## 1 Introduction to the Tracking and Imaging Radar TIRA  
One of the world's largest radar domes (radomes), visible from afar, is situated in Wachtberg near Bonn, Germany. The radome has a diameter of 47.5 m and protects the radar dish of the TIRA system from all weather influences (Figure 1). TIRA is one of the most powerful radar systems for space observation in Europe. Its three main components are the fully movable Cassegrain antenna system with a diameter of 34 m, the tracking radar in the L band and the imaging radar in the Ku band (Figure 2). The 240 t antenna structure rotates with up to 24 deg/s in azimuth. This is necessary to track any artificial space object almost seamlessly, as tilting of the antenna above 90\({}^{\circ}\) in elevation is not possible. With this ability, the pass of a resident space object (RSO) can be observed without interruption close to its zenith. Note, that due to confidentiality, not all radar parameters may be listed in this paper.  
Among the currently operating space radar systems, TIRA is recognized worldwide as a powerful weather-independent sensor for the observation of the near-Earth space environment [1; 2]. For a comprehensive review of this field, see [3] and references therein. Recent advances are highlighted in the Special Issue at hand (_Radar for Space Observation: Systems_, _Methods and Applications_, a special issue of _Remote Sensing_[[https://www.mdpi.com/journal/remotesensing/special_issues/7C43](https://www.mdpi.com/journal/remotesensing/special_issues/7C43) HJ2V2]([https://www.mdpi.com/journal/remotesensing/special_issues/7C43](https://www.mdpi.com/journal/remotesensing/special_issues/7C43) HJ2V2), accessed on 29 October 2024). The development and construction of TIRA started by the end of the 1960s. As an experimental system, TIRA was modified and upgraded continuously over the decades. Its focus changed to space observation in the 1980s, followed by enhancing capabilities, e.g., to measure the space debris population in the 1990s or to increase the bandwidth of the imaging radar in the early 2000s. In addition, individual experiments like measurements on airplanes or jamming were conducted on the side. The latest upgrades were the implementation of a new antenna drive and control system in 2012 and the replacement of the tracking radar transmitter one year later. Finally, after 46 years of operation, the radome was renewed in 2014. For this, the new radome was constructed inside the old one up to a certain height, followed by taking off the cap and replacing it with the new cap, which was pre-assembled on the ground. At the end, the lower part of the old radome was dismantled, revealing the new radome step by step. The new radome consists of statistically distributed triangles with side lengths between 2.1 m and 4.5 m which ensures that interferences are minimized. A membrane between the aluminum rods allows the radar waves to pass through nearly undisturbed.  
In its current configuration, TIRA is able to accurately determine RSO orbits while also generating detailed radar images of them [4; 5; 6; 7]. In the following paragraphs, the present state of the system and its capabilities are introduced. Later, the plans for the complete overhaul of TIRA's two radars are discussed in detail. This transformative system upgrade will be a game changer in space observation with radar.  
Figure 1: Illustration of the Tracking and Imaging Radar, TIRA. The system’s 34 m fully movable parabolic antenna and radar systems are protected by a 47.5 m radome. The surrounding 3-story building houses offices and laboratories associated with FHR’s research activities in SSA as well as technical infrastructure necessary for the operation of the TIRA system.