Differencing strategies for SLR observations at the Wettzell observatory

The precise estimation of geodetic parameters using single- and double-differenced SLR observations is investigated. While the differencing of observables is a standard approach for the GNSS processing, double differences of simultaneous SLR observations are practically impossible to obtain due to the SLR basic principle of observing one satellite at a time. Despite this, the availability of co-located SLR telescopes and the use of the alternative concept of quasi-simultaneity allow the forming of SLR differences under certain assumptions, thus enabling the use of these processing strategies. These differences are in principle almost free of both, satellite- and station-specific error sources, and are shown to be a valuable tool to obtain relative coordinates and range biases, and to validate local ties. Tested with the two co-located SLR telescopes at the Geodetic Observatory Wettzell (Germany) using SLR observations to GLONASS and LAGEOS, the developed differencing approach shows that it is possible to obtain single- and double-difference residuals at the millimetre level, and that it is possible to estimate parameters, such as range biases at the stations and the local baseline vector with a precision at the millimetre level and an accuracy comparable to traditional terrestrial survey methods. The presented SLR differences constitute a valuable alternative for the monitoring of the local baselines and the estimation of geodetic parameters.

Multi-GNSS Slant Wet Delay Retrieval Using Multipath Mitigation Maps

<p>The conventional Global Navigation Satellite System (GNSS) processing is typically contaminated with errors due to atmospheric variabilities, such as those associated with the mesoscale phenomena. These errors are manifested in the parameter estimates, including station coordinates and atmospheric products. To enhance the accuracy of these GNSS products further, a better understanding of the local-scale atmospheric variability is necessary. As part of multi-GNSS processing, station coordinates, carrier phase ambiguities, orbits, zenith total delay (ZTD) and horizontal gradients are the main parameters of interest. Here, ZTD is estimated as the average zenith delay along the line-of-sight to every observed GNSS satellite mapped to the vertical while the horizontal gradients are estimated in NS and EW directions and provide a means to partly account for the azimuthally inhomogeneous atmosphere. However, a better atmospheric description is possible by evaluating the slant path delay (SPD) or slant wet delay (SWD) along GNSS ray paths, which are not resolved by ordinary ZTD and gradient analysis. SWD is expected to provide better information about the inhomogeneous distribution of water vapour that is disregarded when retrieving ZTD and horizontal gradients. Usually, SWD cannot be estimated directly from GNSS processing as the number of unknown parameters exceeds the number of observations. Thus, SWD is generally calculated from ZTD for each satellite and may be dominated by un-modelled atmospheric delays, clock errors, unresolved carrier-phase ambiguities and near-surface multipath scattering.</p><p> </p><p>In this work, we have computed multipath maps by stacking individual post-fit carrier residuals incorporating the signals from four GNSS constellations, i.e. BeiDou, Galileo, Glonass and GPS. We have selected a subset of global International GNSS Service (IGS) stations capable of multi-GNSS observables located in different climatic zones. The multipath effects are reduced by subtracting the stacked multipath maps from the raw post-fit carrier phase residuals. We demonstrate that the multipath stacking technique results in significantly reduced variations in the one-way post-fit carrier phase residuals. This is particularly evident for lower elevation angles, thus, producing a retrieval method for SWD that is less affected by site-specific multipath effects. We show a positive impact on SWD estimation using our multipath maps during increased atmospheric inhomogeneity as induced by severe weather events.</p>

Station Classification and Reference Station Selection

<p>When using a network approach, expressing reliably GNSS position and velocities in a given reference frame (ITRF2014, IGS14, …) requires the identification of ‘stable’ and ‘rebliable’ reference stations. The choice of these reference stations can have a non-negligible impact on the estimated positions and velocities and of course on the derived geodynamic interpretations.</p><p>This study will present the work done to address this issue within EUREF and help the users of the EUREF products (more specifically of the EPN multi-year position and velocity solution) to identify the best reference stations in the EUREF Permanent Network (EPN). To that aim, in addition to a station classification, a web tool (https://epncb.oma.be/_productsservices/ReferenceFrame/) has been developed in order to discover the most suitable EPN reference stations. The primary goal of this tool is to help the user of EUREF reference frame product select suitable EPN reference stations to be added to his network during the preparation of own GNSS processing.</p><p>The tool helps the selection of optimal reference stations:</p><ul><li>by providing a restricted list of reference stations (based on the station classification and the begin and end date of the user processing)</li> <li>by giving access to additional information (number of position or velocity discontinuities, post-seismic deformation,…) and plots (detrended position time series, selection criteria values, velocity variability) for the stations.</li> </ul><p>The web tool as well as the station classification will be presented.</p>

