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<font size="2"><font face="Courier New">Dear colleagues,<br>
<br>
The IGS will soon adopt a new reference frame, called IGS14, as the basis of its<br>
products. IGS14 is closely related to ITRF2014, released in January 2016. An<br>
updated set of satellite and ground antenna calibrations, igs14.atx, will also<br>
become effective at the same time and should be used together with IGS14. The<br>
IGS intends to switch from IGS08/igs08.atx to IGS14/igs14.atx starting with<br>
products of GPS week 1934 (29 January 2017). The IGS08/igs08.atx framework has<br>
been in effect since GPS week 1632 (17 April 2011) and was also used for the<br>
second IGS reprocessing campaign (repro2) back to 1994.<br>
<br>
The following files can already be downloaded from either<br>
ftp://igs-rf.ign.fr/pub/IGS14 or ftp://igs-rf.ensg.eu/pub/IGS14:<br>
- IGS14.snx : IGS14 SINEX file<br>
- IGS14.ssc : IGS14 SINEX file without covariance matrix<br>
- IGS14.png : map of the full IGS14 network<br>
- psd_IGS14.snx : post-seismic deformation models to be used with<br>
IGS14<br>
- soln_IGS14.snx : discontinuity list to be used with IGS14<br>
- IGS14_core.txt : list of IGS14 core stations<br>
- IGS14_core.png : map of the IGS14 core network<br>
- igs14_1926.atx : latest version of igs14.atx (will be updated, if<br>
needed)<br>
- ITRF2014_to_IGS14.txt : position offsets applied to the ITRF2014<br>
coordinates of IGS14 stations affected by ground<br>
antenna calibration updates<br>
- igs08_to_igs14_offsets.txt : position offsets estimated for all IGS stations<br>
affected by ground antenna calibration updates<br>
- lat_models.txt : latitude-dependent models of position offsets<br>
expected from ground antenna calibration updates<br>
<br>
Below, you will find some details about the definition and use of IGS14 and<br>
igs14.atx. IGS14 and igs14.atx were defined using a similar strategy as IGS08<br>
and igs08.atx. Therefore, additional useful information can be found in the<br>
IGS08/igs08.atx documentation:<br>
- [IGSMAIL-6354] Upcoming switch to IGS08/igs08.atx<br>
(https://igscb.jpl.nasa.gov/pipermail/igsmail/2011/006346.html)<br>
- [IGSMAIL-6355] Upcoming switch to IGS08/igs08.atx - Details on igs08.atx<br>
(https://igscb.jpl.nasa.gov/pipermail/igsmail/2011/006347.html)<br>
- [IGSMAIL-6356] Advice for coping with the change to IGS08+igs08.atx<br>
(https://igscb.jpl.nasa.gov/pipermail/igsmail/2011/006348.html)<br>
- IGS08: the IGS realization of ITRF2008<br>
(http://doi.org/10.1007/s10291-011-0248-2)<br>
- Absolute IGS antenna phase center model igs08.atx: status and potential<br>
improvements (http://doi.org/10.1007/s00190-015-0876-3)<br>
<br>
Also note that, starting with the release of GPS week 1934 (IGS17P05.snx), the<br>
IGS cumulative solution will be replaced by a new version based on:<br>
- the weekly combined repro2 solutions (ig2) of GPS weeks 0730-1831,<br>
- the weekly combined operational solutions (igs) of GPS weeks 1832-present,<br>
- a revised discontinuity list, consistent with the ITRF2014 discontinuity list,<br>
which will replace the current list at:<br>
ftp://igs-rf.ign.fr/pub/discontinuities/soln.snx<br>
ftp://igs-rf.ensg.eu/pub/discontinuities/soln.snx<br>
This new IGS cumulative solution will also incorporate the ITRF2014 post-seismic<br>
deformation models. It will be made consistent with the igs14.atx ground antenna<br>
calibrations and aligned to the IGS14 reference frame.<br>
<br>
Please send any further questions to:<br>
- igs-rf@ign.fr for the reference frame aspects,<br>
- schmid@tum.de for the antenna calibration aspects.<br>
<br>
Best regards,<br>
<br>
Paul Rebischung, Ralf Schmid, Tom Herring<br>
<br>
<br>
<br>
* Antenna calibration updates<br>
<br>
- Satellite antenna calibrations:<br>
Because of recent orbit modeling changes (Earth radiation pressure,<br>
antenna thrust), the scale of the IGS products currently differs from the<br>
ITRF2008 scale by about -0.3 ppb. igs14.atx therefore contains new radial<br>
antenna phase center offsets (z-PCOs) for all GPS and GLONASS satellites.<br>
These offsets were obtained from time series of z-PCO estimates,<br>
consistent with the ITRF2014 scale, derived from the daily repro2<br>
solutions of seven IGS analysis centers. Note that the z-PCO time series<br>
were trend-corrected to epoch 2010.0 before computing weighted averages.<br>
The igs14.atx satellite antenna z-PCO values are therefore expected to<br>
give access to the ITRF2014 scale at epoch 2010.0.<br>
Satellite-specific x- and y-PCOs from pre-flight calibrations were<br>
additionally introduced in igs14.atx for the GPS Block IIR satellites.<br>
