G2.3

The precise processing of data derived by several global navigation satellite systems (GNSS) for global and regional networks relies on high-quality and calibrated equipment. Currently, an intensively discussed question in the IGS antenna working group is the best practice for publishing and distributing calibration values for receiver antennas for different systems and frequencies. There is the question of frequency band specific output of calibration values or system specific output, the magnitude of their differences and their impact the estimation parameters that are not yet assessed. We will address these points in our contribution.

Several studies performed and evaluated at our calibration facility demonstrate a systematic impact of the receiver and the implemented signal tracking concept. The expected magnitudes in GNSS processing lead to differences on the coordinate domain of a few millimetres on a short and well-controlled baseline for original observations or frequencies. These effects are superimposed and amplified when forming linear combinations of independent signals and frequencies, which, however, are essential for global GNSS processing tasks such as ionosphere-free linear combination in global GNSS networks.  These amplifications are critical as apparent biases in the coordinate and troposphere estimates are introduced with different magnitudes.

For this reason, we present a quality assessment for different antenna-receiver combinations and provide an in-depth analysis and comparison for the majority of available and existing systems, signals, frequencies and linear combinations. The data were recorded under well-controlled conditions and include GNSS data of more than one week for each of the analysed number of four geodetic and reference station grade antennas. The analysis of the different combinations of antenna-receiver configurations provides metrics for assessing the impact of the receivers on the multi-system GNSS processing and the determination of the geodetic estimates. Consequently, validation with theoretical and expected metrics derived through multiple linear combinations is investigated, with additional focus on coordinate and troposphere estimates. The analysis uses the concepts of relative (baseline processing) and absolute (precise point positioning, PPP) GNSS processing.

How to cite: Kersten, T., Kröger, J., Breva, Y., and Schön, S.: On the Role of GNSS Receivers for Antenna Patterns and Parameter Estimations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3029, https://doi.org/10.5194/egusphere-egu21-3029, 2021.

The next-generation VLBI system called VGOS (VLBI Global Observing System) has been designed and built as a significant improvement over the legacy geodetic VLBI system to meet the accuracy and stability goals set by the Global Geodetic Observing System (GGOS). Improved geodetic products are expected as the VGOS technique transitions from demonstration to operational status, which is underway. Since 2019, a network of nine VGOS stations has been observing bi-weekly under the auspices of the International VLBI Service for Geodesy and Astrometry (IVS) to generate standard geodetic products. These products, together with the mixed-mode VLBI observations that tie the VGOS and legacy networks together will be contributions to the next realization of the International Terrestrial Reference Frame (ITRF2020). Moreover, since 2020 a subset of 2 to 4 VGOS stations has also been observing in a VLBI Intensive-like mode to assess the feasibility of Earth rotation (UT1) estimation using VGOS. Intensives are daily legacy VLBI observations that are run on a daily basis using a single baseline between Kokee Park Geophysical Observatory, Hawaii, and Wettzell Observatory, Germany, made with the goal of near-real-time monitoring of UT1. In this presentation, we will describe the VGOS observations, correlation, post-processing, and preliminary geodetic results, including UT1. We will also compare the VGOS estimates to estimates from legacy VLBI, including estimates from mixed-mode observations, to explore the precision and accuracy of the VGOS products.

How to cite: Mondal, D., Elosegui, P., Barrett, J., Corey, B., Niell, A., Ruszczyk, C., and Titus, M.: A preliminary assessment of the accuracy of the VGOS geodetic products: implications for the terrestrial reference frame and Earth orientation parameters, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13032, https://doi.org/10.5194/egusphere-egu21-13032, 2021.

With the VLBI technique radio sources are observed in dedicated time intervals. The most usual length of these observing sessions are 24 and 1-hour long. 24-hour long experiments usually incorporate a global network of stations, and, thus, are the prominent source of a consistent determination of all Earth Orientation Parameters (EOPs), celestial and terrestrial reference frames. The shorter experiments are designed to determine dUT1 parameter only. The number of short or intensive sessions is growing every year. Also some of them involve 3-4 stations in observation programs instead of standard 2-station mode. This leads to a larger number of observations per session, a better coverage of the Earth, and, consequently more accurate dUT1 estimates.

All 24-hour and 1-hour sessions since 1984 up to now were re-processed by BKG using the most up-to-date modelling within the parameter estimation. This results in new series of consistently estimated EOPs, station coordinates and troposphere parameters.

In this contribution we present our new series and investigate the quality of the obtained geodetic products, especially the EOPs. The work is focused on the consistency between dUT1 parameters derived from 24-hour and 1-hour sessions, respectively. In this study we pinpoint challenges and prospects of the inclusion of 1-hour experiments into the standard analysis of the 24-hour experiments.

