IGS contribution to the ITRF

We examine the contribution of the International GNSS Service (IGS) to the International Terrestrial Reference Frame (ITRF) by evaluating the quality of the incorporated solutions as well as their major role in the ITRF formation. Starting with the ITRF2005, the ITRF is constructed with input data in the form of time series of station positions (weekly for satellite techniques and daily for VLBI) and daily Earth Orientation Parameters. Analysis of time series of station positions is a fundamental first step in the ITRF elaboration, allowing to assess not only the stations behavior, but also the frame parameters and in particular the physical ones, namely the origin and the scale. As it will be seen, given the poor number and distribution of SLR and VLBI co-location sites, the IGS GPS network plays a major role by connecting these two techniques together, given their relevance for the definition of the origin and the scale of the ITRF. Time series analysis of the IGS weekly combined and other individual Analysis Center solutions indicates an internal precision (or repeatability) <2 mm in the horizontal component and <5 mm in the vertical component. Analysis of three AC weekly solutions shows generally poor agreement in origin and scale, with some indication of better agreement when the IGS started to use the absolute model of antenna phase center variations after the GPS week 1400 (November 2006).     

DORIS and the Determination of the Earth’s Polar Motion

DORIS (Détermination d’Orbite et Radiopositionnement Intégrés par Satellite) is a system used for precise orbit determination (POD) and ground-station positioning. It has been implemented on-board various satellites: the SPOT (Système pour l’Observation de la Terre) remote sensing satellites SPOT-2, SPOT-3, SPOT-4, SPOT-5, TOPEX/Poseidon and more recently on its successors Jason-1 and ENVISAT. DORIS is also a terrestrial positioning system that has found many applications in geophysics and geodesy; in particular, it contributes to the realization of the International Terrestrial Reference Frame, ITRF2000 and the forthcoming ITRF2005. Although not its primary objective, DORIS can bring information on Earth orientation monitoring, mainly polar motion and length of day (LOD) variations that complement other astrogeodetic techniques. In this paper, we have analyzed various recent polar motion solutions derived from independent analysis centers using different software packages and applying various analysis strategies. Comparisons of these solutions to the International Earth Rotation and Reference Systems Service (IERS) C04 solution are performed. Depending on the solutions, the accuracy of DORIS polar components are in the range of 0.5–1 mas corresponding to a few centimeters on the Earth’s surface. This is approximately ten times larger than results derived from GPS, which are typically 0.06 mas in both components. This does not allow DORIS results to be taken into account in the IERS–EOP combinations. A gain in the precision could come from technical improvements to the DORIS system, in addition to improvement of the orbit, tropospheric, ionospheric and Earth gravity field modeling.     

Plate kinematics of Nubia–Somalia using a combined DORIS and GPS solution

We have used up to 12 years of data to assess DORIS performance for geodynamics applications. We first examine the noise characteristics of the DORIS time-series of weekly station coordinates to derive realistic estimates of velocity uncertainties. We find that a combination of white and flicker noise best explains the DORIS time-series noise characteristics. Second, weekly solutions produced by the Institut Géographique National/Jet Propulsion Laboratory (IGN/JPL) DORIS Analysis Centre are combined to derive a global velocity field. This solution is combined with two independent GPS solutions, including 11 sites on Nubia and 5 on the Somalia plate. The combination indicates that DORIS horizontal velocities have an average accuracy of 3 mm/year, with best-determined sites having velocity accuracy better than 1 mm/year (one-sigma levels). Using our combined velocity field, we derive an updated plate kinematics model with a focus on the Nubia–Somalia area. Including DORIS data improves the precision of the angular velocity vector for Nubia by 15%. Our proposed model provides robust bounds on the maximum opening rates along the East African Rift (4.7–6.7 mm/year). It indicates opening rates 15 and 7% slower than values predicted by NUVEL-1A for the southern Atlantic Ocean and Indian Ocean, respectively. These differences are likely to arise from the fact that NUVEL-1A considered Africa as a single non-deforming plate, while here we use a more refined approach.     

