Contribution of the Vienna Center for VLBI to ITRF2020

<p>The next realization of the International Terrestrial Reference System, the ITRF2020, is planned to be released in 2021. Our joint VLBI Analysis Center VIE which runs between TU Wien and BEV is one of eleven IVS (International VLBI Service for Geodesy and Astrometry) analysis centres which provide VLBI input to the ITRF2020. The SINEX files submitted to the IVS Combination Center are produced with the Vienna VLBI and Satellite Software VieVS and contain unconstrained normal equation systems for station position, source coordinates and Earth orientation parameters. In this presentation, we document the included sessions and stations in our submission and introduce the Vienna terrestrial reference frame based on our contribution to the ITRF2020. In particular, we highlight special settings in the Vienna solution and assess the impact on the terrestrial reference frame.</p>

Sistemas Geodésicos de Referência: Rumo ao GGRS/GGRF

O estabelecimento de Sistemas Geodésicos de Referência globais integrando características geométricas e físicas é um dos desafios atuais da Geodésia, principalmente devido às demandas de diversas áreas do conhecimento de que as informações relacionadas aos Sistemas de Observação da Terra (EOS – Earth Observation Systems), sejam integradas em Redes Geodésicas de Referência (RGRs) com uma acurácia de 10-9 ou melhor. O surgimento das técnicas de posicionamento espacial trouxe melhora significativa na qualidade posicional e possibilitou a substituição das RGRs clássicas por redes modernas com características globais. Hoje, a questão das coordenadas de caráter geométrico, está bem resolvida com o ITRS/ITRF (International Terrestrial Reference System/International Terrestrial Reference Frame). Todavia, aspectos associados a diversos processos físicos, tais como os reflexos das redistribuições de massa, não são atendidos por referenciais puramente geométricos. A aprovação da resolução para o GGRS/GGRF (Global Geodetic Reference System/Global Geodetic Reference Frame) surge com a visão da integração entre o referencial terrestre, o celeste, um referencial com características físicas para as altitudes e a nova rede global de gravidade absoluta. Esforços têm sido feitos para definição e realização deste referencial global para as altitudes. É uma tarefa complexa em vista das características clássicas dos referenciais verticais, heterogeneidade em termos de qualidade e distribuição espacial de dados necessários, principalmente os relacionados ao campo de gravidade da Terra. Apresentam-se como grandes desafios para o futuro a necessidade de estabelecimento de procedimentos padrão para a integração ao referencial altimétrico global e a precisão necessária para o estabelecimento dos EOS.

Comparison Between IGS and Three ITRS Realizations

Four space geodetic techniques (IGS, SLR, VLBI and DORIS) contribute to the realizations of International Terrestrial Reference System (ITRS). The GNSS-derived terrestrial reference frame generated from the second reprocessing campaign (repro2), named IG2, act as the IGS input to the most recently three realizations (ITRF2014, DTRF2014 and JTRF2014). Its origin and orientation are aligned to the IGb08, and its scale is defined by using the igs08.atx satellite antenna phase center offset (PCO) values. To study the consistencies and discrepancies between IGS solutions and the three ITRS realizations, we corrected the IG2 solutions to be uniform with the IGS14 frame and perform Helmert transformation to compare the IGS frame and the three ITRS realizations. Results indicate that IGS frame is more stable than the two secular frames especially in the periods after 2015. The similarity transformation parameters between the corrected IGS solutions and ITRF2014 show excellent agreement with a notable mean z-offset of around 1 mm. The transformation parameters between the corrected IGS solutions and DTRF2014 show linear discrepancies in the three categories parameters, where the origin offsets are around less than 5.5 mm, rotational alignment is consistent at the level of 4.5 uas/yr (about 0.15 mm/yr) and the scale exhibits a stable offset of 0.16 ppb. Unlike the two secular frames, distinct seasonal signals and interannual variations of translation time series can be observed from the comparison between JTRF2014 and the IGS solutions. The orientation of JTRF2014 is in worst agreement with the IGS solutions, which is related to biased no-net-rotation (NNR) condition due to weekly center of network (CN) variations. Moreover, the scale defined by JTRF2014 suffer from large instability variations over time.

