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GRAJO (GPS and RAdiometric Joint Observations) is a longterm 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 multiple: (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) characterise the soil roughness factor. This paper presents an overview of the GRAJO experiment, describing the setup and measurements strategy.