## 2 The SRT as a Receiver in a Bi-Static Radar Configuration
\begin{table}
\begin{tabular}{c c} \hline _Optics_ & **Gregorian (Shaped) + BWG** \\ \hline \multirow{4}{*}{_Focal Positions_} & Primary: \(f\)/D = 0.33 \\  & Gregorian: \(f\)/D = 2.34 \\  & 2 x BWG I: \(f\)/D = 1.38 \\  & 2 x BWG II: \(f\)/D = 2.81 \\ \hline _Frequency Range_ & \(0.3-116\) GHz \\ \hline _Primary Reflector Diameter_ & 64 m \\ \hline _Secondary Reflector Diameter_ & 7.9 m \\ \hline _BWG Mirrors Diameter_ & \(2.9-3.9\) m \\ \hline _Azimuth and Elevation Speed (wind speed \(<\) 60 km/h)_ & \(0.85\)\({}^{\circ}\)/s (Az) \\  & \(0.5\)\({}^{\circ}\)/s (El) \\ \hline _Azimuth and Elevation Range pointing_ & \(5\div 90^{\circ}\) (Az) \\  & \(-270^{\circ}\div 4270^{\circ}\) (El) \\ \hline _Antenna Gain at 410 MHz_ & 46.6 dBi \\ \hline _Antenna Efficiency at 410 MHz_ & 57.7\% \\ \hline _Half Power Beam Width (HPBW) at 410 MHz_ & \(0.8^{\circ}\) \\ \hline _System noise temperature_ & about 25 K \\ \hline _System noise power_ & about \(-100\) dBm \\ \hline _Typical received SNR level_ & \(>\)10 dB \\ \hline \end{tabular}
\end{table} 
Table 1: Main characteristics of the SRT, focusing on P-band (410 MHz). Figure 2: Picture of the L-P cryogenic receiver (red circled) installed on the SRT primary focus. From the top of the primary mirror, the echo radar signal is then carried down to a shielded room at ground level, by means of a 500 m optical link. Inside the shielded room, the RF signal passes through the Intermediate Frequency (IF) distributor block, characterized by an overall gain of about 20 dB, which allocates the signals coming from the antenna to the different back-ends [13]. The dedicated back-end for space debris observations is based on the Red Pitaya board, a commercial System-on-a-Chip FPGA board provided with a dual-core 800 MHz ARM9 processor, a FPGA with 28,000 logic cells, 512 MB of DDR3 RAM system memory, 2 RF inputs and 2 RF outputs, a sample rate of 125 MSample/s, 14 bit of ADC resolution and a working frequency in the range 0-50 MHz. For this reason, a down-conversion block is necessary, which allows the system to lower the RF signal at 410 MHz to an Intermediate Frequency of 30 MHz and later filtered with a band pass filter centered at 30 MHz with 5 MHz bandwidth and 0.9 dB of insertion loss [13]. After the filtering stage, a digital step attenuator, controlled by the Red Pitaya, is located upstream to the final amplifier in order to avoid the saturation of the system. A digital signal processing chain has been programmed inside the Red Pitaya board, allowing the conversion of the analog input signal in a base-band complex raw signal. This raw signal can be stored and post-processed using a Fast Fourier Transform (FFT) and, finally, displayed in various plot such as spectrogram on the whole acquisition time, data acquisition at the exact epoch of the debris passage and passage duration inside the beam of the SRT. Finally, by analyzing various produced plots, it is possible to extrapolate the Doppler shift measurement, with a resolution bandwidth less than 20 Hz, and the received SNR level [13; 20]. Alternatively to the space debris dedicated channel of Figure 3, a simplified acquisition chain could be used, where the two channels of the P-band receiver, one for each polarization, typically used for radio astronomical measurements, are combined and simply connected to a spectrum analyzer (see Figure 4). This measurement setup has been used during the first space debris experiments with the SRT in April 2014 [1]. In this way, it is possible to see in real time the echo radar with a resolution bandwidth less than 20 Hz, but data processing could be poor, mainly due to the limitations of the spectrum analyzer, i.e., the slow frequency sweep for low resolution bandwidths, thus preventing all data saving in real time. Therefore, thanks to the space debris dedicated receiver channel, or alternatively the configuration with the spectrum analyzer as a back-end, the SRT performs Doppler shift measurements with an expected accuracy of 20 Hz.