GLOBAL REFERENCE SYSTEM AND ITS LOCAL REALIZATION – RUSSIAN STATE COORDINATE SYSTEM GSK-2011

The article highlights the issue of interpreting reference stations networks as a local realization of the global reference system. The substantiation of the proposed approach is given and its advantages are shown. The rationale for the proposed approach is given and its advantages are shown. In particular, the top block in the structure of the formation of the Russian state coordinate system (GSK‑2011) is the fundamental astronomical and geodetic network. It is a regional realization of a global reference system. The creation of GSK‑2011 was carried out with a focus on the global International Terrestrial Reference System (ITRS) however geodynamic processes affecting the displacement of reference points relative to the center of the Earth's masses play a different role in the time evolution of systems. Such processes in GSK‑2011 are not subject to accounting, since the system was created to conduct various types of applied geodetic and cartographic activities in it. In this case, taking into account the constant change in the coordinates of reference points is almost never implied. In this regard, the asynchronous movement of the Russian state coordinate system (GSK‑2011) with the global reference system (ITRS) began to lead to inconsistencies in the results of high-precision positioning performed at different times, by different methods. Based on this, the necessity to find a way of matching GSK‑2011 to ITRS is urgent. The article presents the rates of change of the match parameters of the above-mentioned systems. These parameters make it possible to match the results of high-precision positioning performed in different reference systems using different methods of positioning for different epochs of the GNSS observations. The experiment carried out in the second part of the article confirms this.

Determination of the relationship between Vietnam national coordinate reference system (VN-2000) and ITRS, WGS84 and PZ-90

In July 2000, Hanoi-72 reference system was replaced by the Vietnam reference system, namely as VN-2000 as an official geodetic background system in Vietnam. Ministry of Natural Resources and Environment of Vietnam has reported the transformation parameters between VN-2000 and WGS84. Nevertheless, there is a need to estimate a new transformation parameter set between VN-2000 and WGS84 because WGS84 has been updated. In addition, there is now a lack of an accurate published set of parameters for transformation from VN-2000 to not only the International Terrestrial Reference System ITRS but also PZ-90. In this study, coordinate transformation parameters between ITRS and VN-2000 are estimated through the use of a least square approach and the common points with known coordinates in both systems. These set of parameters was then deployed to determine the link between VN-2000 and WGS84 as well as PZ-90. The results denoted that the derived transformation parameters, on the basis of the results at the checkpoints, could generated station positions with the accuracy at several cm level for transformation from VN-2000 to the new realizations of ITRS, WGS84 and PZ90 and reversely. These achievements reveals that the set of parameters is great significance for many applications related to positioning in Vietnam.

On the determination of transformation parameters between different ITRS realizations using Procrustes approach in Turkey

The International Terrestrial Reference Frame (ITRF) solutions that are published by the International Earth Rotation and Reference Systems Service (IERS) are annual realizations of the ITRS (International Terrestrial Reference System). The results expressed in two different ITRS realizations can be compared using the transformation parameters that provide a link between different ITRF solutions. Generally, the 7-parameter (the three translation parameters, three rotation parameters and one scale factor) Helmert transformation is employed to compute the transformation parameters. However, the number of transformation parameters can be increased for better understanding. For example, 3 different scale factors may be computed instead of one scale factor. In this paper, the 9-parameter (the three translation parameters, three rotation parameters and three scale factors) transformation model and its solution by Procrustes approach is considered. Transformation parameters between ITRF 05 and ITRF 08 for Turkey have been computed in both 7-parameter model and 9-parameter model and a numerical example has been given to understand the difference between two models in a better way. An explanation about the proposed methodology as a flow chart also has been shown in appendix.

