# Crowded Space: A Review on Radar Measurements for Space Debris Monitoring and Tracking
## 4 European Countries' Radar Measurements
A fully steerable 32 m dish parabolic antenna is also present, equipped to operate in L-, S-, C-, X-, and K-band, and it is part of the European Very Long Base-line Interferometry Network (EVN-VLBI) [42]. The Medicina VLBI antenna was used in a joint bistatic experiment together with the Evpatoria RT-70 transmitter (later discussed in this paper) in July and November 2007 [46]. A continuous wave (CW), RHCP, high-power signal (20-40 kW) was transmitted within a beam of approximately 2.6 arcmin. Several suitable targets were selected (inactive geostationary satellites and known debris, among others), and a part of the experiment was dedicated to the detection of uncatalogued objects using beam-park mode. Unfortunately, in [46] no scatter plot or summarizing table is reported to show the complete campaign results, but only an example of one of the several echoes of unknown objects received, which is herein depicted as Figure 21 [46]. \begin{table}
\begin{tabular}{c c c c c} \hline _Object ID_ & _Doppler (Hz)_ & _Altitude (km)_ & _Slant Range (km)_ & _RCS (m\({}^{2}\))_ \\ \hline  
896 & 1497.73 & 3604.45 & 11.5 \\ \hline \end{tabular}
\end{table} 
Table 1: Summary of the debris measurements by the Bistatic Radar for LEO Survey (BIRALES) in 2014. The counterpart of the BIRALES system is the BIRALET, which employs a different receiving segment, represented by the Sardinia Radio Telescope (SRT). The SRT is a 64 m dish fully steerable wheel-and-track parabolic antenna, able to operate in the frequency range 0.3-116 GHz [47; 48], located 35 km north of Cagliari (Sardinia, Italy). Its azimuth and elevation maximum rotation speeds, 0.85\({}^{\circ}\)/s and 0.5\({}^{\circ}\)/s, respectively, allow for potential debris tracking. Jointly with the Northern Cross, the SRT was employed as a receiver in the same 2014 debris experiment [2]. In the same fashion, the list of debris to observe was predetermined, and the observations followed the same rules explained for the BIRALES experiment. Only six objects were detected (two of them shared with the BIRALES observations), some with multiple detections. This was due to the fact that the SRT, devoted mainly to radioastronomical observations, was not optimized for space debris detection. The results of the brief preliminary campaign are reported in Table 2[2]. The objects detected reside in the altitude range 460-980 km with variable RCS from 0.3 to 11.6 m\({}^{2}\). After this first experiment, the SRT was upgraded. In particular, a new space debris-dedicated channel was designed and installed to improve the detection capabilities of the radio telescope [48; 49]. The new receiving channel was first tested during the reentry process of the Chinese space station Tiangong-1 on 29 March 2018 [48]. On the French side, the most well-documented and prominent example of debris sensor is the Grand Reseau Adapte a la Veille Spatiale (GRAVES). The GRAVES is a CW VHF (143.05 MHz) bistatic radar, with its transmitter located near Dijon (France) and its receiver 400 km further south, in Provence [50; 51]. The transmitter is a phased array composed of four linearly polarized 30-element arrays (arranged in panels), each one covering 45\({}^{\circ}\) in azimuth and 30\({}^{\circ}\) in elevation. The layout of the panels allows a 180\({}^{\circ}\) azimuth coverage toward the south. The receiving segment is a 60 m diameter sparse array with 100 circularly polarized elements. The azimuth and elevation detections of the resident objects in the LEO are enabled by the digital beamforming (DBF) [50; 51]. Preliminary trials using only a portion of the phased array were performed between 1997 and 2001. The