Sylvia Dayau

GLORI: A GNSS-R Dual Polarization Airborne Instrument for Land Surface Monitoring

Global Navigation Satellite System-Reflectometry (GNSS-R) has emerged as a remote sensing tool, which is complementary to traditional monostatic radars, for the retrieval of geophysical parameters related to surface properties. In the present paper, we describe a new polarimetric GNSS-R system, referred to as the GLObal navigation satellite system Reflectometry Instrument (GLORI), dedicated to the study of land surfaces (soil moisture, vegetation water content, forest biomass) and inland water bodies. This system was installed as a permanent payload on a French ATR42 research aircraft, from which simultaneous measurements can be carried out using other instruments, when required. Following initial laboratory qualifications, two airborne campaigns involving nine flights were performed in 2014 and 2015 in the Southwest of France, over various types of land cover, including agricultural fields and forests. Some of these flights were made concurrently with in situ ground truth campaigns. Various preliminary applications for the characterisation of agricultural and forest areas are presented. Initial analysis of the data shows that the performance of the GLORI instrument is well within specifications, with a cross-polarization isolation better than −15 dB at all elevations above 45°, a relative polarimetric calibration accuracy better than 0.5 dB, and an apparent reflectivity sensitivity better than −30 dB, thus demonstrating its strong potential for the retrieval of land surface characteristics.

Significance and limits in the use of predawn leaf water potential for tree irrigation

Research in estimating the water status of crops is increasingly based on plant responses to water stress. Several indicators can now be used to estimate this response, the most widely available of which is leaf water potential (ΨLWP) as measured with a pressure chamber. For many annual crops, the predawn leaf water potential (ΨPLWP), assumed to represent the mean soil water potential next to the roots, is closely correlated to the relative transpiration rate, RT. A similar correlation also holds for young fruit trees grown in containers. However, exceptions to this rule are common when soil water content is markedly heterogeneous.Two experimental conditions were chosen to assess the validity of this correlation for heterogeneous soil water content: 1) young walnut trees in split-root containers. The heterogeneity was created by two unequal compartments (20% and 80% of total volume), of which only the smaller was irrigated and kept at a moisture content higher than field capacity (permanent drainage). 2) adult walnut trees in an orchard. In this case, soil water heterogeneity was achieved by limiting the amount of localised irrigation (20% of the irrigated control)which was applied every evening.Values of sap flux and of minimum and predawn leaf water potentials with homogeneous and heterogeneous soil water content were compared for trees grown in the orchard and in containers. In spite of intense drought reflected by very low RT or stem water potential, ΨPLWP of trees under heterogeneous moisture conditions remained high (between -0.2 and -0.4 MPa) both in the orchard and in containers. These values were higher than those usually considered critical under homogeneous soil conditions. A semi-quantitative model, based on the application of Ohms analogy to split-root conditions, is proposed to explain the apparently conflicting results in the literature on the relation between ΨPLWP and soil water potential.

Directional Anisotropy of Brightness Surface Temperature Over Vineyards: Case Study Over the Medoc Region (SW France)

Measurements of directional brightness surface temperature were performed over vineyards using an airborne thermal infrared camera. These measurements reveal differences of up to 15 °C according to the viewing direction and are critically impacted by vine row orientation. The contribution of each element of the canopy (sunlit/shaded soil and vegetation) to the directional anisotropy is illustrated using a simple 3-D model and the POV-Ray ray-tracing tool.

Near-surface remote sensing observations for monitoring deciduous broadleaf forest species phenology

Since several years, more and more studies aim at developing phenology products from satellite time-series at high temporal frequency such as those provided by the VEGETATION or MODIS sensors. Reflectance times-series at high spatial resolution, such as those that will be obtained from SENTINEL2, will soon available to monitor the phenology response of forests under climate change at the level of the forest stand or the tree species. There is a great need for continuous in situ monitoring of phenology to calibrate and validate the current and future remotely-sensed phenology products. In this context, we proposed a method based on near surface remote sensing techniques to monitor the seasonal change in LAI (Leaf area Index) and date key stages of the leaf phenology. As it is important to use a network of sensors with autonomous recording systems at a minimal cost in order to maximize the spatial sampling, we selected a method based on the transmittance continuous measurement of photosynthetic active radiation (PAR). We evaluated and validated the method for a deciduous forest species (Quercus petraea) over two sites encompassing a large variation in the timing of spring leafing, where direct visual phenology observations were performed. A specific preprocessing and modeling of the measured temporal signal was developed. The performances for dating the leaf unfolding in spring are satisfying: the bias is lower than ~1 day and the RMSE is (generally) lower than ~ 4 days.

Performance of GNSS-R GLORI data for biomass estimation over the Landes forest

Abstract The Above-Ground Biomass (AGB) is a key parameter used for the modeling of the carbon cycle. The aim of this study is to make an experimental assessment of the sensitivity of Global Navigation Satellite System (GNSS) reflected signals to forest AGB. This is based on the analysis of the data recorded during several GLORI airborne campaigns in June and July 2015, over the Landes Forest (France). Ground truth measurements of tree height, density and diameter at breast height (DBH), as well as AGB, were carried out for 100 maritime pine forest plots of various ages. The GNSS-R data were used to obtain the right-left (Γ RL ) and right-right (Γ RR ) reflectivity observables, which are geo-referenced in accordance with the known positions of relevant GPS satellites and the airborne receiver. The correlations between forest AGB and the GNSS-R observables yield the highest sensitivity at high elevation angles (70°-90°). In this case, for (Γ RL ) and the reflectivity polarization ratio (PR = Γ RL /Γ RR ) estimated with a coherent integration time Tc = 20 ms, the coefficients of determination R 2 are equal to 0.67 and 0.51, with a sensitivity of −0.051 dB/[10 6 g (Mg) ha −1 ], and −0.053 dB/[Mg ha −1 ], respectively. The relationships between AGB and the observables are confirmed through the use of a 5-fold cross validation approach, with several different coherent integration times.

Results from the GLORIE GNSS-R airborne campaign: Agricultural areas

The GLORIE Campaign was performed in June-July 2015 in order to investigate the sensitivity of airborne GNSS-R measurements to land parameters. In this paper we present the first results focusing on agricultural areas. For this purpose ground truth measurements of soil moisture, roughness, plant water content, leaf area index and plant height were measured over 20 agricultural plots of various crops (cereals, vegetables, bare soil). The correlation with GNSS reflectivity in LHCP polarization confirms noticeable sensitivity to soil moisture, and plant-related parameters especially vegetation cover height.

Impacts of surface bounadry layer turbulence on surface temperature measurements at different spatial resolutions

Surface temperature displays temporal fluctuations driven by turbulence intensity of both surface (SBL) and planetary (PBL) boundary layers. An experiment conceived to evaluate the error induced by these fluctuations on instantaneous satellite measurements at different spatial resolutions is described. It is based on the reconstruction of surface temperature time series from sequences of thermal images acquired using a TIR camera aboard an helicopter in stationary flight over 5 different surface types. It is shown that the spatial averaging performed over the pixel allows one to reduce the contribution of the high frequency SBL turbulence to the error on Ts measurements. This error rapidly decreases with pixel size, and is found to be around ±0.5°C at 50m resolution for the cases we studied. Unfortunately the experimental set up revealed not as well suited to evaluate the contribution of low frequency PBL turbulence to Ts error, and our recommendations are given for future experiments.