# Recent Advances of the BIRALET System about Space Debris Detection
## 2 The BIRALET System  
The BIRALET system is a bistatic radar configuration located in Sardinia (Italy). The transmitter of the BIRALET is the TRF (Trasmettitore a Radio Frequencz--Radio Frequancy Transmitter), a 7 m fully steerable wheel-and-track parabolic antenna with a primary focus configuration, located in the region \"Salto di Quirra\" (Northeast from Cagliari). The azimuth and elevation range of the transmitting antenna are 0-360 deg and 0-90 deg, respectively, with an accuracy of 0.1 deg and a speed of 3 deg/s in both directions [8]. The TRF is capable of transmitting a continuous wave (CW) signal of 10 kW at 410 MHz. The receiver of the BIRALET system is the SRT, a 64 m fully steerable wheel-and-track parabolic antenna (Figure 1) devoted mainly to radio astronomical observations, located in Pranu Sanguini (Northeast from Cagliari), operating in the frequency range from 0.3 to 116 GHz. In addition to the 64 m primary parabolic mirror, the antenna is equipped with a 7.9 m secondary mirror and the beam waveguide system (BWG), which includes two 2.9 m mirrors and one 3.9 m mirror, for a total of four focal positions [16; 17; 18; 19; 20]. To better clarify the mirrors and foci configuration of the SRT, a sketch of the antenna is reported in Figure 2, whereas Table 1 summarizes the key features of the radio telescope. One of the peculiarities of the SRT is the active surface system that allows the modification of the profile of the primary mirror (by means of an electro-mechanical control) in order to compensate for deformations tied to gravitational load, pressure of the wind, and thermal gradients [21]. Figure 1: Photo of the Sardinia Radio Telescope (SRT). Figure 2: Sketch of the Sardinia Radio Telescope. The sensitivity of the bistatic system was computed in terms of scattered power from the targets, using the well-known radar equation [22] for the calculation of the received power and Equation (1) for the evaluation of the noise floor:  
\[\mathrm{N_{F}=k_{B}T_{sys}B_{N}} \tag{1}\]  
where \(\mathrm{k_{B}}\) is the Boltzmann's constant; \(\mathrm{T_{sys}}\) is the system temperature (as sum of the antenna and receiver temperature); and \(\mathrm{B_{N}}\) is the noise bandwidth, evaluated as the bandwidth of the output filter of the receiver. Figure 3 shows the simulated performances of the BIRALET system in terms of received power versus RCS for the range of values from 200 to 2000 km. The estimated signal-to-noise ratio (SNR) for a specific RCS can be derived easily by the difference between the received power and the noise floor. It is worth mentioning that the latter represents an ideal estimation of the SNR received, because, in a real measurement, the noise floor tends to be higher. The receiver employed for the space debris observations is the P-band receiver, working between 305 and 410 MHz [13; 23]. The receiver block is composed of five main sections: the coaxial feed, the cryogenic front-end, the linear to circular polarizer, the noise calibrator and antenna unit injection, and the filter selector. The radio frequency (RF) signal is received by the coaxial feed and sent to the cryogenic front end, directly connected to the \begin{table}
\begin{tabular}{c c} \hline \hline
**Optics** & **Gregorian (Shaped) + BWG** \\ \hline \multirow{4}{*}{Focal Positions} & Primary: f/D = 0.33 \\  & Gregorian: f/D = 2.34 \\  & 2 \(\times\) BWG I: f/D = 1.38 \\  & 2 \(\times\) 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 \textless{} 60\) km/h) & 0.85 \(\circ\)/s (Az) \\  & 0.5 \(\circ\)/s (El) \\ \hline Antenna Gain at 410 MHz & 46.6 dBi \\ \hline Antenna Efficiency at 410 MHz & 57.7\% \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Key features of the Sardinia Radio Telescope (SRT). BWG, beam waveguide system.