The goal of this study is to test the feasibility of gravimetric measurements with a small Inertial Navigation System (INS) unit in order to use it with various platforms (drone, plane, satellite) in the future. The INS operated in this study is the SPATIAL model developed by Advanced Navigation which has 3-axis accelerometers of precision 2.10−4m/s2. To start we shall compare its gravimetric capabilities with an LPS-1 seismometer (Micro-g LaCoste) during an in situ experiment over a long period of time at the Géosciences Environnement Toulouse (GET) laboratory in order to see if the INS can detect the varying gravimetric signals at least related to the Earth tides. Then we'll compare its precision with the measurements made by a CG-5 Autograv gravimeter (Scintrex) during an excursion between the GET laboratory and the Cammazes dam, which has an altitude difference of more than 400 meters so the gravity variation should be detectable.

This study presents an analysis of radar signature at Ku-band for incidences ranging from 0° to 10° over the major bioclimatic zones, soil and vegetation types encountered in West-Africa, using data from Jason-3 and SWIM. Time-series of radar responses were built over the following environments: stone and sand deserts, Sahelian savannah and floodplain, flooded and non-flooded equatorial forests. Deserts and non-flooded equatorial forest exhibit almost constant responses, decreasing as the incidence angle increases. Similar seasonal variations of the backscattering coefficient between the dry and the wet season are observed at nadir for Jason-3 and SWIM with a decrease in dry season level and amplitude with the increase of the incidence angle. Backscattering at Ku-band can be related to soil roughness, vegetation cover and soil wetness.

Radar altimetry is commonly used to derive water levels over inland water bodies. If lakes and rivers are increasingly covered with radar altimetry virtual stations, wetlands and floodplains are still poorly monitored using this technique. In this study, an unsupervised classification of Ku-band radar altimetry backscattering coefficients from ENVISAT and Jason-2 is performed in the Congo Cuvette Centrale. Comparisons performed against a classification map of the study area shows a good agreement between the water and vegetation classes of the two datasets. Based on these results, radar altimetry-derived water levels are automatically derived over the water classes. Comparisons against radar altimetry-based water stages from Hydroweb database exhibits also a good agreement.

The Local Inverse Barometer (LIB) effect is a complex phenomenon which consists in the response in sea level surface variations to barometric changes. This contribution aims to study the evolution of the impact of this phenomenon on sea level in different sites in coastal areas. Four GNSS stations were used all over the world (Port-Tudy on Groix island (France), Tarifa in Spain, Fort -de- France on Martinique island (overseas France) and Lyttelton in New-Zealand) in order to retrieve coastal sea level height through SNR - based GNSS reflectometry. This sea level height estimates are then extensively compared to oceanic tides and surge effects due to pressure variations. The results show that the impact of the LIB effect on sea level is strongly dependent on the location but also on the spatial extension and timescales.

We propose to use a regional orthogonal Slepian Functions (SF) decomposition to represent and analyse the surface water mass variations sensed by GRACE satellite over the Niger basin. As band-limited Spherical Harmonics (SH) remain not adapted for describing hydrological signals in small geographical areas, Slepian functions can solve the signal energy loss by searching the concentration of information in both spatial and spectral domains of various size and shape. Another advantage of using such regionally-defined basis of orthogonal functions is the drastic reduction of the problem dimensions, so that much faster computation of only a few coefficients, instead of thousands of SH. We propose to use SF to adjust the corresponding 10-day SF coefficients for the semi-arid Niger basin from real geopotential data of the GRACE gravimetry mission. Multi-year time series of the Niger coefficients retrieve the seasonal cycle including a short-wet season every end of summer.

Signal to Noise Ratio (SNR) time series acquired by a single Global Navigation System Satellites (GNSS) antenna are used to retrieve water levels during asymmetric tides and tidal bores in the Garonne River. As height variations are important, a dynamic SNR approach was applied, instead of the static case, to estimate the relative antenna height from the dominant frequencies. Evaluation of the results is performed against pressure measurements. Results on asymmetric tides are quite good overall$\text{(ubRMSD}=0.55\mathrm{m}$), and way better in the falling tide$(\text{ubRMSO}=0.12\mathrm{m}$), where the vertical velocity of the reflecting surface$h$. is lower than in the rising part$\text{(ubRMSD}=0.59\mathrm{m}$). The dynamic SNR method is limited for high h values as it is not able to extract the correct frequencies. For this reason, extremely rapid variations of the river surface during the tidal bore could not be retrieved.

This study presents the potential of GNSS Reflectometry (GNSS-R) for the monitoring of water levels in the Mekong river delta (Vietnam). A geodetic GNSS station was installed at the Tran De port, Soc Trang province, over 23-30 August 2016. The method used to retrieve water levels from GNSS-R data is based on the Interference Pattern Technic of the Signal-to-Noise Ratio (SNR). To validate our measurements, we used different radar altimetry data from Jason-2, SARAL, Jason-3 and Sentinel-3A missions during the same period for measuring sea surface height (SSH) and in-situ tide gauge station as reference for both tides and floods. The aim of this study is to evaluate the performances GNSS-R, in complex environment, for monitoring at the same time river/tide stages and the possibility to use this technique for calibration/validation of altimetry data. Early results show the good agreement between in -situ gauge and GNSS-R based estimates with correlations which reach ~ 0.85.