A model of present-day tectonic plate motions from 12 years of DORIS measurements

In the frame of the International DORIS Service (IDS), the Laboratoire d’Etudes en Géophysique et Océanographie Spatiales (LEGOS)/Collecte Localisation Satellites (CLS) Analysis Center (LCA) processes DORIS measurements from the SPOT, TOPEX/Poseidon and Envisat satellites and provides weekly station coordinates of the whole network to the IDS. Based on DORIS measurements, the horizontal and vertical velocities of 57 DORIS sites are computed. The 3D positions and velocities of the stations with linear motion are estimated simultaneously from the 12-year (1993–2004) combined normal equation matrix. We include 35 DORIS sites assumed to be located in the stable zones of 9 tectonic plates. For the motion of these plates, we propose a model (LCAVEL-1) of angular velocities in the ITRF2000 reference frame. Based on external comparison with the most recent global plate models (PB2002, REVEL, GSRM-1 and APKIM2000) and on internal analysis, we estimate an average velocity error of the DORIS solution of less than 3 mm/year. The LCAVEL-1 model presents new insights of the Somalia/Nubia pair of plates, as the DORIS technique has the advantage of having a few stations located on those two plates. We also computed (and provide in this article) the horizontal motion of the sites located close to plate boundaries or in the deformation zones defined in contemporary models. These computations could be used in further analysis for these particular regions of the Earth not moving as rigid plates.     

Plate Motion of India and Interseismic Strain in the Nepal Himalaya from GPS and DORIS Measurements

We analyse geodetically estimated deformation across the Nepal Himalaya in order to determine the geodetic rate of shortening between Southern Tibet and India, previously proposed to range from 12 to 21 mm yr^?1. The dataset includes spirit-levelling data along a road going from the Indian to the Tibetan border across Central Nepal, data from the DORIS station on Everest, which has been analysed since 1993, GPS campaign measurements from surveys carried on between 1995 and 2001, as well as data from continuous GPS stations along a transect at the logitude of Kathmandu operated continuously since 1997. The GPS data were processed in International Terrestrial Reference Frame 2000 (ITRF2000), together with the data from 20 International GNSS Service (IGS) stations and then combined using quasi- observation combination analysis (QOCA). Finally, spatially complementary velocities at stations in Southern Tibet, initially determined in ITRF97, were expressed in ITRF2000. After analysing previous studies by different authors, we determined the pole of rotation of the Indian tectonic plate to be located in ITRF2000 at 51.409±1.560° N and ?10.915±5.556°E, with an angular velocity of 0.483±0.015°. Myr^?1. Internal deformation of India is found to be small, corresponding to less than about 2 mm yr^?1 of baseline change between Southern India and the Himalayan piedmont. Based on an elastic dislocation model of interseismic strain and taking into account the uncertainty on India plate motion, the mean convergence rate across Central and Eastern Nepal is estimated to 19±2.5 mm yr^?1, (at the 67% confidence level). The main himalayan thrust (MHT) fault was found to be locked from the surface to a depth of about 20 km over a width of about 115 km. In these regions, the model parameters are well constrained, thanks to the long and continuous time-series from the permanent GPS as well as DORIS data. Further west, a convergence rate of 13.4±5 mm yr^?1, as well as a fault zone, locked over 150 km, are proposed. The slight discrepancy between the geologically estimated deformation rate of 21±1.5 mm yr^?1 and the 19±2.5 mm yr^?1 geodetic rate in Central and Eastern Nepal, as well as the lower geodetic rate in Western Nepal compared to Eastern Nepal, places bounds on possible temporal variations of the pattern and rate of strain in the period between large earthquakes in this region.     

Defining a DORIS core network for Jason-1 precise orbit determination based on ITRF2000: methods and realization

In view of the future adoption of the new precise orbit determination (POD) standards for the TOPEX/Poseidon and Jason-1 satellites, we propose a method to evaluate terrestrial reference frames for POD. We applied this method to the ITRF2000 realization of the DORIS network using local geodetic ties, plate motion models, the recent DORIS IGN04D02 cumulative solution and DORIS weekly time-series of coordinates. We propose to adopt a selection of the ITRF2000 realization based on specific criteria that we define here, and to extend it with ground stations for which we propose new coordinates and velocities. Only 13 out of 131 stations were considered to be inappropriate for POD activities. The result is a robust and well-distributed DORIS core network of 118 stations (DPOD2000) suitable for POD during the 1993–2008 period considered here.     

