## 1 Introduction  
Soil moisture (SM) is an essential climate variable that plays an important role in both the Earth's water and carbon cycles [1]. SM regulates the net ecosystem energy exchange at the land-atmosphere boundary layer over the continents through the heat fluxes (latent and sensible) and its feedback on precipitation. Also, it influences hydrological processes, runoff generation, flood or drought development, and agricultural productivity [2]. A better understanding of these processes and their couplings is needed for the improvement of meteorological and climate forecasts, and for a more sustainable use of water resources. The scientific community considers L-band (1-2 GHz) the optimal bandwidth for measuring the Earth's global surface SM, penetrating approximately the top 5 cm of the soil. At these frequencies, the amount of thermal radiation naturally emitted by the Earth's surface depends mainly on its physical temperature and the soil dielectric constant (\(\epsilon\)), which for the near surface soil layer is primarily a function of the soil water content [3]. Active sensors (radars) consist of a transmitter and a receiver that measures the signal that is reflected, refracted or scattered; while passive sensors (radiometers) consist only of a receiver that measures the radiation naturally emitted from the scene under observation. L-band radiometers are based on the large contrast between the dielectric properties of dry soil (\(\epsilon\sim 4\)) and water (\(\epsilon\sim 80\)); the Earth's surface emissivity decreases with increasing SM [4; 5]. Other advantages of L-band are: (i) it is significantly less affected by rain and atmospheric effects than higher microwave frequencies, with the atmosphere being nearly transparent [6]; (ii) the radiation emitted by the Earth's surface can pass through sparse and up to moderate canopies (i.e., those with Vegetation Water Content (VWC) \(\leq\) 5 kg/m\({}^{2}\), corresponding to 70% of non-frozen land areas on Earth, excluding dense forest), but even vegetation with lower VWC attenuates the signal and, consequently, it needs to be accounted for in SM retrieval algorithms [7]; and (iii) since L-band is part of the microwave frequency range, the measurements are also independent of solar illumination. Currently, there are two L-band satellite missions in orbit specifically devoted to monitoring the Earth's global surface SM: (i) the Soil Moisture and Ocean Salinity (SMOS, 2009-2017) mission from the European Space Agency (ESA); and (ii) the Soil Moisture Active Passive (SMAP, 2015-2018) mission from the U.S. National Aeronautics and Space Administration (NASA). SMOS, launched on 2 November 2009, is the first satellite to globally measure surface SM [8] and sea surface salinity [9]. Its single payload is the two-dimensional Microwave Imaging Radiometer with Aperture Synthesis (MIRAS), an interferometric radiometer which provides multi-angular (0\({}^{\circ}\)-65\({}^{\circ}\)) and full-polarimetric observations with a spatial resolution of \(\sim\)35-50 km [10]. SMAP was launched on 31 January 2015, to globally measure surface SM and the freeze/thaw state of the soil [11]. It includes a real aperture radiometer and a Synthetic Aperture Radar (SAR), providing full-polarimetric observations at a single look angle (40\({}^{\circ}\)) with spatial resolutions of \(\sim\)36 km and \(\sim\)3 km, respectively [12]. SMAP planned to provide a 9-km SM product using a disaggregation algorithm that combines active-passive observations [13]. However, due to a failure on the data transmission of the SMAP SAR on 7 July 2015, the duration of active observations is limited. Still, there are ten weeks of combined measurements to allow new insights in active-passive SM remote sensing. Land surface temperature (LST) is also a key climate variable. Remotely sensed LST, acquired with thermal infrared (TIR) sensors, are routinely used in many operational applications, including weather forecasting.