Swedish researchers use GNSS signals to measure sea level change

Researchers from Chalmers University of Technology in Sweden have developed and tested a GNSS (Global Navigation Satellite System) tide gauge that measures the sea level, said the university in a statement on Wednesday. The GNSS tide gauge uses radio signals from satellites in orbit around the Earth that are part of satellite navigation systems like GPS and Glonass (Russia's equivalent of GPS). "We measure the sea level using the same radio signals that mobile phones and cars use in their satellite navigation systems," Johan Lofgren, researcher from the university was quoted as saying in the statement. "As the satellites pass over the sky, the instrument 'sees' their signals -- both those that come direct and those that are reflected off the sea surface," Lofgren added. Two antennas, covered by small white radomes, measure signals both directly from the satellites and signals reflected off the sea surface, and by analyzing these signals together, the sea level and its variation can be measured, up to 20 times per second. As the sea level time series is rich in physical phenomena such as tides (caused mostly by the gravitational pull of the Moon and the Sun), meteorological signals (high and low pressure), and signals from climate change, these signals can be studied further through advanced signal processing. The new ways of measuring sea level could become important tools for testing climate models and for investigating how the sea level along the world's coasts is affected by climate change. "The global mean sea level is rising because of climate change, but the change depends on where you are in the world," Rudiger Haas, another researcher in the program was quoted as saying in the statement. "Now we can measure the sea level both relative to the coast and relative to the centre of the Earth, which means we can clearly tell the difference between changes in the water level and changes in the land", said Lofgren. This summer the tide gauge station at Onsala Space Observatory will be completed, in collaboration with the Swedish Meteorological and Hydrological Institute. Other high-precision instruments will be installed to work together with the GNSS tide gauge.