Precision coordinate transformations for Thai national geodetic infrastructure

The International Terrestrial Reference Frame (ITRF), the Earth Orientation Parameter (EOP) time series, and the International Celestial Reference Frame are obtained separately and may thus present inconsistencies. To solve this problem, a first step has been made with the latest ITRF realization (ITRF2005), which has been computed, for the first time, with consistent EOP time series. Another approach to better understand this issue is to directly estimate, in the same process, station positions and EOP time series, from all the space-geodetic measurements.

The observations of the four space-geodetic techniques (GNSS, SLR, VLBI, DORIS) are used for the computation of the International Terrestrial Reference Frame (ITRF) and for the realisation of its geodetic datum as well as (except VLBI) for the precise orbit determination of Earth-observing satellites. The ITRF itself is fundamental for a broad variety of scientific and societal applications such as Earth monitoring and positioning, relevant for satellite companies, logistics, and finally new techniques like autonomous driving. However, all four techniques still suffer from unknown systematic measurement or modelling errors which makes the estimation of bias parameters inevitable.The Geodetic Observatory Wettzell (GOW), Germany, with its unique and ideal test environment comprising multiple GNSS antennas, three VLBI antennas, two SLR telescopes, a DORIS beacon, and numerous other sensor systems provides the opportunity to systematically identify, quantify, understand, and compensate system-specific measurement errors. The installation of a so-called common target (CT) in 2017 and the realisation of a common clock (CC) enables a profound analysis of space-geodetic measurements. The CT is connected to the clocks of the space-geodetic techniques via delay-compensated fibre links which allows so-called ‘closure in time’ experiments.Within the recently established DFG research unit ‘Clock Metrology: Time as a New Variable in Geodesy’, one project at DGFI-TUM focuses on the investigation of time-related and technique-specific errors using ‘closure in time’ experiments at the GOW. Therefore, completed experiments involving the CT are analysed and enhanced experiments will be planned and conducted. With the aid of the CC, the source of present measurement discrepancies will be investigated and resolved. We present the current state of our work with the main focus on the analysis of VLBI and SLR observations of the two SLR and three VLBI systems at GOW.

Until today the products of the International Earth Rotation and Reference Systems Service (IERS), like International Terrestrial Reference Frame (ITRF), International Celestial Reference Frame (ICRF) and Earth Orientation Parameters (EOP), are combined independently, neither intra-technique nor inter-technique combinations, including the full variance-covariance information are performed. To overcome this deficiencies in the present IERS product generation the IERS implemented a new structure in January 2001. This includes the new IERS Combination Research Centres (CRC) and the IERS Analysis Coordinator (AC). He is responsible for the long-term and internal consistency of the IERS products. To achieve the highest accuracy and consistency, it is crucial to proceed towards a fully rigorous combination of all the parameters common to more than one space geodetic technique. Since 2001 the IERS AC initiated and coordinated many different projects and campaigns towards this overall goal. Now the results of the last three years build the theoretical and practical base for the latest project, the Combination Pilot Project (CPP). This project will prepare the generation of a combined IERS product on a routine basis.

The International Laser Ranging Service (ILRS) was established by the International Association of Geodesy (IAG) in 1998 to support programs in geodesy, geophysics, fundamental constants and lunar research, and to provide the International Earth Rotation Service with data products that are essential to the maintenance and improvement in the International Terrestrial Reference Frame (ITRF), the basis for metric measurements of changes in the Earth and Earth–Moon system. Other scientific products derived from laser ranging include precise geocentric positions and motions of ground stations, satellite orbits, components of Earth’s gravity field and their temporal variations, Earth Orientation Parameters, precise lunar ephemerides and information about the internal structure of the Moon. Laser ranging systems are already measuring the one-way distance to remote optical receivers in space and are performing very accurate time transfer between remote sites in the Earth and in Space. The ILRS works closely with the IAG’s Global Geodetic Observing System. The ILRS develops (1) the standards and specifications necessary for product consistency, and (2) the priorities and tracking strategies required to maximize network efficiency. The service collects, merges, analyzes, archives and distributes satellite and lunar laser ranging data to satisfy a variety of scientific, engineering, and operational needs and encourages the application of new technologies to enhance the quality, quantity, and cost effectiveness of its data products. The ILRS works with (1) new satellite missions in the design and building of retroreflector targets to maximize data quality and quantity, and (2) science programs to optimize scientific data yield. Since its inception, the ILRS has grown to include forty laser ranging stations distributed around the world. The ILRS stations track more than ninety satellites from low Earth orbit (LEO) to the geosynchronous orbit altitude as well as retroreflector arrays on the surface of the Moon. Applications have been expanded to include time transfer, asynchronous ranging for targets at extended ranges, free space quantum telecommunications, and the tracking of space debris. Laser ranging technology is moving to lower energy, higher repetition rates (kHz), single-photon-sensitive detectors, shorter pulse widths, shorter normal point intervals for faster data acquisition, and increased pass interleaving, automated to autonomous operation with remote access, and embedded software for real-time updates and decision making. An example of pass interleaving is presented for the Yarragadee station (see Fig. 4); tracking of LEO satellites is often accommodated during break in LEO and GNSS passes. New satellites arrays provide more compact targets and work continues on the development of lighter less expensive arrays for satellites and the moon. The service now provides operational ITRF products including daily/weekly station positions and daily resolution Earth orientation products; the flow of weekly combination of satellite orbit files for LAGEOS/Etalon-1 and -2 has recently been established. New products are under testing through a pilot project on systematic error monitoring currently underway. The article will give an overview of activities underway within the service, paths forward presently envisioned, and current issues and challenges.

