Direct Geocentric Realisation of the American reference frame by combination of geodetic observation Techniques (DIGERATI)

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The project shall investigate the possibility of realising directly and epoch-wise regional geocentric reference frames (RGRF) without the usual transformation onto a global reference system. Such reference frames are of particular importance in seismic active regions, where strong earthquakes (e.g. Chile 2010) produce large surface deformations (up to several meters), damage existent reference systems with station coordinates at a fixed epoch and linear coordinate changes, and disable their use. Also seasonal variations of air pressure and water storage produce sub-annual coordinate changes on the centimetre-level and complicate the application of existing reference systems in the interpretation of environmental changes. Regional reference frames are at present exclusively realised through measurements of the Global Navigation Satellite Systems (GNSS), which enable the local access to the global geocentric reference systems because of their large number. In this project epoch RGRF shall be computed directly by combining GNSS with the geodetic techniques Satellite Laser Ranging (SLR) and Very Long Baseline Interferometry (VLBI). The origin of the epoch solutions will be realised by SLR, the scale by a weighted mean of SLR and VLBI. An epoch-wise realisation entails a variable SLR-Network configuration from epoch to epoch. This effect on geocentricity and a stable datum realisation need to be investigated. The project will be applied in practice to the geocentric reference frame for the Americas (SIRGAS). Regarding GNSS, it has to be studied how far the network for the combination needs to be extended beyond the present SIRGAS network in order to transfer the datum from SLR/VLBI optimally to SIRGAS. The project will be performed in close cooperation with scientists of the Argentine CONICET in the frame of a project for the installation of the Argentine-German Geodetic Observatory (AGGO). AGGO assembles the important geodetic observation techniques GNSS, SLR and VLBI and thus will be an important co-location station in the combination. A particular focus will be put on the benefit of AGGO for SIRGAS. In the frame of the collaboration, the Argentine side will establish an operational GNSS-SLR-VLBI combination centre. DGFI will provide the SLR and VLBI software. At DGFI, primarily the strategy of the direct combination in regional reference frames will be investigated. After completion of this project the computation of the epoch RGRF shall be continued in Argentina on a routine basis following the combination strategy derived from the findings of this project. DIGERATI has high scientific impact for investigations of the effect of environmental changes on global change, in particular in the topical regions of South America, which require highly precise reference systems. There is also a huge practical benefit in the common application of the SIRGAS coordinates, e.g., in regional geodesy, cadastre geographical information systems and land surveying.