Establishment of an Official Tide Gauge Station at the Onsala Space Observatory

The Onsala Space Observatory has been active in performing geodetic and astrometric VLBI observations since 1968. Currently about 40–50 sessions per year are observed in IVS programs. In April 2012 we received a decision for funding of a twin telescope at Onsala, to be part of the VLBI2010 Global Observing System (VGOS) network. With plans for a significant improvement in accuracy within the VGOS project, the need for complementary instrumentation and observations will increase. Particularly at the Onsala site, being located at the coast, loading effects on the Earth's crust are important. The modelling of the sea level in Kattegatt is complicated and the available tide gauge data today are from Gothenburg and Ringhals at a distance of 33 km and 18 km, respectively. This together with the fact that the Onsala site will be very well determined in the International Terrestrial Reference Frame (ITRF) motivates the installation of an official tide gauge station of the Swedish national network. This network is operated by the Swedish Meteorological and Hydrological Institute (SMHI). An agreement for a joint responsibility in constructing and operating a tide-gauge station at the Onsala site was signed in 2013. Since 2010 we operate a so-called GNSS tide gauge utilizing reflected GNSS signals. This was complemented by three pressure sensors submerged into the sea at the same location in the summer of 2011. In the autumn of 2013 a pneumatic tide gauge was installed. At a new location that will host the official tide gauge mentioned above. When the construction is completed also a radar sensor and an additional pneumatic sensor will be mounted in a special concrete culvert with heating, in order to ensure operating conditions also when the sea is covered by ice. In this presentation we will give an overview of the status of the installation of the tide gauge station, including preliminary results. These include comparisons between different sensors. They quality will be assessed in terms of estimates of measurement uncertainty, repeatability, and linearity