# Role of Radio Telescopes in Space Debris Monitoring: Current Insights and Future Directions
## 3 Radio Telescope Measurements with Active Transmitters
### BIRALES  
Both the BIRALES and BIRALET systems utilize a common transmitter, the Radio Frequency Transmitter (TRF), which consists of a 7 m dish wheel-and-track steerable parabolic antenna. The transmitter is situated in the Italian Joint Test Range within the region of Cagliari, Sardinia, Italy. TRF is capable of generating a peak power of 10 kW and operates with a 410-415 MHz bandwidth [43]. The receiving antenna is part of the Northern Cross radio telescope, which is currently one of the largest UHF-capable antennas worldwide [44], as shown in Figure 7. The components of the BIRALES receiver include eight parabolic cylindrical antennas located along the north-south (N-S) arm with a combined collection area of approximately 1400 square meters, enabling the detection of objects as small as 10 cm at a slant range of 2000 km [45]. BIRALES has achieved notable success in tracking debris owing to its higher sensitivity and strategic location in central Italy. The current 32-receiver multibeam configuration of BIRALES allows the angular track of the space object to be calculated to complement the bistatic range and Doppler measurements [46; 47]. This innovative technique outperforms conventional single-beam radar because when an object passes through the field of view of the radar, it is illuminated by radio waves, and the beam illumination sequence helps define the angular trajectory of the target, as explained in [46]. BIRALES began its measurement campaigns with initial trials in 2014, during which the bistatic radar successfully detected around 20 objects at altitudes between 300 and 850 km. A comprehensive summary of the experimental measurements performed by BIRALES in 2014 is reported in the comprehensive analysis by [PERSON] et al. [15]. BIRALES is designed to simultaneously receive Continuous Wave (CW) and chirp signals from the TRF for Doppler and range measurements, respectively [48]. With the help of the multibeam approach, the time profiles of the range, Doppler, and angular position can be used to perform initial orbit determination (IOD) for unknown objects and reconstruct the track [49]. Consequently, an IOD algorithm was developed, particularly according to BIRALES system measurements, to tackle the problem of element spacing longer than half the wavelength [44]. Researchers have reported a range accuracy of 50 m and an angular accuracy of \(10^{-3}\) deg, which were utilized by the orbit determination block to perform statistical orbit determination using measurement data from BIRALES during single- or multipass observations [50]. In 2018, BIRALES participated in tracking the re-entry of Tiangong-1 with observations conducted on 29 and 31 March. The re-entry time was predicted by propagating the state estimates, with the 31 March observation aligning well within the European Space Agency's forecasted window [51]. Figure 7: BIRALES receiving antenna, part of the Northern Cross radio telescope located at the Medicina Observatory in Bologna, Italy. It consists of eight parabolic cylindrical antennas arranged along the north—south arm depicted in red box, with a total collection area of approximately 1400 m\({}^{2}\)[44]. Reprinted from Acta Astronautica, Copyright 2020, with permission from Elsevier. Another study involving BIRALES radar observation and reconstruction of the Cosmos 1408 fragmentation event, which occurred on 15 November 2021, following an anti-satellite missile test, was conducted by a team from Politecnico di Milano, the Italian Space Agency, and the Italian National Institute of Astrophysics [52]. In a recent article [53], results from the third Long March 5 B re-entry campaign, which took place from 31 October to 4 November 2022, were derived. BIRALES observations were combined with measurements from the MFDR-MR sensor, a tracking radar involved in the campaign and managed by the Italian Air Force.