3.3.2. Local ties.
Full benefit from co-located space geodetic instruments can only be achieved when the inter-technique 3D vectors (the so-called 'local ties') between the reference points of the individual techniques are known. Usually, site surveys provide these local ties which can then be used to relate the reference points of co-located space geodetic instruments to each other. Doing so, one can assure that two or more co-located space geodetic instruments, i.e. placed together at the same observation site, reveal the same site movement over time. Thus it is crucial that local tie measurements are carried out with utmost high precision and are transformed into the terrestrial reference frame so that they can be applied to connect the different co-located techniques. Local ties are usually provided by the agencies that host the space geodetic instruments and are made public by the ITRF (International Terrestrial Reference Frame) center of the IERS (International Earth Rotation and Reference Frame Service). The c5++ software can use the provided local tie information as an independent observation for combination purposes. A detailed discussion on the weighting of local tie information and its impact of combining two techniques on the observation level is given in [15]. However, since VCE has been implemented recently in c5++, it is now also possible to determine the weights of local tie vectors directly and automatically within the estimation process. Although the realization of such an estimation strategy is straightforward, one needs to consider the fact the there is a huge unbalance between the number of space geodetic observations and the number of local tie vectors. Since the precision of local tie measurements could vary from station to station it would in principle be necessary to solve for a variance component of each of the local ties. However, as this leads to an even further unbalance in the VCE it has been decided to estimate only one single variance component that scales the formal errors of all local ties in the station network.
3.3.2 การท้องผูกFull benefit from co-located space geodetic instruments can only be achieved when the inter-technique 3D vectors (the so-called 'local ties') between the reference points of the individual techniques are known. Usually, site surveys provide these local ties which can then be used to relate the reference points of co-located space geodetic instruments to each other. Doing so, one can assure that two or more co-located space geodetic instruments, i.e. placed together at the same observation site, reveal the same site movement over time. Thus it is crucial that local tie measurements are carried out with utmost high precision and are transformed into the terrestrial reference frame so that they can be applied to connect the different co-located techniques. Local ties are usually provided by the agencies that host the space geodetic instruments and are made public by the ITRF (International Terrestrial Reference Frame) center of the IERS (International Earth Rotation and Reference Frame Service). The c5++ software can use the provided local tie information as an independent observation for combination purposes. A detailed discussion on the weighting of local tie information and its impact of combining two techniques on the observation level is given in [15]. However, since VCE has been implemented recently in c5++, it is now also possible to determine the weights of local tie vectors directly and automatically within the estimation process. Although the realization of such an estimation strategy is straightforward, one needs to consider the fact the there is a huge unbalance between the number of space geodetic observations and the number of local tie vectors. Since the precision of local tie measurements could vary from station to station it would in principle be necessary to solve for a variance component of each of the local ties. However, as this leads to an even further unbalance in the VCE it has been decided to estimate only one single variance component that scales the formal errors of all local ties in the station network.
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