The 2008 DGFI realization of the ITRS: DTRF2008

A new realization of the International Terrestrial System was computed at the ITRS Combination Centre at DGFI as a contribution to ITRF2008. The solution is labelled DTRF2008. In the same way as in the DGFI computation for ITRF2005 it is based on either normal equation systems or estimated parameters derived from VLBI, SLR, GPS and DORIS observations by weekly or session-wise processing. The parameter space of the ITRS realization comprises station positions and velocities and daily resolved Earth Orientation Parameters (EOP), whereby for the first time also nutation parameters are included. The advantage of starting from time series of input data is that the temporal behaviour of geophysical parameters can be investigated to decide whether the parameters can contribute to the datum realization of the ITRF. In the same way, a standardized analysis of station position time series can be performed to detect and remove discontinuities. The advantage of including EOP in the ITRS realization is twofold: (1) the combination of the coordinates of the terrestrial pole—estimated from all contributing techniques—links the technique networks in two components of the orientation, leading to an improvement of consistency of the Terrestrial Reference Frame (TRF) and (2) in their capacity as parameters common to all techniques, the terrestrial pole coordinates enhance the selection of local ties as they provide a measure for the consistency of the combined frame. The computation strategy of DGFI is based on the combination of normal equation systems while at the ITRS Combination Centre at IGN solutions are combined. The two independent ITRS realizations provide the possibility to assess the accuracy of ITRF by comparison of the two frames. The accuracy evaluation was done separately for the datum parameters (origin, orientation and scale) and the network geometry. The accuracy of the datum parameters, assessed from the comparison of DTRF2008 and ITRF2008, is between 2–5?mm and 0.1–0.8?mm/year depending on the technique. The network geometry (station positions and velocities) agrees within 3.2?mm and 1.0?mm/year. A comparison of DTRF2008 and ITRF2005 provides similar results for the datum parameters, but there are larger differences for the network geometry. The internal accuracy of DTRF2008—that means the level of conservation of datum information and network geometry within the combination—was derived from comparisons with the technique-only multi-year solutions. From this an internal accuracy of 0.32?mm for the VLBI up to 3.3?mm for the DORIS part of the network is found. The internal accuracy of velocities ranges from 0.05?mm/year for VLBI to 0.83?mm/year for DORIS. The internal consistency of DTRF2008 for orientation can be derived from the analysis of the terrestrial pole coordinates. It is estimated at 1.5–2.5?mm for the GPS, VLBI and SLR parts of the network. The consistency of these three and the DORIS network part is within 6.5?mm.     

The International DORIS Service: genesis and early achievements

All space-geodetic techniques are now organized as separate services of the International Association of Geodesy (IAG), supporting the first pilot project “Global Geodetic Observing System (GGOS)”. The International DORIS (Détermination d’Orbite et Radiopositionnement Intégrés par Satellite) Service (IDS) was created in mid-2003 to organize a DORIS contribution to this project and to foster a larger international cooperation on this topic. The goal of this paper is to summarize the key steps that were taken to create this structure and to present its current organization and recent results. At present, more than 50 groups from 35 different countries participate in the IDS at various levels, including 43 groups hosting DORIS stations in 32 countries all around the globe. Four Analysis Centres (ACs) provide results, such as estimates of weekly or monthly station coordinates, geocentre variations or Earth polar motion, that will soon be used to generate IDS-combined products for geodesy and geodynamics. As a first test, a preliminary combination was performed for all the 2004 data from these four ACs. Three of them show RMS of weighted station residuals with respect to this combination solution between 1 and 2 cm. The main topic under investigation is a discrepancy in the scale factor of the terrestrial reference frame (TRF) to map the individual solutions into the combination solution, which reaches 6 cm (multiplying the unit-less scale factor by the Earth radius to get convert scale to millimetre in vertical at the Earth’s surface). Finally, foreseen improvements of the DORIS technology are discussed as well as future improvements concerning the service organization itself and the accuracy and reliability of its scientific products.     

