Global warming is affecting the sea level by, for example, melting of large masses of ice in polar and subpolar regions, and thermal expansion of seawater. Monitoring and understanding the local response to a global sea level rise is crucial to the safety of the population in coastal and low-lying regions.
A tide gauge measures variations in the sea level. There are different methods to do this. Traditional tide gauges measure the sea level relative to the Earth’s crust. The Earth’s crust is, however, continuously moving and in order to fully understand sea level change processes, measurements of sea level in relation to the Earth’s centre of gravity are necessary. There are two tide gauges at Onsala Space Observatory to achieve this, both connected with millimeter precision to the geodetic instruments at the observatory.
The super tide gauge, inaugurated on 17 September 2015, was built in cooperation with the Swedish Meteorological and Hydrological Institute, SMHI. It is part of SMHI:s national network, and must have a high reliability. Therefore, it measures sea level with two methods simultaneously: with a radar and with a so called bubble sensor. The sea level is measured relative to the Earth's crust.
The super tide gauge at Onsala Space Observatory.
Copyright: Onala Space Observatory/Lars Wennerbäck
It is also important to measure sea level relative to the Earth's centre of gravity. One way to do this is by using GNSS (global navigation satellite systems) receivers. Different techniques exist. In Onsala, an experimental GNSS tide gauge using two GNSS receivers and two antennas has been installed close to the sea. One of the antennas is up-looking for direct signals, and one is down-looking for signals reflected from the sea surface. The receivers track the phase of the GNSS signal, and the sea level can be derived using traditional geodetic phase data analysis.
The GNSS tide gauge at Onsala Space Observatory.
The super tide gauge:
Email: email@example.com, Phone: +46 31-772 5565
The GNSS tide gauge:
Email: firstname.lastname@example.org, Phone: +46 31-772 5530
Permanent Service for Mean Sea Level (PSMSL):
Swedish Meteorological and Hydrological Institute (SMHI):