# Crowded Space: A Review on Radar Measurements for Space Debris Monitoring and Tracking
## 6 Conclusions and Future Perspectives  
The topicality of the space debris problem forces worldwide space agencies to build or reinforce radar sensor networks. While US holds primacy in efficiency and extension of the SSN network, other countries are gearing up in order to face the planetary threat. In addition, the interest of researchers in the field is getting higher, despite there not being many works available in the open literature. In this respect, this review aims to gather and assemble as much information as possible on radar measurements on space debris, displaying them in an informative and understandable fashion. The findings provide interesting considerations about the topic of debris measurement. Firstly, it is common to note the requalification of many facilities (in particular, former US and Russian antimissile structures during Cold war), which are now dedicated to space debris detection. Another common trait is the different primary mission of the many radar facilities, since plenty of the analyzed structures are radio telescopes devoted mainly to radioastronomy, that embrace space debris detection uniquely as a secondary objective. Among the radar types investigated, one of the most used layouts is easily the parabolic reflector, given its high gain, but common solutions also include phased arrays, which avoid mechanical pointing by means of digital beamforming networks. In order to help the reader to easily navigate through the analyzed radar configurations, a summary table (Table 3) is presented, reporting name of the radar, geographic location, configuration, operating frequency, peak/average power, type of transmission, and minimum detectable size (if available). From the works herein presented, it can be argued that the majority of the space debris radar measurements are performed in LEO; this is due to two main reasons, the first one being that LEO is surely the most crowded orbit, and the second one being the fact that in order to allow the EM echo to travel back from the debris to the receiver the power budget must be considerably high (in the order of magnitude of tens of kW and higher). This means that reaching higher orbits would require even more transmission power. Measurements on space debris are carried out employing different radar configurations, such as monostatic (where the transmitter and the receiver are the same antenna), bistatic (where transmitter and receiver are separated), or multistatic (with multiple transmitting/receiving sites). The mode of the observation also changes depending on the necessity, thus a beam-park mode is used to detect a large number of objects, including uncatalogued ones, whereas the tracking mode is used to focus on a particular object and retrieve more information about its orbit. The results provided by the articles on the subject are not limited to the number of objects observed but include different important information, such as range, altitude, RCS, effective size, and Doppler, outlining a detailed profile of the resident space objects. To sum up, even in the scientific research field, the attractiveness of space debris is growing by the day, which is proven by the fact that new papers on the topic increased in number in the last two decades. This trend is expected to stay unaltered in the years to come. The radar monitoring of space debris is a rather complex and multifaceted process, which can be carried out in multiple ways, but the common objective is to define the current status of the debris population in order to mitigate the jeopardy associated with their presence in space. As a result, taking advantage of technological evolution, radar systems for space debris monitoring and tracking will have improved performance with the availability of new hardware and software components on the market.