Zuheir Altamimi

ITRF2005: A new release of the International Terrestrial Reference Frame based on time series of station positions and Earth Orientation Parameters

[1] Unlike the past International Terrestrial Reference Frame (ITRF) versions where global long-term solutions were combined, the ITRF2005 uses as input data time series (weekly from satellite techniques and 24-h session-wise from Very Long Baseline Interferometry) of station positions and daily Earth Orientation Parameters (EOPs). The advantage of using time series of station positions is that it allows to monitor station non-linear motion and discontinuities and to examine the temporal behavior of the frame physical parameters, namely the origin and the scale. The ITRF2005 origin is defined in such a way that it has zero translations and translation rates with respect to the Earth center of mass, averaged by the Satellite Laser Ranging (SLR) time series spanning 13 years of observations. Its scale is defined by nullifying the scale and its rate with respect to the Very Long Baseline Interferometry (VLBI) time series spanning 26 years of observations. The ITRF2005 orientation (at epoch 2000.0) and its rate are aligned to the ITRF2000 using 70 stations of high geodetic quality. The estimated level of consistency of the ITRF2005 origin (at epoch 2000.0) and its rate with respect to the ITRF2000 is respectively 0.1, 0.8, 5.8 mm and 0.2, 0.1, 1.8 mm/yr along the X, Yand Z-axis. We estimate the formal errors on these components to be 0.3 mm and 0.3 mm/yr. We believe that this low level of agreement between the two frame origins is most probably due to the poor SLR network geometry and its degradation over time. The ITRF2005 combination involving 84 co-location sites revealed a scale inconsistency of 1 ppb (6.3 mm at the equator), at epoch 2000.0, and 0.08 ppb/yr between the SLR and VLBI long-term solutions as obtained by the stacking of their respective time series. Possible causes of this inconsistency may include the poor SLR and VLBI networks and their co-locations, local tie uncertainties, systematic effects and possible inconsistent model corrections used in the data analysis of both techniques. For the first time of the ITRF history, the ITRF2005 rigorous combination provides self-consistent series of EOPs, including Polar Motion from VLBI and satellite techniques and Universal Time and Length of Day from VLBI only. A velocity field of 152 sites with an error less than 1.5 mm/yr is used to estimate absolute rotation poles of 15 tectonic plates that are consistent with the ITRF2005 frame. This new absolute plate motion model supersedes and significantly improves that of the ITRF2000 which involved six major tectonic plates. Citation: Altamimi, Z., X. Collilieux, J. Legrand, B. Garayt, and C. Boucher (2007), ITRF2005: A new release of the International Terrestrial Reference Frame based on time series of station positions and Earth Orientation Parameters, J. Geophys. Res., 112, B09401,

The ITRF96 realization and its associated velocity field

This text intends to present and to analyse the quality of the new 1996 edition of the International Terrestrial Reference System, called the ITRF96. The ITRF96 represents a key step forward in the activity of the IERS terrestrial system determinations: a new physical combination model has been developed as well as a new stochastic model. For the first time, the level of accuracy has reached 1 cm (or better) in position for about 50% of ITRF96 stations and 30% of the stations have their velocities determined, free of any aditional tectonic plate model assumption, with an uncertainty better than 3 mm/yr. The ITRF96 also provides a more complete solution: a new software has been developed which makes available the ITRF96 full variance-covariance matrix whereas this information was restricted to station positions in the previous ITRF94 edition. Moreover, the network coverage is clearly the largest set of globally distributed terrestrial Space Geodesy stations that have ever been computed: 521 stations located among 290 sites. In addition, station velocities are also determined at a very accurate level allowing possible determination of kinematic plate motion models free of any geological assumption.

Intraplate deformation in western Europe deduced from an analysis of the International Terrestrial Reference Frame 1997 (ITRF97) velocity field

