# Role of Radio Telescopes in Space Debris Monitoring: Current Insights and Future Directions
## 3 Radio Telescope Measurements with Active Transmitters
### Long Baseline Bistatic Radar (LBBR)
RD analysis was performed, and four targets were detected using all three antennas, as can be seen in Figure 11. The study also highlighted the challenges and complexities of long-baseline bistatic radar, including timing synchronization, phase coherence, and the need for an accurate target ephemeris. To the best of our knowledge, orbit determination results are not included in the study. The same RTG performed monostatic and bistatic radar imaging experiments on real on-orbit tumbling rocket bodies by utilizing the same network of transmitters and receivers [64; 65; 66; 67]. These studies demonstrated advanced bistatic Doppler characterization across diverse imaging geometries and highlighted the successful implementation of specialized Doppler processing techniques, such as the Doppler superpulse algorithm. The study in [64] also outlines the methodology for accurately determining the rotation periods and maximum observed length extent of tumbling objects in space. Figure 12 displays the bistatic Doppler tomography reconstructions for the Atlas 5 Centaur rocket body in the L-band with an 8 MHz bandwidth based on WSRT data collection. The PP and OP channels are shown in Figure 12a,b, respectively. Table 4 compares the rotation period estimates for Delta-4 and Atlas 5, derived from multiple receivers and different processing methods. Recent studies have presented a novel range-Doppler-time (RDT) tensor processing technique for GEO characterization with narrow-band radar, which provides detailed Doppler information [68]. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Name** & **Country of Origin** & **Peak Power** & **Operating Frequency** \\ \hline TIRA & Germany & 1.6 MW & 1.33 GHz \\ \hline Millstone Hill Radar & USA & 3 MW & 1.295 GHz \\ \hline Haystack Radar & USA & 400 kW & 10 GHz \\ \hline Goldstone Radar & USA & 440 kW & 8.56 GHz \\ \hline EISCAT \({}^{*}\) UHF Radar & Sweden & 2 MW & 930 MHz \\ \hline EISCAT Svalbard Radar (ESR) & Norway & 1 MW & 500 MHz \\ \hline \hline \end{tabular}  
* European Incoherent Scatter Scientific Association.  
\end{table} 
Table 3: List of high-power radar transmitters with the capability to detect GEO targets. The table includes radar systems that are either currently used for GEO monitoring or are suitable for such applications. Figure 11: Range—Doppler map generated from the TIRA–Knockin bistatic radar data using a CPI of 512 pulses. The map clearly reveals four distinct targets detected during the January 2020 experimental campaign [62]. Figure 10: **(a,b)** Range—time intensity plots for objects 43039 and 27683, recorded during a single CPI in January 2020 using the MHR transmitter and WSRT receiver. These plots show signal strength variations over time and range. (**c,d**) Corresponding range–Doppler (RD) maps for the same targets. The origin (0,0) in each map is aligned with the expected range and Doppler shift of the respective object, aiding in accurate target identification and motion characterization [30]. In another study, [PERSON] et al. [69] discussed the performance analysis of ground-based long baseline radar for SSA by examining targets at different altitudes. The study examines the performance of a multistatic system considering one transmitter in three scenarios: a cluster of receivers, receivers spread throughout the world, and a combination of both. The authors used the multiple-input-multiple-output (MIMO) ambiguity function to analyze the system's performance, and simulations were performed for targets in LEO and GEO to check the feasibility of the proposed radar systems.