# Role of Radio Telescopes in Space Debris Monitoring: Current Insights and Future Directions
## 4 Radio Telescope Measurements with Illuminators of Opportunity
### Murchison Widefield Array  
The Murchison Widefield Array (MWA), a member of the trio of SKA Precursor telescopes, is situated at the Murchison Radio-astronomy Observatory in the Murchison Shire, located in the mid-west of Western Australia. This site was specifically chosen because of its exceptionally low radio-frequency interference. Operating in the low radio frequency range of 80-300 MHz, the MWA features a processed bandwidth of 30.72 MHz per linear polarization. It comprises 128 aperture arrays or tiles spread across an area with a diameter of approximately 3 km, as illustrated in Figure 14[17; 75]. The primary objectives of MWA include detecting neutral hydrogen from the epoch of re-ionization, investigating Earth's ionosphere, and creating maps of both galactic and extragalactic radio sources. Earlier demonstrations used MWA to detect ISS and the moon using reflected FM radio emissions, but observations were limited to angular domain (i.e., right ascension and declination) only [76; 77]. The experimental campaign conducted in April 2015 used both the MWA and a Defense Science and Technology group line-of-sight receiver in Adelaide. The primary target was the ISS, which was illuminated by nearby FM radio transmissions, as described in the scenario outlined in [78]. Researchers in [78; 79] developed a passive radar signal processing technique to derive the bistatic range and Doppler shift with a coherent processing interval of 1 s. This technique improved the detection performance by 25-40 dB compared with the previous case where only angle observations were possible [80]. Additionally, azimuth and elevation measurements were derived through interferometric processing. These four measurements were combined to perform IOD and tracking by using the Markov chain Monte Carlo algorithm. Figure 14: Layout of the Murchison Widefield Array (MWA). Each receiver is represented by a blue marker. The array consists of multiple tiles spread across the site to form a large interferometric baseline. Reproduced from [17]. [PERSON] et al. [81] introduced the concept of Orbit Determination Before Detect (ODBD) for MWA, which constrains the search space to realistic orbital trajectories and improves detection efficiency. This algorithm incorporates orbital parameters into the radar ambiguity function and applies matched filtering to single-beam radar systems. The authors further validated the algorithm by applying it to a real measurement campaign at MWA conducted in 2018 and comparing it with the standard IOD method, such as Herrick-Gibbs, demonstrating the practical implementation of the algorithm, which further enhances the accuracy of debris detection [82]. While the authors proposed ways to constrain the search space, it may still require significant processing power for practical implementation. Another measurement campaign was conducted in December 2019 using four transmitters (Geraldton, Perth, Albany, Mount Gambier) [17], and one of the detected targets, COSMOS 1707, was tracked for almost 90 s. The tracking results from the multistatic measurements are shown in Figure 15, with the top row as the covariance of position and velocity, and the bottom row shows the error estimates of position and velocity in comparison with the TLE ephemeris. The authors also mentioned that these results are typical for most of the objects detected at the MWA in a bistatic configuration. The measurements were combined at the orbit determination stage, and it can be seen in Figure 15 that the joint result from the Perth and Albany transmitters shows significant improvement in both position and velocity covariance. The mean error values, especially in the velocity, were reduced, which shows that the orbit accuracy improved with the combined data. In the same campaign, the tracking results of other targets, OPS 5721 (9415) and NADEZHDA (25567), are also published [17].