## 2 PAU-RAD Overview  
The _Passive Advanced Unit_ (PAU) _for ocean monitoring_[10] is an instrument that combines, in a single receiver and without time-multiplexing, an L-band microwave radiometer (PAU-RAD) [11] and a GPS-reflectometer (PAU-GNSS/R) [12-14] which, in conjunction with an infra-red radiometer (PAU-IR) [15], simultaneously provide the sea state information and surface temperature needed to accurately retrieve the SSS. Furthermore, PAU-RAD, as it has been designed, is a concept demonstrator ground-based instrument, which is expected to measure multi-angular incidence ocean brightness temperature from a terrain elevation, for instance a cliff. Measurements of the ocean brightness temperature from a cliff have previously been successfully conducted, such as in [14], where the height above sea level was 110 meters. PAU-RAD is a digital radiometer with DBF and polarization synthesis capabilities. That is, any polarization can be generated by a proper combination of signals collected by the two orthogonal polarizations of the antenna. The DBF consists of a \(4\times 4\) rectangular array with triangular illumination. A simplified sketch of the PAU-RAD architecture is presented in Figure 2, where:  
* \(\mathrm{S_{aVH}}\), \(\mathrm{S_{w}}\) are the antenna signal, from vertical and horizontal polarization, and the Wilkinson's signal, respectively;
* CNS stands for Controlled Noise Source; and
* LPF stands for Low Pass Filter. The PAU-RAD radiometric architecture is based on a new kind of correlation radiometer (Figure 2) equivalent to a Dicke radiometer in terms of radiometric output (\(\alpha\,T_{a}\,-\,T_{w}\) ) and to a total power radiometer in terms of radiometric sensitivity (\(T_{sys}\)/\(\sqrt{B\tau}\) ) [10], where \(T_{a}\) is the antenna temperature, \(T_{w}\) is the temperature of the [PERSON]'s power splitter, \(T_{sys}=(T_{a}\,+\,T_{w})\,/\,2+\,T_{REC}\) is the system temperature, \(T_{REC}\) is the receiver noise temperature and \(B\tau\) is the product of the equivalent noise bandwidth with the integration time. To clarify the PAU-RAD radiometric concept, Figure 2 shows a simplified model, which uses only a dual-polarization receiver and two channels per polarization (a total of four channels per receiver). The outputs of the PAU-RAD are the complex correlation between the signals at the outputs of a [PERSON] power splitter (upper and lower channels), which are proportional to the antenna temperature minus the [PERSON]'s resistor physical temperature. Since the output is a complex value, it has modulus and phase. The phase of this complex value is the differential phase between both channels and is zero only if the upper and lower channels have exactly the same phase. Otherwise, the resulting phase will be relevant for calibration purposes (Section 3.2). The correlation of the signalsfrom the two orthogonal polarizations is proportional to the third (real part) and fourth (imaginary part) Stokes parameters. The results of PAU-RAD are the four Stokes parameters as it is a full-polarimetric radiometer. The Stokes parameters characterize the polarization status of a surface's polarized brightness temperature. This polarization relates to different phenomena, such as the incidence angle, the SSS, the sea surface temperature, the sea surface roughness, wind direction and many others. The first and the second Stokes parameters are proportional to the sum and the difference of the brightness temperatures at vertical and horizontal polarizations. It is known that the sea surface salinity can be retrieved from these two parameters [3]. The third and the fourth parameters characterize the complex correlation between these two orthogonal polarizations [16]. A first analysis, using only one receiver demonstrator, of the PAU-RAD calibration algorithms and their performance was described in [11]. Then, the first results, analysis and digital beamforming antenna diagrams where presented in [17]. This work is the continuation, verification and extension of these preliminary works.