By the Beautiful Sea

A panorama from the GNSS tide gauge at Onsala Space Observatory. When satellites pass over the sky, the GNSS tide gauge uses signals direct from the satellite and signals reflected off the sea surface to measure the sea level. Photo: Johan Löfgren

New Tide Gauge Uses GNSS to Measure Sea-Level Change

A new way of measuring and monitoring sea level — an important facet of researching climate change — has been implemented by scientists at Chalmers University of Technology in Sweden using existing coastal GPS stations.

When satellites pass over the sky, the GNSS tide gauge uses signals direct from the satellite and signals reflected off the sea surface to measure the sea level. Photo: Johan Löfgren

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. Researchers at Chalmers University of Technology have studied new ways of measuring sea level that 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.

Johan Löfgren and Rüdiger Haas, scientists at Chalmers Department of Earth and Space Sciences, have developed and tested an instrument that measures the sea level using a GNSS tide gauge.

“The global mean sea level is rising because of climate change, but the change depends on where you are in the world,” said Rüdiger Haas. “We want to be able to make detailed measurements of sea level so that we can understand how coastal societies will be affected in the future.”

The GNSS tide gauge uses GPS and GLONASS signals. BeiDou and Galileo will be added in the future.

“We measure the sea level using the same radio signals that mobile phones and cars use in their satellite navigation systems,” said Johan Löfgren. “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.”

Antenna Setup. Two antennas, covered by small white radomes, measure signals both directly from the satellites and signals reflected off the sea surface. By analyzing 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.

Schematic drawing of the GNSS tide gauge for SNR analysis (left) and phase-delay analysis (right). For the SNR analysis, the satellite signal with elevation ε reflects off the sea surface and interferes with the direct satellite signal at the antenna, creating an interference pattern in the recorded SNR observable that can be related to the reflector height, hr. For the phase delay analysis, the phase delays of the direct and the reflected signals are recorded separately, and through geodetic analysis of the phase delay, the baseline between the antennas can be determined and related to the height of the nadir-looking antenna over the sea surface, ha, and the vertical distance between the antenna phase centers, d.

The scientists’ initial study compared sea-level solutions from two analysis methods: signal-to-noise ratio (SNR) analysis and phase-delay analysis. The SNR analysis uses multipath signals observed with an upward-looking antenna, and the phase delay analysis uses the phase delay for both an upward- and a downward-looking antenna (see diagram).

Both GPS and GLONASS L1 and L2 signals were recorded, and the results were compared to independent measurements of sea level from a co-located pressure tide gauge. The GNSS-derived sea level showed a high correlation with the tide-gauge sea level for both analysis methods. Correlation coefficients for the phase-delay analysis and for the SNR analysis using frequency L1 were 0.95 to 0.97, whereas the correlation coefficients for the SNR analysis using frequency L2 were 0.86 to 0.87. 

The phase-delay analysis shows a better agreement with the independent tide gauge sea level than the sea level from SNR analysis. Expressed as RMS differences, the phase-delay analysis achieves values of 3.5 cm (GPS) and 3.3 cm (GLONASS), whereas the SNR analysis achieves 4.0 cm (GPS) and 4.7 cm (GLONASS). The scientists concluded that, for the phase-delay analysis, it is possible to use both frequency bands, and for the SNR analysis, frequency band L2 should be avoided if other signals are available.

The GNSS tide gauge at Onsala Space Observatory uses signals from satellite navigation systems like GPS to measure the sea level. Photo: Johan Löfgren

Land and Sea. Unlike traditional tide gauges, the new GNSS tide gauge can measure changes in both land and sea at the same time, in the same location. That means 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 center of the Earth, which means we can clearly tell the difference between changes in the water level and changes in the land,” said Johan Löfgren.

This summer, other high-precision instruments are being installed to work with the Onsala GNSS tide gauge, in collaboration with SMHI, the Swedish Meteorological and Hydrological Institute.

“Our tide gauge station will become part of a network of stations along the coast of Sweden that will be able to monitor changes in the water level to millimeter precision well into the future,” said Gunnar Elgered, professor at Chalmers Department of Earth and Space Sciences.

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 Johan Löfgren.

This work was previously reported in these publications: Larson, K.M., J. Lofgren, and R. Haas, “Coastal Sea Level Measurements Using A Single Geodetic GPS Receiver,” Adv. Space Res., Vol. 51(8), 1301-1310, 2013, doi:10.1016/j.asr.2012.04.017, 2013; and Larson, K.M., R. Ray, F. Nievinski, and J. Freymueller, “The Accidental Tide Gauge: A Case Study of GPS Reflections from Kachemak Bay, Alaska,” IEEE GRSL, Vol 10(5), 1200-1205, doi:10.1109/LGRS.2012.2236075, 2013.