# Role of Radio Telescopes in Space Debris Monitoring: Current Insights and Future Directions
## 2 Why Use Radio Telescopes?  
One of the main reasons behind using radio telescopes as receivers is their capability to detect small debris that could not be observed by traditional monostatic radars. This can be understood by the well-known radar Equation in (1) ([18] pp. 1.11-1.13).  
\[SNR=\frac{P_{t}G_{t}G_{r}\lambda^{2}\sigma}{(4\pi)^{2}R_{t}^{2}R_{r}^{2}LkT_{ sys}B} \tag{1}\]  
where \(P_{t}\) denotes transmitted power. \(G_{t}\), \(G_{r}\) are the transmitter and receiver gains, \(\lambda\) is the wavelength, \(\sigma\) is the radar cross-section (RCS) of the debris. \(R_{t}\), \(R_{r}\) are the range from the target to the transmitter and receiver, respectively, \(k\) is [PERSON]'s constant, \(T_{sys}\) represents the system noise temperature, \(B\) represents the bandwidth of the system, and  
Figure 3: This flowchart presents the overall layout of the review article, including classification of radio telescope experiments based on the type of transmitter used and the radio telescopes involved in the past experiments.  
SNR represents the signal-to-noise ratio. The benefits of using a radio telescope as a bistatic receiver are summarized as follows:  
1. The high sensitivity of radio telescopes arises from their large collecting areas. According to antenna theory, receiver antenna gain (\(G_{r}\)) in Equation (1) can be expressed as \[G_{r}=\frac{4\pi A_{eff}}{\lambda^{2}}\] (2) where \(A_{eff}\) denotes the effective reception area. The larger the effective area, the larger the signal-to-noise ratio. Hence, the large aperture size of the radio telescopes significantly enhances their detection range.
2. The lower system noise temperature (\(T_{sys}\)) of radio telescopes is another key factor contributing to their higher SNR, as it is inversely proportional. This improved sensitivity is achieved by cryogenically cooling the receiver front-end, including the low-noise amplifiers, to temperatures as low as 4 Kelvin using liquid helium. Such cryogenic cooling significantly reduces thermal noise and enhances the overall sensitivity of the system [19].
3. In a bistatic configuration, separating the transmitter and receiver enhances angular diversity, minimizing blind spots and improving detection capabilities. The total bistatic range, given by \(R_{b}=R_{t}+R_{r}\), provides greater flexibility in observation geometry. This configuration complements traditional monostatic radar systems by enhancing coverage and redundancy [20].
4. Radio telescopes can also contribute to debris characterization by analyzing radar cross-sections \(\sigma\) at multiple scattering angles, providing detailed information about the size, shape, and material composition of debris, as \(\sigma\) is a function of \(f(\theta_{inc},\theta_{sct},\phi,\lambda)\), where \(\theta_{inc}\) is the incident angle, \(\theta_{sct}\) is the scattering angle, and \(\phi\) is the polarization. This detailed profiling is vital for understanding the physical properties of debris and assessing its potential threats [21].
5. Interferometric capabilities from modern phased array radio telescopes such as the Square Kilometre Array (SKA) enable high-resolution imaging of space debris, allowing precise visualization of debris clouds and their spatial distribution.
6. Using these radio telescopes in a multistatic configuration becomes beneficial, as the systems already work in synchronization for Very Long Baseline Interferometry (VLBI) or similar projects.
7. Another benefit of using a radio telescope is to utilize the existing infrastructure, primarily designed for other purposes, which makes the overall system cost-effective.  
These features, coupled with the ability to leverage the existing infrastructure, make radio telescopes indispensable tools for enhancing SSA.