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Scientists open altimeter satellites' 'eyes' to ice-covered blind spots of the Arctic and Antarctic

Researchers from the Technical University of Munich (TUM), part of ESA's Climate Change Initiative Sea Level project, have developed an algorithm for satellites that enables the properties of oceans beneath the ice caps to be accurately determined.

This is a major advance, as until now the radar "eyes" of current satellites have been blind where the oceans are ice covered. The ability to map sea level and ocean currents is an important variable understanding the climate.

The team published their findings in the journal Remote Sensing of Environment, which explains how oceans beneath the ice caps are obscured from view, and that observing and analysing the few cracks in the ice is insufficient.

The ocean east of Greenland is covered by ice all year round (the white line shows the boundary of the oceanic ice). The water underneath is subject to a dynamic seasonal process and is influenced by the currents of the Atlantic ocean. Credit: Marcello Passaro and Felix Müller / DGFI-TUM

The blind spot of the radar "eye"

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 radar signals of existing altimeter satellites, which have been surveying the surface 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," according to lead author, Marcello Passaro.

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.

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

Publication:

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 https://www.sciencedirect.com/science/article/pii/S0034425718300920