## 1 Introduction  
Space debris is defined as all inactive, human-made objects, including fragments, with variable sizes and shapes, orbiting the Earth or reentering the atmosphere. In near-Earth space, such as Low Earth Orbit (LEO), between 200 and 2000 km of altitude, this debris is more significant than natural meteoroids. The presence of this kind of objects generates two types of risk: the in-orbit collision risk for every manned and unmanned spacecraft and the risks from reentering debris [1]. Collision avoidance procedures represent an important mitigation measure but they require an accurate knowledge of the state of the orbiting objects. Within this framework, a network of dedicated sensors (radars, telescopes and lasers) is necessary for surveillance and tracking of space debris. Thanks to the Space Surveillance Network (SSN), an extensive network of optical, lasers and various types of radar sensors (such as phased arrays, reflector antennas and multi-static fences), the United States Strategic Command (USSTRATCOM) is able to maintain the largest database of catalogued objects in LEO [2], which is composed by more than 24,000 debris of assorted sizes larger than 1 centimeter. A detailed list of US SSN sensors is reported in [3,4]. The US Joint Space Operations Center (JSpOC) acts as coordinator for the large amount of data coming from the SSN, performing orbit determination with its own software and elaborating the orbital parameters for making them available in a universal suitable format, i.e., the two-line element set (TLE) [5]. Survey and tracking of objects in Earth orbit is one area where the European Space Surveillance and Tracking (SST) Support Framework and the European Space Agency (ESA) Space SituationalAwareness (SSA) programmes are active, and the implementation of a European network of sensors represents one of the main objectives [2]. Consequently, Europe also can count on a large network of optical [6], laser and radar sensors. Focusing on radars, an important system available 24 hours a day for observations in survey mode is the Grand Reseau Adapte a la Veille Spatiale (GRAVES), a military continuous wave (CW) bi-static phased array radar operating at 143.05 MHz, located in Dijon (France) [7]. Another radar system, named European Incoherent Scatter Scientific Association (EISCAT), is composed by three separate sites [8]: the mono-static VHF radar, located near Tromso (Norway), which operates at 224 MHz; the mono-static double antenna EISCAT Svalbard radar, located in Longyearbyen (Svalbard), which operates at 500 MHz; and the tri-static UHF EISCAT radar operating at 930 MHz, with transmission site located in Tromso and receiving sites located in Tromso, Kiruna (Sweden) and Sodankyla (Finland). Another European radar sensor, available for space objects monitoring, is a powerful pulsed bi-static radar consisting of the FGAN Tracking and Imaging Radar (TIRA), located near Bonn (Germany), as a transmitter, and the Max-Planck-Institute Effelsberg Radio Telescope as a receiver [9]. The system works in L-Band, allowing the detection of objects with minimum size of 1 cm. In addition, Germany is developing the German Experimental Surveillance and Tracking Radar (GESTRA) [10], a close-monostatic pulsed phased array working in L-band (1280-1400 MHz), able to perform digital beamforming for improving performances on space objects observations and designed to detected objects up to 3000 km altitude. The group of dedicated radar sensors involved in the European SST network includes two Italian radars based on two different radio telescopes as a receiver (RX): the BI-static RAdar for LEo Survey (BIRALES) system [11; 12] and the BI-static RAdar for LEo Tracking (BIRALET) system [1; 13; 14]. Each of these systems, which are included in the European monitoring program for Space Surveillance and Tracking (SST), uses the same transmitter (TX), named Radio Frequency Transmitter (TRF), located in Sardinia in the region \"Salto di Quirra\" (Lat. 39.6050\({}^{\circ}\)N - Log.