Coherent GNSS-Reflections Characterization Over Ocean and Sea Ice Based on Spire Global CubeSat Data

This article assesses the coherency of Global Navigation Satellite System (GNSS) signals reflected off the oceans and sea ice under grazing angle geometries and received aboard low Earth orbit (LEO) CubeSats for precision altimetry applications. The coherency is characterized as a function of ocean surface conditions and reflected signal parameters based on Spire Global CubeSat data collected from January to April 2019. The data contain 50-Hz GPS L1 and L2 carrier phase estimations obtained by open-loop tracking. Indicators based on the circular statistics of the excess-phase noise are developed to identify coherent and semicoherent reflections. Based on these indicators, we found that ~1% and 44% of GPS reflections over the ocean and sea ice, respectively, have potential for precision altimetry. The coherent and semicoherent reflection rates reach 23% in areas less than 200 km from the coastline and under calm sea conditions. Over young sea ice over the Arctic, this rate can be as high as 70%. There is a strong relationship between coherency and signal strength, and the coherency occurrence rate improves as the grazing angle decreases. The quality of the L1 and L2 coherent reflections is similar over sea ice, while, for reflections over the ocean, L1 signals are predominantly noisier and less coherent than the L2 signals. Using a postprocessing filtering method, the semicoherent reflections can achieve a similar level of altimetry precision as that of the coherent ones, thereby increasing the along-track length of the retrieved altimetry profile.