# Role of Radio Telescopes in Space Debris Monitoring: Current Insights and Future Directions
## 3 Radio Telescope Measurements with Active Transmitters
### Birallet  
BIRALET operates in the P-band at 410-415 MHz, has a bistatic configuration composed of the same transmitting antenna used in BIRALES and SRT as the receiver, with a baseline of approximately 20 km [13]. The SRT is a 64-m fully steerable wheel-and-track antenna located near San Basilio (Cagliari, Sardinia, Italy). In the initial phase, SRT partic  
\begin{table}
\begin{tabular}{l l l l l l} \hline  & \(a\) [km] & \(e\) & \(i\) [deg] & \(\Omega\) [deg] & \(\omega\) [deg] \\ \hline BIRALES 1 & 6530.3 & 0.020 & 41.5 & 77.4 & 179.6 \\ MFDR-MR & 6533.6 & 0.003 & 41.6 & 82.3 & 158.2 \\ BIRALES 2 & 6517.4 & 0.004 & 41.6 & 82.1 & 172.6 \\ \hline Reference & 6516.1 & 0.002 & 41.6 & 82.8 & 201.0 \\ \hline \end{tabular}
\end{table}
Table 1: Keplerian parameters for the Orbit determination results obtained on 4 November from BIRALES and MFDR observations, along with parameters computed from the last available TLE [53]. Reprinted from Advances in Space Research, Copyright 2025, with permission from Elsevier.  
pated in the debris monitoring campaign in 2014 jointly with the Northern Cross radio telescope and was able to detect six objects, as listed in Table 2. The authors described three possible hardware configurations of the system to perform Doppler shift and range measurements [25]. The first configuration utilizes the spectrum analyzer as a backend that permits Doppler shift measurements, whereas the second configuration is based on the electronic Red Pitaya board. The final configuration uses the National Instruments USRP board as a back end. The hardware has been constantly upgraded to perform space debris monitoring, including the re-entry of the Chinese space station Tiangong-1 together with other Italian SST sensors in 2018 [57; 58].  
Between 13 December 2018 and 10 October 2019, the BIRALET system was able to observe a total of 33 objects in the beam-parking mode using the dedicated back-end. The estimated and measured Doppler shifts for the detected debris between 13 December 2018 and 8 October 2019, can be found in the following literature [25; 26]. Figure 8 shows a scatter plot of the RCS versus the range for the detected objects with respect to the minimum detectable size using the BIRALET system. These measurement campaigns serve as a solid foundation for monitoring space debris using radio telescopes.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Object ID** & **Name** & **Doppler (Hz)** & **Altitude (km)** & **Slant Range (km)** & **RCS (m\({}^{2}\))** \\ \hline
22565 & COSMOS 2237 (First passage) & not measured & 853.6 & 1732 & 11.6 \\ \hline
22565 & COSMOS 2237 (Second passage) & \(-\)5200 & 852.7 & 2836 & 11.6 \\ \hline
33320 & HJ-1A (First passage) & 6200 & 628.7 & 3640 & 1.5 \\ \hline
33320 & HJ-1A (Second passage) & 4600 & 629.3 & 1832 & 1.5 \\ \hline
32783 & CARTOSAT 2A & 5200 & 629.4 & 2056 & 2.3 \\ \hline
13552 & COSMOS 1408 & not measured & 523.7 & 1093 & 8.4 \\ \hline
16206 & COSMOS 1375 & 3400 & 984.8 & 2326 & 0.5 \\ \hline
38047 & VESSELSAT 2 & 5200 & 460.7 & 1664 & 0.3 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Summary of the debris measurements by Bistatic Radar for LEO Tracking (BIRALET) in 2014, using the Sardinia Radio Telescope (SRT) as the receiver. During this campaign, six objects in LEO were successfully detected. Reproduced from [13].  
Figure 8: Scatter plot of radar cross-section (RCS) versus range for the detected objects, with reference to the minimum detectable size of the BIRALET system. Reproduced from [26].