## 3 Operational Retrievals of SSM from EO  
Monitoring of the Earth system from the vantage point of space-borne instruments is nearing its half-century mark. For the most part of this half-century, the instruments have mostly measured in the visible and infrared regions of the spectrum where reflected sunlight and terrestrial thermal emissions are strong and detectable. About mid-way through the half-century, advanced microwave instruments were introduced, adding new perspectives to the study of the Earth's system. This is especially true for the hydrological cycle, since microwave sensors have allowed for the first time to measure the Earth's SSM. A variety of methods have been explored for the estimation of soil moisture using EO data from long-heritage optical and thermal infrared sensors and from relatively new microwave sensors [21; 46; 47]. Among them, theoretical and experimental evidences support the idea that satellite microwave observations are best suited for the retrieval of the Earths' surface soil moisture on an operational basis. This review will focus on the retrievals of SSM from EO and the operational products available. The main advantage of microwaves is that they allow the quantitative measurement the Earths' surface soil moisture as the direct relationship of soil emissivity (or reflectivity) with soil water content and that the atmosphere is almost transparent at these frequencies. In contrast, optical and thermal sensors are only able to provide an indirect estimation [21; 47]. Microwave remote sensing encompasses both active and passive forms, depending on the sensor and its mode of operation. While active sensors (scatterometers or Synthetic Aperture Radars-SAR) measure the energy reflected from the land surface after transmitting a pulse of microwave energy, passive sensors (radiometers) measure the self-emission of the land surface. At microwave L-band (1-2 GHz frequency), or \"the water frequency channel,\" the signal is sensitive to the top 5 cm soil moisture and penetrates through low to moderate layers of vegetation canopy (see recent review by [48]). At higher frequencies (C and X-bands, at 4-8 & 8-12.5 GHz frequency respectively) the signal is sensitive to shallower soil moisture layers and is able to penetrate only through low vegetation densities (C and X-bands) [49]. Dedicated inversion algorithms have been developed to retrieve surface soil moisture tailored to the frequency, measurement concept and characteristics of the different microwave sensors set on space. This intense activity has led to a suite of EO microwave satellite soil moisture data products spanning since 1978 to present. At the time of writing (December 2017), five quasi-operational, i.e., available either in near real time (NRT) or few days after sensing, satellite surface soil moisture products are available from: (1) the Advanced SCATerometer (ASCAT) on-board Metop-A and Metop-B satellites, starting from January 2007 [50]; (2) the Soil Moisture and Ocean Salinity (SMOS), Earth Explorer mission, starting from January 2010 [51]; (3) the Advanced Microwave Scanning Radiometer 2 (AMSR2) on-board the Global Change Observation Mission for Water, GCOM-W starting from July 2012 [42]; (4) the Soil Moisture Active and Passive (SMAP) mission (L-band radiometer) starting from April 2015 [52]; and (5) the disaggregation of SMOS soil moisture using Aqua/Terra MODIS fine-scale information, starting from January 2010. The BEC L4 product is currently providing 1 km soil moisture maps in NRT over the Iberian Peninsula [53; 54]. An example of this product is shown in Figure 3 below (adopted from [54]). Recent research studies have shown that its coverage and temporal resolution could be enhanced using MSG SEVIRI data [55] and that finer spatial scales could be reached by using finer optical information [56; 57].