# Crowded Space: A Review on Radar Measurements for Space Debris Monitoring and Tracking
## 3 US SSN Radar Measurements  
A first glimpse on the US space debris surveillance activity that can be found in the literature dates back to the 90s, when the Reagan Test Site (RTS) sensors were involved in the 1991 NASA Multifrequency Space Debris Radar Tests (MSDRT). The RTS is a former missile testing facility located on Kwajalein Atoll (central western Pacific) [11] and the home of one of the most important US radar sites, the Kiernan Reentry Measurements Site (KREMS). The KREMS is one of the collateral sensors of the US SSN, meaning that its primary mission is not related to space debris monitoring, but includes the SSA program as a secondary objective [12]. The heart of the KREMS are the four powerful radar systems: the Advanced Research Projects Agency (ARPA)-Lincoln C-band Observables Radar (ALCOR), the ARPA Long-Range Tracking and Instrumentation Radar (ALTAIR), the Target Resolution and Discrimination Experiment (TRADEX), and the Millimeter-Wave (MMW) system. In 1991, during NASA MSDRT experiment, the TRADEX was employed in beam park mode or stare mode; that indicates an operational mode in which the pointing direction of the radar is fixed and the antenna is capable of detecting objects passing through its beam. TRADEX is a monostatic 25 m parabolic dish equipped with a dual-frequency feed. It is able to transmit right circular polarization in both the L-band (1.320 GHz) with an average power of 300 kW and the S-band (2.950 GHz) with an average power of 60 kW, and receive either right and left circular polarization signals [11; 13]. The azimuth and elevation maximum speeds are equal to 12.5 deg/sec allowing also a tracking mode. During the 4.4 h of 1991 campaign, the TRADEX was able to detect a total of 19 objects with estimated size of 3-4 cm [13]. The positive outcome of this campaign led to another experiment in 1994, this time involving all the KREMS sensors. In particular, TRADEX was employed in stare-and-chase mode (or mixed mode), which is a combination of beam park mode and tracking mode (where the sensor actively follows the target), ALTAIR was used in tracking mode and for a multispectral debris characterization, and all the KREMS sensors were combined in a multisensor session performed in tracking mode [11; 14]. The TRADEX was used in stare-and-chase mode for about 40 h during the 1994 campaign, with the purpose of detecting and tracking uncatalogued objects between 300 and 1200 km of altitude. Thanks to its sensitivity (it is capable of detecting an object with a radar cross section (RCS) of approximately 3 cm (\(-\)40 dBsm) at 500 km range) the TRADEX was able to detect and track 77 objects, 39 of which were uncorrelated with the SSN catalog. Figure 1 shows the summary of the radar measurements of the TRADEX during the 1994 debris campaign [14]. Most of the detections are grouped in a range between 750 and 1200 km, with a secondary cluster at 500 km. ALTAIR is a 46 m steerable parabolic antenna with dual-frequency operation capabilities, at VHF (0.162 GHz) and UHF (0.422 GHz), able to transmit right circular polarized signal with an average power of 120 kW [15]. It is more sensitive than TRADEX, being capable of detecting an object with 3 cm RCS at 1000 km range. During the 1994 debris campaign, ALTAIR was able to track at least 100 object, 51 of which were uncatalogued. A total of 120 tracks were performed on these new objects, gathering information about RCS, inclination and eccentricity [14]. A summary of the radar measurements performed by ALTAIR is reported in Figure 2.  
The distribution has its peak at around \(-\)31 dBsm RCS, which corresponds to a sphere of about 8.5 cm diameter. Finally, the multisensor activity, involving all the KREMS sensors (also optical), was organized into six different evening sessions for a joint campaign of about 21.5 h of tracking time. A total of 55 objects were tracked using two or more sensors and 5 piece of debris were jointly tracked by radar and optics [14].