## 2 Instrument Developments  
In order to test the feasibility of using GNSS-R over the sea surface to make sea state corrections to the radiometric data, the _PAU_ (_Passive Advanced Unit for ocean monitoring_) project was proposed to the European Science Foundation in 2003, and it was granted in 2004 within the EURYI program. _PAU_ is also a test bed of new technological demonstrators such as real aperture radiometers with digital beamforming and polarization synthesis, and fully-digital synthetic aperture radiometers _etc._ In the _PAU_ system a suite of three different instruments operate in a synergetic way:  
(1) _PAU/RAD_: an L-band radiometer to measure the brightness temperature of the sea surface,  
(2) _PAU/GNSS-R_: a GPS-reflectometer using the L1 C/A code to measure the sea state, and  
(3) _PAU/IR_: two infrared radiometers to measure the sea surface temperature,  
and both _PAU/RAD_ and _PAU/GNSS-R_ share the same RF front-end.  
Figure 4: Measured (not normalized) 1 s incoherently integrated DDM with the threshold applied to compute its volume. Noise is well below the threshold [9].  
Figure 3: (left) GNSS-R geometry of observation from Garraf coast (South of Barcelona). Red: isorange annuli (same delay), Yellow: isoDoppler hyperbolae (same Doppler), Green dots with same delay and Doppler. (right) measured DDM.  
A number of _PAU_ system prototypes have been developed:  
(1) _PAU-Real Aperture_ instrument with a 4 \(\times\) 4 element array with digital beamforming and polarization synthesis that uses of an innovative pseudo-correlation radiometer topology to avoid the classical input switch in a Dicke radiometer and  
(2) _PAU-Synthetic Aperture_ instrument, which is also used to test potential new technological developments and algorithms for future SMOS missions.  
In addition to these two, and in order to advance the scientific studies relating the GNSS-R and radiometric observables other _PAU_ demonstrators have been developed:  
(3) _PAU-OR_ with just one element for ground tests and algorithms development, and _griPAU_, an improved _PAU-OR_ instruments fully automated,  
(4) _PAU-ORA_, a lighter version of _PAU-OR_ for aircraft operations from a remote controlled plane, and  
(5) _MERITXELL_ (_Multi-frequency Experimental Radiometer With Interference Tracking For Experiments Over Land And Litoral_) a classical Dicke radiometer, that includes not only L-band, but S-, C- X-, K-, Ka-, and W-bands, plus a multi-spectral camera, in addition to the _PAU/IR_ and _PAU/GNSS-R_ units.  
In these technology demonstrators the input signals (_PAU/RAD_ and _PAU/GNSS-R_) are the same: the L1-GPS band. This is not critical, since, due to the scattering on the sea surface, the scattered GPS signal is at least 23 dB below of the thermal noise signal (to be measured by _PAU/RAD_). Thanks to the 30.1 dB correlation gain, _PAU/GNSS-R_ can detect the GPS signal when the correct C/A code is applied, and the error introduced in _PAU-RAD_ observables is negligible, and it only occurs when the specular reflection is coming from the antenna boresight. Taking into account that the beam can be electronically steered the chance of having a significant interference is very remote.  
Signals' bandwidth are limited to the GPS bandwidth (2.2 MHz) and are demodulated at an intermediate frequency (IF) of 4.309 MHz. Signals are then digitalized at 8 bits at a sampling frequency of 5.745 MHz, to allow the use of digital demodulation, using band-pass sampling techniques.  
These different instruments are explained in the following sections.