THE GPS AND RADIOMETRIC JOINT OBSERVATIONS EXPERIMENT AT THE REMEDHUS SITE (ZAMORA-SALAMANCA REGION, SPAIN) A. Monerris ⋆† , N. Rodr´ ıguez- ´ Alvarez ⋆ , M. Vall-llossera ⋆† , A. Camps ⋆† , M. Piles ⋆† , J. Mart´ ınez-Fern´ andez ‡ N. S´ anchez-Mart´ ın ‡ , C. P´ erez-Guti´ errez ‡ , G. Baroncini-Turricchia ‡ , R. Acevo ⋆ , and A. Aguasca ⋆ ⋆ RSLab/UPC, c/ Jordi Girona 1-3, D3, E-08034 Barcelona, Spain † SMOS-BEC and IEEC-CRAE/UPC, Barcelona, Spain ‡ CIALE/USal, Salamanca, Spain E-mail: sandra.monerris@tsc.upc.edu ABSTRACT GRAJO (GPS and RAdiometric Joint Observations) is a long- term field experiment over land which is being conducted since November 2008 at the REMEDHUS site, Zamora, Spain. REMEDHUS has been identified as a cal/val site for ESA’s SMOS mission. The objectives of GRAJO are multi- ple: (i) validate and calibrate SMOS-derived soil moisture, (ii) study the variability of soil moisture within the SMOS footprint, (iii) test pixel disaggregation techniques to improve the spatial resolution of SMOS observations, (iv) determine the optical depth and vegetation water content of barley and grass and assess their influence on soil moisture estimates from radiometric and GNSS-R measurements, and (v) char- acterise the soil roughness factor. This paper presents an overview of the GRAJO experiment, describing the setup and measurements strategy. Index Terms— Microwave radiometry; GNSS-R; Soil moisture; Barley; Grass; Bare soil 1. INTRODUCTION The increasing interest of the scientific community in soil moisture mapping at global scale, with which climate and hydrologic models could be improved, has lead to the pro- posal of ESA’s Soil Moisture and Ocean Salinity (SMOS) [1], and NASA’s Soil Moisture Active and Passive (SMAP) [2]. After launch, both missions will need an intensive cali- bration/validation of soil moisture products and, thus, in situ data concurrent with observations will be necessary. The REMEDHUS (REd de MEDici´ on de la HUmedad del Suelo) soil moisture network in the semi-arid area of the The GRAJO experiment is being funded by the Spanish projects MIDAS-5 ESP2007-65667-C04-02, and AYA2008-05906-C02-01/ESP, and the participating organisations of the EURYI 2004 award and the EC Sixth Framework Program project “Passive Advanced Unit (PAU): A Hybrid L- band Radiometer, GNSS-Reflectometer and IR-Radiometer for Passive Re- mote Sensing of the Ocean”. The SMOS-BEC is a joint initiative of CSIC and UPC mainly funded by the Spanish Ministry of Education and Science through the National Program on Space. Duero basin, Zamora, Spain, has been selected as a calibra- tion/validation site for SMOS, and has been proposed as a potential site for SMAP. Its area is of 40 km × 30 km, close to the size of SMOS field-of-view, and it is mainly covered by farmland (cereals and vineyards) with small areas of bare soil and pine forests. REMEDHUS has a continental and semi- arid climate, with cold winters and warm summers (12 ◦ C an- nual mean temperature and 400 mm mean rainfall). This paper presents the GPS and RAdiometric Joint Ob- servations one-year experiment (GRAJO), which is being car- ried out from November 2008 at REMEDHUS. At the plot- scale, the goal of this experiment is to jointly use radiometry and GPS-reflectometry data to study: (i) the influence of the vegetation on the retrieval of geophysical parameters [3, 4], and (ii) to characterise the effective roughness parameter to be used in the land emission models. At the mid-scale, disag- gregation and soil moisture retrieval algorithms will be tested using air-bone radiometric measurements over REMEDHUS acquired with the ARIEL radiometer [5]. In parallel to the GRAJO experiment, simulations of high resolution brightness temperature using in-situ auxiliary data are being performed with the SMOS End-to-end Performance Simulator (SEPS) and then aggregated to be compared to future SMOS data. A Level 2 Soil Moisture processor has also been developed so that SMOS-derived and in-situ soil moisture over REMED- HUS could be compared in near real time. 2. MATERIALS AND METHODS 2.1. Experiment Site The experiment site is located in a farm within the REMED- HUS network located at Vadillo de la Guare˜ na, Zamora, Spain (41.18 ◦ N, 5.22 ◦ W, 716 m altitude). Figure 1 shows a dia- gram of the site, which consists of three 4 m × 10 m plots of grass, bare soil, and barley which are oriented West, North, and East respectively. Soil at the site has a 72.6% sand and 16.2% clay. Soil roughness at the bare soil plot is periodically