## 2 The Front-End of the Italian BIRALET Radar
### The Sardinia Radio Telescope  
The receiver system of the BIRALET radar is the SRT antenna, located close to a Sardinian town named San Basilio, in the Pranu Sanguni area (Lat. 39.493072\({}^{\circ}\) N-Long. 9.24515\({}^{\circ}\) E, Cagliari, Sardinia, Italy) [25]. SRT is a versatile instrument, designed for radio astronomy, geodynamical studies and space science, which is also utilized for applications of space situational awareness from 2014 [26]. The antenna is a multi-reflector system with a quasi-Gregorian configuration, composed of a parabolic 64 m primary mirror and a 7.9 m elliptical secondary mirror. SRT is equipped with three other mirrors (two with a diameter of 2.9 m and one with a diameter of 3.9 m), which form the Beam Wave-Guide (BWG) system. Consequently, the telescope has six focal positions (the primary focus, the Gregorian focus and the four BWG foci) and it is able to host up to twenty remotely controllable receivers and to observe the sky with high efficiency in the frequency range between 300 MHz and 110 GHz. Currently, in the telescope there is the availability of three INAF receivers (which works in the L-P band, C-band and K-band) and one Italian Space Agency (ASI) receiver (X-band), but in the next future the number of receivers will increase and it will also cover more high frequencies up to 110 GHz. SRT is one of the largest radio telescopes in the world that provides an active surface system [29]. In particular, this technology is composed of a total of 1116 electromechanical actuators that control the 1008 aluminum panels that make up the primary mirror, which is supported by a rear frame. The actuators are fundamental to compensate for possible undesired deformations of the primary mirror surface due to gravitational loads, wind pressure, and thermal gradients. Regarding the performances of the telescope with a view to space debris application, one important aspect concerns the azimuth and elevation mechanical maximum speeds, which are 0.85 and 0.5 degrees per second, respectively. Since, as we explain above, the transmitter works in P-band at 410-415 MHz, SRT has an antenna gain of about 46 dBi and a HPBW of about 0.8 degrees in this frequency range [23]. These features allow observing of most space objects orbiting in LEO, that have a typical angular speed of \(<\)0.1 degrees per second, also in tracking mode [30]. The front-end utilized for radar observations of space objects is the dual-feed L-P receiver (see Figure 2), installed in the primary focus of the telescope. It is a cryogenically cooled (with a system temperature typically less than 20 K) coaxial receiver with two channels, one for the P-band (with a main receiver frequency response at 305-410 MHz, useful for radio astronomy applications) and the other one for the L-band (with the largest bandwidth at 1300-1800 MHz) [23]. The dual-feed L-P receiver is made up of five main blocks: the coaxial feed, the cryogenic front-end, the linear to circular polarizer, the noise calibrator and antenna unit injection, and the filter selector block (see Figure 3a). Considering a typical space debris radar observation, the coaxial feed receives the Radio Frequency (RF) radar echo and sends it to the cryogenic front end, directly connected with the antenna unit injection and noise calibration, which performs the calibration of the whole system. Afterward, the received RF signal is sent to the linear to circular polarizer, which allows the system to have both linear and circular polarizations. The last stage in the path of the received RF signal is the selection of a suitable filter, according to the type of application. In particular, for resident space objects monitoring, a band pass filter (BPF) has been recently installed in the filter selector block (see Figure 3b). In particular, this BPF is a tubular filter (model 3B110-410/T15-O/O from K&L (K&L Microwave, Salisbury, MD, USA) [31]) centered at 410 MHz with a 3-dB bandwidth 399-419 MHz and an insertion loss of 1.1 dB.