# Role of Radio Telescopes in Space Debris Monitoring: Current Insights and Future Directions
## 3 Radio Telescope Measurements with Active Transmitters
### Italian Multistatic Radar  
A multibistatic radar system consists of multiple spatially separated antennas, typically with one serving as the transmitter and several others as receivers, or vice versa. The spatial diversity provided by such configurations enhances detection, tracking, and IOD capabilities. In recent years, the Radar and Surveillance Systems National Laboratory in Pisa initiated simulation-based studies on multibistatic radar systems. [PERSON] et al. [27] proposed a configuration involving three Italian radio telescopes: Medicina, NOTO, and SRT. The Medicina observatory is the same one that is used in the BIRALES experiment, while SRT is the receiver in BIRALET [48]. The third radio telescope, NOTO, located in Sicily, is operated by the Istituto di Radioastronomia di Bologna. A simulation tool was developed to evaluate system performance for SSA missions under various scenarios. To manage system complexity, the authors proposed decentralized processing with track fusion, and addressed synchronization challenges using GPS-based clocks [28]. Different signal processing techniques were implemented to extract range and Doppler measurements by combining data from multiple sensors. Consequently, IOD was performed using a multilateration technique, while orbit tracking was carried out using an Unscented Kalman Filter (UKF). Additionally, the system was designed to associate newly detected tracks or correlate them with existing ones. Figure 9a shows the target ground track on a flat Earth crossing over considered sensors, and Figure 9b presents the tracking results in Earth-Centered Earth-Fixed (ECEF) coordinates, comparing the true trajectory (blue), multilateration output (red), and UKF-based estimation (green). In a recent study [59], the same research group demonstrated that the proposed multibistatic configuration can significantly enhance tracking performance. Their findings underscore the potential of this architecture to support not only space situational awareness (SSA) but also mission planning and navigation for low-thrust transfer trajectories.  
Interestingly, these three Italian radio telescopes (Medicina, NOTO, and SRT), along with the Effelsberg radio telescope in Germany, have demonstrated the capability to detect Near-Earth Objects (NEOs) in a collaborative experiment with NASA [60]. This effort was part of a European Space Agency (ESA) project aimed at defining functional requirements for a future planetary radar system and assessing the suitability of existing European infrastructure for NEO observations. In parallel, the Planetary Radio Interferometry and Doppler Experiment (PRIDE), also implemented in ESA's JUICE mission, represents a sophisticated evolution of multibistatic radar concepts [61]. PRIDE uses VLBI by coordinating globally distributed radio telescopes to derive high-precision spacecraft state vectors. Although originally developed for deep-space and planetary science applications, the PRIDE framework presents a promising approach for long-baseline, passive multibistatic tracking of high-altitude targets, such as GEO satellites. Although NEO and planetary surveillance are important applications of SSA, they lie outside the scope of this paper. Its potential for future extension to space debris monitoring is particularly relevant, as similar concepts inspire the long-baseline GEO surveillance approach discussed in the following section.