# Crowded Space: A Review on Radar Measurements for Space Debris Monitoring and Tracking
## 3 US SSN Radar Measurements
Reprinted from Proceedings of the 4 th European Conference on Space Debris, Copyright 2005, with permission from NASA. the information gathered, 599 element sets were generated, increasing the available material on catalogued objects. For the all the uncatalogued objects detected during the campaign, Figure 13 shows RCS vs. inclination. A further analysis of the size of the objects confirmed that most debris families created from breakup events shared similar profiles [28]. Outside of the canonical US SSN network, some experiments have been performed using radar sensors belonging to a different US government organization, the National Science Foundation (NSF), in particular. An old preliminary test on the radar detection of centimeter-sized orbital debris was made using the Arecibo Observatory radar. The Arecibo Radio Telescope is a 305 m dish located in Arecibo, on the island of Puerto Rico [29]. The massive antenna (for years the largest in the world), mainly employed for radioastronomical research in the ionosphere, is characterized by a huge spherical Gregorian primary reflector hanging above the parabolic dish. The radar works in UHF at 430 MHz and can transmit a pulsed signal with a peak power of 2.5 MW. In late June-early July 1989, during a debris radar experiment, the Arecibo observatory collected nearly 19 h of data and detected a total amount of 90 objects with variable size between 5 and 20 mm diameter [30]. The experiment involved the Arecibo radar as the transmitting antenna and a 3.5 m reflector auxiliary antenna (located near the main one) with a baseline of about 10 km. Transmitting and receiving antenna stared at a common point at 575 km altitude. A plot with a comparison with a prediction model of the smallest objects detected (between 0.5 and 2 cm) during the experiment is presented in Figure 14 [30]. Sadly, in late 2020, the NSF declared the Arecibo Radio Telescope closed due to some structural flaws. As if that were not enough, the antenna structure dramatically collapsed on itself on December 2020. The other notable radar for space debris form NSF is the Advanced Modular Incoherent Scatter Radar (AMISR). The AMISR system is a modular, remotely operated, UHF phased array radar built in a panel-like fashion that is deployed in various locations: one of them operates at the Poker Flat Research Range in Alaska (it is named the Poker Flat Incoherent Scatter Radar of PFISR); two systems are located in Resolute Bay in Canada; and smaller AMISR are used in Gakona (Alaska) and at the Jicamarca Radio Observatory (Peru) [31; 32]. The AMISR systems are highly modular; each panel is composed of 32 Antenna Element Units (AEUs), in the form of resonant crossed-dipoles, and is connected to Panel Control Unit (PCU) for state control and monitoring. For every 16 panels within the system, the feeding power is provided through a Utility Distribution Unit (UDU), so that the peak power scales with the number of panels used. The whole system is controlled via an Operation and Control Center (OCC) that houses data acquisition channels, transmission waveform generators and RF signal receivers. The AMISR system works in UHF in the 430-450 MHz range, each panel is characterized by a 16 kW peak power, a 5.6 m\({}^{2}\) antenna aperture, and a 20-dBi gain, and it is capable of detecting a 13 m  
Figure 3.5 Distribution of the debris RCS vs. inclination of the uncatalogued objects detected by the AN/FPS-85 during May 2000 campaign [28]. Reprinted from Acta Astronautica, Copyright 2003, with permission from Elsevier. object at 500 km range. A standard AMISR face is comprised of 128 panels and its detection capabilities are enhanced to a 3 cm object at 500 km range [31; 32]. To demonstrate the modular radar capabilities of the AMISR, the 128-panel PFISR was employed on July and August 2005 for a brief debris campaign.