GNSS antenna calibration tables evaluated by means of large volume metrology

<p>We have developed a novel method in which a pair of GNSS antennas with similar characteristics are used to evaluate hidden systematic errors in existing GNSS calibrations with the help of high-end industrial metrology equipment. We tilt the calibrated antennas out of parallel and sort the observations in individual antenna reference frames rather than epoch time. With the combined and compared measurements, we can sort out the different elevation dependent uncertainties in the GNSS observations and quantify the errors of the calibration methods. We show the extent to which the calibration method error systematically maps as troposphere and height components in the GNSS processing and in the worst case found this to be > 1 cm in the vertical when using the ionosphere-free frequency combination L3. While showing results in the presentation for the full elevation range in 5° elevation cells, we report here the 1σ uncertainties of our method for 30° elevation at ±0.38 mm on L1 and ±0.62 mm on L3 with respect to the antenna phase centers. Once uncertainties have been characterized at this level, the etalon antennas can be deployed as space geodetic anchor points at core sites without compromising existing installations.</p>

Impact of solutions on the identification of tectonic transients: A case study using the last decade of GNSS data in South America

<p>GNSS derived displacement time series are used to track plate tectonics and the associated motions across major plate boundaries. With a growing number of continuous GNSS observations, it is becoming increasingly apparent that plate trajectories rarely conform to standard trajectory models. The deviations from these expected trajectories can be considered as transient motions, some being tectonically related, and others being related to geophysical fluid loading, local site effects, and artifacts of the GNSS processing. As we increasingly inspect the transient motions of GNSS displacement time series, we have to take care that the GNSS processing choices, such as the reference frame<!-- The reference frame is already the realization of the system. Just a terminology fineness. -->, are not introducing non-tectonically related artifacts into the eventual isolated tectonic signals.</p><p>Here we explore the effects that different solutions <!-- Take up the abstract title. -->and processing strategies have on our eventual daily GNSS displacement time series - the aim being to explain how artifacts arise and to determine which strategies best mitigate these artifacts. We compare displacement time series made from both Precise Point Positioning and network (double-differenced) solutions that are provided in the latest official reference frame solution ITRF2014, and in JTRF2014 based on Kalman filtering.</p><p>In our analyses, we use approximately one hundred GNSS stations from South America, with a focus being to identify transient tectonic activity related to the subduction of the Nazca plate under Chile over the past decade.</p><p> </p>

On the Role of GNSS Receivers for Antenna Patterns and Parameter Estimations

<p>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.</p><p>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.</p><p>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.</p>

Sub-daily polar motion from GPS, GLONASS, and Galileo

We derive an empirical model of the sub-daily polar motion (PM) based on the multi-GNSS processing incorporating GPS, GLONASS, and Galileo observations. The sub-daily PM model is based on 3-year multi-GNSS solutions with a 2 h temporal resolution. Firstly, we discuss differences in sub-daily PM estimates delivered from individual GNSS constellations, including GPS, GLONASS, Galileo, and the combined multi-GNSS solutions. Secondly, we evaluate the consistency between the GNSS-based estimates of the sub-daily PM with three independent models, i.e., the model recommended in the International Earth Rotation and Reference Systems Service (IERS) 2010 Conventions, the Desai–Sibois model, and the Gipson model. The sub-daily PM estimates, which are derived from system-specific solutions, are inherently affected by artificial non-tidal signals. These signals arise mainly from the resonance between the Earth rotation period and the satellite revolution period. We found strong spurious signals in GLONASS-based and Galileo-based results with amplitudes up to 30 µas. The combined multi-GNSS solution delivers the best estimates and the best consistency of the sub-daily PM with external geophysical and empirical models. Moreover, the impact of the non-tidal spurious signals in the frequency domain diminishes in the multi-GNSS combination. After the recovery of the tidal coefficients for 38 tides, we infer better consistency of the GNSS-based empirical models with the new Desai–Sibois model than the model recommended in the IERS 2010 Conventions. The consistency with the Desai–Sibois model, in terms of the inter-quartile ranges of tidal amplitude differences, reaches the level of 1.6, 5.7, 6.3, 2.2 µas for the prograde diurnal tidal terms and 1.2/2.1, 2.3/6.0, 2.6/5.5, 2.1/5.1 µas for prograde/retrograde semi-diurnal tidal terms, for the combined multi-GNSS, GPS, GLONASS, and Galileo solutions, respectively.