# Crowded Space: A Review on Radar Measurements for Space Debris Monitoring and Tracking
## 3 US SSN Radar Measurements
On the other hand, in Figure 6b, the overlap occurs for diameters from about 4 to 70 cm. The radar measurements from the Haystack and HAX radars have been compared with those from a facility that is not canonically recognized to be part of the debris SSN: the Goldstone radar. The Goldstone radar has already been employed for several debris campaigns. The Goldstone observatory consists of several parabolic dish antennas, the most relevant of which is the Goldstone Solar System Radar (GSSR), a powerful 70 m dish transmitter working in X-band (8.56 GHz) that provides a circularly polarized signal with a peak power of about 500 kW [19]. Between October 1994 and May 1997, using the GSSR and a 35 m parabolic antenna as a receiver (one of the receivers located at Goldstone complex) in beam-park mode, about 110 h of observation and 3476 detections were collected [20, 21]. The results of the campaign, in terms of debris RCS and debris range rate vs. altitude, are summarized in Figure 7. It can be noted that there is a lack of sensitivity of the bistatic configuration below a 300 km altitude. This is due to the geometry of the intersecting beams; in fact, the overlapping of the latter starts from a 280 km altitude [21]. A similarity with a family of debris detected also by the Haystack radar was spotted near a 900 km altitude. This probably led to the following comparison between the Haystack radar, HAX, and Goldstone radar debris measurements, proposed in [22]. The three radars cover different debris size intervals with partial superposition: the Haystack radar generally detects debris in the size range of 5 mm to several meters, the HAX radar from 2 cm to several meters, and finally the Goldstone radar observes objects between 2 mm and 1 cm [22]. The debris flux over the debris diameter for all three radars during three different years tied to solar activity (1998, 2001, and 2003) is presented. Solar activity is believed to play an important role in the reentry of orbital debris resident in LEO due to its influence on atmospheric drag [22]. All the measurements were performed in beam-park mode, with Haystack and HAX radar pointing east at 75\({}^{\circ}\) elevation, and the Goldstone radar pointing near the zenith. The results of the joint campaign are reported in Figures 8-10. Figure 7: Scatter plot of the objects detected by the Goldstone radar during 1994–1997, showing altitude vs. debris RCS (**a**) and altitude vs. debris range rate (**b**) [21]. Reprinted from Advances in Space Research, Copyright 1999, with permission from Elsevier. Figure 3.0 Debris flux versus diameter at different altitudes for the Goldstone, Haystack, HAX radars during fiscal year 1998 [22]. Reprinted from Advances in Space Research, Copyright 2009, with permission from Elsevier. Figure 3.1 Debris flux versus diameter at different altitudes for the Goldstone, Haystack, HAX radars during fiscal year 2001 [22]. Reprinted from Advances in Space Research, Copyright 2009, with permission from Elsevier. There are some clarifications to be made. Since there is no data derivable from Goldstone radar from 2002 to 2004, in Figure 10 the Haystack and HAX radars' data are compared with 2005 Goldstone data. The global data is also compared with the 10 cm catalogued population provided by the US SSN. Moreover, despite the premise, the data of the fiscal year 1998 from HAX radar was collected from a point near the zenith. This explains the lack of detections by HAX radar between 400 and 800 km altitude. A lower sensitivity in detecting small objects can be noticed for all radars; however, a major impact is observable for the Goldstone radar data above 8 mm when compared to the Haystack. This is due to a signal saturation problem. It must also be taken into account that there is a reduced capability to detect objects smaller than 1 cm and 4 cm, respectively, for the Haystack and HAX radars in 2003 data, due to some malfunction in the data acquisition system (which the two facilities share).