# Role of Radio Telescopes in Space Debris Monitoring: Current Insights and Future Directions
## 5 Conclusions and Future Directions  
An extensive review of past space debris monitoring campaigns using radio telescopes is conducted in this study. The challenges of monitoring space debris are compounded by the need to track fragments as small as 5 mm, as even at this size, they can cause significant damage to active satellites. Globally, more than 50 radar systems are currently dedicated to monitoring space objects and debris, and efforts are underway to expand this network of radar systems designed for space surveillance. Among these, bistatic radars with radio telescopes provide a balanced approach that combines high sensitivity and cost-effectiveness for space debris monitoring. In this study, an important perspective is explored by organizing the debris monitoring campaigns into two segments based on the type of transmitter used. A comparative summary of the radio telescopes discussed in this study is available in Table 5, listing relevant attributes including location, setup, transmission features, and detection performance. The analysis presented in Table 5 and the review of existing literature reveal that more campaigns have been conducted with active transmitter configurations, likely due to the advantage of higher transmitted power. Nevertheless, opportunistic transmitters are also favored because of their all-time availability and long exposure durations. This study included measurement campaigns for both LEO and GEO orbital regions, but most of the campaigns were conducted to monitor the LEO orbit, as it is highly populated with space debris. However, monitoring GEO targets is particularly challenging because of their large distance from the Earth, and it involves coordinating multiple countries to establish the necessary bistatic configuration and requires high transmission power, as emphasized in Section 3.7. Some of the radio telescopes discussed also operate in a multistatic configuration, which, despite introducing additional system complexity, significantly enhances overall sensitivity. This improvement, along with the transition from bistatic to multistatic configurations, provides advantages in terms of precise localization and measurement ambiguity reduction, as highlighted in Sections 3.6, 3.7 and 4.1 (indicated by \({}^{\dagger}\) in Table 5). Figure 16: (**a**) A zoom on cross -ambiguity function (CAF) near-zero bistatic range and velocity before direct-path interference (DPI) and clutter cancellation. (**b**) A zoom on cross-ambiguity function (CAF) near zero bistatic range and velocity after direct-path interference (DPI) and clutter cancellation, showing improved target signal clarity in the bistatic domain. The data were obtained using the LOFAR PL610 station with a DVB-T transmitter as an illuminator of opportunity for ISS detection [92]. Reprinted from IET Radar, Sonar, and Navigation, Copyright 2023, with permission from John Wiley and Sons. There are a few limitations associated with conventional radio telescopes for space debris monitoring, and they have been highlighted in this manuscript. A common issue encountered when using radio telescopes is obtaining precise angular measurements, as most radio telescopes provide only range and Doppler measurements. Determining the orbit of an uncataloged object using only these two types of measurement can be challenging. However, some of the radio telescopes discussed here have addressed this problem through unique designs such as multibeam antennas and advanced data processing algorithms, as detailed in Sections 3.4 and 4.1, respectively. Single-beam radio telescopes still face difficulties in deriving accurate angular measurements, which could be an area of future research and improvement. Several studies [11; 97; 98; 99] suggested that integrating space-based radar and radio telescopes with existing space situational awareness programs can significantly improve debris tracking and collision avoidance strategies.