How to cite: Girdiuk, A., Engelhardt, G., Ullrich, D., Thaller, D., and Hellmers, H.: BKG’s new series of 24-hour and 1-hour VLBI experiments, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8950, https://doi.org/10.5194/egusphere-egu21-8950, 2021.

We present the current activities of the Federal Agency for Cartography and Geodesy (BKG) towards a combined processing of VLBI and GNSS data.  The main goal of the combined analyses of the two different space-geodetic techniques is the improvement of the consistency between the techniques through common parameters, i.e., mainly Earth Rotation Parameters (ERPs), but also station coordinates and tropospheric parameters through local ties and atmospheric ties, respectively.

Based on our previous combination studies using GNSS data and VLBI Intensive sessions on a daily and multi-day level, we generate a consistent, low-latency ERP time series with a regular daily resolution for polar motion and dUT1. We achieved in this way a significant accuracy improvement of the dUT1 time series and a slight improvement of the pole coordinates time series, comparing ERPs from the combined processing with the individual technique-specific ERPs.

In our recent studies, we extend the combination of GNSS and VLBI Intensive sessions by adding VLBI 24-hour sessions in order to exploit the benefit of the combination to its maximum extend. We analyse the impact of the combination on the global parameters of interest, i.e., mainly dUT1, polar motion and LOD, but also on station coordinates.

BKG’s primary interest is the combination of GNSS and VLBI data on the observation level. However, the current combination efforts are based on the normal equation level using technique-specific SINEX files as a starting point.

How to cite: Lengert, L., Flohrer, C., Girdiuk, A., Hellmers, H., and Thaller, D.: Combination of GNSS and VLBI data for consistent estimation of Earth Rotation Parameters, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10768, https://doi.org/10.5194/egusphere-egu21-10768, 2021.

The terrestrial and celestial reference frames are linked by the Earth Orientation Parameters (EOP), which describe the irregularities of the Earth's rotation and are determined by the space geodetic techniques, namely, Very Long Baseline Interferometry (VLBI), Satellite Laser Ranging (SLR), Global Navigation Satellite Systems (GNSS), and Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS). The satellite geodetic techniques (SLR, GNSS, and DORIS) cannot determine the UT1-UTC or celestial pole offsets (CPO), rendering VLBI the only technique capable of determining full EOP set. On the other hand, the GNSS technique provides precise polar motion estimates due to the continuous observations from a globally distributed network. Integrating VLBI and GNSS provides the full set of EOP and guarantees a superior accuracy than any single-technique solution.

In this study we focus on the integrated estimation of the full EOP set from GNSS and VLBI. Using five VLBI continuous observing campaigns (CONT05–CONT17), the GNSS and VLBI observations are processed concurrently in a common least-squares estimator. The impact of applying global ties (EOP), local ties, and tropospheric ties, and combinations thereof is investigated. The polar motion estimates in integrated solution are dominated by the huge GNSS observations, and the accuracy in terms of weighted root mean squares (WRMS) is ~40 μas compared to the IERS 14 C04 product, which is much better than that of the VLBI-only solution. The UT1-UTC and CPO in the integrated solution also show slight improvement compared to the VLBI-only solution. Moreover, the CPO agreement between the two networks in CONT17, i.e., the VLBA and IVS networks, shows an improvement of 20% to 40% in the integrated solution with different types of ties applied.

How to cite: Wang, J., Balidakis, K., Ge, M., Heinkelmann, R., and Schuh, H.: Earth Orientation Parameter Estimation by Integrating VLBI and GNSS on the Observation Level, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-616, https://doi.org/10.5194/egusphere-egu21-616, 2021.

The Earth orientation parameters (EOP), the regular products of IERS Earth Orientation Centre, are computed at daily bases by combination of EOP solutions using different astro-geodetic techniques. At SYRTE we have developed a strategy of combination of the Global Navigation Satellite Systems (GNSS) and Very Long Baseline Interferometry (VLBI) techniques at normal equation level using Dynamo software maintained by CNES (France). This approach allows to produce the EOP at the daily bases, which contains polar coordinates (x,y) and their rates (x_r,y_r), universal time UT1 and its rate LOD, and corrections from IAU2000A/2006 precession-nutation model (dX,dY), and in the same run station coordinates constituting the terrestrial frame (TRF). The recorded EOP solutions obtained from GNSS and VLBI combination at weekly bases is recently maintained by SYRTE.

