In preparation for the International Terrestrial Reference Frame 2020, the International GNSS Service analysis centers released the results of the third reprocessing campaign (IGS Repro 3) of all the GNSS network solutions backwards starting from 1994. For the first time, the IGS reprocessing products included not just GPS and GLONASS, but also the Galileo constellation. In this study, we describe the methodology and results of the orbit combination provided by the IGS Analysis Center Coordinator (IGS ACC) at Geoscience Australia. The quality of the combined orbit products was cross-checked with the individual IGS Repro3 Analysis Center (AC) contributions. The internal consistency of the individual Analysis Center (AC) solutions with the combined orbits was assessed based on the root mean square of the 3D orbit differences. In 2020, the mean consistency of the combination is at the level of 9, 23, and 15 mm for GPS, GLONASS, and Galileo, respectively. The external validation of the orbits was performed using Satellite Laser Ranging (SLR). We proposed a novel approach to handling detector-specific biases in the results of SLR validation, which reduced the standard deviation of SLR residuals by up to 13% for Galileo FOC satellites. This method is based on bias aligning the offsets to single-photon SLR stations that were treated as a reference. The proposed approach increased the internal consistency of the SLR dataset, facilitating the detection of orbit modeling issues. The standard deviation of SLR residuals of the best individual solution versus the combined solution equals 13/13, 15/17, 17/17, 18/19 mm for Galileo-FOC, -IOV, GLONASS-K1B, -M, respectively. Therefore, the combined solution can be considered equal in quality compared to the best individual AC solutions. Searching for patterns in SLR residuals for different satellite-Sun-Earth geometries revealed that some orbit modeling issues are not fully diminished for individual ACs. Eventually, our findings suggest that the delivered combined orbit product may be considered the best solution overall, as it benefits from the best individual solutions for each satellite type.

All realizations of the European Vertical Reference System (EVRS) computed so far are solely based on geopotential differences obtained by spirit leveling/gravimetry. As such, there are no direct connections between height benchmarks separated by large water bodies. In this study, such connections are added by means of model-based hydrodynamic leveling resulting in a new, yet unofficial realization of the EVRS. The model-derived mean water levels used in computing the hydrodynamic leveling connections were obtained from the Nemo-Nordic (Baltic Sea) and 3D DCSM-FM (northwest European continental shelf) hydrodynamic models. The impact of model-based hydrodynamic leveling on the European Vertical Reference Frame is significant, especially for France and Great Britain. Compared to a solution which only uses spirit leveling/gravimetry, the differences in these countries reach tens to hundreds of hbox {kgal}, hbox {mm}. We also observed an improved agreement with normal heights obtained by differencing GNSS and the European gravimetric quasi-geoid 2015 (EGG2015) heights. In Great Britain, the south-north slope of 48 mm hbox {deg}^{-1} present in the solution which uses only spirit leveling/gravimetry data reduced to 2.2 mm hbox {deg}^{-1}. In France, the improvement is confined to the southwest. The choice of the period over which water levels are averaged has an impact on the results as it determines, among others, the set of tide gauges available to establish the hydrodynamic leveling connections. When using an averaging period that can be considered as the least preferred choice based on three established criteria, the positive impact for France has gone. For Great Britain, the estimated south-north slope became 12.6 mm hbox {deg}^{-1}. This is larger than the slope obtained using the most preferred averaging period but still substantially lower compared to the slope associated with a solution that uses only spirit leveling/gravimetry.

We describe a new harmonic tidal analysis method, which constrains the solution to be near a reference model. This regularization stabilizes the linear regression, allowing us to infer model parameters for each tidal harmonic. This overcomes the need to create a priori groupings of harmonics. The inversion is done iteratively by adjusting the reference model to reduce the data misfit. The frequency dependence of the solution is thus data-driven. We find models for the different spherical degrees independently. Our procedure allows narrow-band variations of the tidal admittance. We test the hypothesis that some of the temporal variations of tidal parameters found in previous studies were caused by inappropriate body tide models in combination with a priori wave grouping. We determine a local response model from 11.5 years of data recorded by the superconducting gravimeter SG056 at Black Forest Observatory (BFO, Schiltach). Using this as an a priori model in a non-regularized moving window analysis of wave groups composed from summed harmonics, we find that periodic variations of groups M1, K1, mu 2, N2, L2, and S2 are reduced by up to a factor of 7 compared to earlier studies. Some variations previously seen in the M2 group are captured as well.

