Coherence Analysis of CYGNSS Land Reflections and Potential Application to Inland Water and Wetlands Remote Sensing

Abstract:

Global navigation satellite system reflectometry (GNSS-R) is a method of remote sensing wherein passive receivers utilize existing GNSS satellite transmissions that are reflected from the surface of Earth. In recent years, GNSS-R measurements from the Cyclone GNSS (CYGNSS) mission have been used for a variety of land applications, such as estimating soil moisture and performing flood mapping. An emerging trend in GNSS-R is the use of precise carrier phase measurements, which require that the reflected signal be sufficiently coherent. Coherent reflections also yield a finer spatial resolution (hundreds of meters) than the incoherent reflections captured in the standard CYGNSS data. Amongst other applications, leveraging these capabilities could produce a novel source of height and extent data for inland water. However, the degree to which various surface characteristics impact coherence is unclear. In this study, we explore how surface water, topography, soil moisture, and vegetation water content impact the coherence of reflected GNSS signals. Carrier phase measurements are obtained by tracking the raw intermediate frequency (IF) data that is occasionally collected by CYGNSS satellites. In total, several hundred raw IF collections between 2017 and 2019 are analyzed. Coherence, quantified by applying circular statistics to the phase measurements, is compared to the corresponding land characteristics on a per-track basis and across the entire dataset. This analysis establishes a baseline for the degree of coherence to be expected for varying surface conditions, giving guidance to further development of GNSS-R land remote sensing applications.