# Crowded Space: A Review on Radar Measurements for Space Debris Monitoring and Tracking
###### Abstract  
Space debris monitoring is nowadays a priority for worldwide space agencies, due to the serious threat that these objects present. More and more efforts have been made to extend the network of available radar systems devoted to the control of space. A meticulous review has been done in this paper, in order to find and classify the considerable amounts of data provided by the scientific community that deal with RADAR measurement for the debris monitoring and tracking. The information gathered is organized based on the volume of found data and classified taking into account the geographical location of the facilities.  
space debris; SSN; radar measurements +
Footnote †: journal: Applied Sciences  
0000-0002-2999-2000-2999-2000-2999-2000-2999-2000-2999-2000-2999-2000-2999-2000-2999-2000-2999-20000-2999-2000-2999-20000-2999-20000-2999  
Center (CSpOC, former known as JSpOC) acts as coordinator for the large amount of data coming from the SSN, elaborating the orbital parameters and making them available in a suitable format, i.e., a two-line element set (TLE). Following the steps taken by the US, other countries have started space surveillance programs, coordinated by their respective space agencies, like Roscosmos for Russia, the Japan Aerospace Exploration Agency (JAXA), and the European Space Agency (ESA) for the European countries, just to name some prominent examples [5; 6; 7]. It is worth clarifying that the data collected from space debris measurements are usually not the prerogative of a single country but are shared for the sake of international cooperation. However, since space debris is a relatively new topic in the field of research, there are very few works in the scientific literature that address results deriving from a space debris campaign, and most of them are conference papers. As a consequence, an extensive and complete review of the measurement campaigns performed by the space surveillance sensors is still missing from the open literature.  
Since it would be overly complicated considering every type of sensor for space debris detection, i.e., radar, optical and laser, this review aims to accomplish at least part of this task, taking into account only the radar sensors. An overview of radar principles and configurations typically used in space debris monitoring and tracking is reported in Section 2.  
This review paper aims to gather information about the radar measurements of space debris performed by worldwide space agencies, and make them available in a coherent and comprehensible manner, focusing on the characteristics of the radar sensors and on the key results of the measurement campaign. The results are divided in three major categories, based on the volume of the available material in literature, the first one being devoted to the US SSN, the second one to the European countries' radar measurements, and the third and last one to the efforts made by other countries. This classification choice aims to identify a specific sensor within a wider network, and to highlight its belonging to the local space agency and/or government organization, thus helping the reader to understand the operating context of these sensors.