GNSS Reflectometry for Remote Sensing of Ross Ice Shelf Surface Topography

This study presents surface topography results for the Ross Ice Shelf in Antarctica, derived from space-borne Global Navigation Satellite System Reflectometry (GNSS-R) measurements. The spaceborne GNSS-R method utilizes reflected GNSS signals of opportunity collected on a platform in low-Earth orbit, allowing for widespread data coverage with improved temporal resolution compared to conventional remote sensing technologies. Remote sensing of the cryosphere is a relatively new application for GNSS-R, but the effectiveness of the method has been demonstrated for sea ice extent (J. Cartwright et al., “Sea Ice Detection Using GNSS-R Data From TechDemoSat-1,” JGR: Oceans, 2019) and altimetry (V. Nguyen et al., “Ini-tial GNSS Phase Altimetry Measurements From the Spire Satellite Constellation”, GRL, 2020), and more recently land ice altimetry (R. Buendia et al., “Ice sheet height retrievals from Spire grazing angle GNSS-R,” Remote Sensing of Environment, 2023). In this study, two separate GNSS-R processing techniques are used to determine the roughness and precise relative surface elevation of the reflecting ice shelf surface. Coherency of the reflected signal can be used to separate different types of terrain (e.g., mountainous areas versus flatter ice areas versus ocean) and as a proxy for the roughness of the surface at GNSS wavelengths. Coherency level can be quantified using circular length and circular kurtosis, defined respectively as the length of the sum of the phase rate unit vectors
$\delta \phi$
and the kurtosis of
$\delta\dot{\phi}$
. Both are close to 1 for coherent reflections and close to 0 otherwise, and an empirical threshold can be employed to differentiate coherent and incoherent reflections. Reflected signal coherency is a prerequisite for the second portion of the inves-tigation. For coherent reflections, we use carrier phase-based altimetry to retrieve precise surface height relative to a reference elevation model according to Equation 1, where
$\Delta h$
is the retrieved height difference,
$\theta$
is the elevation angle, and
$\triangle\Phi_{\text{iono}}$
-free is the difference between the direct and reflected ionosphere free filtered residual phases. In addition to accounting for ionospheric effects using the dual-frequency signal, troposphere delay, cycle slips, clock and orbit errors, and dynamics are removed from
$\Delta \Phi_{\text{iono}}$
-free before height calculation.