## 1 Introduction  
Since 1957, when the space age started, rockets and satellites have been sent to space for several missions of telecommunication, remote sensing, navigation, meteorology, climate research and human-space exploration. Most of these objects have lost their originally activity over time, but still orbiting out of control around the Earth or reentering the atmosphere as space debris [1]. In fact, nowadays only about 24% of the cataloged objects are satellites (less than a third of which are operational), and about 18% spent upper stages and mission-related objects [2]. Both active satellites and space debris in orbit around the Earth, are defined as resident space objects. Space debris represent a serious risk for every active satellites or spacecraft, because collisions between in-orbit objects can occur from every direction with speed up to 10 km/s. The fragments created by a collision can generate a cascading process, known as the \"Kessler syndrome,\" in which each collision between objects generates more space debris, which increases the probability of further collisions and consequently it could create new debris [2, 3, 4]. Moreover, during the re-entering process, most space debris burn up in the atmosphere, but larger objects can reach the ground intact. The monitoring of all resident space objects is necessary to protect valuable space assets and, consequently, characterizing them is an essential task for space objects classification and risk assessment. In this context, the availability of efficient techniques for accurate estimation of motion and orbit parameters is extremely important. In general, the most densely populated orbit is the Low Earth Orbit (LEO), between 200 km and 2000 km of altitude, where pieces of debris are denser than meteoroids. The United States Strategic Command (USSTRATCOM), which benefits from its Space Surveillance Network (SSN) [5], maintains the largest database in the world of cataloguedobjects in LEO [6], with about 24,000 trackable objects with sizes greater than 10 cm [7]. Periodically, about every 8 h, the United States Joint Space Operations Center (JSpOC) analyzes the data collected by the SSN and performs orbit determination using its software based on Simplified General Perturbation 4 (SGP4), with the aim to create a file with a universal suitable format as well-known as the Two-Line Element set (TLE), which contains the orbital parameters of the object under consideration [8; 9]. Public release of the TLE catalog has been accomplished for many years, first through NASA, and more recently through the Space-Track web site [10]. In addition, Celestrak has maintained a web site for obtaining the TLE catalog for several decades [11]. In 2015, with the aim to align with the United States, the European Commission started a dedicated framework for European Space Surveillance and Tracking (EUSST) in the space situational awareness program [12]. The SST services assess the risk of in-orbit collisions and uncontrolled re-entry of space debris into the Earth's atmosphere, and detect and characterize in-orbit fragmentations. Within this framework, a network of dedicated sensors (radars, telescopes and lasers) has been designed. Lasers and optical telescopes are employed for observations in Medium Earth Orbit (MEO), between 2000 km and 35,786 km of altitude, and in Geosynchronous Equatorial Orbit (GEO), above 36,000 km. Whereas, the principal technique used for the LEO debris monitoring is based on radar systems. The analysis of the state-of-the-art shows that Europe can count on a large network of radar sensors with different technical features. The European Incoherent Scatter Scientific Association (EISCAT), for instance, operates three incoherent scatter radar systems, at 224 MHz, 931 MHz in Northern Scandinavia and one at 500 MHz on Svalbard [13; 14]. All the EISCAT radars work in beam-parking mode and they are able to provide a peak power of few Megawatts, with a duty cycle of about 10-20% [14].