In the following paragraphs, we describe some basics of microwave radiometry, the requirements of the SMOS mission, and the corresponding research activities. The design and the main characteristics of the ELBARA II instruments are described in Sections 2 and 3. The Appendix contains a list of the abbreviations used and the specifications of the electronic components used in the radiometer design.1.1. Measurement PrincipleMicrowave radiometry is a passive remote-sensing technique that measures thermal radiation. The radiance TBp emitted from a terrestrial surface at horizontal (p = H) or vertical (p = V) polarization depends on the surface temperature TS, and on the surface reflectivity Rp. The latter can be used as a proxy for the remote retrieval of soil moisture or sea salinity.

In the microwave range, the Planck function of thermal radiation is linear with the absolute temperature. In this so-called Rayleigh-Jeans approximation, the upwelling brightness temperature of the emitted radiation above a surface is TS?(1 ? Rp). Since downwelling radiation Tsky also contributes to the observed radiation by the fraction reflected at the surface, the total radiation TBp received by a radiometer oriented towards the surface can be expressed by:TBp=(1?Rp)TS+RpTsky(1)The value of Tsky is determined by the cosmic background temperature of ��2.7 K and enhanced by an atmospheric contribution. At 1.4 GHz, this enhancement is almost constant, leading to 4 K < Tsky < 5 K. Since the terrestrial surface temperature is much larger than Tsky, the brightness temperature TBp has a strong sensitivity to Rp.

The sensitivity of TBp with volumetric soil water content WC [m3 m?3] is established through Rp, being dependent on the relative dielectric constant. The latter is a strong function of WC due to the marked contrast between the permittivity of free water (��80) and dry soil (��3 to 5). This allows the soil surface-water content to be determined from its reflectivity by applying dielectric Cilengitide mixing (e.g., [9�C11]) and radiative transfer models. Typically, TBp of a very dry bare soil can be 150 K higher than for the same soil in the saturated moisture state.Two different soil-depth ranges are of relevance: First, TS represents an effective soil temperature averaged over the emission depth of the microwave radiation in the soil [12]. For a dry soil this can be 1 m or even more at 1.

4 GHz, whereas for a wet soil the emission depth may be as little as a few centimeters [13]. Second, Rp represents an effective surface reflectivity as a result of the dielectric transition from air to bulk soil with a more or less constant permittivity. In the simplest case of a homogeneous soil with a flat surface, the Fresnel equations [14] can be used to represent Rp at polarization p = H, V and for a certain observation angle. At 1.