## 1 Introduction
Germany has available a powerful L-band bi-static radar composed of the FGAN Tracking and Imaging Radar (TIRA), located near Bonn, as transmitter antenna, and the Max Planck Institute Effelsberg Radio Telescope, as receiver system [15]. In this configuration, the radar works in beam-parking mode and can transmit up to 2 MW peak power pulses, permitting the detection of objects with size down to 1 cm [16]. In addition, the German Space Administration (DLR) is developing the German Experimental Surveillance and Tracking Radar (GESTRA), a close-monostatic pulsed phased array working in L-band (in the frequency range 1280-1400 MHz), able to carry out digital beamforming and designed to observe space objects up to 3000 km of altitude [17; 18]. On the French side, the main radar employed all day for space surveillance in survey mode is the Grand Reseau Adapte a la Veille Spatiale (GRAVES), a military bi-static phased array radar operating at 143.05 MHz in continuous wave (CW) mode, located in Dijon [19]. As concerns Spain, the Spanish Space Surveillance and Tracking (S3T) system is currently equipped with two radars: the Monostatic Space Surveillance Radar (MSSR), a close-monostatic L-band radar, located in the Santorcz military naval base, and the new S3T Surveillance Radar (S3 TSR) [20]. In the United Kingdom, the Chilbolton Advanced Satellite Tracking Radar (CASTR) was developed starting from the Chilbolton radar, a fully steerable antenna, with a 25 m reflector, operating in S-band (at 3 GHz) with a peak power of 700 kW, located near Winchester (Hampshire). The CASTR is able to efficiently observe objects with Radar Cross Section (RCS) greater than 0.5 m\({}^{2}\) in LEO [21]. All these European systems, although they present different technologies, are able to collect important information about space objects, such as slant-range and Doppler shift measurements. Italy is also part of the EUSST program and has available two bi-static radars, based on a radio telescope as receiver, that operate in survey and tracking mode, respectively. They are the BIRALES system [22] (Bi-Static Radar for LEO Survey), and the BIRALET system [23] (Bi-Static Radar for LEO Tracking). Both bi-static radars have the same transmitter, named the Radio Frequency Transmitter (TRF) that works in the P-band at 410-415 MHz and supplies a transmitting power up to 10 kW. BIRALES is used for monitoring the space environment in survey mode. The receiver of the BIRALES radar is the Northern Cross Radio Telescope, located in the Medicina Radio Astronomical Station, near Bologna, in Northern Italy. The Northern Cross is a multi-beam system that performs Doppler shift, illumination time and received signal-to-noise ratio (SNR) measurements. It permits an estimation of the angular track of the object, that transits through its field of view, analyzing the beams illumination sequence. This innovative configuration allows the sensor to collect fundamental data useful to perform initial orbit determination [24]. BIRALET, on the other hand, allows the space environment monitoring in both beam-parking and tracking modes. The receiver of the BIRALET radar is the Sardinia Radio Telescope (SRT), a 64-m dish fully steerable wheel-and-track antenna, located near San Basilio (Cagliari, Sardinia, Italy). It represents a flexible instrument utilized for radio astronomy research and space science, developed to operate in a large frequency range between 300 MHz and 110 GHz [25]. The antenna is a multi-reflector system with a quasi-Gregorian configuration, with a 64-m parabolic primary mirror and a 7.9 m elliptical secondary mirror. The telescope has three other mirrors (two with a diameter of 2.9 m and one with a diameter of 3.9 m), which provide the Beam Wave-Guide (BWG) system. Thanks to these reflectors, the system has available 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 0.3-116 GHz.