VLBI terrestrial reference frame contributions to ITRF2008

In late 2008, the Product Center for the International Terrestrial Reference Frame (ITRF) of the International Earth Rotation and Reference Systems Service (IERS) issued a call for contributions to the next realization of the International Terrestrial Reference System, ITRF2008. The official contribution of the International VLBI Service for Geodesy and Astrometry (IVS) to ITRF2008 consists of session-wise datum-free normal equations of altogether 4,539 daily Very Long Baseline Interferometry (VLBI) sessions from 1979.7 to 2009.0 including data of 115 different VLBI sites. It is the result of a combination of individual series of session-wise datum-free normal equations provided by seven analysis centers (ACs) of the IVS. All series are completely reprocessed following homogeneous analysis options according to the IERS Conventions 2003 and IVS Analysis Conventions. Altogether, nine IVS ACs analyzed the full history of VLBI observations with four different software packages. Unfortunately, the contributions of two ACs, Institute of Applied Astronomy (IAA) and Geoscience Australia (AUS), had to be excluded from the combination process. This was mostly done because the IAA series exhibits a clear scale offset while the solution computed from normal equations contained in the AUS SINEX files yielded unreliable results. Based on the experience gathered since the combination efforts for ITRF2005, some discrepancies between the individual series were discovered and overcome. Thus, the consistency of the individual VLBI solutions has improved considerably. The agreement in terms of WRMS of the Terrestrial Reference Frame (TRF) horizontal components is 1 mm, of the height component 2 mm. Comparisons between ITRF2005 and the combined TRF solution for ITRF2008 yielded systematic height differences of up to 5 mm with a zonal signature. These differences can be related to a pole tide correction referenced to a zero mean pole used by four of five IVS ACs in the ITRF2005 contribution instead of a linear mean pole path as recommended in the IERS Conventions. Furthermore, these systematics are the reason for an offset in the scale of 0.4 ppb between the IVS’ contribution to ITRF2008 and ITRF2005. The Earth orientation parameters of seven series used as input for the IVS combined series are consistent to a huge amount with about 50 μas WRMS in polar motion and 3 μs in dUT1.     

A corrective model for Jason-1 DORIS Doppler data in relation to the South Atlantic Anomaly

The DORIS Doppler measurements collected by Jason-1 are abnormally perturbed by the influence of the South Atlantic Anomaly (SAA). The DORIS ultra-stable oscillators on-board Jason-1 are not as stable as they should be; their frequency is sensitive both to the irradiation rate and to the total irradiation encountered in orbit. The consequence is that not only are the DORIS measurement residuals higher than they ought to be, but also large systematic positioning errors are introduced for stations located in the vicinity of the SAA. In this paper, we present a method that has been devised to obtain a continuous observation of Jason-1 frequency offsets. This method relies on the precise determination of the station frequency and troposphere parameters via the use of other DORIS satellites. More than 3 years of these observations have then been used to construct a model of response of the oscillators of Jason-1 to the SAA. The sensitivity of the Jason-1 oscillators to the SAA perturbations has evolved over time, multiplied by a factor of four between launch and mid-2004. The corrective performances of the model are discussed in terms of DORIS measurement residuals, precise orbit determination and station positioning. The average DORIS measurement residuals are decreased by more than 7 % using this model. In terms of precise orbit determination, the 3D DORIS-only orbit error decreases from 5 to 4.2 cm, but the DORIS+SLR orbit error is almost unaffected, due to the already good quality of this type of orbit. In terms of station positioning, the model brings down the average 3D mono-satellite monthly network solution discrepancy with the International Terrestrial Reference Frame ITRF2000 from 11.3 to 6.1 cm, and also decreases the scatter about that average from 11.3 to 3.7 cm. The conclusion is that, with this model, it is possible to re-incorporate Jason-1 in the multi-satellite geodetic solutions for the DORIS station network.     

