An Assessment of Sentinel-3A Measurements and Fully-Focused SAR Processing over Arctic Sea Ice

Launched February 16, 2016, the European Sentinel-3A mission will provide critical measurements of sea ice freeboard and sea surface height, providing coverage to a latitudinal limit of 81.5 °N. Discrimination of waveform returns from floes and leads is required to determine freeboard, that portion of an ice floe floating above the local sea surface. Since 2002, the NOAA Laboratory for Satellite Altimetry has cooperated with ESA, EUMETSAT, and NASA to conduct airborne validation experiments over the Arctic Ocean to assess the use of satellite altimetry for measuring sea ice freeboard. During its Spring Arctic campaigns of 2016, 2017, and 2018, NASA's Operation IceBridge (OIB) mission, utilizing multi-instrumented aircraft , conducted surveys over Arctic sea ice timed to coincide with an overpasses of the Sentinel-3A satellite. Spatially and temporally coincident data were collected in the eastern Beaufort Sea, in a study area that comprised large sea ice floes, interspersed with open and refrozen leads. The OIB instrument suite, including laser and radar altimeters, and high-resolution visible and infrared camera systems, provides measurements of sea ice freeboard, snow depth, and sea ice morphology.

Here we present an evaluation of Sentinel-3A data over Arctic sea ice with coincident OIB data. For our analysis we have processed the Sentinel-3 SAR mode data in a fully-focused SAR (FF-SAR) fashion. FF-SAR is a novel SAR altimetry data processing technique, that accounts for the phase evolution of the targets in the scene, making it possible to focus the complex echoes along the aperture. This process, similar to SAR imaging systems, can potentially reduce the along-track resolution to the theoretical limit equal to L/2, where L is the antenna length. The improved resolution allows better representation of radar backscatter changes on the surface and therefore detection of more sea-ice leads, improving freeboard estimates when compared to the standard delay/Doppler processing technique.

In this paper, we verify sea ice lead and floe delineations with the Sentinel-3A FF-SAR data. We also assess the accuracy of Sentinel-3A surface elevation measurements and evaluate Sentinel-3A sea ice freeboard estimates via comparison with independent OIB freeboard measurements.