## 2 Instrument Developments
### PAU-One Receiver Airborne  
Figure 11a shows the _PAU_-One Receiver Airborne on the bay of a remote controlled aircraft. Details on the control, telemetry, data links and data storage can be found in [20-22]. Figure 11b shows the DDMs measured when the direct (RHCP) and reflected (LHCP) signals are collected simultaneously using two different antennas connected to the inputs of a non-resistive 2-way power combiner. The left-hand side peak corresponds to the direct signal, which has a larger amplitude, while the right-hand side one corresponds to the reflected one, which is attenuated in the scattering and in the longer signal path. The separation between peaks is 21 samples, which correspond to \(\sim\)770 m, since in this implementation of the instrument, the sampling frequency is 8.1838 MHz. Therefore, since the antenna was pointing to the nadir direction and the GPS satellite was close to the zenith, the estimated height is \(\sim\)385 m, which is very close to the flight height (379 m). This design offers several advantages over the previous ones (just measuring the reflected signal) since it intrinsically provides absolute calibration of the scattering coefficient (ratio of peaks between direct and reflected DDM), if offer altimetry capabilities, and sea state determination using the full DDM, as shown in Section 3. Figure 11: (a) _PAU_–One Receiver Airborne instrument block mounted on the bay of a Remote Control aircraft. (b) DDMs obtained collecting simultaneously the direct (RHCP) and reflected (LHCP) signals. Brightness temperature maps measured over (c) Vadillo de la Guaréña (Zamora, Spain) and (d) Marquesa Beach (Ebro river mounth, Tarragona, Spain). The Multifrequency Experimental Radiometer with Interference Tracking for Experiments over Land and Littoral (MERITXELL)  
The _MERITXELL_ microwave radiometer is a step forward advancing our understanding of the potentials of combining data from different sensors: microwave radiometers, multi-spectral and TIR cameras, and GNSS Reflectometers. It will also be used in testing radio frequency interference (RFI) detection and mitigation algorithms for microwave radiometry. The _MERITXELL_ microwave radiometer is a multi-band dual-polarization Dicke radiometer covering eight protected bands used for passive remote sensing: L, S, C, X, K, Ka, and W (see Table 2). To add flexibility and simplify the design, a spectrum analyzer is used as IF stage, for filtering and power detection for all bands. This allows an easy reconfiguration of the band and/or frequency response shape, since the antennas and amplifiers response exceed those indicated in table 2. Antennas are 4 \(\times\) 4 dual-polarization path arrays at L, S, and C bands, and horn antennas with a lens in the aperture to provide a quasi Gaussian beam for the other bands. In addition, _MERITXELL_ includes a thermographic camera (320 \(\times\) 240 pixels) operating in the 8-14 \(\upmu\)m range, a multi-spectral camera (640 x 480 pixels) with four spectral bands: red (\(\lambda_{0}\)= 0.62 \(\upmu\)m), green (\(\lambda_{0}\)= 0.54 \(\upmu\)m), blue (\(\lambda_{0}\)= 0.45 \(\upmu\)m) and Near Infra-Red (\(\lambda_{0}\)= 0.80 \(\upmu\)m), and a _PAU/GNSS-R_ unit. \begin{table}
\begin{tabular}{c c c c c} \hline \multirow{2}{*}{**Band**} & **Central** & \multirow{2}{*}{**Bandwidth**} & **Antenna** & **Main beam** \\  & **frequency** & & **beamwidth** & **efficiency** \\ \hline L & 1.4135 GHz & 27 MHz & \(\sim\)25\({}^{\circ}\) & 98 \% \\ S & 2.695 GHz & 10 MHz & \(\sim\)25\({}^{\circ}\) & 98 \% \\ C & 7.185 GHz & 90 MHz & \(\sim\)25\({}^{\circ}\) & 98 \% \\ X & 10.69 GHz & 20 MHz & \(\sim\)5\({}^{\circ}\) & 95 \% \\ K & 18.7 GHz & 20 Figure 12a shows the schematic of the _MERITXELL_ microwave radiometer, and Figure 12b shows the front view with the antennas mounted (cameras and _PAU/GNSS-R_ will be placed in the hole in the center of the top row).