[...] Studies regarding the theory of this correlation and how to handle it when estimating sub-daily polar motion together with nutation are included in the thesis.Furthermore, it is demonstrated that the common estimation of the terrestrial reference frame and EOPs can be employed to identify a mismatch between the local tie values and the estimated coordinate differences at co-located sites. Thus, the EOPs offer another independent evaluation of the LT values, as mentioned already above for the troposphere gradients.For the studies described in this thesis, it is of vital importance that continuous observations are available for all observation techniques. Therefore, the so-called CONT campaigns of the International VLBI Service for Geodesy and Astrometry (IVS) are well-suited. Such special campaigns with continuous VLBI observations for several days are initiated from time to time in order to demonstrate the full capabilities of VLBI. The studies described in the thesis at hand are based on the two-week CONT campaign scheduled in October 2002, named CONT02.

The World Geodetic System (WGS) was conceived as a practical geodetic reference system that maintains consistency with the best scientific terrestrial reference system at the time but also retains some stability. It has evolved from its beginnings as WGS 60 to its present manifestation as WGS 84. A significantly improved Earth Gravitational Model and global geoid were released in 1996, and the realization of the WGS 84 reference frame is consistent with the International Terrestrial Reference Frame (ITRF) 1994 at the 5-cm level. Evaluations of WGS 84 relative to ITRF94 have been made by comparing International GPS Service (IGS) GPS orbits with the National Imagery and Mapping Agency’s precise orbits and by computing WGS 84 positions for a globally distributed set of IGS stations with known ITRF94 coordinates. The results indicate that there is no practical difference between ITRF94 and WGS 84 for mapping, charting, navigation and many survey applications.

The National Geographic Institute of Spain (IGN Spain) develops, operates, and exploits the Spanish national geodetic networks and their associated infrastructure. This includes the permanent networks of reference GNSS (Global Navigation Satellite System) receivers, VLBI (Very Long Baseline Interferometry) telescopes, and a new SLR (Satellite Laser Ranging) station under construction at the Yebes Observatory. In order to fully exploit the opportunities offered by the availability of these space geodetic techniques, IGN has been operating Analysis Centres of GNSS and VLBI for a number of years, with the recent addition of an Associated Analysis Centre of the International Laser Ranging Service. IGN Spain is a EUREF Analysis Centre since 2001, contributing with their weekly and daily coordinate solutions to the realisation of the European Terrestrial Reference System. The activities of the AC include projects such as IBERRED, for geodynamic purposes, and the participation in the European E-GVAP programme for meteorological applications. Over the last years, IGN Spain has expanded its contribution to geodetic VLBI analysis, starting in 2019 the operational analysis of VLBI sessions and the submission of the respective solutions (containing consistently estimated Earth Orientation Parameters, station coordinates and source positions) to the International VLBI Service for Geodesy and Astrometry (IVS). Additionally, the reprocessing of the historical VLBI data since 1979 is ongoing, which is the basis for future contributions to the IVS combination series that will be provided for future realisations of the international terrestrial reference frame (ITRF). The latest addition to IGN Spain analysis capabilities is the IGN-Yebes Associated Analysis Centre. Highlights of its activities are the participation in the ITRF2020 reprocessing in collaboration with the Analysis Centre NSGF in the UK, and the computation of SLR centre-of-mass corrections for spherical geodetic satellites. IGN strives to further exploit the synergies between these groups and work towards the combined analysis of the data. A description of the analysis activities of IGN Spain, along with its future prospects, is presented.

Abstract. A dynamic datum denotes a coordinate datum in real-time linked with the International Terrestrial Reference Frame (ITRF) in order to provide a dynamic ITRF-like datum to the users. The ITRF is dynamic and updating every few years as its stations’ coordinates consider the motion of earth’s tectonic plate and other deformations. This paper is an effort to review the implementation of dynamic geocentric datum techniques from a few countries. An overview of dynamic geocentric datum implements Malaysia, Australia, New Zealand, Uzbekistan, Israel and Brunei will be summarized to support the future application. Thus, a review consists of a type of datum; datum parameters, reference frame and epoch will be discussed and outlined. This initiative is the significance for the advancement of the future datum development.

