# Crowded Space: A Review on Radar Measurements for Space Debris Monitoring and Tracking
## 5 Other Countries' Radar Measurements
Investigations on the suitability of the powerful transmitter in a bistatic configuration for debris measurements were conducted in the form of four trial experiments between 2001 and 2003 performed in Geostationary, Highly Elliptical and Medium Earth Orbits (GEO, HEO and MEO) [68]. During these experiments the RT-70 was employed in a differential interferometer radar technique using as a receiving segment other radars from the VLBI network, including Noto RT-32 (Italy), Torun RT-32 (Poland), Shanghai RT-25 (China), Urumqi RT-25 (China), and Bear Lakes RT-64 (Russia) [68]. The first experiment (23-29 May 2001) was focused on the detection of GEO large inactive objects with small inclination, tracked by the RT-70 and their echoes recorded on magnetic tapes at the receiving locations. An example of the echo registered at Bear Lakes is reported in Figure 25 showing the power intensity of the echo signal as a function of the observation interval. Figure 24: Range vs. RCS of satellite/debris detected by the MWA in late 2014 [66]. Copyright from [PERSON]; [PERSON]; [PERSON]; [PERSON] The Development of Non-coherent Passive Radar Techniques for Space Situational Awareness with the Murchison Widefield Array. Publications of the Astronomical Society of Australia 2020, 37, e010, reproduced with permission. Figure 25: Echo intensity of the object 1990-061A registered at Bear Lakes during the first differential interferometer experiment in 2001 [68]. Reprinted from Advances in Space Research, Copyright 2004, with permission from Elsevier. A total of seven objects were tracked and recorded during this campaign. A first successful attempt at the correlation processing of one specific object to retrieve the Doppler shift was also made; however, the measurement lacked of accuracy. The second experiment (14-19 December 2001) aimed at the observation of HEO and GEO objects with sizes smaller than 1 m. Of the six objects detected, two belonged to HEO and the rest to GEO. This campaign also served as a test for GEO beam-park mode between Evpatoria RT-70 and Bear Lakes RT-64. The third experiment (23-29 July 2002) included the first attempt to track objects in MEO and a second attempt of the _beam-park_ mode, this time between Evpatoria and Noto, but the main goal was the ground testing of a new real-time interface designed in Noto. Once again, the number of detected objects was seven: two in HEO and five in GEO. Moreover, a new correlation processing was tested, improving the frequency resolution of the measurements. The fourth and final experiment (23-29 July 2003) included the observation of GEO, HEO and MEO objects, and a third test on the beam-park mode in order to detect small fragments in GEO, involving Evpatoria, Bear Lakes, Noto, and Urumqi. The number of observed objects was 10. All the four campaigns displayed the results as registered echoes from the receiving station, in the same fashion shown in Figure 25. The overall number of detected objects (GEO, HEO, and MEO population) was 30. Regrettably, despite being probably the second most fruitful source on space debris information, there are no relevant published works representing the Russian radar network. Nonetheless, there are some clues suggesting that one of the major Russian radars, i.e., the Don-2N phased array radar [6], participated in one of the so-called Orbital Debris Radar Calibration Spheres (ODERACS) experiments in 1994 [69]. The ODERACS experiments were a series of tests, sponsored by NASA, conducted between 1992 and 1995. They consisted of the release of three pairs of highly polished metal spheres with different diameters for calibration purposes and in order to identify which radar facility was able to detect small-sized debris [69, 70]. The Don-2N phased array radar (a.k.a. Pill Box) is a 16 m diameter 4-face antenna working in S-band (4 GHz) with a peak power of 25 MW and an azimuth coverage of 360\({}^{\circ}\)[71]. This radar was part of the early warning antiballistic missile sites of the former USSR that became fully operational in 1989 [71].