Although tectonic deformation in western Europe is essentially concentrated in the Appenines and Alpine (Alps, Pyrenees) mountain ranges, several large historical and instrumentally recorded earthquakes (M>6) are known in the supposedly “stable” part of Europe. Because of its accuracy and internal consistency at a global scale, the International Terrestrial Reference Frame 1997 (ITRF97) velocity field allows testing of intraplate rigidity in western Europe at a millimeter per year level. Using the full statistical information available on the ITRF97 velocities, we identify a subset of sites located in central Europe that satisfies a rigid cap rotation with residual velocities <1 mm/yr and therefore provide a stable Europe reference frame (SERF). In this reference frame we find residual velocities at European ITRF sites that are consistent with known active tectonic features. We identify a northward motion at sites located in Italy, with internal deformation of the Adriatic block rather than rigid plate motion and a westward motion of the westernmost part of Europe of the order of 1–2 mm/yr relative to central Europe. The relative motion of the Adriatic block and western Europe agrees with the current extension known in the Tyrrhenian sea and the Apennines. In central Europe, we find active deformation <1 mm/yr in the eastern Alps and western Carpathians. In the Alpine range our results indicate E-W extension across the western Alps and N-S compression across the central and eastern Alps, in agreement with the strain regime deduced from seismotectonic observations. In Belgium and the Netherlands we find residual velocities of 1–1.5 mm/yr to the northwest at all the sites, most likely accommodated along the Upper and Lower Rhine Graben structures. An important outcome of this study is the identification of internal deformation of the order of 1–2 mm/yr in an area usually interpreted as “stable” Europe. This result should be further checked as new, denser, and more accurate space geodetic data sets with longer observation time spans, become available for Europe.

Accuracy of the International Terrestrial Reference Frame origin and Earth expansion

The International Terrestrial Reference Frame (ITRF) is a fundamental datum for high?precision orbit tracking, navigation, and global change monitoring. Accurately realizing and maintaining ITRF origin at the mean Earth system center of mass (CM) is critical to surface and spacecraft based geodetic measurements including those of sea level rise and its sources. Although ITRF combines data from satellite laser ranging (SLR), Very Long Baseline Interferometry (VLBI), Global Positioning System (GPS), and Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS), its origin is currently realized by the single technique of SLR. Consequently, it is difficult to independently evaluate the origin accuracy. Also, whether the solid Earth is expanding or shrinking has attracted persistent attention. The expansion rate, if any, has not been accurately determined before, due to insufficient data coverage on the Earths surface and the presence of other geophysical processes. Here, we use multiple precise geodetic data sets and a simultaneous global estimation platform to determine that the ITRF2008 origin is consistent with the mean CM at the level of 0.5 mm yr?1, and the mean radius of the Earth is not changing to within 1? measurement uncertainty of 0.2 mm yr?1.

New trends for the realization of the international terrestrial reference system

Abstract With the advent of Space geodesy techniques in early eighties, global terrestrial reference frames became available whose precision is still improving parallel to measuring and modeling advances. As a global reference, the realization of the International Terrestrial Reference System (ITRS), known as the International Terrestrial Reference Frame (ITRF), maintained by the International Earth Rotation Service, has sustained substantial improvement and enhancement. One of the major new trends is the 2000 ITRS realization, to be considered as a standard solution for a wide user community (geodesy, geophysics, astronomy, etc.). The ITRF2000 comprises on one hand primary core stations observed by VLBI, LLR, GPS, SLR and DORIS techniques and, on the other hand, significant extension provided by regional GPS networks for densifications as well as other useful geodetic markers tied to space geodetic ones. The ITRF2000 combination and implementation strategy are described in this paper. Important results in terms of datum definition as well as quality assessment of the ITRF2000 are presented.

The worldwide centimetric terrestrial reference frame and its associated velocity field

Abstract Space-geodetic techniques allow, on a global basis, a centimetric realization of a Conventional Terrestrial Reference System through a Conventional Terrestrial Reference Frame. These techniques are in particular: VLBI, LLR, SLR and GPS. The actual Terrestrial Reference Frame (TRF) needs global coverage of observing sites for describing the motion and deformation of the whole Earth. In addition to geodetic and Earth Rotation aspects, geophysical phenomena requiring a global TRF include tectonic motion and deformation, change in global sea level and post-glacial rebound. The work on the terrestrial frame of the International Earth Rotation Service (IERS) and in particular the implementation of its TRF, called ITRF and based on combination of several data sets coming from space-geodetic techniques, will be presented in terms of quality and internal consistency. This work will be viewed from the perspective of an International Global Network.

KALREF—A Kalman filter and time series approach to the International Terrestrial Reference Frame realization