Altimetry-derived sea surface heights (SSH) from the recently launched Jason-3 and Sentinel-3A missions were validated against records from seven tide gauges located along the French Atlantic coast in the southern Bay of Biscay. Comparisons between Jason-2 and Jason-3 based SSH were performed along-track when they were orbiting 80 seconds apart during the tandem mission that lasted more than six months. The benefits of the Synthetic Aperture Radar (SAR) acquisition mode on Sentinel-3A is compared to the classical Low Resolution Mode (LRM) on Jason-2 and 3 missions are analyzed.

In this letter, Global Navigation Satellite System Reflectometry (GNSS-R) delay Doppler maps (DDMs) acquired by the U.K. TechDemoSat-1 (TDS-1) mission are analyzed to identify sea ice infesting waters in circumpolar areas. First, the sensitivity of actual DDM to sea ice is investigated using reference information provided by a Sentinel-1 synthetic aperture radar imagery and a Special Sensor Microwave/Imager based sea-ice concentration map. Then, a new product is proposed that consists of normalized radar cross section measurements derived from actual DDMs and arranged in gridded format. This product is shown to provide spatial information on the sea ice transition that cannot be obtained exploiting the conventional DDM product. Hence, this new product can be used to improve/complement conventional remote sensing techniques for sea ice observation.

Global Navigation Satellite Systems - Reflectometry (GNSSR) is an innovative and promising tool for remote sensing purposes. Several applications have been developed to extract geophysical information of the reflected scene from the measured delay waveforms and Delay-Doppler Maps (DDMs). Recently, techniques have been presented to deconvolve the DDMs to reconstruct the Normalize Radar Cross Section (NRCS) field of the observed area in order to provide added value products for imaging remote sensing. In this paper an effective reconstruction scheme, based on the 2-D Truncated Singular Value Decomposition (TSVD), is proposed and succesfully tested on actual Delay Doppler Maps (DDMs) acquired by TechDemoSat-1 (TDS-1) mission.

We performed a detailed time series Interferometric Synthetic Aperture Radar (InSAR) analysis using an enhanced Small Baseline Subset (SBAS) approach on COSMO-SkyMed data acquired between 2011 and 2014 over Piton de la Fournaise volcano in the Réunion Island. 2-Dimensional components of mean velocities were calculated from ascending and descending orbits. Clear time-dependent displacement was observed both at the Central Cone and on the Eastern Flank. Our results show the persistence of large-scale downslope displacement of the Eastern Flank triggered by the March-April 2007 eruption.

The floods are an annual phenomenon on the Pacific coast of Ecuador and can become devastating during El Niño years, especially in the Guayas watershed (32,300 km2), the largest drainage basin on the South American western side of the Andes. In this study, we used ENVISAT ASAR GM SAR images with a spatial resolution of 1 km to map the flooded areas between 2004 and 2008 and study the spatio-temporal dynamics of floods in the Guayas Basin. Maximum of likelihood supervised classification performed on ASAR images acquired during four consecutive dry seasons allowed us to identify five classes of land cover consistent with land use map. From the four wet seasons, we computed standardized anomalies of backscattering coefficient to detect changes between dry and wet season and tested different thresholds to identify flooded areas.

Multipaths are one of the major source of error for positioning applications using Global Navigation Satellite Systems (GNSS). During the last decade, several studies demonstrated the potential of the inversion of Signal to Noise Ratio (SNR) due to multipaths for estimating environmental parameters such as sea level, soil moisture, and snow depth variations, vegetation growth and biomass quantification. Here, we propose i) a modeling approach contribution of the multipath signals to the SNR, ii) an inversion technique of the receiver height based on the modeled SNR.

As multipaths still represent a major problem for reaching precise GNSS positioning, the mitigation of their influence has been widely investigated. However, previous studies have lately proposed to use these interferences of GNSS electromagnetic waves to estimate parameters related to the reflecting surface (e.g., antenna heights, rugosity,...). Variations of the nature of the surface is likely to modify the properties of the reflected waves, and consequently lead to variations of amplitude / phase of the signal-to-noise ratio (SNR), e.g. recorded at 1 Hz by a GNSS receiver. By analyzing the time variations of SNR measurements linked to the dielectric constant of the surrounding soil, we use a method to recover the local fluctuations of the soil moisture content. It is simply based on the obvious linear correlation between SNR amplitude / phase and retrieved antenna height time series and independent measurements of humidity probe at 2 and 5 cm depths. This method of combination is applied to determine soil moisture in a corn and soya field at Lamasquère, France, for 21 successive days. Results show a good correlation (e.g. 0.96 with GPS PRN-01 satellite) between SNR inversion and humidity probes for most satellites.

Ocean altimetric applications of GNSS-R have particularly been developed through the last decades. Interference Pattern Technique (IPT) based on the analysis of the Signal-to-Noise Ratio (SNR) of a classical GNSS antenna presents the main advantage of being applicable everywhere by using a single geodetic antenna and a classical GNSS receiver, transforming them to real wave/tide gauges. Such a technique has been already tested in various configurations of acquisition of surface-reflected GNSS signals with an accuracy of a few centimeters. Nevertheless, the traditional method for estimating the reflecting surface-antenna height is limited by an approximation: the vertical velocity of the reflecting surface must be negligible. This article presents a significant improvement of the IPT technique to solve this problem and broaden the scope of SNR-based tide monitoring. This method was validated in situ, with an antenna placed at 60 meters above the Atlantic Ocean surface which variations reach ±3 meters, and amplitude rate of the semi-diurnal tide up to 0.5 mm/s. Over the three months of SNR records for sea level determination, we found linear correlations of 0.94 by comparing with a traditional tide gauge. Our SNR-based time series was also compared to the TUGO tide theoretical model [1] and amplitudes and phases of the main astronomical periods (6-, 12- and 24-h) were perfectly well detected.