# 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)  
The study, conducted by the NATO STO Research Task Group (RTG), investigated the use of existing monostatic radars as transmitters in combination with large-aperture radio telescopes as bistatic receivers for GEO monitoring [30; 62]. This approach aimed to enhance the sensitivity of object detection at GEO distances. Monitoring geostationary satellites poses significant challenges due to their considerable distance of approximately 36,000 km from Earth. Addressing these challenges requires high-power radars, typically in the megawatt (MW) range, as well as international collaboration to establish the necessary bistatic configuration for effective detection and tracking of objects at GEO altitudes. Table 3 lists high-power radars that are either currently used for GEO monitoring or have the capability to track GEO targets due to their substantial transmission power. Only radars that can be configured in a bistatic setup with radio telescopes are included. Other high-power phased array radars, such as the Cobra Dane phased array radar with a peak power of 15.4 MW, are not listed here. However, the full list can be found in [15]. The experiments conducted by RTG involved two transmitting radars: the Millstone Hill Radar (MHR) in the USA and TIRA in Germany. These transmitters were paired with multiple receiving radio telescopes in Europe, including SRT in Italy, Westerbork Synthesis Radio Telescope (WSRT) in the Netherlands, and multiple antennas of the e-MERLIN (enhanced MultiElement Remotely Linked Interferometer Network) array in Manchester, United Kingdom. The research team conducted several experiments between 2020 and 2021, targeting known geostationary satellites, to test the feasibility and performance of the bistatic radar setup. In such scenarios, a large-aperture radio telescope is the best option to increase the sensitivity of the system, especially when dealing with large-distance detection. Consequently, the results demonstrate successful continental baseline coherent bistatic radar collections of GEO targets. It should be noted that there was no synchronization between the transmitters and receivers, and the only available time synchronization was provided by GPS, which was presumed sufficient for these experiments. Figure 9: **(a)** Ground track of the target as it passes over the Italian radio telescopes—Medicina, NOTO, and SRT. **(b)** Three-dimensional global tracking results of the target’s truth path (blue), IOD-multilateration path (red), and UKF tracking path (green), demonstrating tracking accuracy and deviations [27]. Reprinted with permission from IEEE Aerospace and Electronic Systems Magazine, Copyright 2020, under license from IEEE. The team was able to perform range-Doppler (RD) processing after performing coherent integration for a long period and then determine the target RCS. The experimental outcomes for two targets (ID 43039 and 27683) in January 2020 using the MHR-WSRT combination are displayed in Figure 10. Specifically, Figure 10a,b present the range-time intensity plot, whereas Figure 10c,d show the RD maps for the same targets. It is important to note that the origin (0, 0) of the RD maps aligns with the expected target range and Doppler measurements. The bias in the range value plots could be a result of the timing difference between the transmit and receive site clocks and the TLE error, which can reach up to 25 km in the along-track direction and 10 km in the cross-track direction for GEO objects [63]. The observed Doppler offset may have resulted from an unmatched bistatic Doppler during pulse compression, leading to a residual phase ramp across the pulses. The research team also provided measurement results from the monostatic radar TIRA and bistatic results combined with the e-MERLIN radio telescope. During this measurement campaign, three antennas (Knockin: 25 m, Pickmere: 25 m, and Cambridge: 32 m) of the e-MERLIN array simultaneously received the TIRA signal at 1.33 GHz in both polarizations.