ITRF2008: an improved solution of the international terrestrial reference frame

ITRF2008 is a refined version of the International Terrestrial Reference Frame based on reprocessed solutions of the four space geodetic techniques: VLBI, SLR, GPS and DORIS, spanning 29, 26, 12.5 and 16?years of observations, respectively. The input data used in its elaboration are time series (weekly from satellite techniques and 24-h session-wise from VLBI) of station positions and daily Earth Orientation Parameters (EOPs). The ITRF2008 origin is defined in such a way that it has zero translations and translation rates with respect to the mean Earth center of mass, averaged by the SLR time series. Its scale is defined by nullifying the scale factor and its rate with respect to the mean of VLBI and SLR long-term solutions as obtained by stacking their respective time series. The scale agreement between these two technique solutions is estimated to be 1.05 ± 0.13 ppb at epoch 2005.0 and 0.049 ± 0.010?ppb/yr. The ITRF2008 orientation (at epoch 2005.0) and its rate are aligned to the ITRF2005 using 179 stations of high geodetic quality. An estimate of the origin components from ITRF2008 to ITRF2005 (both origins are defined by SLR) indicates differences at epoch 2005.0, namely: ?0.5, ?0.9 and ?4.7?mm along X, Y and Z-axis, respectively. The translation rate differences between the two frames are zero for Y and Z, while we observe an X-translation rate of 0.3?mm/yr. The estimated formal errors of these parameters are 0.2?mm and 0.2?mm/yr, respectively. The high level of origin agreement between ITRF2008 and ITRF2005 is an indication of an imprecise ITRF2000 origin that exhibits a Z-translation drift of 1.8?mm/yr with respect to ITRF2005. An evaluation of the ITRF2008 origin accuracy based on the level of its agreement with ITRF2005 is believed to be at the level of 1?cm over the time-span of the SLR observations. Considering the level of scale consistency between VLBI and SLR, the ITRF2008 scale accuracy is evaluated to be at the level of 1.2?ppb (8?mm at the equator) over the common time-span of the observations of both techniques. Although the performance of the ITRF2008 is demonstrated to be higher than ITRF2005, future ITRF improvement resides in improving the consistency between local ties in co-location sites and space geodesy estimates.     

First results of DORIS data analysis at Geodetic Observatory Pecný

In a cooperation between the Astronomical Institute, University of Bern (AIUB), the Geodetic Observatory Pecný (GOPE), and the Institut Géographique National (IGN), DORIS data analysis capabilities were implemented into a development version of the Bernese GPS software. The DORIS Doppler observables are reformulated such that they are similar to global navigation satellite system (GNSS) carrier-phase observations, allowing the use of the same observation models and algorithms as for GNSS carrier-phase data analysis with only minor software modifications. As such, the same algorithms may be used to process DORIS carrier-phase observations. First results from the analysis of 3 weeks of DORIS data (September 2004, five DORIS-equipped satellites) at GOPE are promising and are presented here. They include the comparison of station coordinates with coordinate estimates derived by the Laboratoire d’Etudes en Géophysique et Océanographie Spatiale/Collecte Localisation Satellites analysis centre (LCA) and the Institut Géographique National/Jet Propulsion Laboratory (IGN/JPL), and the comparison of Earth orientation parameters (EOPs) with the International Earth Rotation and Reference Frames Service (IERS) C04 model. The modified Bernese results are of a slightly lower, but comparable, quality than corresponding solutions routinely computed within the IDS (International DORIS Service). The weekly coordinate repeatability RMS is of the order of 2–3 cm for each 3D station coordinate. Comparison with corresponding estimates of station coordinates from current IDS analysis centers demonstrates similar precision. Daily pole component estimates show a mean difference from IERS-C04 of 0.6  mas in X _p and  ? 0.5  mas in Y _p and a RMS of 0.8  mas in X _p and 0.9  mas in Y _p (mean removed). An automatic analysis procedure is under development at GOPE, and routine DORIS data processing will be implemented in the near future.     

CPLat: first operational experimental processing center for SIRGAS in Argentina

The SIRGAS permanent GPS network which is in fact the IGS network densification for the American continent, consists today of more than 200 stations covering the continent and islands. It is currently processed by the IGS RNAAC SIR centre at Deutsches Geodätisches Forschungsinstitut producing weekly free solutions relying on IGS final orbits and EOP that contribute to the ITRF through IGS. By August 2006, the SIRGAS Working Group I had accepted five proposals for experimental processing centers within the region that would collaborate with IGS RNAAC SIR. One of them, Centro de Procesamiento La Plata (CPLat) in Argentina, began processing 60 stations on October 2006. By January 2007 CPLat reached operational capability, delivering weekly free solution SINEX files, with an internal consistency of 1.5 mm average for the horizontal components, and 3 mm in the vertical. Comparisons with IGS global and IGS RNAAC SIR weekly solutions were taken as external consistency indications, showing average RMS residuals of 1.8, 2.4 and 5 mm for the north, east, and vertical component, respectively. Analysis and comparison of adjusted solution time series from CPLat and other processing centers has proved to be highly valuable for solution QC, namely detection and identification of station anomalous behavior or modelling problems. These procedures will ensure the maintenance of the performance specifications for CPLat solutions. Action is being taken in order to guarantee the continuity of this effort beyond the experimental phase.     

