## 1 Introduction  
L-band microwaves have very good advantages in soil moisture remote sensing, for being unaffected by the atmosphere (clouds, aerosols, etc.), and for the ability to penetrate vegetation, except in very dense forests. Using GNSS (Global Navigation Satellite System)-reflected signals for remote sensing applications was put forward by ESA in 1993 ([PERSON] et al., 2010). Extending GNSS-R applications toward soil moisture remote sensing was first proposed by [PERSON] and [PERSON] (2000) in 2000. To measure GNSS signals reflected from the land surface, the well-known flight campaign SME02 was launched by NASA and the University of Colorado in 2002 ([PERSON] et al., 2010). LEiMON (Land Monitoring with Navigation Signals) experimental field campaign was carried out by ESA and Starlab in 2009 to investigate the interaction between land surface parameters such as soil moisture, surface roughness, and vegetation biomass, and the scattered GNSS signal characteristics ([PERSON] et al., 2010). A brand-new method was proposed by [PERSON] (1999); [PERSON] (1999), who verified that multipath signals could be used to infer near-surface soil moisture. A single up-looking antenna was used to receive the multipath combination of direct signal and reflected signals, the latter varying with soil moisture. A physical model of GNSS direct and reflected signal interference was built by [PERSON] and [PERSON] (2007). [PERSON] et al. (2010) presented the measurements applying the interference pattern technique (IPT) for soil moisture and vegetation height retrievals over vegetation-covered soils. The potential of sensing land surface properties with spaceborne GNSS receivers have been demonstrated with the TDS-1 (TechDemoSat-1) and CYGNSS (Cyclone Global Navigation Satellite System) observations ([PERSON], 2009; [PERSON] et al., 2010; [PERSON] et al., 2010).  
L-band radiometry is one of the most efficient techniques to monitor surface soil moisture on a global scale. Thus, the European space agency launched the Soil Moisture and Ocean Salinity (SMOS) mission in 2009 with a passive L-band radiometer dedicated to making global observations of soil moisture over land as well as salinity over oceans ([PERSON] et al., 2010), and the NASA Soil Moisture Active and Passive (SMAP) satellite mission was launched in 2015 to provide global mapping of soil moisture using an L-band (active) radar and an L-band (passive) radiometer ([PERSON] et al., 2010). Both missions have been successful in providing surface soil moisture on a global scale from a space platform. For the ground referencing, the ELBARA-II radiometer was specifically designed for the validation of SMOS data and products and for the further improvement of the radiative transfer models used in the soil moisture retrieval algorithms ([PERSON] et al., 2010). Several ground experiments were carried out with an ELBARA-II radiometer in different regions ([PERSON] et al., 2010; [PERSON] et al., 2010; [PERSON] et al., 2010; [PERSON] et al., 2010). Further steps were given by [PERSON] and [PERSON] (2009) who quantified the relationship between forward scattered GNSS signals recorded by the CYGNSS constellation and the SMAP soil moisture. [PERSON] et al. (2010) also compared microwave radiometry and GNSS-R observations conducted for different soil moisture conditions from an airborne experiment.  
In this work, the SOMOSTA (Soil Moisture Monitoring Station) long-term experiment at the Valencia Anchor Station (2014-2016) is introduced. This experiment is fruitfully framed within the ESA-China joint program of collaboration on GNSS-R. Based on the long-term measurements with Oceanpal GNSS reflectometry and ELBARA-II passive L-band radiometer, the correlation between the two sets of measurements was analyzed.