## 2 The SRT as a Receiver in a Bi-Static Radar Configuration  
The SRT is a fully steerable wheel-and-track multiple-reflectors antenna used mainly for radio astronomical applications (see Figure 1). It is located 35 km Northeast of Cagliari, in Pranu Sanguini (Lat. 39.493072\({}^{\circ}\)N - Long. 9.245151\({}^{\circ}\)E), at an altitude of about 650 m above sea level. The system can operate, with a radiation efficiency of about 60 percent, in the frequency range from 0.3 to 116 GHz. Currently, four receivers are available, covering a portion of P-band (305-410 MHz), L-band (1300-1800 MHz), C-band (5.7-7.7 GHz) and K-band (18-26.5 GHz) [18]. New receivers are currently under development. The SRT is a Gregorian configuration system with the primary parabolic mirror and the secondary mirror diameters of 64 and 7.9 m, respectively. In addition, the telescope is equipped with a Beam Waveguide system (BWG) which includes two 2.9 m mirrors and one 3.9 m mirror, for a total of four additional focal positions [18]. One of the features of the SRT is the active surface system that permits the modification the profile of the primary mirror (by means of an electro-mechanical control) in order to compensate for deformations caused by gravitational loads, pressure of the wind and thermal gradients [19]. The SRT can be pointed from 5 to 90 degrees in elevation and from \(-\)270 to 270 degrees in azimuth, with a rotational speed of 0.5 and 0.85 degrees per second, respectively. Table 1 summarizes the main characteristics of the radio telescope. The antenna is used as a receiver in the Italian BIRALET radar system for space debris observations in P-band [1,13,20]. The front-end employed for space debris radar campaigns is the dual-feed L-P receiver (see Figure 2), a cryogenically cooled (typically less than 20 K) coaxial receiver with two channels, one for the P-band (with a receiver frequency response at 305-410 MHz) and the other one for the L-band (at 1300-1800 MHz). This receiver is installed in the primary focus of the telescope [21] and it is composed by 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 [13]. The coaxial feed intercepts the Radio Frequency (RF) echo radar and sends it to the cryogenic front end, directly  
Figure 1: Picture of the Sardinia Radio Telescope (SRT) in a typical measurement campaign. connected with the noise calibration and antenna unit injection, which carry out the calibration of the whole system. Afterward, the RF signal is sent to the polarizer, which allows the system to keep the linear polarizations but at the same time to collect it in circular polarization. The last stage is the selection of a suitable filter depending on the type of application. In particular, for space debris monitoring, a band pass filter (centered at 410 MHz with a bandwidth 402-418 MHz) has been recently installed in the filter selector block. This filter guarantees a first discrimination of the frequencies of interest, reducing considerably the entire bandwidth of the P-band receiver [13]. The P-band receiver block is the first part of the space debris dedicated channel of the SRT, which is reported in Figure 3.  
After the P-band receiver block, the RF signal enters the focus selector block, which permits the choice of the antenna focus for the observation (the primary focus in the case of space debris observations) and provides an overall gain of about 20 dB.