TUM – Technical University of Munich Menu
Das Meer östlich von Grönland ist das ganze Jahr von Eis bedeckt (die weiße Linie zeigt die Grenze des Meereises). Das Wasser darunter unterliegt einer jahreszeitlichen Dynamik und wird beeinflusst durch die Strömungen im atlantischen Ozean.
Das Meer östlich von Grönland ist das ganze Jahr von Eis bedeckt (die weiße Linie zeigt die Grenze des Meereises). Das Wasser darunter unterliegt einer jahreszeitlichen Dynamik und wird beeinflusst durch die Strömungen im atlantischen Ozean. (Illustration: Marcello Passaro, Felix Müller / DGFI-TUM)
  • Research news

Algorithm improves sea level measurements

Virtual contact lenses for radar satellites

Radar satellites supply the data used to map sea level and ocean currents. However, up until now the radar's "eyes" have been blind where the oceans are covered by ice. Researchers at the Technical University of Munich (TUM) have now developed a new analysis method to solve this problem.

The melting of the polar ice cap would have a drastic effect: Sea level would rise by several meters around the world, impacting hundreds of millions of people who live close to coasts. "This means one of the most important questions of our time is how climate change is affecting the polar regions," explains Dr. Marcello Passaro of the TUM German Geodetic Research Institute.

The blind spot of the radar "eye"

But changes in sea level and ocean currents in the ice-covered regions of the Arctic and Antarctic in particular are very difficult to detect. The reason: The radar signals of the altimeter satellites that have been surveying the surfaces of the earth and oceans for more than two decades are reflected by the ice at the poles. This renders the water underneath the ice invisible.

But ocean water also passes through cracks and openings in the permanent ice, reaching the surface. "These patches of water are however very small and the signals are highly distorted by the surrounding ice. Here standard evaluation methods like those used for measurements made on the open seas are incapable of returning reliable results," Passaro points out. Together with an international team he has now developed a data analysis method which sharpens the focus of the radar's eyes.

An algorithm for all occasions

The core of this virtual "contact lens" is the adaptive algorithm ALES+, (Adaptive Leading Edge Subwaveform). ALES+ automatically identifies the portion of the radar signal which is reflected by water and derives sea level values using this information only.

This makes it possible to precisely measure the altitude of the ocean water which reaches the surface through ice cracks and openings. By comparing several years of measurements, climate researchers and oceanographers can now draw conclusions about changes in sea level and ocean currents.

"The special thing about our method is that it is adaptive," Passaro notes. "We can use one and the same algorithm to measure sea level in both open and ice-covered ocean areas. ALES+ can also be used for coastal waters, lakes and rivers. Here the signals are highly varied, but always exhibit certain characteristic properties which the system then learns." The scientists were able to use a test scenario in the Greenland Sea to demonstrate that ALES+ returns water levels  for ice-covered and open ocean regions which are significantly more precise than the results of previous evaluation methods.

More information:

The research is a part of the Sea Level Climate Change Initiative of the European Space Agency.


Passaro, M., S. Kildegaard Rose, O. Andersen, E. Boergens, F. M. Calafat, D. Dettmering, J. Benveniste: ALES+: Adapting a homogenous ocean retracker for satellite altimetry to sea ice leads, coastal and inland waters. Remote Sensing of Environment, 2018, doi: 10.1016/j.rse.2018.02.074

Müller F.L., Dettmering D., Bosch W., Seitz F.: Monitoring the Arctic Seas: How satellite altimetry can be used to detect open water in sea-ice regions. Remote Sensing, 9(6), 551, doi:10.3390/rs9060551, 2017. https://mediatum.ub.tum.de/doc/1362260/1362260.pdf.

Photos for editorial use:



Dr. Marcello Passaro
Technical University of Munich
German Geodetic Research Institute and Chair of Geodetic Geodynamics
Tel.: +49 (89) 23031-1214

Corporate Communications Center

Technical University of Munich

Article at tum.de

Überflutete Reisfelder im Mekong-Delta in Vietnam. (Foto: iStock / Huy Thoai)

River levels tracked from space

The 4,300 kilometer Mekong River is a lifeline for South-East Asia. If this mighty river system bursts its banks, flooding can affect the lives and livelihoods of millions of people. A new method developed by a team of...

Mit dem "eRay" kreierte das siegreiche Team der TUM ein ultraeffizientes Flugzeug. (Bild: DLR, CC-BY 3.0)

Ultra-efficient aircraft "eRay" from Munich

The German Aerospace Center (DLR) and the National Aeronautics and Space Administration (NASA) in the United States have announced the winners of their joint design competition in Washington DC. As in 2017, students of the...

Radioteleskope am Geodätischen Observatorium Wettzell

New Department of Aerospace, Aeronautics and Geodesy

In response to epoch-making developments in aviation and space flight, the Bavarian Minister President Dr. Markus Söder announced the establishment of a new engineering department at the Technical University of Munich (TUM)...

Gletscherspalten in der West-Antarktis.

Antarctic ice cap melting faster and faster

Melting ice in the Antarctic has resulted in a 7.6 millimeter rise in the global sea level since 1992. Three millimeters of this increase were recorded in the last five years alone, according to a comprehensive study...