## 2 Instrument Developments
### PAU-Synthetic Aperture  
The receivers and frequency plan of _PAU-Synthetic Aperture_ is the same as for _PAU-Real Aperture_, but since the number of receiving elements (Y-shaped array with 8 elements per arm plus a central one) is much larger than in the _PAU-Real Aperture_ case (16 elements), limitations in the FPGAs processing capabilities, forced to retain only one receiver per element and polarization, and the [PERSON] input switch was removed (topology in Figure 5 could not be followed). Another of the objectives of _PAU-Synthetic Aperture_ was to test new techniques, technologies and algorithms that could be eventually used in future SMOS missions. To better understand this, Table 1 summarizes the main parameters of the MIRAS instrument aboard SMOS mission [13] and the _PAU-Synthetic Aperture_ ones. The rightmost column lists the rationale behind each technological decision. Figure 7: (a) _PAU-Real Aperture_ without radome in UPC anechoic chamber for testing beamforming performance, (b) front view of the \(4\times 4\) element array, and (c) inside of the instrument showing the distribution of the dual-polarization receivers, FPGAs for data processing, and the temperature control. \begin{table}
\begin{tabular}{l l l l} \hline _Parameter_ & _MIRAS/SMOS_ & _PAU-Synthetic Aperture_ & _Rationale for changing the parameter_ \\ \hline
**Frequency** & protected band: & GPS L1 (1575.42 MHz) & \(\bullet\) availability of integrated circuits \\
**operation** & 1400–1427 MHz & & \(\bullet\) limitation of integrated circuits and FPGAs \\
**Bandwidth** & 19 MHz & 2.2 MHz & FPGAs \\  & & & \(\bullet\) minimizes spatial decorrelation. \\
**Arm size** & 4 m & 1.3 m & \(\bullet\) operation from a truck \\
**Altitude** & 755 km & ground-based experiments & - \\  & dual-polarization patch & & \(\bullet\) multiply by 2 the integration time \\
**Antenna type** & antenna (non & (simultaneous) & \(\bullet\) eliminate polarization transitions \\
**Number of antennas per arm** & 23 & 8+1 (dummy) & \(\bullet\) dummy antenna at edge for better pattern similarity \\
**Total number of antennas** & 69 & 31 & - \\
**Antenna spacing** & 0.875 \(\lambda\) at 1400 MHz & 0.816 \(\lambda\) at 1575.42 MHz & \(\bullet\) reduced antenna spacing to increase alias-free field-of-view \\  & & 2 per element & \(\bullet\) double integration time \\
**Receiver type** & 1 per element & (1 per polarization) & \(\bullet\) allow full-polarimetric mode without pol-switching schemes \\  & & & [14] \\
**Topology of the LO down-** & distributed LO & centralized reference clock + internal PLL in each receiver for LO generator. & \(\bullet\) avoid correlated noise from common LO leaking to outputs through mixers and generating correlation offset \\
**Converter** & 1 bit IF sampling & 8 bit IF sub-sampling using a external ADC & \(\bullet\) use digital I/Q demodulation, low-pass filtering and power detection \\
**Quantization** & 1 bit IF sampling & 8 bit IF sub-sampling using a external ADC & \(\bullet\) avoid phase quadrature errors and different receivers’ noise temperature between I/Q branches \\  & & & \(\bullet\) match global frequency response \\
**Power measurement** & & & using narrow digital low-pass filters \\  & & & filters \\
**system (PMS)** & & & \\
**Digital Correlator** & & & \\
**Unit** & & & \\
**Imaging** & dual-pol or full-pol & Full-pol & \\
**capabilities** & (sequential) & (non-sequential) & \(\bullet\) avoid detector diodes thermal \\  & & & \\  & uncorrelated/2-level & & drifts \\
**Calibration capabilities** & correlated noise & & \\  & & & \\  & injection & & Pseudo-Random Noise (PRN) sequences \\
**Integration time** & 1.2 s & 1 s, 0.5 s, 100 ms and 10 ms & \(\bullet\) flexibility \\ \hline \end{tabular}
\end{table} 
Table 1: MIRAS instrument and _PAU-Synthetic Aperture_ main parameters. The _PAU_-Synthetic Aperture is composed by a Y-shaped array of eight antennas per arm plus the one in the center, and additional dummy antenna at the end of each arm.