Observing Arctic sea ice freeboard with high-resolution spaceborne grazing angle GNSS-Reflectometry

A consistent monitoring of the sea ice freeboard is crucial for observing changes in sea ice thickness and for improving sea ice forecasts. Mainly, area-wide freeboard heights are determined by using surface elevations from satellite altimetry missions like ESA's Earth Explorer Cryosat-2 and NASA's ICESat-2. A sudden failure or interruption of these altimeter missions would lead to significant data gaps in the central Arctic Ocean. Therefore, additional ways to determine sea level and freeboard based on remote sensing techniques are of great importance.

One of these additional techniques is grazing angle GNSS reflectometry (GA GNSS-R), which uses surface reflections from Global Navigation Satellite System (GNSS) signals to capture elevation information and characterize surface roughness. Here, we utilize reflections collected by up to 15 Spire Global nanosatellites. These nanosatellites record both the directly transmitted signal (line of sight) and the signal reflected from the surface , The signals are used to determine the delays in the measured phase compared to a modelled phase, from which the elevation of a reflecting surface is then derived. Accurate height estimation is feasible when the reflected signal remains phase‑coherent and retains right‑hand circular polarization at grazing incidence angles between approximately 5° and 30°. Under these conditions, smooth surfaces such as sea ice or calm water produce strong coherent reflections, whereas rougher ocean surfaces induce decorrelation and significantly weaker reflected signatures.

As part of the ESA Earthnet Data Assessment Project (EDAP+), a new approach is developed to derive sea ice freeboard from GA GNSS-R observations providing complementary information to satellite altimetry. One key advantage of Spire’ GA GNSS-R constellation is the high geographic density over the poles  and thus continuous coverage of high latitudes without a systematic polar observation gap, as is the case with altimetry missions. Geolocated surface reflections and elevation data provided by Spire Global are used to classify surfaces, to detect water openings within the sea-ice cover (e.g., leads), to retrieve local sea-surface elevations, and subsequently to derive along-track sectional freeboard heights from GA GNSS-R observations.

This study presents first monthly Arctic-wide freeboard maps for a complete winter season (2023/2024) and comparisons with upward-looking sonar measurements and CryoSat-2 freeboard products. Initial comparisons demonstrate that GNSS-R-derived freeboard provides valuable complementary information, particularly in regions with a dense data coverage, and achieves accuracy comparable to altimetry during autumn and early winter.