Following the principles set forth in the Position Paper #3 at the 1998 Darmstadt Analysis Center (AC) Workshop on the new International GPS Service (IGS) International Terrestrial Reference Frame (ITRF) realization and discussions at the 1999 La Jolla AC workshop, a new clock combination program was developed. The program allows for the input of both SP3 and the new clock (RINEX) format (ftp://igsch.jpl.nasa.gov//igscb/data/format/rinex_clock.txt). The main motivation for this new development is the realization of the goals of the IGS/BIPM timing project. Besides this there is a genuine interest in station clocks and a need for a higher sampling rate of the IGS clocks (currently limited to 15 min due to the SP3 format). The inclusion of station clocks should also allow for a better alignment of the individual AC solutions and should enable the realization of a stable GPS time-scale. For each input AC clock solution the new clock combination solves and corrects for reference clock errors/instabilities as well as satellite/station biases, geocenter and station/satellite orbit errors. External station clock calibrations and/or constraints, such as those resulting from the IGS/BIPM timing pilot project, can be introduced via a subset of the fiducial timing station set, to facilitate a precise and consistent IGS UTC realization for both station and satellite combined clock solutions. Furthermore, the new clock combination process enforces strict strict conformity and consistency with the current and future IGS standards. The new clock combination maintains orbit/clock consistency at millimeter level, which is comparable to the best AC orbit/clock solutions. This is demonstrated by static GIPSY precise point positioning tests using GPS week 0995 data for stations in both Northern and Southern Hemispheres and similar tests with the Bernese software using more recent data from GPS week 1081. © 2001 John Wiley & Sons, Inc.

The World Geodetic System 1984 (WGS84) has been consistent with the International Terrestrial Reference Frame (ITRF) since 1994. After the original release of WGS84 in 1987 by the Defense Mapping Agency (since superseded by the National Geospatial-Intelligence Agency, NGA), there have been six subsequent realizations of the frame, designated WGS84 (G730), WGS84 (G873), WGS84 (G1150), WGS84 (G1674), WGS84 (G1762), and WGS84 (G2139). Users of WGS84 often fail to document metadata indicating the realization and epoch related to their data collections. As a result, when transforming between realizations or between other reference frames, the transformation can lead to incorrect results. We aim to bring awareness to the nature of WGS84 and the ambiguity in how it is defined. An internally consistent set of transformation parameters is derived that allows for transformation between the WGS84 realizations and other global reference frames while accounting for nuances and inconsistencies in the published literature. The derivations are based on existing (though not easily discoverable) information. Transformation accuracy is evaluated through comparison to other software and NGA-published GPS tracking station coordinates. After accounting for plate tectonic motion, the estimated uncertainties average ±0.03 m horizontally and ±0.08 m vertically for transformations involving all realizations other than the original WGS84. Uncertainties are at the meter level for transformations that include the original WGS84. If plate motion is ignored, the horizontal uncertainties can increase by several decimeters, depending on location within a plate.

<p>The ITRF2020 is the upcoming realization of the International Terrestrial Reference Frame. As the successor of the ITRF2014, it is based on an inter-technique combination of all four space-geodetic techniques, i.e., VLBI, GNSS, SLR and DORIS, and it is based on contributions from different institutions around the world. In this context, the Combination Centre of the International VLBI Service for Geodesy and Astrometry (IVS) – operated by the Federal Agency for Cartography and Geodesy (BKG, Germany) and the Deutsches Geodätisches Forschungsinstitut (DGFI-TUM, Germany) – generates the VLBI intra-technique combination for ITRF2020 utilizing the individual contributions of multiple IVS Analysis Centres (AC).</p><p>For the contribution to the ITRF2020 solution, sessions containing 24h VLBI observations from 1979 until the end of 2020 are reprocessed by 11 ACs and submitted to the IVS Combination Centre. All individual sessions include datum-free normal equations containing station coordinates and source positions as well as full sets of Earth Orientation Parameters (EOP) in the required SINEX format. For ensuring consistently combined solutions, time series of EOP and station coordinates, as well as a VLBI-only Terrestrial Reference Frame (VTRF), have been investigated.</p><p>This contribution focuses on detailed investigations concerning the session-dependency of the scale and the impact of the individual AC contributions to the combination. Thereby significant differences of the scale estimates from the different session types are investigated. Also, the evaluation of the ACs’ contributions to the combined solution will be presented.</p>

