## 2 Instrument Developments
### PAU-Real Aperture  
One of the technological goals of the project is to demonstrate the feasibility of combining in a single hardware two types of receivers: the radiometer itself (_PAU/RAD_) which, for stability reasons cannot be a total power radiometer, and the GNSS-Reflectometer (_PAU/GNSSR_). In order to be able to use the same receivers for both the radiometer and the GPS-reflectometer, a new radiometer topology has been devised (Figure 5). Another technological goal, as challenging as the first one, is to provide an L-band radiometer with a digital beamforming. This capability is a very useful novelty in microwave radiometry, since it allows one to obtain simultaneously brightness temperature measurements at different incidence angles without mechanical scan.  
As compared to a real aperture radiometer, instead of connecting the antenna output directly to the radiometer receiver, it is connected to the input of a Wilkinson power splitter that divides the signal in two signals that are in phase. However, the 100 ohm resistor of the Wilkinson power splitter that connects the two outputs also adds two noise signals that are 180\({}^{\circ}\) out-of-phase.  
Therefore, the signals at the input of the two channels of the radiometer are the sum and the difference of the antenna signal and the noise generated by the [PERSON] power splitter resistor, which is proportional to the physical temperature of the resistor. Once properly amplified, down-converted, and sampled they are finally cross-correlated leading to an output that is proportional to the difference between the antenna temperature and the physical temperature of the [PERSON] power splitter resistor. That is, the system output is the same as the one of the [PERSON] radiometer, but the input signal is not chopped, so that it can be used to track the GPS-reflected signal.  
The extension to the \(N\)-element case is straightforward although technology challenging. Instead of one antenna, there are \(N\)-dual polarization antennas, whose outputs are connected each to a Wilkinson power splitter. After amplification, down-conversion, and sampling, the \(N\)-outputs at each polarization are combined with the ones from other receivers with the appropriate amplitude and phases so as to digitally form the beam(s). The final output of each beam is obtained by cross-correlating the upper and lower branches, corresponding to the sum and difference signals (Figure 5). If the  
Figure 5: Block diagram of the _PAU/RAD_ concept with just one receiver.  
Figure 6: Schematic of the _PAU-Real Aperture_ with 16 elements [4].  
vertical-polarization signals are cross-correlated, the antenna temperature at vertical polarization is measured. If the horizontal-polarization signals are cross-correlated, the antenna temperature at horizontal polarization is measured. And if the vertical-polarization signal is correlated with the horizontal-polarization one, the third and the fourth Stokes elements are measured, as twice the real and imaginary parts of the complex cross-correlation. Note that, since the vertical and horizontal channels do not share a Wilkinson power splitter, there is no \"Dicke effect\" (output proportional to the difference between the antenna temperature and the physical temperature of a reference) for the third and fourth Stokes parameters. Figure 6 shows the schematic of the whole _PAU-Real Aperture_ instrument.  
In addition, two calibration signals are required for calibration purposes:  
* uncorrelated noise, generated by a matched load at each input channel, to compensate for instrumental biases (cross-correlations must be zero), and
* two different levels of correlated noise generated by a common noise source, to compensate for phase and amplitude mismatches among receivers.  
Details on the calibration techniques and beamforming can be found in [10-12]. Figure 7 shows the _PAU-RA_ instrument: (a) in the UPC anechoic chamber for testing, (b) front-view without cover, and (c) inside view without cover and with foldable sides opened.