrf.ign.fr/ITRF_solutions/2014/tp_14-08.php

  - Ground antenna calibration updates:
      The igs08.atx to igs14.atx ground antenna calibration updates will provoke
      additional coordinate changes for many stations. This effect will mainly
      be antenna-type-dependent, but will also depend on station location.
      The impact of the ground antenna calibration updates from igs08.atx to
      igs14.atx on the coordinates of all affected IGS stations was assessed by
      means of differential PPP analyses. The estimated position offsets are
      given in igs08_to_igs14_offsets.txt.
      For users who would like to assess the impact of the calibration updates
      for specific stations/antennas, a set of latitude-dependent coordinate
      change models is additionally available in lat_models.txt.

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