# Active Control Loop of the BOROWIEC SLR Space Debris Tracking System
## 1 Introduction  
The issue of laser ranging measurements of artificial Earth satellites dates back to 1964. With the launch of the Beacon Explorer (Beacon-B) satellite by the National Aeronautics and Space Administration (NASA), the first laser measurement of the distance between a ground station and an orbital object was carried out [1]. Over the years with the development of laser technology, optics, control and executive systems, the accuracy of laser range measurements on the ground station--satellite path has increased many times and currently ranges from 1 cm to 2 cm. The fact of the progress made in building time and frequency references is also not without significance (hydrogen masers, cesium fountains) [2], thanks to which it is possible to accurately and precisely locate a specific observation in the time domain. The greatest advantage of the laser technique is the direct and absolute measurement of the distance between the ground station and the orbital object (satellite, rocket bodies, space debris). These measurements make a huge contribution to research on the implementation and definition of the terrestrial reference system, the International Terrestrial Reference Frame (ITRF), pole movement, the coordinates of the geocenter of the Earth's gravitational field, the coordinate and velocity changes of the position of ground SLR stations [3, 4], and determining the position of objects orbiting Earth with centimeter accuracy. However, due to the continual migration of continents, this is a complex and multi-stage process (the position of the ground station changes). The key solution to the above is the use of passive geodynamic satellites such as LAGEOS [5] (orbit altitude \(\sim 6000\) km) or ETALON [6] (orbit altitude \(\sim\) 19,000 km), whose location at the MediumEarth Orbit (MEO) ensures the stability of their orbit over time (no influence of atmospheric effects). This constancy enables the determination of the current position of a given SLR station and, consequently, means it is suitable for measurements of other orbital objects. The continuous and almost simultaneous carrying out of this process by all laser ground stations enables the determination of their position, as well as the determination of geodynamic satellites' orbits. In classical terms, laser range measurements are carried out for orbits from the Low Earth Orbit (LEO) region (altitude from 200 km to 2000 km) to Geostationary Orbit (GEO). Currently, due to the dynamic exploration of orbits (LEO to GEO) associated with the launch of new satellites for the needs of industry, science, and defense, there is a need for increased monitoring of the traffic of orbital objects and the deorbiting process of used components such as rocket bodies, satellite fragments, as well as entire satellites [7]. From the point of view of the laser measurements performed by ground stations, orbital objects can be classified as cooperative and uncooperative. Cooperative objects, mainly satellites, are equipped with special systems of cubic prisms (retroreflectors) mounted on the outer casing, whose task is the low-loss reflection of the laser beam emitted from the ground station. Uncooperative objects are therefore all other orbital objects that are not equipped with retroreflectors (satellites, rocket bodies, space debris). This fact forces the use of high-energy laser measurement pulses, which increase the probability of a photon event in the form of light scattering from the object and its subsequent detection. Laser ranging measurements of cooperative satellites in the region from 350 km to 40,000 km are currently not a challenge in terms of tracking, but ranging measurements of objects with uncertain orbits or subject to deorbiting (altitude from 100 km to 200 km) require the use of new solutions in the fields of mechanics, drives, and control algorithms, hence the need to develop new control algorithms and the multiplication of measurement techniques in order to maximize information about the object during the pass over the ground station [8; 9; 10].