The strategy applied to consistently combine the IGS and IVS solutions provided in Sinex format over the time period 2000-2021 are presented and the resulting EOP, station positions (TRF) are analysed and evaluated, differences w.r.t. the individual solutions and the IERS time-series investigated.

How to cite: Richard, J.-Y., Bizouard, C., Lambert, S., and Becker, O.: Earth Orientation Parameters determination by GNSS & VLBI Combination at Normal Equation Level, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2511, https://doi.org/10.5194/egusphere-egu21-2511, 2021.

Satellite Laser Ranging (SLR), i.e., the optical distance measurement to satellites equipped with laser retro-reflectors, has become an invaluable core technique in numerous geodetic applications. For instance, SLR measurements to spherical geodetic satellites, such as LAGEOS-1/2 or Etalon-1/2, form an essential contribution for the determination of geocenter coordinates and global scale in the International Terrestrial Reference Frame (ITRF) realizations.

SLR measurements to active satellites in Low Earth Orbit (LEO) are, on the other hand, up to now mostly used for an independent validation of orbit solutions, usually derived by microwave tracking techniques based on Global Navigation Satellite Systems (GNSS) or Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS). This allows for the analysis of systematic orbit errors (e.g., originating from poorly known satellite center of mass locations or sensor offsets) not only in radial direction, but in three dimensions. A high level of radial orbit reliability is, e.g., key to satellite altimetry applications.

For many of these geodetic SLR applications a mm accuracy and 0.1 mm/year stability is required or at least desired. Unavoidable SLR station biases are a major error source and obstacle to reach the aforementioned accuracy and stability goals. Among the stations of the International Laser Ranging Service (ILRS) there is a large diversity of biases and measurement qualities, and the calibration of these biases for all stations is key to further exploit SLR data for present and future geodetic applications.

In this presentation we demonstrate that the analysis of SLR data to active LEO satellites equipped with GNSS or DORIS receivers is a promising means to analyze SLR biases and their stability. Using three independent selections of Earth observation missions in LEOs with three different SLR analysis software packages (Bernese GNSS Software, Zoom, Napeos), we estimate SLR range biases for all involved tracking stations on a yearly basis. We find that for many of the stations the three independently estimated sets of biases agree on a few-mm level and that the inclusion of satellites from multiple missions allows to render the bias estimation more robust and in particular less prone to geographically correlated orbit errors. This shows that microwave-derived orbits of active LEO satellites, nowadays of very high quality due to numerous advances in modeling and analysis techniques, can serve as interesting sources for SLR station calibration in demanding geodetic applications like, e.g., future ITRF realizations.

How to cite: Arnold, D., Couhert, A., Saquet, E., Peter, H., Mercier, F., and Jäggi, A.: Millimeter-accuracy SLR bias determination using independent multi-LEO DORIS and GNSS-based orbits, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5721, https://doi.org/10.5194/egusphere-egu21-5721, 2021.

The Global Positioning System (GPS) satellite transmitter antenna phase center offsets (PCOs) in z-direction and the scale of the terrestrial reference frame are highly correlated when neither of them is constrained to an a priori value in a least-squares adjustment. The commonly used PCO values offered by the International GNSS Service (IGS) are estimated in a global adjustment by constraining the ground station coordinates to the current International Terrestrial Reference Frame (ITRF). As the scale of the ITRF is determined by other techniques, the estimated GPS z-PCOs are not independent. Consequently, the z-PCOs transfer the scale to any subsequent GNSS solution. To get a GNSS-based scale that can contribute to a future ITRF realization, two methods are proposed to determine scale-independent GPS z-PCOs. One method is based on the gravitational constraint on Low Earth Orbiters (LEOs) in an integrated processing of the GPS satellites and LEOs. The correlation coefficient between the GPS PCO-z and the scale is reduced from 0.85 to 0.3 by supplementing a 54-ground-station network with seven LEOs. The impact of individual LEOs on the estimation is discussed by including different subsets of the LEOs. The accuracy of the z-PCOs of the LEOs is very important for the accuracy of the solution. In another method, the GPS z-PCOs and the scale are determined in a GPS+Galileo processing where the PCOs of Galileo are fixed to the values calibrated on ground from the released metadata. The correlation between the GPS PCO-z and the scale is reduced to 0.13 by including the current constellation of Galileo with 24 satellites. We use the whole constellation of Galileo and the three LEOs of the Swarm mission to perform a direct comparison and cross-check of the two methods. The two methods provide mean GPS z-PCO corrections of -186±25 mm and -221±37 mm with respect to the IGS values, and +1.55±0.22 ppb (part per billion) and +1.72±0.31 ppb in the terrestrial scale with respect to the IGS14 reference frame. The results of both methods agree with each other with only small differences. Due to the larger number of Galileo observations, the Galileo-PCO-fixed method leads to more precise and stable results. In the joint processing of GPS+Galileo+Swarm in which both methods are applied, the constraint on Galileo dominates the results. We also discuss how fixing either the Galileo transmitter antenna z-PCO or the Swarm receiver antenna z-PCOs in the GPS+Galileo+Swarm processing propagates to the respective freely estimated z-PCOs of Swarm or Galileo.

