## 2 Airborne Instruments and Interface with the ATR-42
### CAROLS Radiometer
#### 2.1.1.3 Detailed System Design  
A. Receiver Unit  
a. Analog Front End (AFE) Electronics  
The Analog Front End (AFE) is composed of several microwave components (switch, director coupler, noise source, filter, isolator). The antenna and receiver unit is illustrated in Figure 1.  
\begin{table}
\begin{tabular}{|c|c|} \hline \multicolumn{2}{|c|}{CAROLS: correlation radiometer with direct sampling} \\ \hline Sampling rate & 139.4 MHz \\ Integration time & 1 ms and 1.8 μs \\ Internal calibration & Load and noise diode \\ Sensitivity & 0.1 K for 1 s integration time \\ Stability & 0.1 K \\ System noise temperature & 150 K \\ Physical temperature of AFE & 45 °C \\ Physical temperature of DFE & 90 °C \\ \hline \end{tabular}
\end{table}
Table 1: \(Cont.\)  
Figure 1: Block diagram of the antenna and receiver unit.  
This unit ensures a gain of 81 dB for each channel and an isolation of approximately 80 dB between the H (Horizontal) and V (Vertical) channels. Special attention was paid to the performance of the bandpass filter design. This device, based on cavity properties, provides a center frequency of 1,413 MHz and a 3 dB bandwidth of 24 MHz. The filter was specially tuned to attenuate interfering signals lying outside the bandwidth of interest.  
The performance of the analog front end is strongly dependent on its physical temperature. Therefore, to achieve the desired radiometric sensitivity, the measurement of inside temperatures is very important for the calibration process. The receiver is fitted with approximately ten thermal sensors, placed at various locations. The microwave section is thermally controlled using a PI (Proportional Integral) regulation algorithm. The AFE demonstrated its excellent ability to stabilize the internal temperature at 45 degree: during a 4-hour flight this varies by only 0.1 degree.  
b. Digital Front End (DFE) Electronics  
The analog signal output from the analog receiver is directly digitized by two A/D converters, with a sampling frequency of 139.4 MHz. The unit is designed for sub-harmonic sampling of the analog signals, _i.e._, direct sampling of the L-band signal with no analog down conversion. The signals produced by the converter are fed into a FPGA (Field Programmable Gate Array), in which 1.8 \(\upmu\)s and 1 ms integrations are computed. The 1.8 \(\upmu\)S integration period is useful during post processing, for the removal of Radio Frequency Interferences (RFI).  
B. Antenna Unit  
The antenna unit comprises a horn and an orthomode transducer (OMT). Each horn has an identical pattern and a 37.6\({}^{\circ}\) half-power beam width (HPBW), for both horizontal and vertical polarizations. This large beam and corresponding antenna dimensions were specified in order to fit the lower fuselage apertures of the aircraft. The antenna system (OMT and Potter horns) is characterized by almost ideal radiation patterns with practically no side-lobes and low losses. The performance of the Potter horns was tested at the CNES' (Centre National d'Etudes Spatiales) anechoic chamber, and has been judged to be very satisfactory, with its side and back lobes being suppressed by more than 30 dB, and an even lower cross-polarization level. The performance of each OMT was also measured. Isolation between ports was greater than 49 dB for all frequencies between 1.4 and 1.427 GHz.