# Crowded Space: A Review on Radar Measurements for Space Debris Monitoring and Tracking
## 3 US SSN Radar Measurements
Further important joint campaigns involving the US SSN sensors were organized by the Inter-Agency Space Debris Coordination Committee (IADC), an international association that includes the most important space agencies of the world with the common goal of facing the debris problem [23]. Some of them involved the AN/FPS-108, also known as Cobra Dane. Cobra Dane is a phased array radar, part of the US SSN as a collateral sensor [24], located on Shemya Island (Alaska) [25]. The radar operates in the L-band (1.215-1.400 GHz). It is capable of transmitting a peak power signal of 15.4 MW (0.92 MW average) through its 15,360 active elements. These features allow the detection of a number of objects ranging from 100 to 1000 in 24 h [26]. Cobra Dane was employed in the US 2000 Beam Park Experiment (BPE), together with the TRADEX and a Ground Base Radar-Prototype (GBR-P) at KREMS, and the Haystack and HAX radars at Lincoln Space Surveillance Complex, for 24 h in stare-and-chase mode, and it was able to detect, among other objects, two specific families of debris [26]. The results of the 2000 BPE Cobra Dane campaign are reported in Figure 11 in terms of the altitude and inclination of the debris as a function of the duration of the experiment. In these images, the debris clusters belonging to the families 65-027A and 99-057C are clearly visible, originating, respectively, from the 1965-027A rocket \"Snapshot\", and from the third stage of a rocket that launched the satellite CZ-4 R/B. Another 24 h campaign, also hosted by the IADC, was performed  
Figure 3.2 Debris flux versus diameter at different altitudes for the Goldstone, Haystack, HAX radars during fiscal year 2003 [22]. Reprinted from Advances in Space Research, Copyright 2009, with permission from Elsevier. in September 2004 [25]. This time Cobra Dane was able to detect 3400 objects (some with multiple detections). The results of the 2004 campaign are reported in Figure 12, which shows the range vs. diameter of the debris. In this figure, a dense cluster of debris is noticeable around 1800 km range, which includes fairly large objects (1 m diameter). All the above facilities can be considered as collateral sensors. On the other hand, there is not much available material in the open literature regarding the debris radar measurements performed by the dedicated sensors of the US SSN. One of the few works that provides clear and readable results is focused on the AN/FPS-85 multifunction bistatic phased array radar, located at Eglin Air Force Base in northwest Florida [27,28]. The transmit array of the AN/FPS-85 works in the UHF-band at an operative frequency of 442 MHz with 10 MHz bandwidth. It is composed of 5184 identical transmitter modules lined up in a 72 \(\times\) 72 grid. Its main characteristics include a peak power of 30 MW and average power of 10 kW, a 1.4\({}^{\circ}\) beamwidth at boresight, and a variable pulse length (1, 5, 10, 25, 125, and 250 \(\upmu\)s) in various burst waveforms. The transmitting element is a shortened dipole antenna with parasitic element director. Its receiving counterpart consists of 19,500 crossed dipoles arranged in a square grid, connected to 4660 active, identical receiver modules (the inactive elements that act as dummies are terminated). The rest of the receiving chain includes the calibration beam-steering and Local Oscillator (LO), the distribution assemblies, the calibration equipment, and the beam-forming network [27]. On May 2000, taking advantage of the ground testing of a new software that allowed higher elevation angles, the AN/FPS-85 was employed for a two-day campaign (11 and 12 May). During this time, the phased array radar was able to detect more than 80,000 objects. With  
Figure 3.3 Distribution of the altitude (**a**) and inclination (**b**) of the objects detected by Cobra Dane during 2000 BPE [26]. Reprinted from European Space Agency Publications, Copyright 2001, with permission from NASA. Figure 3.4 Results from Cobra Dane collected during the IADC 24 h campaign conducted in September 2004 [25].