With the advancement of multi-GNSS systems, the field calibration of GNSS receiver antennas has been updated at Wuhan University. Benefiting from the use of a six-axis robot that can change its position and attitude precisely, multisession calibration experiments were implemented for several antennas of two types. The calibrations show a high stability of 1 mm for both the phase center offset and phase variation estimation. Compared to the models disclosed in igs14.atx and igsR3.atx, phase center correction differences at the 1 mm level can be obtained for most signals for elevation angles above 15°. For lower elevations, the consistency with the reference model increases to 2–3 mm or more. The consistency of calibrations with different receivers was investigated, and root mean square of differences between these models was better than 0.15 mm. In a short-baseline positioning experiment, the coordinate discrepancies introduced by an antenna phase center (APC) model between GPS and BDS-3 signals could be significantly reduced to the 1 mm level. Compared to the reference coordinates, the positioning accuracies for GPS and BDS-3 were both less than 2 mm with the adoption of the calibrated APC model. The multi-GNSS calibration system tested in this experiment is preliminarily proven reliable and could be applied to future antenna calibration for multi-GNSS applications.

We seek to quantify and understand the residual signal in GPS and GLONASS estimates of ocean tide loading displacements (OTLDs) after removing state-of-the-art model estimates. To consider contributions over a broad spatial scale, we estimate OTLD over the Australian continent using sim 5.5 years of continuous GPS and GLONASS data from 360 sites. We compare these with modelled estimates, with a focus on the lunar semidiurnal M_2 and diurnal O_1 constituents. We observe spatially coherent patterns of residual OTLD in each of the east, north, and up coordinate components after the removal of tidal loading using elastic models. We subsequently assess the impact of including anelastic dispersion in the model and show a 0.2 mm reduction in range of the up component residuals at coastal sites. A similar reduction at all sites is observed in the east and north components. Of the seven ocean tide models used, we find that three recent models, FES2014b, GOT4.10c and TPXO9.v1, perform similarly, noting these comparisons are made in the CE frame. However, we show that the latter contains centre of mass (CoM) biases in amplitude up to 0.2 mm and 0.5 mm for M_2 and O_1, respectively, due to the assimilated altimetry data having not been corrected for geocentre motion. We find OTLD estimates are sensitive to the chosen orbit and clock products used in our analysis, with differences of up to 0.5 mm in the east component between solutions using the JPL and either of ESA or CODE products (GPS-only). Our analysis shows that current GNSS estimates of OTLD over Australia are typically accurate to sim 0.2 mm at which point we are unable to explain spatially coherent residuals when compared to modelled quantities. These may depend on the appropriate treatment of the CoM variation, anelasticity and/or three-dimensional Earth structure. As such, we recommend great care is taken when interpreting OTLD at the level of 0.1 or 0.2 mm, even if it is regionally coherent.

We investigate the impact of the reduction of non-tidal loading (NTL) in the computation of secular terrestrial reference frames (TRFs) from Very Long Baseline Interferometry (VLBI) observations. There are no conventional models for NTL in the geodetic community yet, but the Global Geophysical Fluid Center prepared a set of corresponding site displacements for the 2020 realizations of the International Terrestrial Reference System. We make use of these data, which comprise the total NTL consisting of non-tidal atmospheric, oceanic, and hydrological loading. The displacement series contain linear trends (i.e., offsets plus drifts), and since these affect the estimated linear station positions and the realized geodetic datum in a secular TRF, we remove the trends before reducing the NTL in our computations. The displacements are applied at two different levels of the parameter estimation process: the observation and the normal equation level. This way, we can analyze whether the latter offers a suitable approximation if the original observations have not been reduced by NTL. We find that the TRF statistics are hardly affected by the NTL. The largest impact is given for the secular motion of antennas with short observation time spans. The application level is basically irrelevant for the linear antenna positions, but it leads to differences in the rates of the jointly estimated Earth orientation parameters (EOPs). Secular TRF solutions and session solutions deviate with respect to the parameterization of the antenna coordinates, and thus also with respect to the correlations between the estimated antenna parameters and the EOPs. Due to this, the consistently estimated EOP series also show differences. However, for both solution types the reduction of the NTL leads to a change of the annual signal in the EOP series.