The current International Terrestrial Reference Frame is based on a piecewise linear site motion model and realized by reference epoch coordinates and velocities for a global set of stations. Although linear motions due to tectonic plates and glacial isostatic adjustment dominate geodetic signals, at todays millimeter precisions, nonlinear motions due to earthquakes, volcanic activities, ice mass losses, sea level rise, hydrological changes, and other processes become significant. Monitoring these (sometimes rapid) changes desires consistent and precise realization of the terrestrial reference frame (TRF) quasi-instantaneously. Here, we use a Kalman filter and smoother approach to combine time series from four space geodetic techniques to realize an experimental TRF through weekly time series of geocentric coordinates. In addition to secular, periodic, and stochastic components for station coordinates, the Kalman filter state variables also include daily Earth orientation parameters and transformation parameters from input data frames to the combined TRF. Local tie measurements among colocated stations are used at their known or nominal epochs of observation, with comotion constraints applied to almost all colocated stations. The filter/smoother approach unifies different geodetic time series in a single geocentric frame. Fragmented and multitechnique tracking records at colocation sites are bridged together to form longer and coherent motion time series. While the time series approach to TRF reflects the reality of a changing Earth more closely than the linear approximation model, the filter/smoother is computationally powerful and flexible to facilitate incorporation of other data types and more advanced characterization of stochastic behavior of geodetic time series.

Accuracy Assessment of the ITRF Datum Definition

One of the main objectives of the International Terrestrial Reference Frame (ITRF) is to provide a standard global reference frame having the most attainable accuracy of its datum definition in terms of its origin, scale and the time evolution of its orientation. This latter should satisfy, by convention, the no net rotation condition. The accuracy of the ITRF datum specifications are obviously dependent on the quality and the internal consistency of the solutions contributing to its elaboration and definition. In this paper, we examine and review the quality of the current ITRF datum definition with an accuracy assessment based on the ITRF2005 results and by consistency evaluation with respect to ITRF2000. The availability of time series of station positions and Earth Orientation Parameters, used now as input for the current ITRF construction, will facilitate the accuracy assessment. When rigorously stacking the time series of a given technique to estimate a long-term frame solution, the 7 transformation parameters of each individual temporal set of station positions are also estimated. By applying dynamically internal constraints (equivalent to minimum constraints approach) over the time series of the 7 parameters, we then preserve some physical “natural” parameters as for instance the scale and the origin from VLBI and SLR, respectively. Our conservative evaluation of the estimated accuracy of the ITRF datum definition is that the origin and its rate are accurate at the level of 5 mm and 2 mm/yr, the scale and its rate are at the level of 1 part per billion (ppb) and 0.1 ppb/yr and the No-Net-Rotation condition implementation is at the level of 2 mm/yr.

Local Ties and Co-Location Sites: Some Considerations After the Release of ITRF2008

Tie vectors (TVs) measured at co-location sites carry fundamental information for the computation of the International Terrestrial Reference Frame (ITRF). The combination of the different frames stemming from each space geodetic (SG) technique relies on the availability and accuracy of the relative positions between reference points of co-located SG instruments, i.e. TV. If, on the one hand, TVs accurate at 1mm level are sought to preserve the accuracy of the global frame and fulfill the requirements of the global geodetic observing system (GGOS), on the other hand, the assessment of TVs accuracy is not easy. Their accuracies are often questioned on the base of their agreement within the combination of SG solutions and the combination residuals. Though, the final discrepancies highlighted by the combination residuals do not depend uniquely on the accuracy of the TVs but are influenced by several factors of different origin. In this paper, we identify some of these factors and investigate their possible origin adopting different perspectives: local ties and terrestrial surveying, SG techniques and frames combination. Our purpose is to highlight some of the possible systematic errors in terrestrial and SG data analysis as well as to identify actions to be taken in the near future to mitigate the biases highlighted by the residuals of the combination. In contrast to what is commonly assumed, we show that the residuals are potentially influenced by a combination of biases affecting the TVs, their alignment and the SG solutions. Therefore, an objective evaluation of the error sources is necessary for each SG technique in order to improve their results as well as the combined SG products.

GGOS working group on ground networks and communications

Properly designed and structured ground-based geodetic networks materialize the reference systems to support sub-mm global change measurements over space, time and evolving technologies. Over this past year, the Ground Networks and Communications Working Group (GN&C WG) has been organized under the Global Geodetic Observing System (GGOS) to work with the IAG measurement services (the IGS, ILRS, IVS, IDS and IGFS) to develop a strategy for building, integrating, and maintaining the fundamental network of instruments and supporting infrastructure in a sustainable way to satisfy the long-term (10–20 year) requirements identified by the GGOS Science Council. Activities of this Working Group include the investigation of the status quo and the development of a plan for full network integration to support improvements in terrestrial reference frame establishment and maintenance, Earth orientation and gravity field monitoring, precision orbit determination, and other geodetic and gravimetric applications required for the long-term observation of global change. This integration process includes the development of a network of fundamental stations with as many co-located techniques as possible, with precisely determined intersystem vectors. This network would exploit the strengths of each technique and minimize the weaknesses where possible.