Abstract
The tie vectors at co-location sites are required for combining the individual space geodetic solutions and computing the International Terrestrial Reference Frame. Their provision to the combination centres entails a proper and accurate alignment of the tie vector into the global geocentric frame. In order to assess the impact of different alignment strategies on the space geodetic combination, we test four alignment approaches—two similarity transformations and two transformations based on the local values of the deflection of the vertical—and focus on the residuals of the combination. Our results clearly show that the alignment strategy not only controls the orientation on the tie vector in the global frame—determining its global coordinates and components—but also impacts the magnitude of the tie vector’s combination residuals to an extent that can exceed the 1 cm level. In practice, the information carried by the same tie vector is affected by the alignment strategy—a step in the tie vectors’ production that we prove is critical and that can impact the combination of space geodetic techniques. At both sites, the alignment strategy (local topocentric to the global geocentric ITRF frame) that ensures higher consistency (i.e. smaller residuals) in the computation is the similarity transformation that makes use of all the available space geodetic reference points and no information about the DoV.