Estimated SLR station position and network frame sensitivity to time-varying gravity

This paper evaluates the sensitivity of ITRF2008-based satellite laser ranging (SLR) station positions estimated weekly using LAGEOS-1/2 data from 1993 to 2012 to non-tidal time-varying gravity (TVG). Two primary methods for modeling TVG from degree-2 are employed. The operational approach applies an annual GRACE-derived field, and IERS recommended linear rates for five coefficients. The experimental approach uses low-order/degree $4\times 4$ coefficients estimated weekly from SLR and DORIS processing of up to 11 satellites (tvg4x4). This study shows that the LAGEOS-1/2 orbits and the weekly station solutions are sensitive to more detailed modeling of TVG than prescribed in the current IERS standards. Over 1993–2012 tvg4x4 improves SLR residuals by 18 % and shows 10 % RMS improvement in station stability. Tests suggest that the improved stability of the tvg4x4 POD solution frame may help clarify geophysical signals present in the estimated station position time series. The signals include linear and seasonal station motion, and motion of the TRF origin, particularly in Z. The effect on both POD and the station solutions becomes increasingly evident starting in 2006. Over 2008–2012, the tvg4x4 series improves SLR residuals by 29 %. Use of the GRGS RL02 $50\times 50$ series shows similar improvement in POD. Using tvg4x4, secular changes in the TRF origin Z component double over the last decade and although not conclusive, it is consistent with increased geocenter rate expected due to continental ice melt. The test results indicate that accurate modeling of TVG is necessary for improvement of station position estimation using SLR data.     

Quality assessment of GPS reprocessed terrestrial reference frame

The International GNSS Service (IGS) contributes to the construction of the International Terrestrial Reference Frame (ITRF) by submitting time series of station positions and Earth Rotation Parameters (ERP). For the first time, its submission to the ITRF2008 construction is based on a combination of entirely reprocessed GPS solutions delivered by 11 Analysis Centers (ACs). We analyze the IGS submission and four of the individual AC contributions in terms of the GNSS frame origin and scale, station position repeatability and time series seasonal variations. We show here that the GPS Terrestrial Reference Frame (TRF) origin is consistent with Satellite laser Ranging (SLR) at the centimeter level with a drift lower than 1 mm/year. Although the scale drift compared to Very Long baseline Interferometry (VLBI) and SLR mean scale is smaller than 0.4 mm/year, we think that it would be premature to use that information in the ITRF scale definition due to its strong dependence on the GPS satellite and ground antenna phase center variations. The new position time series also show a better repeatability compared to past IGS combined products and their annual variations are shown to be more consistent with loading models. The comparison of GPS station positions and velocities to those of VLBI via local ties in co-located sites demonstrates that the IGS reprocessed solution submitted to the ITRF2008 is more reliable and precise than any of the past submissions. However, we show that some of the remaining inconsistencies between GPS and VLBI positioning may be caused by uncalibrated GNSS radomes.     

A new combined European permanent network station coordinates solution

The EUREF [International Association of Geodesy (IAG) Reference Frame Sub-Commission for Europe] network of continuously operating GPS stations (EPN) was primarily established for reference frame maintenance, and also plays an important role for geodynamical research in Europe. The main objective of this paper is to obtain an independent homogeneous time series of the EPN station coordinates, which is also available in SINEX format. A new combined solution of the EPN station coordinates was computed. The combination was performed independently for every week, in three steps: (1) the stated constraints on the coordinates were removed from the individual solutions of the Analysis Centers; (2) the de-constrained solutions were aligned to ITRF2000; (3) the resulting solutions were combined using the Helmert blocking technique. All the data from GPS weeks 900 to 1302 (April 1997–December 2004) were used. We investigated in detail the behavior of the transformation parameters aligning the new combined solution to ITRF2000. In general, the time series of the transformation parameters show a good stability in time although small systematic effects can be seen, most likely caused by station instabilities. A comparison of the new combined solution to the official EUREF weekly combined solution is also presented.     

