# Crowded Space: A Review on Radar Measurements for Space Debris Monitoring and Tracking
## 4 European Countries' Radar Measurements
The main cluster around 900 km corresponds to a 65\({}^{\circ}\) inclination which indicates the RORSAT orbit; thus, it was probably an indication of the NaK droplets. Figure 19: Target count vs. RCS for EISCAT UHF radar 2010 campaign [36]. © 2012 IEEE. Reprinted, with permission, from [PERSON]; [PERSON]; [PERSON]; [PERSON] Observation debris RCS with the tristatic radar. 10 th International Symposium on Antennas, Propagation and EM Theory, Xi’an, China, 22–26 October 2012. New campaigns were planned and undertaken in 2000 and 2001, this time with improved data-processing capabilities thanks to the extension in the observation windows of the radar, covering altitudes from 350 to 2000 km [39]. During the 24 h 2000 campaign, nearly 500 objects were detected. The results of the campaign, in terms of altitude vs. Doppler inclination, are reported in Figure 6 of [39]. In that figure, the very same cluster of NaK droplets detected in the 1996 COBEAM experiment can be spotted at 900 km altitude, and, in addition, the Globalstar constellation (a satellite constellation for mobile phone and low-speed data communications), can be spotted at 1400 km altitude. The TIRA-Effelsberg system is one of the major staples in space debris monitoring for Europe and for Germany, and it will be soon joined by the German Experimental Space Surveillance and Tracking Radar (GESTA), a dedicated L-band phased-array radar capable of detecting small objects at altitudes below 3000 km [40]. In recent times, the European space debris network has been enhanced by the addition of two new radar sensors located in Italy: the Bistatic Radar for LEO Survey (BIRALES) and the Bistatic Radar for LEO Tracking (BIRALET). The two systems share the same transmitter, the Radio Frequency Transmitter (TRF), a 7 m dish wheel-and-track steerable parabolic antenna, located in the Italian Joint Test Range in the region \"Salto di Quirra\" (Cagliari, Sardinia, Italy) [41]. The transmitter can supply a peak power of 10 kW within the bandwidth 410-415 MHz. The receiver of the BIRALES system is the Northern Cross, a UHF (408 MHz) T-shaped array transit antenna composed of two major branches. The East/West (E/W) branch is 564 m long and it is composed of a single 35 m wide cylindrical  
Figure 20: Detection rate vs. altitude for the monostatic and bistatic configuration (**a**) and altitude vs. Doppler inclination (**b**) for the 1996 COBEAM experiment [38]. Reprinted from Advances in Space Research, Copyright 1999, with permission from Elsevier. antenna, whereas the North/South (N/S) branch consists of 64 parallel antennas, each one characterized by a total length of 23.5 m and width equal to 7.5 m [41; 42]. The actual portion of the overall antenna which is devoted to the BIRALES system includes 16 parabolic cylindrical antennas of the N/S branch, renamed BEST-2, that provides a collecting area of about 2800 m\({}^{2}\) and a Field of View (FoV) of 6.6\({}^{\circ}\)\(\times\) 2.2\({}^{\circ}\)[41]. The Northern Cross is located at the Medicina Radioastronomical Station, near Bologna (Italy). The first experiment with the BIRALES was conducted in 2014, when the bistatic radar was able to spot around 20 objects in the altitude range 300-850 km. The observations were made in a sort of beam-parking mode, meaning that the list of objects to detect was predetermined (and provided by the Italian Air Force) and each time the antenna was pointed at the object coordinates waiting for its passage [43]. Table 1 shows a summary of the experimental measurements performed by the BIRALES in 2014, reporting altitude, slant range, RCS, and measured Doppler frequency. Thanks to its ability to rapidly switch from multibeaming observation mode to ranging mode, the Northern Cross is able to refine orbital parameters of known objects or to perform a preliminary orbit determination [44; 45]. The Northern Cross is not the only antenna located at the Medicina Facility.