First Precise Spaceborne Sea Surface Altimetry With GNSS Reflected Signals

Abstract

The precision of sea surface altimetry using bistatically reflected signals of the Global Navigation Satellite System (GNSS) is typically one to two orders of magnitude worse than dedicated radar altimeters. However, when the scattering is coherent, the electromagnetic phase of the carrier signal can be tracked, providing precise ranging measurements. Under grazing angle (GA) geometries, the conditions for coherent scattering are maximized, enabling carrier phase-delay altimetric techniques over sea waters. This work presents the first implementation of GA carrier phase sea surface altimetry using data acquired from a spaceborne platform (NASA Cyclone GNSS mission) and transmitted from both GPS and Galileo constellations. The altimetric results show that the measurement system precision is 3/4.1 cm (median/mean) at 20 Hz sampling, cm level at 1 Hz, comparable to dedicated radar altimeters. The combined precision, including systematic errors, is 16/20 cm (median/mean) precision at 50 ms integration (a few cm level at 1 Hz). The wind and wave requirements to enable coherent scattering at GA geometries appear to be below 6 m/s wind and 1.5 m significant wave height, although only 33% of tracks under these conditions present sufficient coherence. Given that this technique could be implemented by firmware updates of existing GNSS radio occultation missions, and given the large number of such missions, the study indicates that the resulting precision and spatio-temporal resolution would contribute to resolving some submesoscale ocean signals.

Initial GNSS Phase Altimetry Measurements From the Spire Satellite Constellation

“…An RMS difference between the retrieved estimates and the model sea surface height is computed to be 13.8 cm, which agrees with the values obtained by Cardellach et al. (2020) using grazing angle GNSS‐R measurements from the CYGNSS satellite mission. A larger RMS difference is expected due to the rough ocean scattering surface compared to sea ice and the corresponding decrease in coherent reflected signal.…”

“…(2017) and Cardellach et al. (2020), which had to rely on orbit estimates accurate to only tens to hundreds of meters. POD of each Spire receiver is possible due to dual‐frequency GNSS phase measurements collected through the zenith‐pointing antenna.…”

Space-Borne GNSS-R Ionospheric Delay Error Elimination by Optimal Spatial Filtering

“…The altimetry accuracy reached 3 cm for median values and 4.1 cm for mean values at a 20 Hz sampling frequency. This result was comparable to the accuracy of traditional altimetry satellites [ 12 ].…”

Sea Surface Height Estimation by Ground-Based BDS GEO Satellite Reflectometry

“…There is a strong coherence between the direct and reflected signals of BDS GEO satellites that brings great advantage for the relative SSH estimation. Furthermore, paper [31] explains that the wind and wave requirements to enable sufficient coherence appear to be below 6 m/s wind and 1.5 m SWH. Our experiments were conducted under this condition.…”