## 2 PAU-RAD Overview
### The PAU-RAD Instrument  
Each of the 4 \(\times\)4 array elements (Figure 2) has a dual-polarization antenna (horizontal and vertical). Each polarization signal (S\({}_{\mathrm{a}\mathrm{v}}\) and S\({}_{\mathrm{a}\mathrm{H}}\)) is divided using a Wilkinson power splitter, followed by a pair of receiving channels (upper and lower ones). After signal conditioning at Radio Frequency (RF), the input signals are down-converted at an Intermediate Frequency (IF) of 4.309 MHz with a 2.2 MHz bandwidth (L1-GPS bandwidth). Once each channel is down-converted to IF, these signals are digitized at 8 bits using band-pass sampling techniques with a sampling frequency of 5.745 MHz. This frequency plan simplifies signal processing operations since the digital frequency is 0.25. Digitized signals are properly low-pass filtered, I/Q demodulated and processed in an ALTERA Field Programmable Gate Array (FPGA), where the full digital radiometer is embedded [11] (described in VHDL-97 digital description language [18]). Finally, after data calibration and through data processing, values proportional to the Stokes parameters can be achieved as follows: The first and the second Stokes parameters are obtained adding and subtracting the results of the same polarization channels correlation (Figure 2). On the other hand, the third and fourth parameters are twice the real and imaginary parts of the correlation of the two orthogonal polarizations.  
To achieve the best performance, the L-band radiometer should operate in the 1,400-1,427 MHz \"protected\" band and the GPS reflectometer at 1,575.42 MHz (GPS-L1 band), where open C/A codes can be used to generate the local replicas of the GPS signal. However, in the PAU instrument demonstrator, the GPS reflectometer and the L-band radiometer share the same front-end: A commercial GPS down-converter (Zarlink GP2015 [19]). As a result, both operate at 1,575.42 MHz. This architecture reduces the required hardware resources, using only one RF-IF front end instead of two or a dual-band one for each receiver [20]. Although sharing the same RF-IF front-end provides a significant hardware reduction, the system has to deal with some incurred handicaps. The first one concerns the bandwidth, the radiometer uses only the 2.2 MHz GPS band, instead of the nearly 27 MHz available at the protected L-band. Obviously, it has an impact on the radiometric resolution, which can only be compensated by increasing the integration time. The second handicap concerns the interference that GPS signals can incur on the radiometric measurements. Although the spread-spectrum modulation GPS signal is lower than the noise level (~27 dB below the noise level), the impact of the GPS interference on the radiometer measurements has to be quantified [11]. For this purpose, two extreme scenarios are distinguished considering a sea surface reflection coefficient of \(\Gamma=0.7\), a height of h = 110 m (the same height as in [14]) and an antenna's effective area of A\({}_{\mathrm{eff}}=0.16\) m\({}^{2}\) (D = 17.5 dB, where D stands for antenna directivity) for both cases. The worst situation occurs when the specular reflection point is at the antenna's boresight. In this case the antenna temperature increases due to the GPS interference is \(\Delta\)T = 2.3 K (equivalent to ~4.6 psu salinity error in warm water). On the other hand, the best possible scenario occurs when the reflection point comes 90 \({}^{\circ}\) out of boresight (and the attenuation due to the antenna pattern is ~15 dB), the radiometric measurement contribution is then \(\Delta\)T \(\sim\) 0.03 K (~0.06 psu salinity error in warm water), below the 0.1 psu requirement of the Global Ocean Data Assimilation Experiment (GODAE) for open ocean [21]. Taking into account that the radiometer can create different beams using the DBF processing technique and that the specular reflection points are known and can be tracked, the chances of having a significant interference level are negligible.