# Role of Radio Telescopes in Space Debris Monitoring: Current Insights and Future Directions
## 1 Introduction
### Structure of the Review  
The aim of this section is to clarify the boundaries of the review and explain how the content is organized. Firstly, the suitability of radio telescopes for space debris monitoring, along with their advantages, is discussed in Section 2. Secondly, based on the available  
Figure 1: Locations of radio telescopes around the world involved in past space debris monitoring campaigns. Blue squares represent systems used with active narrow beam transmitters, while red square indicate passive systems that utilize illuminators of opportunity as transmitters (generated using MATLAB R2024a, www.mathworks.com; accessed on 6 March 2025).  
literature, the radio telescope experiments have been categorized into two main state-of-the-art configurations, classified by the type of transmitter involved.
* Active systems are those in which the radio telescope acts as a receiver in conjunction with a separate high-power narrow-beam radar transmitter. These systems emit controlled signals and analyze the reflections from space objects. However, the narrow beams of the radar and receiver limit the coverage area. This is consistent with the operation of most space-surveillance radar systems that use single narrow beams or fence configurations. One example of this kind of system is the Effelsberg radio telescope with Tracking and Imaging Radar (TIRA) as the transmitter [16]. Figure 2a illustrates an active bistatic configuration of the TIRA-Effelsberg system, where a narrow beam transmitter TIRA illuminates the target and the Effelsberg radio telescope captures the reflected signal.
* Passive bistatic radar systems rely on existing sources of illumination, such as FM and DVB signals. Unlike active radar systems, passive radar does not require a dedicated transmitter, making it the most cost-effective alternative. In this setup, single or distributed arrays of radio telescopes are paired with broadband transmitters located at significant distances. The wider field of view of the receiver, combined with the broad broadcast coverage of the transmitter, results in a significantly larger surveillance volume. Although this setup decreases the power concentration on space objects, it offers the benefit of a longer detection time for space debris, allowing precise orbital calculations without requiring previous data. Moreover, a distributed ground broadcast network allows several simultaneous sources of illumination. One such example is the Murchison Widefield Array (MWA) in Australia [17]. Figure 2b illustrates a passive radar configuration, where MWA functions as a receiving system capturing signals reflected from a space object, using non-cooperative transmitters (illuminators of opportunity).  
The structure of the following sections reflects this division, with separate discussions on experiments with active transmitters and passive setups using illuminators of  
Figure 2: Types of configurations based on the nature of the transmitter used: (**a**) An example of a narrow-beam active transmitter system: TIRA used as the transmitter and Effelsberg as the receiver, reproduced from [16]. (**b**) An example of a passive configuration using illuminators of opportunity: Murchison Widefield Array (MWA) operating as a receiver with external transmitters, reproduced from [17].  
opportunity. Section 3 reviews experiments conducted with narrow beam transmitters, and Section 4 then discusses experiments carried out involving illuminators of opportunity. Figure 3 illustrates the structure of the review with classification of radio telescope experiments based on the type of transmitter used and lists the notable telescopes involved in experiments conducted under each configuration. Lastly, the conclusions and discussion are presented in the final section of the article. In summary, this article discusses the outcomes of the experiments conducted and presents a broad overview of the current state of research, along with a discussion of future scope.