Sea surface height (SSH) in the Arctic Ocean is usually measured by a few tide gauges along the coasts and with the help of satellite altimeter missions. The latter are also relevant for monitoring sea ice freeboard and thickness. However, only few altimeter missions cover polar latitudes above 81° N. Currently, these are only the ESA Earth Explorer radar altimeter Cryosat-2 and the NASA laser altimeter ICESat-2. An interruption of these missions would lead to significant data gaps in the central Arctic Ocean. Therefore, additional means of determining sea level and freeboard based on remote sensing techniques are of great importance. One such technique is Grazing Angle GNSS Reflectometry (GA GNSS-R), which uses surface reflections from the Global Navigation Satellite System (GNSS) to capture elevation information or surface roughness characterizations.

In GA GNSS-R, both the directly transmitted (line-of-sight) signal and the signal reflected from the surface are captured by nanosatellites and used to determine the delays in the measured phase compared to a modeled phase, from which the height of a reflecting surface is then derived. Height determination works when the phase information remains coherent and is right-handed circularly polarized at small angles that graze the Earth's surface between 5 and 30°. Reflections from calm water surfaces and sea ice are particularly strong, while rougher surfaces, such as the open sea, result in weaker signals. In addition to SSH determination, this offers the possibility of developing algorithms for detecting open water areas such as channels (leads) or polynyas and using them to estimate the freeboard of sea ice. The leading data provider for GA GNSS-R is Spire Global Inc., which operates dozens of nanosatellites with GNSS receivers and has been providing multi-GNSS amplitude and phase observations at 50 Hz since 2020. In the current ESA project “Investigation of the potential of Spire GA GNSS-R data to infer Arctic sea ice freeboard”, part of the broader ESA Earthnet Data Assessment Project (EDAP+), Spire observations are being used and analyzed to assess the possibility of detecting leads/polynias at 50 Hz and to compare GA GNSS-R with existing SSH from satellite altimetry during the winter months. DGFI-TUM is evaluating the potential use of GA GNSS-R, using open water elevations determined with CryoSat-2 and Sentinel-3, as well as optical and imaging radar techniques (e.g., Sentinel-1/2) for comparison.