SIRGAS data processing at DGFI-TUM

Each week, ten SIRGAS analysis centres deliver loosely constrained weekly solutions including station positions for a certain set of SIRGAS stations. The distribution of the SIRGAS stations among the SIRGAS analysis centres ensures that each station is processed by three analysis centres.

The SIRGAS analysis centres follow unified standards for the computation of loosely constrained weekly solutions for the station positions. These standards are generally based on the conventions outlined by the International Earth Rotation and Reference Systems Service (IERS) and the GNSS-specific guidelines defined by the International GNSS Service (IGS); with the exception that in the individual SIRGAS solutions the satellite orbits and clocks as well as the Earth orientation parameters are fixed to the final weekly IGS values (SIRGAS does not estimate these parameters), and positions for all stations are constrained to ±1 m (to generate the loosely constrained solutions in the SINEX format).

The individual solutions are combined by the SIRGAS combination centres to generate a unified set of weekly station positions aligned to the same reference frame applied by the IGS to computed GNSS satellite orbits.

DGFI-TUM is responsible for processing the SIRGAS-C core network and for combining the core network with the individual solutions delivered by the other SIRGAS analysis centres. The SIRGAS-C network is the primary densification of ITRF in Latin America. It presents a good continental coverage and comprises stabile site locations to ensure high long-term stability of the reference frame. The SIRGAS-C network is the backbone for the consistent integration of the Latin American national reference frames into the continental and global reference frame.

The main processing characteristics applied by DGFI-TUM for the analysis of the SIRGAS-C network are:

• Software: Bernese GNSS Software 5.2, Dach et al. 2015 (doi: 10.7892/boris.72297).
• Basic observable ionosphere-free linear combination.
• Sampling rate 30 sec.
• Elevation cut-off angle 3°.
• Elevation-dependent weighting cos(z)**2.
• Satellite orbits, satellite clock offsets, and Earth orientation parameters are fixed to the combined IGS weekly solutions, https://igs.org/products/, Johnston et al. 2017 (doi: 10.1007/978-3-319-42928-1_33).
• Satellite antenna to centre of mass offsets spacecraft-specific z-offsets and block-specific x- and y-offsets from the model igs14.atx, https://files.igs.org/pub/station/general/.
• Phase centre variations (PCV) absolute model for receiver and satellite antennae, model igs14.atx, https://files.igs.org/pub/station/general/.
• Antenna radome calibrations applied, if given in igs14.atx. Otherwise, radome effects are neglected and the standard antenna model (radome NONE) is used.
• Marker to antenna eccentricities (dN, dE, dU) according to the site logs (ftp://ftp.sirgas.org/pub/gps/DGF/station/log/).
• Phase ambiguities are solved as follows:
  • Direct L1 and L2 ambiguity solution for baselines from 0 km to 20 km
  • L3 and L5 ambiguity solution for baselines from 18 km to 200 km
  • Wideline strategy for baselines from 180 km to 9000 km
  • Quasi ionosphere free (QIF) strategy for baselines from 18 km to 5600 km
  • In the ambiguity solution, the ionosphere models of CODE (Centre for Orbit Determination in Europe) are provided as input to increase the number of ambiguities solved, http://ftp.aiub.unibe.ch/CODE/, Dach et al. 2020 (doi: 10.7892/boris.75854.4).
• Troposphere modelling: the a-priori zenith delay is modelled using the Vienna Mapping Function (Boehm et al. 2006, doi: 10.1029/2005JB003629) and further atmospheric parameters are estimated in a 1-hour interval within the network adjustment using also the Vienna Mapping Function. In addition, horizontal gradient parameters are estimated to model azimuthal asymmetries (model described in Chen and Herring 1997, doi: 10.1029/97JB01739). The gridded VMF1 coefficients are provided by J. Boehm, TU Vienna, at https://vmf.geo.tuwien.ac.at/trop_products/GRID/.
• Tidal corrections for solid Earth tide, permanent tide and solid Earth pole tide as described in the IERS Conventions 2010 (Petit and Luzum, 2010). Ocean tide loading reduced with the FES2014b model (Lyard et al. 2021, doi: 10.5194/os-17-615-2021) and atmospheric tide loading for S1 and S2 reduced with the model of Van Dam and Ray 2010 (https://geophy.uni.lu/atmosphere/tide-loading-calculator/). The reduction coefficients for the ocean tide loading are provided by M.S. Bos and H.-G. Scherneck at http://holt.oso.chalmers.se/loading/. The reduction coefficients for the atmospheric tide loading are provided by T. van Dam at https://geophy.uni.lu/atmosphere/tide-loading-calculator/ATM1OnlineCalculator/.
• Ocean tide geocentre coefficients are not applied since this correction is already contained in the final IGS products.
• Non-tidal loadings as atmospheric pressure, ocean bottom pressure, or surface hydrology are not reduced.
• Daily free normal equations are computed by applying the double difference strategy. The baselines are created taking into account the maximum number of common observations for the associated stations.
• Daily free normal equations are combined for computing a loosely constrained weekly solution for station positions (all station coordinates are loosely constrained to +/-1 m).
• Stations with large residuals in the weekly combination (more than +/-10 mm  in the N-E component, and more than +/-20 mm in the height component) are removed from the normal equations.
• The DGFI-TUM loosely constrained solutions are made available to be combined with the corresponding solutions delivered by the other SIRGAS Processing Centres. They are given in SINEX format and are identified with the file names DGFwwww7.SNX.