How to cite: Huang, W., Männel, B., Brack, A., and Schuh, H.: GPS z-PCO and GNSS-based scale determined by integrated processing with LEOs and Galileo, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4798, https://doi.org/10.5194/egusphere-egu21-4798, 2021.

The distance between the observatories on Earth and the retro-reflectors on the Moon has been regularly observed by the Lunar Laser Ranging (LLR) experiment since 1970. In the recent years, observations with bigger telescopes (APOLLO) and at infra-red wavelength (OCA) are carried out, resulting in a better distribution of precise LLR data over the lunar orbit and the observed retro-reflectors on the Moon, and a higher number of LLR observations in total. Providing the longest time series of any space geodetic technique for studying the Earth-Moon dynamics, LLR can also support the estimation of Earth orientation parameters (EOP), like UT1. The increased number of highly accurate LLR observations enables a more accurate estimation of the EOP. In this study, we add the effect of non-tidal station loading (NTSL) in the analysis of the LLR data, and determine post-fit residuals and EOP. The non-tidal loading datasets provided by the German Research Centre for Geosciences (GFZ), the International Mass Loading Service (IMLS), and the EOST loading service of University of Strasbourg in France are included as corrections to the coordinates of the LLR observatories, in addition to the standard corrections suggested by the International Earth Rotation and Reference Systems Service (IERS) 2010 conventions. The Earth surface deforms up to the centimetre level due to the effect of NTSL. By considering this effect in the Institute of Geodesy (IfE) LLR model (called ‘LUNAR’), we obtain a change in the uncertainties of the estimated station coordinates resulting in an up to 1% improvement, an improvement in the post-fit LLR residuals of up to 9%, and a decrease in the power of the annual signal in the LLR post-fit residuals of up to 57%. In a second part of the study, we investigate whether the modelling of NTSL leads to an improvement in the determination of EOP from LLR data. Recent results will be presented.

How to cite: Singh, V. V., Biskupek, L., Müller, J., and Zhang, M.: Effect of non-tidal station loading on Lunar Laser Ranging observatories and on the estimation of Earth orientation parameters, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7827, https://doi.org/10.5194/egusphere-egu21-7827, 2021.

Whether utilizing consistent models to describe weather-dependent effects on geodetic observations or a collection of models that yield accurate results individually, has remained an unanswered question in space geodesy. We study the superimposed effect of atmospheric refraction and environmental loading on GNSS and VLBI data analysis. Variable atmospheric refraction and site displacements induced by non-tidal geophysical loading constitute a large contribution to the modern space geodetic data analysis error budget. State-of-the-art weather models such as ECMWF‘s operational analysis and the atmospheric reanalysis ERA5 have proven to be an accurate forcing data set to drive relevant measurement corrections. Since the effects of these phenomena (refraction and loading) on geodetic observables exhibit non-trivial correlations with each other at a multitude of spatio-temporal scales, employing inconsistent data sets may deteriorate the geodetic results, such as the station coordinates and the Earth rotation parameters. The purpose of this contribution is twofold: (i) present our strategy towards consistent weather-dependent models and explore the merits stemming from the adoption thereof, and (ii) evaluate atmospheric delay and geophysical loading models consistently derived from ERA5 via the reanalysis of GNSS and VLBI data. To identify the extent to which the application of inconsistently forced reduction models causes discrepancies in the geodetic adjustment, we carried out a series of Monte Carlo runs. GNSS and VLBI observations were simulated employing ERA5-driven data (ray-traced delays and loading displacements), but reduced by applying a version thereof subjected to systematic and random noise driven from the performance of state-of-the-art models, at the observation equation level. To evaluate the model coupling with real data, we conduct a GNSS and VLBI repro and compare our new solutions to the GFZ‘s contribution to ITRF2020.

How to cite: Balidakis, K., Zus, F., Dobslaw, H., Männel, B., Thomas, M., and Schuh, H.: Consistent Weather-Dependent Corrections for Space Geodesy; Validation via GNSS and VLBI Analysis, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-182, https://doi.org/10.5194/egusphere-egu21-182, 2021.