# Development of the First Portuguese Radar Tracking Sensor for Space Debris
## 1 Introduction
Referring to existing operational tracking radars for in-orbit object detection (and disregarding the ones based on phased arrays, which usually also have surveillance capability), these can be found all over the world and with different capabilities. On the US side, the development of these radars has a long history with the development of several radars, most of them for the Ballistic Missile Defense program, supported by DARPA (Defense Advanced Research Projects Agency), which were closely related to the detection of objects in orbit [6]. Notwithstanding the huge variety of radars that were developed and used (some of them have been decommissioned), two of the most interesting and relevant radars used for Space Debris detection, tracking, and characterization are the Haystack and the HAX (Haystack Auxiliary Radar) radars. The HAX radar has a 12.2 m antenna, operating at 16.7 GHz and is much smaller than Haystack which has a 36.6 m antenna operating at 10 GHz [11], both in a Cassegrain configuration. HAX had twice the bandwidth of Haystack, being capable of producing correspondingly sharper images of near-earth orbit objects. The cost of building HAX, which became operational in 1994, was reduced by using a refurbished antenna and by sharing signal processing and data processing systems with the Haystack Long Range Imaging Radar (LRIR) (means that the two radars could not operate simultaneously). The Haystack radar has been operational since 1964, but, from 2010 to 2014, it underwent several upgrades becoming the HaystackUltra-Wideband Satellite Imaging Radar (HUSIR), able to also operate in the W-band with a bandwidth up to 8 GHz, which allowed the HAX to detect objects down to 3 cm and the HUSIR down to 3 mm at an altitude up to 1000 km [12]. The Space Debris networks around the world includes facilities from other major players, such as China [13] and Russia and other spacefaring nations such as India [14, 15], Iran, Brazil [16, 17], Japan, or North Korea. However, most of those nations projects' and infrastructures are linked to the military and involved in deep secrecy. It is highly probable that radio-telescopes and other related ground-based sensors support missions including intelligence collection, counterspace targeting, ballistic missile early warning, spaceflight safety, satellite anomaly resolution, and also space debris tracking [18]. In the European continent, the development of tracking radars for orbit objects' detection has evolved for some time and nowadays there are around 50 different radio telescopes. Just to mention some that have played a relevant role in space debris and satellite detection, it is worth mentioning the German Tracking and Imaging Radar (TIRA) system, with a 34 m antenna, located near Bonn. In a monostatic configuration, it is able to detect objects of diameters down to 2 cm at 1000 km, but in a bistatic configuration, in synergy with the Effelsberg Radio Telescope of the Max Planck Institute for Radio Astronomy, which has a 100 m dish antenna, the system can detect objects down to 1 cm diameter [19, 20]. The TIRA radar has been one of the most useful radars in Europe in the SST context and its accuracy and imaging capabilities, along its continued upgrades, made it a valuable asset. The Sardinia Radio Telescope (SRT) is an Italian radio telescope facility with recent radar capabilities with a 64 m antenna located in the Sardinia island, operating only since 2012. Compared to equivalently sized radars, it has a fairly good speed in azimuth and elevation tracking (0.85\({}^{\circ}\)/s and 0.5\({}^{\circ}\)/s, respectively) and is able to operate from 300 MHz up to 15 GHz. It is intended to work in a bistatic configuration, using the Flight Terminator System (FTS), about 40 km apart from the first, whose configuration is named BIRALET (BIstatic Radar for LEo Tracking) [21]. BIRALET has been used in the context of the European SST for LEO detection, but its capabilities, namely the SRT antenna, envisage the possibility to go beyond this orbit.