New system uses GPS signals to measure sea level change

Scientists have developed an instrument that measures sea level using signals from satellite navigation systems, such as GPS.
Researchers at Chalmers University of Technology in Sweden found that sea level and its variation can easily be monitored using existing coastal Global Positioning System (GPS) stations.
Measuring sea level is an increasingly important part of climate research, and a rising mean sea level is one of the most tangible consequences of climate change.
Johan Lofgren and Rudiger Haas, scientists at Chalmers Department of Earth and Space Sciences, have developed and tested an instrument that measures the sea level using radio signals from satellite navigation systems: a GNSS tide gauge (GNSS stands for Global Navigation Satellite System).

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," said Lofgren.

"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 said.
Two antennas, covered by small white radomes, measure signals both directly from the satellites and signals reflected off the sea surface.

By analysing these signals together, the sea level and its variation can be measured, up to 20 times per second.
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. Through advanced signal processing, these signals can be studied further.
The new GNSS tide gauge has the advantage that it can measure changes in both land and sea at the same time, in the same location.
That means that both long-term and short-term land movements (post-glacial rebound and earthquakes) can be taken into consideration.

"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.
The scientists have also shown that existing coastal GNSS stations, installed primarily for the purpose of measuring land movements, can be used to make sea level measurements.
"We've successfully tested a method where only one of the antennas is used to receive the radio signals. That means that existing coastal GNSS stations - there are hundreds of them all over the world - can also be used to measure the sea level," said Lofgren.