From arturo.villiger at aiub.unibe.ch Tue Jul 26 14:58:06 2022 From: arturo.villiger at aiub.unibe.ch (Arturo Villiger) Date: Tue, 26 Jul 2022 16:58:06 +0200 Subject: [IGSMAIL-8238] Upcoming switch to IGS20/igs20.atx and repro3 standards Message-ID: <8f636901-2e7e-fe99-29aa-00413fbfbd06@aiub.unibe.ch> 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.