[IGSMAIL-8238] Upcoming switch to IGS20/igs20.atx and repro3 standards
Arturo Villiger
arturo.villiger at aiub.unibe.ch
Tue Jul 26 14:58:06 UTC 2022
Dear colleagues,
The IGS will soon adopt a new reference frame, called IGS20, as the
basis of its
products. IGS20 is closely related to ITRF2020, released in April 2022. An
updated set of satellite and ground antenna calibrations, igs20.atx,
will also
become effective at the same time and should be used together with
IGS20. The
IGS intends to switch from IGb14/igs14.atx to IGS20/igs20.atx starting
with the
products of GPS week 2230 (2 October 2022).
The following files are already available for download at
ftp://igs-rf.ign.fr/pub/IGS20:
- IGS20.snx.gz : IGS20 SINEX file (gzip compressed)
- IGS20.ssc : IGS20 SINEX file without covariance matrix
- psd_IGS20.snx : post-seismic deformation models to be used
with IGS20
- soln_IGS20.snx : discontinuity list to be used with IGS20
- ITRF2020_to_IGS20.txt : position offsets applied to the ITRF2020
coordinates
of IGS20 stations affected by ground antenna
calibration updates from igsR3.atx to igs20.atx
A first release of igs20.atx can be found at:
http://ftp.aiub.unibe.ch/awg/igs20.atx.gz
while future updates will be available at:
https://files.igs.org/pub/station/general/igs20.atx
At the same time as the switch to IGS20/igs20.atx, the IGS will also
adopt for
its operational products the same conventions and models as used in its
third
reprocessing campaign (repro3; see
http://acc.igs.org/repro3/repro3.html). This
includes in particular:
- the processing of Galileo observations, in addition to GPS and
GLONASS, by a
number of IGS Analysis Centers,
- the adoption of the new IERS secular pole model,
- the rotation of phase center corrections to the actual orientations of
antennas not oriented North,
- the adoption of ***NEW LONG FILE NAMES FOR THE IGS PRODUCTS***
(see http://acc.igs.org/repro3/Long_Product_Filenames_v1.0.pdf).
Details on the definition and use of IGS20 and igs20.atx, as well as on
the main
expected impacts of the switch to IGS20/igs20.atx and the repro3
standards on
user results, can be found below.
Note that test products based on IGS20/igs20.atx and the repro3
standards will
be generated for a 2-month period before the switch (GPS weeks 2222 - 2229;
7 August - 1 October 2022) in parallel with the official
IGb14/igs14.atx-based
products. These test products will be available at:
ftp://igs-rf.ign.fr/pub/igs20_tests
Also note that a new version of the IGS cumulative solution, consistent with
IGS20/igs20.atx and the repro3 standards, will be released with the
products of
GPS week 2230. It will initially be based on the IGS repro3 daily
combined SINEX
solutions of years 1994-2020. The weekly updates of the IGS cumulative
solution
will then halt for a few months, while the repro3 product series will be
extended until 1 October 2022. It is expected that the weekly updates of
the IGS
cumulative solution resume in December, based on a complete up-to-date
series of
solutions compliant with the repro3 standards.
For any question, please contact:
- paul.rebischung at ign.fr for the reference frame aspects,
- arturo.villiger at aiub.unibe.ch for the antenna calibration aspects.
Best regards,
Paul Rebischung, Arturo Villiger, Salim Masoumi, Tom Herring
* Antenna calibration updates
- Ground antenna calibrations:
Regarding ground antenna calibrations, igs20.atx is a minor
update to the
ANTEX file used in repro3, igsR3.atx, which is itself a major
update to
igs14.atx.
From igs14.atx to igsR3.atx, 37 legacy phase center correction
patterns
covering the L1 and L2 frequencies only were replaced by new multi-
frequency patterns provided by Geo++ (36) and the University of
Bonn (1).
From igsR3.atx to igs20.atx, 15 multi-frequency patterns provided
by Geo++
were further introduced. Besides, 2 type-mean multi-frequency
patterns
already present in igsR3.atx were updated with the results of recent
individual calibrations.
Details on all these updates can be found in the header of igs20.atx.
- Satellite antenna calibrations:
The radial phase center offsets (z-PCOs) of the GPS, GLONASS and
Galileo
satellites have been updated in igs20.atx to become consistent
with the
ITRF2020 scale. The new values were obtained from time series of
z-PCO
estimates derived from the daily repro3 solutions of 9 Analysis
Centers.
A single common correction was estimated and applied to the
calibrated
z-PCOs of all Galileo satellites (except E102) provided by GSC.
Another
single common correction was estimated and applied to the calibrated
z-PCOs of all GPS Block III satellites provided by Lockheed-Martin.
Satellite-specific z-PCOs were estimated for all other satellites.
Note that the terrestrial scale implied by the GPS, GLONASS and
Galileo
satellite z-PCOs in igs20.atx matches the ITRF2020 scale at epoch
2015.0,
but departs from the ITRF2020 scale at a rate of about +0.1 mm/yr.
* IGS20 reference frame definition and use
- Station selection:
IGS20 is a subset of 332 stable, well-performing IGS stations from
ITRF2020. Station stabilities were assessed from the ITRF2020 results
(WRMS of residual time series, formal errors of ITRF2020 coordinates
propagated over expected lifetime of IGS20). Compared to IGb14, IGS20
contains 98 new stations, mostly in previously sparsely covered
areas.
On the other hand, 25 previous IGb14 stations were not selected
in IGS20.
