Abstract
Reflectometry of Global Navigation Satellite Systems (GNSS) signals from the ocean surface has provided a new source of observations to study the ocean-atmosphere interaction. We investigate the sensitivity and performance of GNSS-Reflectometry (GNSS-R) data to retrieve sea surface roughness (SSR) as an indicator of sea state. A data set of one-year observations in 2016 is acquired from a coastal GNSS-R experiment in Onsala, Sweden. The experiment exploits two sea-looking antennas with right- and left-hand circular polarizations (RHCP and LHCP). The interference of the direct and reflected signals captured by the antennas is used by a GNSS-R receiver to generate complex interferometric fringes. We process the interferometric observations to estimate the contributions of direct signals and reflections to the total power. The power estimates are inverted to the SSR using the state-of-the-art model. The roughness measurements from the RHCP and LHCP links are evaluated against match-up wind measurements obtained from the nearest meteorological station. The results report on successful roughness retrieval with overall correlations of 0.76 for both links. However, the roughness effect in LHCP observations is more pronounced. The influence of surrounding complex coastlines and the wind direction dependence are discussed. The analysis reveals that the winds blowing from land have minimal impact on the roughness due to limited fetch. A clear improvement of roughness estimates with an overall correlation of 0.82 is observed for combined polarimetric observations from the RHCP and LHCP links. The combined observations can also improve the sensitivity of GNSS-R measurements to the change of sea state.
Highlights
T HE characterization and monitoring of sea surface roughness (SSR) are important for understanding air–sea interactions
We have investigated the response of Global Navigation Satellite Systems (GNSS) reflectometry observations to the SSR during different wind conditions
A coastal GNSS-R experiment has been used to assess the performance of polarimetric observations for estimating the roughness
Summary
T HE characterization and monitoring of sea surface roughness (SSR) are important for understanding air–sea interactions. This parameter is considered as one of the indicators of the sea state. The difficulty of making direct SSR measurements due to the complexity and random behavior of sea surface fluctuations has cleared the way for remote sensing techniques [1]. A robust observation resource of SSR can be acquired from the reflectometry of the Global Navigation Satellite Systems (GNSS) signals. The GNSS-Reflectometry (GNSS-R) is an all-weather operating technique offering high temporal resolution observations based on low-cost passive instrumentation. The anomalies of surface roughness over mesoscale ocean eddies can reveal the presence of these oceanic features and the ongoing air–sea interaction [4]
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