# Crowded Space: A Review on Radar Measurements for Space Debris Monitoring and Tracking
## 4 European Countries' Radar Measurements  
After US radar sensors, the most significant amount of work retrievable in the open literature about radar measurements of space debris is unquestionably related to the European countries. The European Incoherent Scatter Scientific Association (EISCAT) system is the primary source of this material. The EISCAT consists of three separate radars: the monostatic VHF radar, located near Tromse (Norway); the monostatic VHF double antenna EISCAT Svalbard Radar (ESR) located in Longyearbyen, Svalbard; and the tristatic EISCAT UHF radar with transmitter located in Tromse; and receivers located in Tromse, Kiruna (Sweden), and Sodankyla (Finland) [33]. Among these three radars, the most used for space debris activities are the ESR and the EISCAT UHF radar. The ESR consists of two parabolic dishes, a steerable 32 m and a fixed 42 m antenna with gains of 42.5 and  
Figure 14: Results of the 1989 Arecibo experiment compared with a prediction model for small-sized debris [30]. Reprinted from Geophysical Research Letters, Copyright 1992, with permission from [PERSON] Wiley and Sons. Figure 15: Results of the 2005 AMISR-PFISR space object detection experiment [32]. Reproduced with permission from [PERSON] of SRI International and from the 2015 Advanced Maui Optical and Space Surveillance Technologies Conference (AMOS), Maui, Hawaii—www.amostech.com. 45.3 dBi, respectively [34; 35]. The radar, operating at 500 MHz, is characterized by a single transmission system with a peak power of 1 MW and a duty cycle of 25%, and it can be switched to either of the two antennas on a pulse-to-pulse basis. The ESR was employed for a two-year campaign during the International Polar Year (IPY) from March 2007 to March 2009, attempting to increase the canonical 2000 h per year dedicated to space debris observation to a continuous coverage [34]. Within this campaign, the EST was able to record 5060 h detecting 23,9000 events. From this huge pool of events, a Quality Set (QS) was extracted, consisting of 101 complete 24 h debris observations in beam-park mode. The QS was comprised of measurements made from March 2007 to February 2008, including 95,000 events. The effective sensitivity of the ESR within the QS is indicated by the smallest objects detected at various altitudes, in this case 3.5 cm-sized debris in the 700-1000 km altitude range, and 5 cm-sized debris in the 1300-1600 km altitude range. A time-altitude distribution of the events at the beginning and at the end of the QS during the IPY campaign is given in Figure 16 [34]. Two big clusters can be seen in Figure 16b at about a 1300 km altitude, characterized by a day to day drifting, as they do not represent a sun-synchronous ring [34]. The EISCAT UHF radar, in contrast, is characterized by a 32 m diameter fully steerable parabolic dish working at 929 MHz, with Cassegrain optics, which represents the transmitting segment [33; 35]. The EISCAT UHF transmitter consists of a programmable radar controller that generates the pulse patterns at DC level, later converted in RF signals and amplified by two klystrons to reach a peak power of 2.5 MW. However, space debris measurements' power supply is usually in the range of 1-2 MW [33; 35]. The receiver used for space debris measurement is also connected to the EISCAT UHF radar (in this case the radar consists solely of the monostatic parabolic antenna in Tromse), and is a special digital receiver back-end capable of capturing a large and complex amount of analog IF data via a single digital stream [33; 35]. In this configuration, the EISCAT UHF radar was employed in 2005 in two different campaigns, for an ESA contract: the Manda campaign, 11-13 August and 17-20 November 2005, and the Cp2 campaign, 7-29 September 2005. The radar collected almost 700 h of measurements, detecting about 800 debris events [35]. Both campaigns were performed in beam-park mode. Figures 17 and 18 show the results of the double 2005 campaign.