Coherent GNSS reflections over the sea surface - A classification for reflectometry

The exploitation of GNSS signals for reflectometry opens several fields of application over the ocean, land and in the cryosphere. Coherence of the reflection allows precise measurements of the carrier phase and signal amplitude for accurate sea surface altimetry and sea ice characterisation. A coherence condition can be set by a threshold of the signal-to-noise power ratio (SNR). Previous simulations suggest that an SNR > 30 dB will ensure a coherent processing of the signal.

This paper presents reflectometry measurements that provide signal coherence information. The measurements have been conducted on two research vessels: R/V Lance and R/V Polarstern. The objective is to reveal the required conditions for coherent reflectometry depending on sea state and sea ice occurrence. Three data sets from expeditions of the two research vessels to Fram Strait, the Northern Atlantic and the Arctic Ocean are analysed.

On both ships a GORS (GNSS Occultation Reflectometry Scatterometry) receiver with three antenna links has been installed. A common up-looking link is dedicated to direct signal observations. Two additional side-looking links allow sampling the reflected signal with right- and left-handed polarization (RHCP and LHCP). The respective setups have suitable positions to observe grazing sea surface reflections (< 30 deg elevation angle). The antennas are mounted on Lance and Polarstern about 24 m and 22 m above sea level, respectively.

Reflection events are recorded continuously covering more than 70 days. Each event comprises a track of the satellite signal in the grazing angle elevation range. On average 2-3 reflection events were recorded in parallel. The results of the analysis show that in coastal waters (German Bight and Svalbard fjords) up to 44%, 37% (RHCP, LHCP) of the measurements meet the coherence condition. On the high sea it is rarely met, only <0.5% of RHCP and LHCP records fulfill the coherence condition there. The rate of coherent observations increases up to 14%, 13% (RHCP, LHCP) in case of sea ice occurrence.

It can be concluded that the sea state plays an important role for coherent reflectometry. Applications of coherent reflectometry over the ocean may concentrate on the retrieval of sea ice properties and altimetry in coastal waters. For the early data set, recorded in Fram Strait 2016, the estimation of sea concentration has been demonstrated. At present the Polarstern setup continues reflectometry measurements in the MOSAiC expedition with unique opportunities for sea ice observations in the central Arctic.

The limits of coherent reflectometry at high sea became clear. However, it is worth noting that the direct signal link meets the SNR condition also at high sea with an average rate of 55%. This result motivates further investigations to exploit the direct link of shipborne GNSS for atmospheric and ionospheric soundings on the sparsely covered ocean using coherent phase delay measurements.