# Active Control Loop of the BOROWIEC SLR Space Debris Tracking System
## 3 Active Control Loop  
### Passive Optical Acquisition Active Control Loop  
Due to the positional uncertainty of the TLE ephemeris (Section 2.2) used to track space objects, an appropriate algorithm was developed and implemented, combining the passive optical acquisition technique with the process of correcting the telescope motion parameters (Section 2.1) [20]. In the case of objects from the LEO region (h \(<\) 2000 km), due to the relatively small distance between the SLR station and the orbital object, it is possible to determine the position of an orbital object illuminated by the Sun (day/night terminator) using the wide-angle photographic acquisition technique (a digital camera with an additional telescope). The information obtained in the form of images can be transformedinto the real coordinates of the orbital object (\(Az^{\prime\prime}/El^{\prime\prime}\)), in relation to the ephemeris position (\(Az/El\)). The use of high-speed planetary cameras, which are characterized by a short exposure time (0.001 s-1 s) and fast data transfer (x \(\ast\) MB/s), leads to obtaining information about the position of the object in close to real-time. Using postprocessing, it is possible to extract the position of the object from the image frame and relate the vector of position changes to the current movement of the telescope (Figure 6).  
It should be mentioned that passive optical acquisition also enables the reference of the object's position to the recorded star trails (astrometry) [21]. Such information can therefore be treated as a correction of the theoretical ephemeris position, which is introduced into the control loop of the \(Az/El\) axes of the telescope mount. A more accurate method of active control correction is carried out by analyzing the images obtained from the main focus of the receiving telescope, which is characterized by a smaller field of view, resulting from the large focal length of the instrument. For this purpose, the receiving optical path, whose main task is to detect photons scattered by the orbital object from the previously emitted laser beam, shares this with a digital camera. Due to the simultaneous emission of the laser beam (532 nm--green or 1064 nm--infrared), it is impossible to acquire an image without the use of an appropriate method of optical signal filtering. For this purpose, a dichroic optical filter is used, which splits the orthogonally detected signal in two directions. The split point is chosen so as to separate the optical signal of the laser beam from the optical path of the camera. For laser sources operating at 532 nm, a split point above or below this value is applied. In this way, two signal-separated optical bands are obtained. In the case of laser sources operating at a wavelength of 1064 nm, a split point is used in the range of 900 nm-1000 nm. Then, the entire visible spectrum can be used for photographic acquisition using a digital camera. The method due to the large focal length of the telescope used (main mirror diameter over 50 cm) requires the use of digital cameras equipped with large matrices (e.g., 50 mm \(\times\) 50 mm) to increase the image detection area. On the other hand, the relatively small field of view of the camera together with the telescope determines the control loop intervals below the standard value of 100 ms.