As the International Terrestrial Reference Frame (ITRF) is constructed by combination of space geodesy technique solutions together with terrestrial local ties in co-location sites, it then inherits strengths and weaknesses of the combined data. This paper uses the ITRF2005 results in order to illustrate the critical aspects of the combination that impact the ITRF datum definition: the origin, the scale and the No Net Rotation condition. The main ITRF2005 results, using as input data time series of station positions, indicate a drift of 1.8 mm/yr in the origin Z-translation component with respect to ITRF2000, while SLR solutions are used to define the origins of both frames. A scale bias of about 1 ppb between VLBI and SLR solutions is also detected. Possible causes of this inconsistency include the poor SLR and VLBI networks and their co-locations, systematic errors and possible inconsistent model corrections used in the data analysis of both techniques. As conclusion of this paper, some recommendations for improvements of future ITRF solutions are addressed.KeywordsReference SystemsReference FramesITRF

Satellite Laser Ranging (SLR) is currently the unique technique to determine station coordinates, Earth Orientation Parameters (EOPs) and Stokes coefficients of the Earth’s gravity field from one observation equation with a high accuracy. These parameters form the so-called ‘three pillars’ (Plag and Pearlman, 2009) of the Global Geodetic Observing System (GGOS). In its function as an official analysis center of the International Laser Ranging Service (ILRS) (Pearlman et al., 2002), DGFI is developing and maintaining software to process SLR observations called ‘DGFI Orbit and Geodetic parameter estimation Software’ (DOGS). The software is used to analyze SLR observations and to compute multi-satellite solutions. In this study, up to 10 satellites (ETALON1/2, LAGEOS1/2, STELLA, STARLETTE, AJISAI, LARETS, LARES and BLITS) with different orbit characteristics (e.g., inclination and altitude) are combined. The relative weighting of the satellites is done using a variance component estimation. The diverse orbits allow to decrease the correlation of parameters such as gravity field coefficients (GFCs) and EOPs. Beside the Earth’s gravity field (weekly GFCs with degree and order ≤ 6 and monthly GFCs with degree and order ≤ 20) and rotation (terrestrial pole coordinates, UT1-UTC extrapolated with LOD), 3-D station coordinates are estimated weekly or monthly. Different combined solutions are compared to LAGEOS-only solutions. If LARES is combined with LAGEOS1/2, a significant improvement in the GFCs, the EOP and the TRF can be achieved. If more satellites are combined, the variation of the pole coordinates w.r.t. the IERS 08 C04 time series can be reduced up to 56%. Furthermore, systematics in LOD are nearly completely eliminated. The variation of the station coordinates w.r.t. SLRF2008 (http://www.ilrs.gsfc.nasa.gov/science/awg/SLRF2008.html, 2013) can be reduced by about 30%.

We present the vectors of rotation of 10 major lithospheric plates, estimated from continuous GPS observations at 192 globally distributed stations; 71 stations were selected as representing stable plate regions. All days for the period 1995.0–2007.0 were included in the analysis. In contrast to previous GPS plate models, our model is independent of international terrestrial reference frames (ITRF). The origin of our plate‐consistent reference frame is the center of plate rotation (CP) rather than the center of mass of the entire Earth's system (CM) as in recent versions of ITRF. We estimate plate rotations and CP by minimizing the misfit between the horizontal velocities predicted by the plate model and the observed GPS velocities. If any version of ITRF is used as the reference frame, the drift of the ITRF origin relative to CP cannot be neglected in estimation of plate rotation vectors and plate‐residual station velocities. The model of the plate kinematics presented here addresses the problem debated since the beginning of the space geodesy: how big are disagreements between the current plate motions and the motions averaged over several million years? We compare the vectors of relative plate rotations estimated here with the published vectors from GPS and geologic models. We also discuss the integrity of individual plates as exhibited by plate‐residual station velocities. For seven largest plates, the RMS value of plate‐residual station velocities in stable plate interiors is 0.5–0.9 mm/a; this value is an upper bound on deviation of real plates from infinite stiffness.

The BeiDou Navigation Satellite System (BDS) has used the BeiDou Coordinate System (BDCS) as its geodetic reference system since December 2017. After a brief description of the background, definitions of the origin, scale, and orientation of BDCS are given, which are consistent with China’s Geodetic Coordinate System 2000 (CGCS2000). Then the first realization of the BDCS aligning with the International Terrestrial Reference Frame 2014 (ITRF2014) is addressed. Coordinates at epoch 2018.0 and velocities at eight BDS monitoring stations were obtained achieved by processing four-phase GPS data for a global navigation satellite system (GNSS) network, including the monitoring stations, and by the frame alignment and linear regression fitting of station coordinate series. Assessment of the precision of the coordinates and Helmert parameters with respect to ITRF2014 reveals that the differences between the first frame of BDCS and ITRF2014 are less than 1 cm, which is compatible with WGS84 (G1762), PZ-90.11, and the latest Galileo terrestrial reference frame.