- Coordinate corrections due to ground antenna calibration updates:
While ITRF2020 station coordinates are consistent with the set of
ground
antenna calibrations used in repro3 (igsR3.atx), the IGS20 station
coordinates were made consistent, when needed, with the updated
igs20.atx
antenna calibrations. For that purpose, the impact of the antenna
calibration updates on the positions of the affected IGS20
stations were
assessed by four IGS Analysis Centers (ACs) by means of
differential PPP
analyses. Weighted averages of the AC position offset estimates
were then
applied to the ITRF2020 coordinates of the affected IGS20
stations. The AC
position offset estimates and their weighted averages can be
found at:
ftp://igs-rf.ign.fr/pub/IGS20/ITRF2020_to_IGS20.txt
- Transformation parameters:
Although some station coordinates differ between ITRF2020 and
IGS20 due to
the ground antenna calibration updates from igsR3.atx to
igs20.atx, both
frames share the same underlying origin, scale and orientation. The
transformation parameters between ITRF2020 and IGS20 are thus
zero. The
transformation parameters between ITRF2020/IGS20 and past ITRF
realizations can be found at:
https://itrf.ign.fr/docs/solutions/itrf2020/Transfo-ITRF2020_TRFs.txt
- Discontinuities:
IGS20 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:
ftp://igs-rf.ign.fr/pub/IGS20/soln_IGS20.snx
The discontinuity list in soln_IGS20.snx is based on the ITRF2020
discontinuity list but also includes recent discontinuities and
will be
continuously updated as new breaks occur. Because IGS20 does not
contain
coordinates for solns starting after 31 December 2020 (end of
ITRF2020
input data), stations affected by such recent discontinuities
cannot be
used as reference frame stations anymore.
- Post-seismic deformation models:
Like in ITRF2020, the reference coordinates of some IGS20
stations include
post-seismic deformation models in the form of exponential and/or
logarithmic functions. These models are given in:
ftp://igs-rf.ign.fr/pub/IGS20/psd_IGS20.snx
Details on their application can be found at:
https://itrf.ign.fr/ftp/pub/itrf/itrf2020/ITRF2020-PSD-model-eqs-IGN.pdf
The IGS20 positions+velocities of the affected stations cannot be
used
without the post-seismic deformation models.
- Seasonal terms:
While coefficients of annual and semi-annual station
displacements were
provided with ITRF2020, it was agreed among IGS ACs not to
implement these
seasonal terms for the time being. IGS20 is thus provided without
seasonal
terms. Users wishing to experiment with the ITRF2020 seasonal
terms may
nevertheless add them to the IGS20 piecewise linear coordinates +
post-
seismic deformation models.
- IGS20 core network:
For the purpose of aligning global GNSS solutions to IGS20, a well-
distributed sub-network of IGS20 stations, called IGS20 core
network, was
designed. It is composed of 55 clusters of stations (i.e., 55 primary
stations, each with possible substitutes) selected to ensure a
homogeneous
global distribution and the best possible temporal stability of
the core
network. The list of IGS20 core stations can be found at:
ftp://igs-rf.ign.fr/pub/IGS20/IGS20_core.txt
* Impact on user results
- Reference frame change:
The alignment of the IGS products to IGS20 instead of IGb14 will
induce
small global effects due to the reference frame change from
ITRF2014 to
ITRF2020. The corresponding transformation parameters can be
found at:
https://itrf.ign.fr/docs/solutions/itrf2020/Transfo-ITRF2020_TRFs.txt
- Ground antenna calibration updates:
The ground antenna calibration updates from igs14.atx to
igs20.atx will
cause additional apparent position changes for many stations.
This effect
will mainly be antenna-type-dependent, but will also depend on
the station
locations. The apparent position changes can be expected to reach
up to
several mm in horizontal; ~1 cm in vertical.
- New IERS secular pole model:
While current IGS operational products make use of the mean pole
model
from the original IERS Conventions, the new IERS secular pole
model was
adopted in repro3 and will be adopted in the IGS operational products
after GPS week 2230. This model change induces an apparent,
time-variable,
degree-2, order-1 deformation pattern of the terrestrial frame, whose
amplitude is relatively small until ~2010, but then increases
linearly. At
the time of the switch to IGS20/igs20.atx and the repro3 standards,
apparent station position changes of up to ~4 mm in vertical; ~1
mm in
horizontal thus have to be expected.
- PCO/PCV rotations for antennas not oriented North:
Although certain ground GNSS antennas are not oriented North, the IGS
practice until repro3 had been to ignore these mis-orientations
and use
the phase center corrections from the IGS ANTEX files as if all
antennas
were oriented North. In repro3 however, the phase center
corrections have
been rotated when necessary to match the actual antenna orientations
reported in the site logs. The same practice will be adopted for
the IGS
operational products after GPS week 2230. These PCO/PCV rotations
have
impacts of up to a few cm in horizontal / a few mm in vertical on the
estimated positions of the affected stations. Antenna orientations,
extracted from the site logs, will be reported in an extra column
in the
SITE/ANTENNA blocks of the IGS SINEX files.
Other minor impacts can be expected from the inclusion of Galileo
observations
in the analyses of several ACs, as well as from the adoption of a certain
number of models used in repro3, including:
- latest-generation ocean tide loading models,
- refined solar radiation pressure models,
- the sub-daily EOP tide model from Desai & Sibois (2016;
https://doi.org/10.1002/2016JB013125),
- time-variable gravity models.
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