## 4 Conclusions/Outlook  
We compared three different methods for estimating the DoV components at two ITRF sites hosting a VLBI-GNSS co-location. There, the knowledge of the DoV components plays a fundamental role to align the tie vectors measured using terrestrial geodetic instruments to the ITRF.  
The GNSS-LEV method and the QDaedalus system gave results with similar high precision levels. In both cases, the standard deviations of the estimated DoV components meet the 1-arcsec threshold and can be implemented in classic networks without compromising the precision of angular measurements, even with high precision instruments and procedures.  
The astro-geodetic measurement using the QDaedalus system has significant advantages: first of all, it allows further improving its precision by greater redundancy and longer observation periods, while being decidedly less onerous than the GNSS-LEV method, from the operational point of view. Furthermore, QDaedalus can measure the DoV components in any desired position, with no need of _a priori_ assumptions about the spatial trends of the DoV, as required by the GNSS-LEV method. Conversely, _ad hoc_ special equipment is necessary that is still not widely available on the market.  
Indeed, almost every surveying company owns the equipment that is required for the GNSS-LEV method, but it is always considerably more onerous in terms of time and staff involved. In addition, site dependent features, such as quality, composition and shape of the topographic surface, may limit or prevent its efficient use, especially when high level of precision must be achieved.  
The ITALGEO2005 grid values showed statistically significant consistencies with the other two methods but associated to standard deviations exceeding the 1-arcsec threshold required to satisfy the angular precision of high precision local networks. Nevertheless, in absence of local DoV measurements, the use of a reliable regional geoid model is recommended anyway.  
It is also worth stressing that the use of regional geoid models is promising as their precision is constantly increasing worldwide, thanks to the investigations carried out at a global level by different satellite-based gravity missions, e.g., CHAMP, GRACE and GOCE.  
**Acknowledgments:** The authors wish to thank [PERSON] (ETH Zurich) for the contributions and suggestions he offered during the editing phase of the article. The authors also wish to thank [PERSON], [PERSON] and [PERSON] (IRA-INAF) for the assistance they offered during the measurements.  
**Author Contributions:** [PERSON]: contributed to all phases of data acquisition, estimated the GNSS-LEV DoV components and contributed in writing the manuscript. [PERSON]: contributed to all phases of data acquisition, estimated the GNSS-LEV DoV components and contributed in writing the manuscript. [PERSON]: contributed to all phases of data acquisition, estimated the GNSS-LEV DoV components and contributed in writing the manuscript. [PERSON]: operated the QDaedalus system, contributed to the GNSS-LEV data acquisition in Noto, estimated the QDaedalus Dov components and contributed in writing the manuscript. [PERSON]: computed the DoV components based on the gravity data and contributed in writing the manuscript. [PERSON]: processed the GNSS data. [PERSON]: provided guidance to the use of the QDaedalus system.