COMPATIBILIZAÇÃO DE REFERENCIAIS DE COORDENADAS E VELOCIDADES COM ESTIMATIVA DE PRECISÃO

Resumo:As transformações de coordenadas e velocidades entre as realizações do ITRS (International Terrestrial Reference System), bem como a atualização de coordenadas, tornar-se-ão tarefas rotineiras em levantamentos geodésicos devido ao emprego de sistemas de referência dinâmicos e ao movimento das placas tectônicas. Neste estudo foram realizadas compatibilizações de sistemas de referência (ITRF2000, ITRF2005 e ITRF2008) das coordenadas e velocidades com suas respectivas estimativas de precisão, via propagação de variância, de 11 estações distribuídas nas placas tectônicas Norte e Sul Americanas e da Eurásia. Foi verificado com base nas coordenadas nos ITRF2008, época 2005 e ITRF2005, época 2000, obtidas pelo IERS (International Earth Rotation and Reference System Service), que 54,55% das discrepâncias planimétricas são centimétricas e 45,45% decimétricas. Comparando as coordenadas no ITRF2008, época 2005,0 e no ITRF2000, época 1997,0 verificou-se discrepâncias planimétricas da ordem do decímetro para todas as estações. Confrontando as coordenadas ITRF2000, época 1997,0 obtidas pelo IERS com as calculadas com base nas transformações entre sistemas de referência e atualizações verifica-se discrepâncias planimétricas da ordem do milímetro em 72,73% dos casos e da ordem do centímetro em 27,27%. As análises realizadas confirmam a necessidade de atualizar e compatibilizar o referencial das coordenadas e velocidades para aumentar a acurácia do posicionamento

Integração de redes GNSS: uma proposta metodológica de densificação da rede SIRGAS na América do Sul

A característica continental do Brasil implica na necessidade de constantes desafios científicos em pesquisas espaciais e tecnológicas, que visam contribuir ao engajamento de centros de pesquisas, fornecendo instrumentos às atividades espaciais no país e no mundo, ampliando as perspectivas de aplicação de técnicas geodésicas como o GPS (Global Positioning System). Neste contexto, destaca-se a contribuição ao IGS (International GNSS Service) dos centros de locais de análise SIRGAS (Sistema de Referência Geocêntrico para as Américas) e o IGS-RNAACSIR (IGS Regional Network Associate Analysis Centre for SIRGAS), que disponibilizam soluções semanais no formato SINEX (Software INdependent EXchange Format), referenciadas ao ITRS (International Terrestrial Reference System), que posteriormente são integradas no contexto de uma solução regional pelo DGFI (Deutsches Geodätiches Forschungsinstitut), denominada IGS-RNAACSIR. Nesta perspectiva, este projeto apresenta uma metodologia de integração de redes GNSS locais no contexto de uma solução regional para o Brasil, com base em dados gerados pelas redes ativas RBMC (Rede Brasileira de Monitoramento Contínuo), GNSS SP (Rede GPS Ativa do Oeste do Estado de São Paulo, e em soluções SIRGAS no formato SINEX, utilizando o princípio da combinação das equações normais pelo MMQ (Método dos Mínimos Quadrados), contribuindo com a manutenção e densificação da rede SIRGAS na América Latina.

Definition and realization of the celestial intermediate reference system

The transformation between the International Terrestrial Reference System (ITRS) and the Geocentric Celestial Reference system (GCRS) is an essential part of the models to be used when dealing with Earth's rotation or when computing directions of celestial objects in various systems. The 2000 and 2006 IAU resolutions on reference systems have modified the way the Earth orientation is expressed and adopted high accuracy models for expressing the relevant quantities for the transformation from terrestrial to celestial systems. First, the IAU 2000 Resolutions have refined the definition of the astronomical reference systems and transformations between them and adopted the IAU 2000 precession-nutation. Then, the IAU 2006 Resolutions have adopted a new precession model that is consistent with dynamical theories and have addressed definition, terminology or orientation issues relative to reference systems and time scales that needed to be specified after the adoption of the IAU 2000 resolutions. These in particular provide a refined definition of the pole (the Celestial intermediate pole, CIP) and the origin (the Celestial intermediate origin, CIO) on the CIP equator as well as a rigorous definition of sidereal rotation of the Earth. These also allow an accurate realization of the celestial intermediate system linked to the CIP and the CIO that replaces the classical celestial system based on the true equator and equinox of date. This talk explains the changes resulting from the joint IAU 2000/2006 resolutions and reviews the consequences on the concepts, nomenclature, models and conventions in fundamental astronomy that are suitable for modern and future realizations of reference systems. Realization of the celestial intermediate reference system ensuring a micro-arc-second accuracy is detailed.