<br>
- Ground antenna calibrations:<br>
For 17 antenna types, additional robot calibrations provided by different<br>
institutions could be added to igs14.atx. Moreover, 19 type-specific phase<br>
center corrections were updated with the results of recent individual<br>
calibrations. As also the AOAD/M_T antenna calibration got updated, all<br>
converted calibrations slightly changed. Detailed antenna names can be<br>
found in the header of igs14.atx.<br>
<br>
* IGS14 definition and use<br>
<br>
- Station selection:<br>
IGS14 is a subset of 252 stable, well-performing IGS stations from<br>
ITRF2014. Station stabilities were assessed from the ITRF2014 results<br>
(WRMS of residual time series, formal errors of ITRF2014 coordinates<br>
propagated over the expected lifetime of IGS14). Compared to IGb08, IGS14<br>
contains 65 new stations, mostly in previously sparsely covered areas. On<br>
the other hand, 48 previous IGb08 stations did not meet the IGS14<br>
selection criteria.<br>
<br>
- Coordinate corrections due to ground antenna calibration updates:<br>
While ITRF2014 coordinates are consistent with the igs08.atx set of<br>
antenna calibrations, IGS14 needs to be consistent with the updated<br>
igs14.atx set. The impact of the igs08.atx to igs14.atx ground antenna<br>
calibration updates on the ITRF2014 station coordinates was therefore<br>
assessed from differential PPP analyses, and the estimated corrections<br>
were applied to the ITRF2014 coordinates of the affected IGS14 stations<br>
whenever exceeding 1 mm in either direction. The coordinate corrections<br>
actually applied are listed in ITRF2014_to_IGS14.txt.<br>
<br>
- Discontinuities:<br>
IGS14 station coordinates are generally represented by piecewise linear<br>
functions, i.e., several position+velocity sets valid over successive time<br>
intervals. Each set is denoted by a solution number (soln) whose validity<br>
period is given in the discontinuity file soln_IGS14.snx. The<br>
discontinuity list in soln_IGS14.snx is based on the ITRF2014<br>
discontinuity list but also includes recent discontinuities and will be<br>
continuously updated as new breaks occur. Because IGS14 does not contain<br>
coordinates for solns starting after 15 February 2015 (end of ITRF2014<br>
input data), stations affected by such recent discontinuities cannot be<br>
used as reference frame stations anymore.<br>
<br>
- Post-seismic deformation models:<br>
Like in ITRF2014, the reference coordinates of some IGS14 stations include<br>
post-seismic deformation models in the form of exponential and/or<br>
logarithmic functions. These models are given in psd_IGS14.snx. More<br>
details on their application can be found at:<br>
http://itrf.ign.fr/ITRF_solutions/2014/psd.php<br>
The IGS14 positions+velocities of the affected stations cannot be used<br>
without the post-seismic deformation models.<br>
<br>
- IGS14 core network:<br>
For the purpose of aligning global GNSS solutions to IGS14, a well-<br>
distributed sub-network of IGS14 stations, called IGS14 core network, was<br>
designed. It is composed of 51 clusters of stations (i.e., 51 primary<br>
stations, each with possible substitutes; see IGS14_core.txt) selected to<br>
ensure a homogeneous global distribution and the best possible<br>
temporal stability of the core network.<br>
<br>
* Impact on user results<br>
<br>
The impact of the switch to IGS14/igs14.atx on station positions estimated<br>
using the IGS products will be twofold:<br>
<br>
- Reference frame change:<br>
The alignment of the IGS products to IGS14 instead of IGb08 will induce<br>
small global effects due to the reference frame change from ITRF2008 to<br>
ITRF2014. The corresponding transformation parameters can be found at:<br>
http://itrf.ign.fr/ITRF_solutions/2014/tp_14-08.php<br>
<br>
- Ground antenna calibration updates:<br>
The igs08.atx to igs14.atx ground antenna calibration updates will provoke<br>
additional coordinate changes for many stations. This effect will mainly<br>
be antenna-type-dependent, but will also depend on station location.<br>
The impact of the ground antenna calibration updates from igs08.atx to<br>
igs14.atx on the coordinates of all affected IGS stations was assessed by<br>
means of differential PPP analyses. The estimated position offsets are<br>
given in igs08_to_igs14_offsets.txt.<br>
For users who would like to assess the impact of the calibration updates<br>
for specific stations/antennas, a set of latitude-dependent coordinate<br>
change models is additionally available in lat_models.txt.<br>
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