On the long-term stability of terrestrial reference frame solutions based on Kalman filtering

The Global Geodetic Observing System requirement for the long-term stability of the International Terrestrial Reference Frame is 0.1 mm/year, motivated by rigorous sea level studies. Furthermore, high-quality station velocities are of great importance for the prediction of future station coordinates, which are fundamental for several geodetic applications. In this study, we investigate the performance of predictions from very long baseline interferometry (VLBI) terrestrial reference frames (TRFs) based on Kalman filtering. The predictions are computed by extrapolating the deterministic part of the coordinate model. As observational data, we used over 4000 VLBI sessions between 1980 and the middle of 2016. In order to study the predictions, we computed VLBI TRF solutions only from the data until the end of 2013. The period of 2014 until 2016.5 was used to validate the predictions of the TRF solutions against the measured VLBI station coordinates. To assess the quality, we computed average WRMS values from the coordinate differences as well as from estimated Helmert transformation parameters, in particular, the scale. We found that the results significantly depend on the level of process noise used in the filter. While larger values of process noise allow the TRF station coordinates to more closely follow the input data (decrease in WRMS of about 45%), the TRF predictions exhibit larger deviations from the VLBI station coordinates after 2014 (WRMS increase of about 15%). On the other hand, lower levels of process noise improve the predictions, making them more similar to those of solutions without process noise. Furthermore, our investigations show that additionally estimating annual signals in the coordinates does not significantly impact the results. Finally, we computed TRF solutions mimicking a potential real-time TRF and found significant improvements over the other investigated solutions, all of which rely on extrapolating the coordinate model for their predictions, with WRMS reductions of almost 50%.     

The IGS-combined station coordinates, earth rotation parameters and apparent geocenter

The International GNSS Service (IGS) routinely generates a number of weekly, daily and sub-daily products. Station coordinates and velocities, earth rotation parameters (ERPs) and apparent geocenter are among these products generated weekly by the IGS Reference Frame Coordinator. They have been determined since 1999 by combining independent estimates from at least seven IGS Analysis Centers (ACs). Two Global Network Associate Analysis Centers (GNAACs) also provide independent combinations using the same AC weekly solutions and they are currently used to quality control the IGS combination. The combined solutions are aligned to an IGS realization (IGS05) of the ITRF2005 using a carefully selected set of the IGS Reference Frame (RF) stations (nominally 132). During the combination process, the contributing solutions are compared and outliers are removed to ensure a high level of consistency of the estimated parameters. The ACs and the weekly combined solution are consistent at the 1–2 and 3–4 mm levels for the horizontal and vertical components. Similarly, the excess Length of Day (LOD), the pole positions and pole rates are consistent at the 10μs, 0.03–0.05 mas and 0.10–0.20 mas/day levels, respectively. The consistency of the apparent geocenter estimate is about 5 mm in the X and Y components and 10 mm in the Z component. Comparison of the IGS-combined ERP estimates with the IERS Bulletin A suggests a small bias of the order of ?0.04 mas and + 0.05 mas (both ±0.05 mas) in the x and y components.     

Strategies to mitigate aliasing of loading signals while estimating GPS frame parameters

Although GNSS techniques are theoretically sensitive to the Earth center of mass, it is often preferable to remove intrinsic origin and scale information from the estimated station positions since they are known to be affected by systematic errors. This is usually done by estimating the parameters of a linearized similarity transformation which relates the quasi-instantaneous frames to a long-term frame such as the International Terrestrial Reference Frame (ITRF). It is well known that non-linear station motions can partially alias into these parameters. We discuss in this paper some procedures that may allow reducing these aliasing effects in the case of the GPS techniques. The options include the use of well-distributed sub-networks for the frame transformation estimation, the use of site loading corrections, a modification of the stochastic model by downweighting heights, or the joint estimation of the low degrees of the deformation field. We confirm that the standard approach consisting of estimating the transformation over the whole network is particularly harmful for the loading signals if the network is not well distributed. Downweighting the height component, using a uniform sub-network, or estimating the deformation field perform similarly in drastically reducing the amplitude of the aliasing effect. The application of these methods to reprocessed GPS terrestrial frames permits an assessment of the level of agreement between GPS and our loading model, which is found to be about 1.5 mm WRMS in height and 0.8 mm WRMS in the horizontal at the annual frequency. Aliased loading signals are not the main source of discrepancies between loading displacement models and GPS position time series.