SNR-Based Water Height Retrieval in Rivers: Application to High Amplitude Asymmetric Tides in the Garonne River

Pierre Zeiger,Philippe Bonneton,Natalie Bonneton,Nicolas Roussel,José Darrozes,Frédéric Frappart,Guillaume Detandt

doi:10.3390/rs13091856

Abstract

Signal-to-noise ratio (SNR) time series acquired by a geodetic antenna were analyzed to retrieve water heights during asymmetric tides on a narrow river using the Interference Pattern Technique (IPT) from Global Navigation Satellite System Reflectometry (GNSS-R). The dynamic SNR method was selected because the elevation rate of the reflecting surface during rising tides is high in the Garonne River with macro tidal conditions. A new process was developed to filter out the noise introduced by the environmental conditions on the reflected signal due to the narrowness of the river compared to the size of the Fresnel areas, the presence of vegetation on the river banks, and the presence of boats causing multiple reflections. This process involved the removal of multipeaks in the Lomb-Scargle Periodogram (LSP) output and an iterative least square estimation (LSE) of the output heights. Evaluation of the results was performed against pressure-derived water heights. The best results were obtained using all GNSS bands (L1, L2, and L5) simultaneously: R = 0.99, ubRMSD = 0.31 m. We showed that the quality of the retrieved heights was consistent, whatever the vertical velocity of the reflecting surface, and was highly dependent on the number of satellites visible. The sampling period of our solution was 1 min with a 5-min moving window, and no tide models or fit were used in the inversion process. This highlights the potential of the dynamic SNR method to detect and monitor extreme events with GNSS-R, including those affecting inland waters such as flash floods.

Highlights

The Global Navigation Satellite System (GNSS) has been used for decades for positioning and navigation purposes
The optimal value of k was empirically determined. It depended on the environment, on the variations of the reflecting surface, the width of the river and the highfrequency phenomena affecting the Signal-to-noise ratio (SNR) time series
SNR time series acquired over a narrow river were analyzed to retrieve the evolution of water height during asymmetric tides

Summary

Introduction

The Global Navigation Satellite System (GNSS) has been used for decades for positioning and navigation purposes. The L2 civilian (L2C) and L5 frequencies offer new tools for sensing atmospheric and Earth surface conditions; the L5 band improves altimetric performances of GNSS and Global Navigation Satellite System Reflectometry (GNSS-R) retrievals [3]. The operating principle for the use of these opportunistic signals was first proposed in 1988 [4], experimented in 1993 for GPS constellation with an ocean altimetry case study [5]. These early experiments used a dual-antenna device, one up-looking to acquire the direct signal, and one down-looking to acquire the multipath reflected on the Earth’s surface

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Discussion

Conclusion