DETECTION OF SOIL MOISTURE CONTENT CHANGES BY USING A SINGLE GEODETIC ANTENNA: THE CASE OF AN AGRICULTURAL PLOT N. Roussel 1,2 , F. Frappart 1,2 , G. Ramillien 2,3 , J. Darrozes 1,2,3 , F. Baup 1,4 , C. Ha 1,2 1 Université Paul Sabatier, 31400 Toulouse, France 2 GET-OMP, GRGS,31400 Toulouse, France 3 CNRS, GET-OMP, 31400 Toulouse, France 4 CESBIO (UPS-CNRS-CNES-IRD), 31400 Toulouse, France ABSTRACT 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 electro- magnetic waves to estimate parameters related to the reflect- ing 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 con- stant 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 ampli- tude / phase and retrieved antenna height time series and in- dependent 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. Index Terms— GNSS-R, soil moisture, remote sensing, IPT, SNR. 1. INTRODUCTION The soil moisture measurements are very important for cli- mate studies, weather predictions, analysis of flood zones or aquifer recharges. In agricultural areas, analyzing in real time the soil water content would allow the farmer to optimize the management of its plots (tillage, treatments, irrigation, ...). Unfortunately, traditional humidity probes are punctual and really heterogeneous and monitoring an entire parcel is re- ally cumbersome. With the advent of satellite remote sensing This work was funded by CNES in the framework of the TOSCA project "Hydrologie, Océnaographie par Réflectométrie GNSS (HORG)" and by the RTRA STAE fundation in the framework of the "Potentialités de la Réflec- tométrie GNSS In Situ et Mobile (PRISM)" project. such as the SMOS mission [1], soil moisture can be monitored at the global scale and systematically but unfortunately with low temporal resolution. Recent studies suggested to take advantage of the waves continuously emitted by the GNSS satellites which reflect upon the Earth surface to monitor dif- ferent geophysical parameters of the reflecting surface. This opportunistic remote sensing technique, known as GNSS re- flectometry (GNSS-R) offers a wide range of applications in Earth science and particularly in soil moisture monitoring and presents the advantage of covering a whole surface around the antenna. The size of this monitored surface is only depen- dent on the height of the antenna above the reflecting surface (for a given satellites constellation and time interval). The aim of this article is to apply a particular GNSS-R technique, know as Interference Pattern Technique (IPT), to an agricul- tural plot located in Lamasquère, France (43°29’14.45”N ; 1°13’44.11”E). IPT is based on the analysis of the Signal- to-Noise Ratio (SNR) of a classical geodetic antenna. SNR data incorporates both direct and reflected signals which inter- acted with the ground while reflecting hence dependence on soil moisture content. The observation site is equipped with a set of meteorological sensors (rainfall, temperature, humidity probes) needed to validate soil moisture retrieved by inver- sion of the SNR signal. Three different parameters are under study: the amplitude, the phase and the frequency of the mul- tipath contribution to SNR. The latter is directly linked to the antenna height above the reflecting surface [2]. 2. METHODOLOGY 2.1. State of the art While the major part of the sending GNSS signal is received directly by the zenith-looking hemisphere of the antenna, a minor part of wave energy arrives from below the horizon, after one or several reflections in the surrounding environ- ment. These so-called multipath signals interfere with the di- rect wave and affect the GNSS observables recorded by the receiver. The effect of the multipath reflections are particu- larly visible in the SNR data constantly collected by GNSS 2008 978-1-4799-7929-5/15/$31.00 ©2015 IEEE IGARSS 2015