# Development of the First Portuguese Radar Tracking Sensor for Space Debris
## 1 Introduction  
The use of sensors (which include telescopes, lasers and radars) has been a growing area of interest for monitoring the space environment around Earth. Due to the growing number of space debris, there is a need to predict and adjust the orbits of the more than 2000 active satellites [1], avoiding collisions with other inactive satellites or debris, to guarantee their long-term operation and investment. These actions are core activities of the Space Surveillance and Tracking (SST) concept or, in a broader sense, are framed within the Space Situational Awareness (SSA) domain, notwithstanding the different concepts acknowledged between a USA concept, strongly related to the Defense, or the European concept, strongly related to a dual-use or purely civil use compliant with the European Union (EU) or the European Space Agency (ESA) guidelines. The Earth near-space environment has been getting polluted with space debris since the onset of the space exploration era in 1957. The envisaged space launching actions to put more satellites in orbit including the announced mega constellations or broadband internetsatellites, e.g., Starlink Space X and One Web, is a growing concern which drives the need to create, maintain and improve accurate sensor networks to collectively contribute to a safe use of space. The safety and security of space assets is an important matter of national interests and central to international treaties and regulations, namely those agreed to in the framework of the United Nations, at the Committee on the Peaceful Uses of Outer Space [2]. Sensor networks comprise both ground based and space based assets, although only a few nations can afford to have space based ones. Regarding the ground based sensors, these are usually divided in three main groups: telescopes, radars, and lasers. Radar usage for SST and SSA purposes is increasing due to its capacity for monitoring the space environment during day and night, and insensitivity both to light pollution and to weather conditions. A tracking radar distinguishes itself not only by recognizing the presence of a target and determining a target location in range and in angle coordinates, but also because it is able to follow the target and observe it over some time range, thus improving the information accuracy about its trajectory [3]. The principle of tracking radar is to use the angular error signal, defined by the difference between the target direction and the reference direction given by the axis of the antenna, to adjust the antenna's pointing direction. The tracking radar attempts to position the antenna with zero angular error (i.e., to locate the target along the reference direction) [4]. The first developments of tracking radars date back from World War II [5], but their biggest developments date from the Cold-War [6]. During those times, both the USA and the Soviet Union built large networks of ground-based tracking facilities that collected data on man-made objects in Earth orbit. They consisted, primarily, of large tracking radars and optical telescopes, often dual-purposed with other military missions, such as ballistic missile warning and tracking. The main focus of space surveillance at that time was on determining the precise orbit of man-made objects around the Earth [7]. Later, they were improved to make them useful for Combat Weapon Systems, such as the Thales STIR-Tracking and Illumination radar [8]. The tracking of distant objects in the higher layers of the atmosphere, in the lower orbits around Earth or even far distant objects throughout the galaxy, continued to develop up to complex phased array systems such as the Square Kilometer Array (SKA) radio telescope [9] or the brand new US Space Fence, costing several million dollars [10], to name a few. Tracking radars are now suitable for Low-Earth Orbit (LEO) tracking or space object catalogue accuracy improvement due to electronic components developments and increased digital processing capability.