Erwan Motte

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.

Reflectometry With an Open-Source Software GNSS Receiver: Use Case With Carrier Phase Altimetry

An open-source GNSS software receiver allows to have full access to the signal processing and to make add-ons to the source code in order to obtain the desired GNSS reflectometry processing. The direct signal is processed in the standard way, its tracking loops replica are tapped to have a robust processing of the reflected signal in a master-slave configuration, with the very same carrier replica used to correlate the reflected signal. In addition, the data bit sign is wiped off, which allows to extend the coherent integration time (CIT) well beyond the usual 20-ms limit on the reflected way. This allows having a straightforward and accurate measurement of the Amplitude Ratio and Differential Carrier Phase between the direct and reflected signals. The possible applications are precise carrier phase altimetry and any application requiring signal amplitude ratio, or reflected signal Delay Map, with single or dual polarization, this include code altimetry, humidity, biomass, soil roughness, ocean surface wind and wave height, and snow and ice characteristics retrieval. This software is intended to be used as a research tool. It has been tested for carrier phase altimetry on real data sets collected in rather calm water conditions: at the 60-m Cordouan Lighthouse, and during a 600- m high ATR42 flight over a lake. In both cases, continuous carrier phase measurement with a centimeter level precision was obtained when extending the CIT up to 500 ms. Increasing the CIT beyond 20 ms is the key to improve carrier phase altimetry robustness.

A 22-GHz Mobile Microwave Radiometer (MobRa) for the Study of Middle Atmospheric Water Vapor

We present a new compact and automated ground- based microwave mobile radiometer dedicated to the study of middle atmospheric water vapor, which is convenient for measurement, intercomparison, and validation campaigns, particularly in remote places. The instrument detects the 616-523 H2O transition line at 22.235 GHz by means of balanced beam-switching observation of the atmosphere. Tipping curves are performed to estimate the tropospheric opacity. The sky is used as a cold load for calibration, minimizing the use of liquid nitrogen. The main technical issues are caused by the small horn antenna (a beamwidth of ~12deg) inducing large bias when using pencil- beam approximation for the calculation of elevation-dependent parameters. Numerical methods have been developed to include this effect in the data calibration process. Preliminary vertical profiles are retrieved with the Microwave Odin Line Estimation and REtrieval radiative transfer and inversion tool from 25 to 55 km with a vertical resolution of 10-20 km increasing with height, for an integration time of ~12 h and a measurement error of ~15% (~0.8 ppmv). Preliminary comparisons with the spaceborne instrument Aura/Microwave Limb Sounder show a good agreement (~5%) in the 35-55-km altitude range, whereas a negative bias is detected below (up to -30% at 25 km).

A 22 GHz mobile microwave radiometer for the study of stratospheric water vapor

We present a new compact ground-based microwave radiometer dedicated to the study of middle atmospheric water vapor. The instrument detects the 616- 523 H2O transition line at 22.235 GHz. This radiometer has been designed to be easily transported and operated during measurement campaigns in remote places. The first retrievals, performed with the MOLIERE inversion and radiative transfer software, show good agreement with MIAWARA, the 22 GHz Radiometer developed at the University of Bern, Switzerland, and the Microwave Limb Sounder instrument onboard the Aura satellite.

Applications of GNSS-R in Continental Hydrology

Abstract: GNSS (Global Navigation Satellite System) Reflectometry (GNSS-R) is a bistatic radar remote sensing technology (transmitters and receivers are not in the same place) that uses microwave signals of opportunity from radio navigation such as the Global Positioning System (GPS). It extends and complements existing techniques for observation of the Earth by multiplying the number of transmitters (satellites in different positioning constellations) and receivers through the use of ground measurement networks, such as the permanent GNSS network in France (Reseau permanent GNSS, RGP) or the Plate Boundary Observatory (PBO) in the western United States, and allowing new measuring geometries.

Optimizing Waveform Maximum Determination for Specular Point Tracking in Airborne GNSS-R

In this study we present techniques that have been developed to optimize the processing of airborne GNSS-R data, with the goal of improving its accuracy and robustness under nonoptimal conditions. This approach is based on the detailed analysis of data produced by the instrument GLORI, which was recorded during an airborne campaign in the south west of France in June 2015. Our technique relies on the improved determination of reflected waveform peaks in the delay dimension, which is related to the loci of the signals contributed by the zone surrounding the specular point. It is shown that when developing techniques for the correct localization of waveform maxima under conditions of surfaces of low reflectivity, and/or contamination from the direct signal, it is possible to correct and extract values corresponding to the real reflectivity of the zone in the neighborhood of the specular point.

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.

Low-Cost GPS Receivers for the Monitoring of Sunflower Cover Dynamics

The aim of this research is to analyze the potential use of Global Navigation Satellite System (GNSS) signals for the monitoring of in situ vegetation characteristics. An instrument, based on the use of a pair of low-cost receivers and antennas, providing continuous measurements of all the available Global Positioning System (GPS) satellite signals is proposed for the determination of signal attenuation caused by a sunflower cover. Experimental campaigns with this instrument, combined with ground truth measurements of the vegetation, were performed over a nonirrigated sunflower test field for a period of more than two months, corresponding to a significant portion of the vegetation cycle. A method is proposed for the analysis of the signal attenuation data as a function of elevation and azimuth angles. A high correlation is observed between the vegetation’s water content and the GPS signals attenuation, and an empirical modeling is tested for the retrieval of signal behavior as a function of vegetation water content (VWC). The VWC was estimated from GNSS signals on a daily basis, over the full length of the study period.

First results from the GLORIE polarimetric GNSS-R airborne campaign dedicated to land parameters estimation

The GLORIE GNSS-R airborne campaign was conducted in the late spring 2015 with the GLORI polarimetric receiver. More than 15 hours or raw data was gathered during 5 flights that spanned over a 3-week period. The aircraft flew over several areas if interest including: 1) agricultural plots with coincident in-situ measurements of soil moisture, vegetation biomass and roughness, 2) in situ monitored forest plots with a wide range of above ground biomass values and 3) inland water bodies in order to test phase altimetry retrievals. Apparent reflectivity was computed from the data, showing a good dynamics above various types of terrains. Phase altimetry was performed over calm water, showing a precision in the range of the centimeter level.