## 1 Introduction
### Measurement Principle  
Microwave radiometry is a passive remote-sensing technique that measures thermal radiation. The radiance \(T_{\mathrm{B}}{}^{p}\) emitted from a terrestrial surface at horizontal (\(p=\mathrm{H}\)) or vertical (\(p=\mathrm{V}\)) polarization depends on the surface temperature \(T_{\mathrm{S}}\), and on the surface reflectivity \(R^{p}\). 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  
Figure 1: The ELBARA II systems mounted above the test site at the Swiss Federal Research Institute WSL, with the radiometer electronics and the power unit installed in the unit on the right. the emitted radiation above a surface is \(T_{\rm S}\):(1 - \(R^{p}\)). Since downwelling radiation \(T_{\rm sky}\) also contributes to the observed radiation by the fraction reflected at the surface, the total radiation \(T_{\rm B}{}^{p}\) received by a radiometer oriented towards the surface can be expressed by:  
\[T_{\rm B}^{p}=(1-R^{p})T_{\rm S}+R^{p}T_{\rm sky} \tag{1}\]  
The value of \(T_{\rm sky}\) is determined by the cosmic background temperature of \(\approx\)2.7 K and enhanced by an atmospheric contribution. At 1.4 GHz, this enhancement is almost constant, leading to 4 K \(<T_{\rm sky}<\) 5 K. Since the terrestrial surface temperature is much larger than \(T_{\rm sky}\), the brightness temperature \(T_{\rm B}{}^{p}\) has a strong sensitivity to \(R^{p}\). The sensitivity of \(T_{\rm B}{}^{p}\) with volumetric soil water content \(WC\) [m\({}^{3}\) m\({}^{-3}\)] is established through \(R^{p}\), 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 (\(\approx\)80) and dry soil (\(\approx\)3 to 5). This allows the soil surface-water content to be determined from its reflectivity by applying dielectric mixing (e.g., [9, 10, 11]) and radiative transfer models. Typically, \(T_{\rm B}{}^{p}\) 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, \(T_{\rm S}\) 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, \(R^{p}\) 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 \(R^{p}\) at polarization \(p=\) H, V and for a certain observation angle. At 1.4 GHz, a requirement for applying the Fresnel equations is a transition depth of \(<\)1 cm. However, more sophisticated models are required to compute \(R^{p}\) if \(T_{\rm B}{}^{p}\) originates from a landscape, e.g., with vegetation. Recent results obtained from several theoretical studies and field experiments dedicated to the retrieval of sea salinity as part of the SMOS mission are presented in [15]. For retrieving ocean salinity from \(T_{\rm B}{}^{p}\) measured at L-band, the principle is similar as applied for retrieving soil moisture. Again \(T_{\rm B}{}^{p}\) can be expressed by Equation (1). However, the dielectric constant of ocean water is in a quite different range. It is the imaginary part of the permittivity that increases with increasing salt content due to the increased conductivity. Sea salinity is measured in Practical Salinity Units (psu) defined as: Sea water with the salinity 35 psu has a conductivity ratio of unity at 15 \({}^{\circ}\)C (and 1 atmosphere pressure) with a potassium chloride (KCl) solution containing 32.4356 g of KCl per kg of solution.